Document And Entity Information - USD ($)	12 Months Ended
Document And Entity Information - USD ($)	Dec. 31, 2018	Mar. 13, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Entity Registrant Name	SI-BONE, INC.
Entity Central Index Key	0001459839
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	false
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		24,458,938
Entity Public Float			$ 0

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
CURRENT ASSETS
Cash and cash equivalents	$ 25,120	$ 22,408
Short-term investments	97,103	0
Accounts receivable, net of allowance for doubtful accounts of $263 and $268 at December 31, 2018 and 2017, respectively	8,486	7,416
Inventory	3,343	2,553
Prepaid expenses and other current assets	1,990	1,252
Total current assets	136,042	33,629
Property and equipment, net	2,154	1,896
Other non-current assets	325	309
TOTAL ASSETS	138,521	35,834
CURRENT LIABILITIES
Accounts payable	2,146	1,814
Accrued liabilities and other	6,860	5,724
Total current liabilities	9,006	7,538
Redeemable convertible preferred stock warrants	0	422
Long-term borrowings	38,963	38,704
Other long-term liabilities	360	0
TOTAL LIABILITIES	48,329	46,664
Commitments and contingencies
Redeemable convertible preferred stock, $0.0001 par value;
Redeemable convertible preferred stock; Authorized: 0 and 12,104,749 shares at December 30, 2018 and December 31, 2017; issued and outstanding: 0 and 11,871,578 shares at December 30, 2018 and December 31, 2017; (Liquidation preference of $0 and $119,194 at December 30, 2018 and December 31, 2017).	0	118,548
STOCKHOLDERS’ EQUITY (DEFICIT)
Preferred stock, $0.0001 par value; Authorized: 5,000,000 and 0 shares at December 31, 2018 and 2017, respectively; no shares issued and outstanding as of December 31, 2018 and 2017.	0	0
Common stock, $0.0001 par value; Authorized: 100,000,000 and 19,333,333 shares at December 31, 2018 and 2017, respectively; issued and outstanding: 24,450,757 and 3,603,140 shares, at December 31, 2018 and 2017, respectively.	3	1
Additional paid-in capital	246,927	9,943
Accumulated other comprehensive income	439	402
Accumulated deficit	(157,177)	(139,724)
TOTAL STOCKHOLDERS’ EQUITY (DEFICIT)	90,192	(129,378)
TOTAL LIABILITIES, REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT)	$ 138,521	$ 35,834

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Allowance for doubtful accounts	$ 263	$ 268
Redeemable convertible preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Redeemable convertible preferred stock authorized (in shares)	0	12,104,749
Redeemable convertible preferred stock issued (in shares)	0	11,871,578
Redeemable convertible preferred stock outstanding (in shares)	0	11,871,578
Redeemable convertible preferred stock, liquidation preference	$ 0	$ 119,194
Preferred stock, par value (in dollars per share)	$ 0.0001
Preferred stock authorized (in shares)	5,000,000
Preferred stock issued (in shares)	0
Preferred stock outstanding (in shares)	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock authorized (in shares)	100,000,000	19,333,333
Common stock issued (in shares)	24,450,757	3,603,140
Common stock outstanding (in shares)	24,450,757	3,603,140

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Statement [Abstract]
Revenue	$ 55,380	$ 47,983
Cost of goods sold	4,833	5,112
Gross profit	50,547	42,871
Operating expenses:
Sales and marketing	44,497	41,646
Research and development	5,376	5,513
General and administrative	12,639	13,062
Total operating expenses	62,512	60,221
Loss from operations	(11,965)	(17,350)
Interest and other income (expense), net:
Interest income	769	175
Interest expense	(5,108)	(6,204)
Other income (expense), net	(1,149)	340
Net loss	(17,453)	(23,039)
Other comprehensive income:
Unrealized gain of marketable securities	10	0
Changes in foreign currency translation	27	(70)
Comprehensive loss	$ (17,416)	$ (23,109)
Net loss per share, basic and diluted (in dollars per share)	$ (2.20)	$ (6.65)
Weighted-average number of common shares used to compute basic and diluted net loss per share (in shares)	7,950,284	3,467,096

CONSOLIDATED STATEMENTS OF CHANGES IN REDEEMABLE CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Stockholders’ Notes Receivable	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Temporary equity, beginning balance (shares) at Dec. 31, 2016	11,330,704
Temporary equity, beginning balance at Dec. 31, 2016	$ 113,121
Increase (Decrease) in Temporary Equity
Issuance of redeemable convertible preferred stock, net of issuance costs (shares)	540,874
Issuance of redeemable convertible preferred stock, net of issuance costs	$ 5,427
Temporary equity, ending balance (shares) at Dec. 31, 2017	11,871,578
Temporary equity, ending balance at Dec. 31, 2017	$ 118,548
Stockholders' equity (deficit), beginning balance (shares) at Dec. 31, 2016		3,446,137
Stockholders' equity (deficit), beginning balance at Dec. 31, 2016	(108,733)	$ 1	$ 8,000	$ (521)	$ 472	$ (116,685)
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of stock options (shares)		152,691
Issuance of common stock upon exercise of stock options	383		383
Issuance of common stock upon exercise of unvested stock options (shares)		4,312
Issuance of common stock upon exercise of unvested stock options	0
Stock-based compensation	1,438		1,438
Repayment of stockholders’ notes receivable	84			84
Forgiveness of stockholders’ note receivable	437			437
Vesting of early exercised stock options	122		122
Foreign currency translation	(70)				(70)
Unrealized gain of marketable securities	0
Net loss	(23,039)					(23,039)
Stockholders' equity (deficit), ending balance (shares) at Dec. 31, 2017		3,603,140
Stockholders' equity (deficit), ending balance at Dec. 31, 2017	$ (129,378)	$ 1	9,943	0	402	(139,724)
Increase (Decrease) in Temporary Equity
Conversion from preferred stock to common stock (shares)	(11,871,578)
Conversion from preferred stock to common stock	$ (118,548)
Temporary equity, ending balance (shares) at Dec. 31, 2018	0
Temporary equity, ending balance at Dec. 31, 2018	$ 0
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of stock options (shares)	395,117	289,077
Issuance of common stock upon exercise of stock options	$ 1,136		1,136
Issuance of common stock upon exercise of unvested stock options (shares)		106,028
Issuance of common stock upon exercise of unvested stock options	0
Conversion from preferred stock to common stock (shares)		12,066,654
Conversion from preferred stock to common stock	118,548	$ 1	118,547
Conversion from preferred stock warrants to common stock warrants	1,248		1,248
Issuance of common stock from warrants exercise (shares)		121,486
Issuance of common stock from warrants exercise	0
Issuance of common stock from IPO Proceeds, net (shares)		8,280,000
Issuance of common stock from IPO Proceeds, net	113,603	$ 1	113,602
Repurchase of unvested early exercised stock options (shares)		(15,628)
Repurchase of unvested early exercised stock options	0
Stock-based compensation	2,312		2,312
Vesting of early exercised stock options	139		139
Foreign currency translation	27				27
Unrealized gain of marketable securities	10				10
Net loss	(17,453)					(17,453)
Stockholders' equity (deficit), ending balance (shares) at Dec. 31, 2018		24,450,757
Stockholders' equity (deficit), ending balance at Dec. 31, 2018	$ 90,192	$ 3	$ 246,927	$ 0	$ 439	$ (157,177)

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities
Net loss	$ (17,453)	$ (23,039)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization	722	1,013
Change in allowance for doubtful accounts	(5)	(36)
Stock-based compensation	2,312	1,438
Change in fair value of redeemable convertible preferred stock warrants	826	(166)
Loss on write-off of property and equipment	52	214
Write-off of debt discount	0	650
Amortization of debt discount	259	285
Write-off of public offering costs	0	1,292
Forgiveness of notes receivable	0	437
Short-term investments accretion	(209)	0
Changes in operating assets and liabilities
Accounts receivable	(1,023)	(1,313)
Inventory	(759)	(980)
Prepaid expenses and other assets	(752)	72
Accounts payable	251	811
Accrued liabilities and other	1,260	1,792
Net cash used in operating activities	(14,519)	(17,530)
Cash flows from investing activities
Purchase of property and equipment	(942)	(478)
Purchase of short-term investments	(96,883)	0
Net cash used in investing activities	(97,825)	(478)
Cash flows from financing activities
Proceeds from initial public offering, net of underwriting discounts and commissions	115,506	0
Proceeds from the exercise of common stock options, net	1,614	383
Repurchase of common stock	(73)	0
Repayment of stockholders’ notes receivable	0	84
Repayment of debt financing	0	(1,119)
Extinguishment of debt financing	0	(29,081)
Proceeds from debt financing	0	40,000
Payment of debt issuance costs	0	(1,540)
Proceeds from the issuance of redeemable convertible preferred stock, net	0	5,427
Payments of public offering costs	(1,897)	(1,292)
Net cash provided by financing activities	115,150	12,862
Effect of exchange rate changes on cash and cash equivalents	(94)	(346)
Net decrease in cash and cash equivalents	2,712	(5,492)
Cash and cash equivalents at
Beginning of period	22,408	27,900
End of period	25,120	22,408
Supplemental disclosure of cash flow information
Cash paid for interest	5,500	4,514
Supplemental disclosure of non-cash information
Conversion of redeemable convertible preferred stock to common stock	118,547	0
Conversion of preferred stock warrants to common stock warrants	1,248	0
Vesting of early exercised stock options	139	122
Purchases of property and equipment included in accounts payable and accrued liabilities	82	97
Public offering costs included in accounts payable and accrued liabilities	$ 7	$ 0

The Company and Nature of Business	12 Months Ended
The Company and Nature of Business	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
The Company and Nature of Business	The Company and Nature of Business SI-BONE, Inc. (the “Company”) was incorporated in the state of Delaware on March 18, 2008 and is headquartered in Santa Clara, California. The Company is a medical device company that has pioneered a proprietary minimally invasive surgical implant system to fuse the sacroiliac joint for treatment of the most common types of sacroiliac joint disorders that cause lower back pain. The Company introduced its iFuse Implant System, or iFuse, in 2009 in the United States, in 2010 in certain countries in the European Union, and in 2015 in certain countries in the rest of the world. Reverse Stock Split In October 2018, the Company's board of directors and stockholders approved a 1-for-18 reverse stock split of the Company's common stock and redeemable convertible preferred stock, which was effected on October 4, 2018. The par value of the common stock and redeemable convertible preferred stock was not adjusted as a result of the reverse split. All issued and outstanding share and per share amounts of common stock, redeemable convertible preferred stock, stock options, and warrants included in the accompanying consolidated financial statements have been adjusted to reflect this reverse stock split for all periods presented. Initial Public Offering On October 16, 2018, the Company’s Registration Statement on Form S-1 (File No. 333-227445) relating to the initial public offering (IPO) of its common stock was declared effective by the Securities and Exchange Commission (SEC). Pursuant to such Registration Statement, the Company sold 8,280,000 shares at an initial public offering price of $15.00 per share for net proceeds of $113.6 million to the Company, net of underwriting discounts and commissions and offering costs. In connection with the IPO, the Company's outstanding shares of redeemable convertible preferred stock were automatically converted into an aggregate of 12,066,654 shares of common stock, and the Company's outstanding warrants to purchase 156,550 shares of redeemable convertible preferred stock were automatically converted into warrants to purchase an aggregate of 160,657 shares of common stock, resulting in reclassification of the related redeemable convertible preferred stock warrant liability of $1.2 million in additional paid-in-capital.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

Basis of Presentation and Principles of Consolidation

The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”). The consolidated financial statements include the Company's accounts, as well as those of the Company's wholly-owned international subsidiaries. All inter-company accounts and transactions have been eliminated.

Use of Estimates

The preparation of financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenue and expenses during the reporting period. Significant accounting estimates and management judgments reflected in the consolidated financial statements include: fair value of assets and liabilities; analysis of the allowance for doubtful accounts; inventory valuation; valuation of deferred tax assets, including related valuation allowances; fair value of common stock and redeemable convertible preferred stock warrants; stock-based compensation; and useful lives of long-lived assets. Estimates are based on historical experience, where applicable and other assumptions believed to be reasonable by the management. Actual results could differ from those estimates.

JOBS Act Accounting Election

As an emerging growth company under the Jumpstart Our Business Startups Act of 2012, or the JOBS Act, the Company is eligible to take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies. The Company has elected to take advantage of the extended transition period for adopting new or revised accounting standards that have different effective dates for public and private companies until such time as those standards apply to private companies.

Segments

The chief operating decision makers for the Company are the Chief Executive Officer and Chief Financial Officer. The Chief Executive Officer and the Chief Financial Officer review financial information presented on a consolidated basis, accompanied by information about revenue by geographic region, for purposes of evaluating financial performance. The Company has one business activity and there are no segment managers who are held accountable for operations, operating results or plans for levels or components below the consolidated unit level. Accordingly, the Company has determined that it has a single reportable and operating segment structure.

The Company derives substantially all of its revenue from sales to customers in the United States. Revenue by geography is based on billing address of the customer. No single country outside the United States accounts for more than 10% of the total revenue during the periods presented. Long-lived assets held outside the United States are immaterial. Following table summarizes the Company's revenue by geography (in thousands):



	Year ended December 31,
	2018		2017
Domestic	$	50,137	$	43,351
International	5,243		4,632
	$	55,380	$	47,983

Foreign Currency

The Company’s foreign subsidiaries use local currency as their functional currency. Assets and liabilities are translated at exchange rates prevailing at the balance sheet dates. Revenue, costs and expenses are translated into U.S. dollars using average exchange rates for the period. Gains and losses resulting from the translation of the Company’s consolidated balance sheets are recorded as a component of accumulated other comprehensive income. Gains and losses from foreign currency transactions are recognized as a component of other income (expense), net.

Concentration of Credit Risk

Financial instruments that potentially subject the Company to a concentration of credit risk consist of cash and cash equivalents. The Company’s cash and cash equivalents are deposited with financial institutions in the United States and in Europe; the majority of the Company’s cash and cash equivalents are deposited with a single financial institution in the United States. Deposits in this institution exceed the amount of insurance provided on such deposits. The Company has not experienced any losses on its deposits of cash and cash equivalents.

The Company’s revenue and accounts receivable are spread across a large number of customers, primarily in the United States, and no one customer accounts for more than 10% of total revenue or gross accounts receivable in any period presented.

Other Risks and Uncertainties

The Company is subject to risks common to medical device companies including, but not limited to, new technological innovations, dependence on key personnel, protection of proprietary technology, compliance with government regulations, product liability, the ability to obtain adequate coverage and reimbursement from third-party payors, uncertainty of market acceptance of products, and the need to obtain additional financing.

The Company is dependent on third-party manufacturers and suppliers, in some cases single source suppliers. The Company currently has limited long term contracts with its key suppliers and is subject to risks such as manufacturing failures, non-compliance with regulatory requirements, price fluctuations, inability to properly meet demand and third-party supplier discontinuation of operations.

Liquidity

As of and for the year ended December 31, 2018, the Company had an accumulated deficit of $157.2 million and used $14.5 million of cash in operations. The Company has not achieved positive cash flow from operations to date. As of December 31, 2018, we had cash and cash equivalents of $25.1 million and short-term investments of $97.1 million. The Company held cash and cash equivalents of $22.4 million as of December 31, 2017.

The Company’s primary cash needs are for the ongoing commercialization of its iFuse products. The Company also has certain debt covenants associated with its current debt agreement. These covenants include a $5.0 million minimum cash balance and revenue targets, which if not met would result in the debt becoming immediately due. The revenue target is assessed quarterly based on the rolling twelve months of revenue, and increases by approximately 2%-4% each quarter. Beginning with the three months ended March 31, 2019, the Company is required to meet either revenue or earnings targets. The Company has met the minimum liquidity and revenue targets as of December 31, 2018; however there can be no assurances that the Company will continue to meet these targets in the future.

Based upon the Company's current operating plan, the Company believes that its existing cash, cash equivalents and short-term investments will enable the Company to fund its operating expenses and capital expenditure requirements through at least the next 12 months. The Company continues to face challenges and uncertainties and, as a result, its available capital resources may be consumed more rapidly than currently expected due to: (a) decreases in sales of our products and the uncertainty of future revenues from new products; (b) changes that the Company may make to the business that affect ongoing operating expenses; (c) changes that the Company may make in its business strategy; (d) regulatory developments affecting the Company's existing products; (e) changes that the Company may make in its research and development spending plans; and (f) other items affecting the Company's forecasted level of expenditures and use of cash resources.

Fair Value of Financial Instruments

Carrying amounts of certain of the Company’s financial instruments, including cash equivalents, short-term investments, accounts receivable, accounts payable, and accrued liabilities, approximate fair value due to their relatively short maturities and market interest rates, if applicable. The carrying value of the Company’s long-term debt also approximates fair value based on management’s estimation that a current interest rate would not differ materially from the stated rate. The carrying amount of the redeemable convertible preferred stock warrants has been marked to fair value such that the carrying amount represents its estimated fair value.

The Company discloses and recognizes the fair value of its assets and liabilities using a hierarchy that prioritizes the inputs to valuation techniques used to measure fair value. Fair value is an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, a three-tier value hierarchy has been established, which prioritizes the inputs used in measuring fair value as follows:

Level 1: Quoted prices (unadjusted) in active market that are accessible at measurement date for assets or liabilities. The fair value hierarchy gives the highest priority to Level 1 inputs.

Level 2: Observable prices that are based on inputs not quoted on active markets, but corroborated by market data.

Level 3: Unobservable inputs are used when little or no market data is available. The fair value hierarchy gives the lowest priority to Level 3 inputs.

Assets and liabilities measured at fair value are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The Company’s assessment of the significance of a particular input to the fair value measurement in its entirety requires management to make judgments and considers factors specific to the asset or liability.

The Company’s cash equivalents consist of money market funds as of December 31, 2018 and December 31, 2017. The money market funds are classified as Level 1 of the fair value hierarchy. The Company's marketable securities are classified as Level 1 or Level 2 of the fair value hierarchy. The Company’s redeemable convertible preferred stock warrants are classified within Level 3 of the fair value hierarchy. The redeemable convertible preferred stock warrants have been valued using a Black-Scholes valuation model and are subsequently marked to fair value at each reporting period.

Cash and Cash Equivalents

The Company considers all highly liquid investments with remaining maturities at the date of purchase of three months or less to be cash equivalents.

Marketable Securities

The Company's marketable securities primarily consist of investments in U.S. treasury securities, corporate bonds, and commercial paper. We classify our marketable securities as available-for-sale at the time of purchase and reevaluate such classification as of each balance sheet date. Based upon maturity dates, all marketable securities as available for use in current operations, and therefore we classify these securities as current assets in the accompanying consolidated balance sheets. All marketable securities are recorded at their estimated fair value. Unrealized gains and losses on available-for-sale securities are recorded in Accumulated other comprehensive income (loss) ("OCI"). We evaluate our investments to assess whether those in unrealized loss positions are other-than-temporarily impaired. The Company considers impairments to be other-than-temporary if they are related to deterioration in credit risk or if it is likely we will sell the securities before the recovery of their cost basis. Realized gains and losses and declines in value judged to be other-than-temporary are determined based on the specific identification method and are reported in Other income (expense), net on the consolidated statements of operations.

If quoted prices for identical instruments are available in an active market, marketable securities are classified within Level 1 of the fair value hierarchy. If quoted prices for identical instruments in active markets are not available, fair values are estimated using quoted prices of similar instruments and are classified within Level 2 of the fair value hierarchy. To date, all of the Company's marketable securities can be valued using one of these two methodologies.

Accounts Receivable and Allowance for Doubtful Accounts

Accounts receivable are recorded at the invoiced amount and do not bear interest. The Company generally does not require collateral or other security in support of accounts receivable. Allowances are provided for individual accounts receivable when the Company becomes aware of a customer’s inability to meet its financial obligations, such as in the case of bankruptcy, deterioration in the customer’s operating results or change in financial position. If circumstances related to customers change, estimates of the recoverability of receivables would be further adjusted. The Company also considers broad factors in evaluating the sufficiency of its allowance for doubtful accounts, including the length of time receivables are past due, significant one-time events, creditworthiness of customers and historical experience. Account balances are charged off against the allowance after all means of collection have been exhausted and the potential for recovery is considered remote.

Inventory

Inventory is stated at lower of cost or net realizable value. The Company establishes the inventory basis by determining the cost based on standard costs approximating the purchase costs on a first-in, first-out basis. The excess and obsolete inventory is estimated based on future demand and market conditions. Inventory write-downs are charged to cost of goods sold. As of December 31, 2018 and December 31, 2017, inventory consisted entirely of finished goods.

Property and Equipment

Property and equipment are stated at cost less accumulated depreciation and amortization. All property and equipment is depreciated on a straight-line basis over the estimated useful lives of the assets, which are as follows:



Computer and office equipment	3 – 5 years
Machinery and equipment	3 – 5 years
Furniture and fixtures	7 years

Leasehold improvements are amortized over the lesser of their useful lives or the life of the lease. Upon sale or retirement of the assets, the cost and related accumulated depreciation are removed from the consolidated balance sheet and the resulting gain or loss is recognized in the consolidated statement of operations. Maintenance and repairs are charged to operations as incurred.

Impairment of Long-Lived Assets

The Company reviews long-lived assets for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset might not be recoverable. When such an event occurs, management determines whether there has been an impairment by comparing the anticipated undiscounted future net cash flows to the related asset’s carrying value. If an asset is considered impaired, the asset is written down to fair value, which is determined based either on discounted cash flows or appraised value, depending on the nature of the asset. Through December 31, 2018 and December 31, 2017, the Company has not experienced impairment losses on its long-lived assets.

Public Offering Costs

Specific incremental costs (i.e. consisting of legal, accounting and other fees and costs) directly attributable to a proposed or actual offering of securities may properly be deferred and charged against the gross proceeds of the offering. In the event a planned IPO does not occur or is significantly delayed, all of the costs will be expensed. There were no capitalized offering costs as of December 31, 2018 and 2017 included in other non-current assets on the consolidated balance sheets. Costs incurred and capitalized during the year ended December 31, 2018 amounting to $1.9 million were transferred to additional paid-in capital upon completion of the IPO on October 16, 2018. Offering costs of $1.3 million were also incurred and expensed in the December 31, 2017, as a result of delays in the IPO process during the period.

Common Stock Warrants

The Company accounts for warrants for shares of common stock as equity in accordance with the accounting guidance for derivatives. The accounting guidance provides a scope exception from classifying and measuring as a financial liability a contract that would otherwise meet the definition of a derivative if the contract is both (i) indexed to the entity’s own stock and (ii) classified in the stockholders’ deficit section of the consolidated balance sheet. The Company determined that the warrants for shares of common stock issued in connection with its prior debt arrangements are required to be classified in equity. Warrants classified as equity are recorded as additional paid-in capital on the consolidated balance sheet and no further adjustments to their valuation are made.

Redeemable Convertible Preferred Stock Warrants

Warrants and other similar instruments related to shares that are contingently redeemable are classified as liabilities on the consolidated balance sheet at their estimated fair value because the shares underlying the warrants may obligate the Company to transfer assets to the holders at a future date under certain circumstances such as a deemed liquidation event. The warrants are exercisable into the Company’s redeemable convertible preferred stock and are classified as liabilities on the consolidated balance sheet. The warrants, measured at fair value, are subject to re-measurement at each balance sheet date and the change in fair value, if any, is recognized as other income (expense), net. The Company adjusted the liability for changes in fair value until the the closing of our initial public offering, at which time certain preferred stock warrants were converted into warrants to purchase common stock and the liability was reclassified to additional paid-in capital.

The Company estimated the fair value of these liabilities using option pricing models and assumptions that were based on the individual characteristics of the warrants on the valuation date, as well as assumptions for future financings, expected volatility, expected life, yield, and risk-free interest rate.

Revenue Recognition

The Company’s revenue is derived from the sale of its products to medical groups and hospitals through its direct sales force and distributors throughout the United States and Europe.

In accordance with ASC Topic 605, Revenue Recognition (“ASC 605”), the Company recognizes revenue when persuasive evidence of an arrangement exists, title and risk of loss has transferred to the customer, the sales price is fixed or determinable, and collectability is reasonably assured. For the majority of product sales where the Company’s sales representative delivers the product at the point of implantation at hospitals or other medical facilities, the Company recognizes revenue related to product sales upon completion of the procedure and authorization by the customer. Revenue is recognized upon receipt of a purchase agreement or agreement on pricing terms with the customer and when all other revenue recognition criteria are met. For the remaining sales, which include distributor and hospital sales where the product is ordered in advance of a procedure and a valid purchase order has been received, the Company recognizes revenue based upon shipping or delivery terms, which represents the point in time when the customer has taken ownership and assumed risk of loss and the required revenue recognition criteria are met. Such customers are obligated to pay within specified terms regardless of when or if they ever sell or use the products, and the Company has no post-delivery obligations.

Warranty Program

In January 2017, the Company implemented a warranty program which provides a purchaser a one-time replacement of any iFuse implant at no additional cost for a revision procedure within a one-year period following the original procedure and is accounted for as a warranty accrual. The Company also provides a purchaser with a one-time credit equal to the purchase price paid for use on future purchases for any revision procedure within the one-year period following an original procedure where an implant is not required. These one-time credits are accounted for as sales reserves. Sales and warranty reserves from the warranty program were immaterial as of both December 31, 2018 and 2017.

Medical Device Excise Tax

In accordance with the Patient Protection and Affordable Care Act, effective January 1, 2013, the Company began to incur an excise tax on sales of medical devices in the United States. The medical device excise tax is included in cost of goods sold in the consolidated statements of operations and comprehensive loss for all the periods presented. Effective December 2015, the Act was amended to include a provision to suspend the tax on medical devices through 2017. In January 2018, the suspension on the tax on medical devices was further extended through 2019.

Shipping and Handling Costs

Shipping and handling costs are expensed as incurred and are included in cost of goods sold.

Research and Development

Research and development costs are charged to operations as incurred and consist of costs incurred by the Company for the development of the Company’s product which include (1) employee-related expenses, including salaries, benefits, travel and non-cash stock-based compensation expense (2) external research and development expenses (3) other expenses, which include direct and allocated expenses for facilities and other costs.

Advertising Expenditures

The cost of advertising is included in Sales and Marketing expense in the consolidated financial statements and expensed as incurred. Advertising costs totaled $0.7 million and $0.8 million for the year ended December 31, 2018 and 2017, respectively.

Stock-Based Compensation

The Company measures its stock-based awards made to employees based on the estimated fair values of the awards as of the grant date using the Black-Scholes option-pricing model. The model requires management to make a number of assumptions including expected volatility, expected life, risk-free interest rate and expected dividends. Stock-based compensation expense is recognized over the requisite service period using the straight-line method and is based on the value of the portion of stock-based payment awards that is ultimately expected to vest. As such, the Company’s stock-based compensation is reduced for the estimated forfeitures at the date of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates.

Equity instruments issued to nonemployees are recorded at their fair value on the measurement date and are subject to periodic adjustments as the underlying equity instruments vest. The Company believes that the estimated fair value of the stock options is more readily measurable than the fair value of the services received. Stock-based compensation related to stock options granted to nonemployees is recognized as the stock options are earned.

In the event the underlying terms of stock options are modified on which stock-based compensation was granted, additional expense is recognized for any modification that increases the total fair value of the share-based payment arrangement at the modification date.

Income Taxes

The Company accounts for income taxes under the asset and liability method, whereby deferred tax assets and liabilities are determined based on the difference between the financial statement and tax bases of assets and liabilities using the enacted tax rates in effect for the year in which the differences are expected to affect taxable income. A valuation allowance is established when necessary to reduce deferred tax assets to the amounts expected to be realized.

The Company adopted the accounting guidance for uncertainties in income taxes, which prescribes a recognition threshold and measurement process for recording uncertain tax positions taken, or expected to be taken, in a tax return in the financial statements. The guidance also prescribes treatment for derecognition, classification, accounting in interim periods and disclosure requirements for uncertain tax positions. The Company accrues for the estimated amount of taxes for uncertain tax positions if it is more likely than not that the Company would be required to pay such additional taxes. An uncertain tax position will not be recognized if it has a less than 50% likelihood of being sustained.

Net Loss per Share of Common Stock

The Company calculates basic and diluted net loss per common share attributable to shareholders in conformity with the two-class method required for companies with participating securities. The Company considers all series of redeemable convertible preferred stock and early exercised stock options to be participating securities as the holders are entitled to receive dividends on a pari passu basis in the event that a dividend is paid on common stock. Under the two-class method, the net loss attributable to common stock is not allocated to the redeemable convertible preferred stock and early exercised stock options as the holders of redeemable convertible preferred stock and early exercised stock options do not have a contractual obligation to share in losses.

Basic net loss per common share is calculated by dividing the net loss by the weighted-average number of common shares outstanding during the period, without consideration for potentially dilutive securities. Diluted net loss per share is computed by dividing the net loss by the weighted-average number of common shares and potentially dilutive securities outstanding for the period. For purposes of the diluted net loss per share calculation, redeemable convertible preferred stock and common stock options and warrants are considered to be potentially dilutive securities. Because the Company has reported a net loss in all periods presented, redeemable convertible preferred stock and common stock options and warrants are anti-dilutive and therefore diluted net loss per common share is the same as basic net loss per common share for those periods.

Comprehensive Loss

Comprehensive loss represents all changes in the stockholders’ equity (deficit) except those resulting from distributions to stockholders. The Company’s unrealized foreign currency translation income (losses) and unrealized gain (losses) of marketable securities represent the two components of other comprehensive income that are excluded from the reported net loss for each of the reporting periods and has been presented in the consolidated statements of operations and comprehensive loss.

Recent Accounting Pronouncements

Recently Adopted Accounting Standards

In July 2015, the FASB issued ASU 2015-11, Inventory (Topic 330): Simplifying the Measurement of Inventory. ASU 2015-11 simplifies the guidance on the subsequent measurement of inventory, excluding inventory measured using last-in, first out or the retail inventory method. Under the new standard, in scope inventory should be measured at the lower of cost and net realizable value. The new standard will become effective for fiscal years, including interim periods within those fiscal years, beginning after December 15, 2016, for public companies. For all other entities, the new standard is effective for fiscal years beginning after December 15, 2016, and interim periods within fiscal years beginning after December 15, 2017, with early adoption permitted. The Company has adopted this standard for the fiscal year ended December 31, 2017, which did not have a material impact on the Company’s consolidated financial statements.

In November 2015, the FASB issued ASU 2015-17, Balance Sheet Classification of Deferred Taxes. ASU 2015-17 specifies that deferred tax assets and liabilities shall be classified as non-current, or long-term, in a classified statement of financial position. The new standard is effective for public entities for fiscal years beginning after December 15, 2016, and interim periods within those fiscal years. For private entities, the new standard is effective for fiscal years beginning after December 15, 2017, and interim periods within fiscal years beginning after December 15, 2018. Earlier application is permitted for all entities as of the beginning of an interim or an annual reporting period. The Company has early adopted this standard for the fiscal year ended December 31, 2017, which did not have a material impact on the Company’s consolidated financial statements.

In March 2016, the FASB issued ASU 2016-09, which simplified several aspects of accounting for stock-based compensation transactions. The areas for simplification in this update involve several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. The new guidance is effective for public entities for fiscal years beginning after December 15, 2016 and interim periods within those years. Other entities must apply the new guidance in fiscal years beginning after December 15, 2017 and in interim periods within fiscal years beginning after December 15, 2018, with early adoption permitted. The Company early adopted this standard in the first quarter of 2017 by recording the cumulative impact of applying this guidance to retained earnings, which was not material. The Company elected to continue to estimate the number of awards that are expected to vest.

In January 2016, the FASB issued ASU 2016-01. Recognition and Measurement of Financial Assets and Financial Liabilities, which amends Accounting Standards Codification, or ASC, Subtopic 825-10, Financial Instruments - Overall, and includes updates on certain aspects of recognition, measurement, presentation and disclosure of financial instruments and applies to all entities that hold financial assets or owe financial liabilities. The new standard is effective for the Company’s annual period beginning after December 15, 2018, with early adoption permitted beginning after December 15, 2017. The Company has adopted this standard for the fiscal year ending December 31, 2018, which did not have any impact on the Company’s consolidated financial statements.

In May 2017, the FASB issued ASU 2017-09, Compensation—Stock Compensation (Topic 718): Scope of Modification Accounting, which clarifies when to account for a change to the terms or conditions of a share-based payment award as a modification. Under the new guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award (as equity or liability) changes as a result of the change in terms or conditions. The standard is effective for all entities for fiscal years, including interim periods within those fiscal years, beginning after December 15, 2017. Early adoption is permitted. The Company has adopted this standard for the fiscal year ending December 31, 2018, which did not have any impact on the Company’s consolidated financial statements.

Recently Issued Accounting Standards Not Yet Effective

In May 2014, the Financial Accounting Standards Board (“FASB”) issued Accounting Standards Update (“ASU”) 2014-09, Revenue from Contracts with Customers, which required an entity to recognize the amount of revenue to which it expects to be entitled for the transfer of promised goods or services to customers. ASU 2014-09 will replace most existing revenue recognition guidance in U.S. GAAP when it becomes effective. The new standard is effective for fiscal years beginning after December 15, 2017 for public companies, and for fiscal years beginning after December 15, 2018, and interim periods beginning after December 15, 2019, for private companies. Early application is permitted. The standard permits the use of either the retrospective or cumulative effect transition method. In March 2016, the FASB issued ASU 2016-08, Revenue from Contracts with Customers (Topic 606): Principal versus Agent Considerations (Reporting Revenue Gross versus Net), which clarifies the implementation guidance on principal versus agent considerations in ASU 2014-09. In April 2016, the FASB issued ASU 2016-10, Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing, which clarifies certain aspects of identifying performance obligations and licensing implementation guidance. In May 2016, the FASB issued ASU 2016-12, Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients, which relates to disclosures of remaining performance obligations, as well as other amendments to guidance on collectability, non-cash consideration and the presentation of sales and other similar taxes collected from customers. These standards have the same effective date and transition date as ASU 2014-09.

The Company continues to evaluate the impacts of adoption of the new standard on its accounting policies, processes, and system requirements and has assigned internal resources, in addition to the engagement of third party service providers, to assist in the evaluation. At this time, as it relates to product sales where the Company's sales representative delivers the product at the point of implantation at hospital or other medical facilities, the Company expects revenue to continue to be recognized upon completion of the procedure and authorization by the customer. Additionally, the new standard requires the capitalization of costs to obtain a contract, primarily sales commissions, and amortization of these costs over the contract period or estimated customer life. The Company expects to continue expensing all sales commissions as incurred. Management will adopt the standard using the modified retrospective method for the fiscal year ending December 31, 2019.

In February 2016, the FASB issued its new lease accounting guidance. Under the new guidance, ASU 2016-02, Leases (Topic 842), lessor accounting is largely unchanged. The new lease guidance simplifies the accounting for sale and leaseback transactions primarily because lessees must recognize lease assets and lease liabilities. Under the new guidance, lessees will be required to recognize a lease liability, which is a lessee’s obligation to make lease payments arising from a lease, measured on a discounted basis; and a right-of-use asset, which is an asset that represents the lessee’s right to use, or control use of, a specified asset for the lease term for all leases (with the exception of short-term leases) at the adoption date. The new guidance is effective for fiscal years, and for interim periods within those fiscal years, beginning after December 15, 2018 for public companies and beginning after December 15, 2019 for private companies. Early adoption is permitted for any interim or annual financial statements net yet issued. Lessees (for capital and operating leases) and lessors (for sales-type, direct financing and operating leases) must apply a modified retrospective approach for all leases existing at, or entered into after, the beginning of the earliest comparative period presented in the financial statements. The Company is currently evaluating the impact of this standard on its consolidated financial statements, and anticipates adopting the standard for the fiscal year ending December 31, 2020.

In August 2016, the FASB issued ASU 2016-15 “Statement of Cash Flows (Topic 230), Classification of Certain Cash Receipts and Cash Payments”. ASU 2016-15 provides guidance on the following eight specific cash flow classification issues: (1) debt prepayment or debt extinguishment costs; (2) settlement of zero-coupon debt instruments or other debt instruments with coupon interest rates that are insignificant in relation to the effective interest rate of the borrowing; (3) contingent consideration payments made after a business combination; (4) proceeds from the settlement of insurance claims; (5) proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies; (6) distributions received from equity method investees; (7) beneficial interests in securitization transactions; and (8) separately identifiable cash flows and application of the predominance principle. Current GAAP does not include specific guidance on these eight cash flow classification issues. The amendments of this ASU are effective for reporting periods beginning after December 15, 2017 for public companies, and fiscal years beginning after December 15, 2018 and interim periods within fiscal years beginning after December 15, 2019 for private companies, with early adoption permitted. The Company is currently evaluating the impact of adopting ASU 2016-15 on its consolidated financial statements, and anticipates adopting the standard for the fiscal year ending December 31, 2019.

In July 2017, the FASB issued ASU 2017-11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480), Derivatives and Hedging (Topic 815) I. Accounting for Certain Financial Instruments with Down Round Features II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception. Part I applies to entities that issue financial instruments such as warrants, convertible debt or convertible preferred stock that contain down-round features. Part II replaces the indefinite deferral for certain mandatorily redeemable noncontrolling interests and mandatorily redeemable financial instruments of nonpublic entities contained within ASC Topic 480 with a scope exception and does not impact the accounting for these mandatorily redeemable instruments. For public business entities, the amendments in Part I of this update are effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. For all other entities, the amendments in Part I of this Update are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted for all entities, including adoption in an interim period. The Company is currently evaluating the impact that the adoption of this standard will have on its consolidated financial statements, and anticipates adopting the standard for the fiscal year ending December 31, 2020.

In February 2018, the FASB issued ASU 2018-02, Income Statement—Reporting Comprehensive Income (Topic 220). This update provides companies with the option to reclassify stranded tax effects caused by the 2017 Tax Cuts and Jobs Act (the 2017 Tax Act) from accumulated other comprehensive income to retained earnings. This standard is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted. The Company is currently evaluating the impact that the adoption of this standard will have on its consolidated financial statements, and anticipates adopting the standard for the fiscal year ending December 31, 2019.

In June 2018, the FASB issued ASU 2018-07, Improvements to Nonemployee Share-Based Payment Accounting. ASU 2018-07 expands the scope of Topic 718, Compensation—Stock Compensation, to include share-based payments issued to nonemployees for goods or services. Consequently, the accounting for share-based payments to nonemployees and employees will be substantially aligned. ASU 2018-07 supersedes Subtopic 505-50, Equity—Equity-Based Payments to Non-Employees. For public business entities, the amendments are effective for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. For all other entities, the amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within fiscal years beginning after December 15, 2020. Early adoption is permitted, but no earlier than a company’s adoption date of ASC 606. The Company is currently evaluating the impact that the adoption of this standard will have on the consolidated financial statements, and anticipates adopting the standard for the fiscal year ending December 31, 2020.

Marketable Securities

12 Months Ended

Dec. 31, 2018

Investments, Debt and Equity Securities [Abstract]

Marketable Securities

At December 31, 2018, marketable securities consisted of the following (in thousands):



	Amortized Cost		Unrealized Gains		Unrealized Losses			Aggregate Fair Value
U.S. treasury securities	$	65,491	$	2	$	(4	)	$	65,489
Corporate bonds	19,708		15		(3		)	19,720
Commercial paper	11,894		—		—			11,894
Short-term investments	$	97,093	$	17	$	(7	)	$	97,103

U.S. treasury securities	$	1,000	$	—	$	—		$	1,000
Commercial paper	6,635		—		—			6,635
Money market funds	15,223							15,223
Cash equivalents	$	22,858	$	—	$	—		$	22,858

Total marketable securities	$	119,951	$	17	$	(7	)	$	119,961

All marketable securities of at December 31, 2017 of $22.1 million consisted of money market funds which were classified as cash equivalents.

Fair Value Measurement

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Fair Value Measurement

The following table sets forth the fair value of the Company’s financial assets and liabilities measured at fair value on a recurring basis based on the three-tier fair value hierarchy (in thousands):



	December 31, 2018
	Level 1		Level 2		Level 3		Total
Assets
Money market funds^[1]	$	15,223	$	—	$	—	$	15,223
U.S. treasury securities	66,489		—		—		66,489
Corporate bonds	—		19,720		—		19,720
Commercial paper	—		18,529		—		18,529

Liabilities
Redeemable convertible preferred stock warrants	$	—	$	—	$	—	$	—

	December 31, 2017
	Level 1		Level 2		Level 3		Total
Assets
Money market funds^[1]	$	22,115	$	—	$	—	$	22,115

Liabilities
Redeemable convertible preferred stock warrants	$	—	$	—	$	422	$	422


[1]	Included in cash and cash equivalents on the consolidated balance sheet

The following table sets forth a summary of the changes in the fair value of the redeemable convertible preferred stock warrants, the Company’s Level 3 financial liability, which is measured on a recurring basis (in thousands):




Balances at December 31, 2017	$	422
Change in fair value recorded in other (income) expense, net	826
Conversion of preferred stock warrants to common stock warrants	(1,248		)
Balances at December 31, 2018	$	—

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies Disclosure [Abstract]

Commitments and Contingencies

Operating Leases

In August 2012, the Company entered into a new four-year non-cancelable operating lease for its existing office building space in San Jose, California which commenced in January 2013. In February 2014, the Company expanded the existing lease space and extended the lease terms through June 2017. In May 2016, the Company entered into another extension of the lease with its lessor for additional 12 months beginning in July 2017. Effective May 2018, the Company entered into an early termination agreement on an operating lease for its San Jose office. No early termination fees were incurred and all previously agreed-to rent payments were released, with no further obligations. In February 2018, the Company entered into a new seven-year non-cancelable operating lease for an office building space in Santa Clara, California which commenced in April 2018.

The Company also entered into non-cancelable operating leases for its office building space in Gallarate, Italy and Mannheim, Germany which both expire in November 2024. Further, the Company also leases vehicles under operating lease arrangements for certain of its sales personnel in Europe which expire various times in 2019 to 2021.

Rent expense is recorded over the lease terms on a straight-line basis. Rent expense charged to operations under operating leases totaled approximately $1.2 million and $1.0 million for the years ended December 31, 2018 and 2017, respectively.

The aggregate future minimum lease payments under all leases as of December 31, 2018 are as follows (in thousands):



Year Ending December 31,
2019	$	1,035
2020	1,033
2021	914
2022	842
2023	844
Thereafter	1,184
	$	5,852

Contingencies

From time to time, the Company may have certain contingent liabilities that arise in the ordinary course of business activities. The Company accrues a liability for such matters when it is probable that future expenditures will be made and such expenditures can be reasonably estimated.

Purchase Commitments and Obligations

The Company has certain purchase commitments related to inventory used in normal course of business. These commitments totaled $0.2 million and $0.1 million at December 31, 2018 and December 31, 2017, respectively. The amounts paid under these arrangements may be less in the event that the arrangement is renegotiated or canceled.

Legal Proceedings

On February 6, 2019, a putative class action captioned Eric B. Fromer Chiropractic, Inc. v. SI-BONE, Inc. (Civil Action No. 5:19-cv-633-SVK), was filed in the United States District Court, Northern District of California. The complaint alleges violations of the Telephone Consumer Protection Act (the “TCPA”) on behalf of an individual and putative classes of persons alleged to be similarly situated. The complaint alleges that the Company sent invitations to an educational dinner event to health care providers by way of facsimile transmission. The TCPA prohibits using a fax machine to send unsolicited advertisements not including proper opt-out instructions or to send unsolicited advertisements to persons with whom the sender did not have an established business relationship. The Company believes that it has meritorious defenses and intends to vigorously defend itself in the action. It is too early in this matter to reasonably predict the probability of the outcomes or to estimate the range of possible loss, if any.

Indemnification

The Company enters into standard indemnification arrangements in the ordinary course of business. Pursuant to these arrangements, the Company indemnifies, holds harmless, and agrees to reimburse the indemnified parties for losses suffered or incurred by the indemnified party, in connection with any trade secret, copyright, patent or other intellectual property infringement claim by any third-party with respect to the Company’s technology. The term of these indemnification agreements is generally perpetual. The maximum potential amount of future payments the Company could be required to make under these agreements is not determinable because it involves claims that may be made against the Company in the future, but have not yet been made.

The Company has entered into indemnification agreements with its directors and officers that may require the Company to indemnify its directors and officers against liabilities that may arise by reason of their status or service as directors or officers, other than liabilities arising from willful misconduct of the individual.

The Company has not incurred costs to defend lawsuits or settle claims related to these indemnification agreements. No liability associated with such indemnifications has been recorded to date.

Borrowings

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Borrowings

The Company had $39.0 million and $38.7 million of outstanding debt, net of debt discounts, as of December 31, 2018 and December 31, 2017, respectively.

Term Loan

In August 2016, the Company entered into a Term Loan facility and a revolving line of credit with Silicon Valley Bank, or SVB and Oxford Finance LLC, or Oxford for $35.2 million. The Term Loan had interest only periods for six months up to October 1, 2017 and was then to be repaid over 25 months of equal principal payments plus interest. The maturity date of the term loan was December 1, 2019, and it carried an interest rate equal to the greater of 11% or the WSJ Prime rate plus 7.75%. The Term Loan borrowings were senior unsecured obligations of the Company, ranking equally and ratably among themselves and with the Company’s existing and future unsecured and unsubordinated debt.

In conjunction with the above Term Loan agreement, the Company issued redeemable convertible preferred stock warrants (refer to Note 9 for details).

In October 2017, the Company extinguished the Term Loan with SVB and Oxford. Concurrently, the Company entered into a New Term Loan with Pharmakon Advisors, or Pharmakon. As a result of the extinguished debt, the Company paid $29.1 million in principal payments to SVB and Oxford. The Company also paid $1.5 million in early termination fees and recorded $0.7 million of unamortized debt discounts. This loss on extinguishment of $2.2 million is reflected as interest expense in the consolidated statement of operations.

The Company entered into the New Term Loan with Pharmakon for $40.0 million in October 2017. Debt issuance costs of $1.3 million were recorded as a direct deduction from the carrying amount of the New Term Loan on the consolidated balance sheet, and are being amortized over the period of the New Term Loan using the effective interest method to interest expense in the consolidated statement of operations. The New Term Loan includes an interest-only period for 35 months through September 2020 and is then repaid in equal quarterly principal payments plus interest through December 2022, and is classified as long-term borrowings on the consolidated balance sheet. The New Term Loan carries a fixed interest rate of 11.5% and a closing fee of 1.5% of the funded amount, or $0.6 million. The New Term Loan includes a pre-payment fee equal to the interest due for the first 30 months of the agreement if it is prepaid within the first 30 months, a 2% prepayment penalty for months 31-48, and a 1% penalty for months 49-60. The New Term Loan required the Company to maintain a minimum cash balance of $5.0 million and to achieve certain revenue targets through December 31, 2018. Beginning with the three months ended March 31, 2019, the Company is required to meet either revenue or earnings targets. Under the New Term Loan, the Company also has a second tranche of $10.0 million available through January 2019, contingent upon the achievement of certain revenue milestones. The Company did not draw upon the second tranche. The New Term Loan is a senior obligation secured with a blanket first lien on the assets of the Company. As of December 31, 2018 and December 31, 2017, the total loan balance, net of debt discount, was $39.0 million and $38.7 million, respectively, with an effective interest rate of 12.3% and 12.0%, respectively, and the Company was in compliance with all debt covenants.

Approximate annual future minimum principal payments under the loan agreements as of December 31, 2018 are as follows (in thousands):



Year Ending at, December 31,
2019	$	—
2020	4,444
2021	17,778
2022	17,778
Total future minimum payments	40,000
Less:
Amount representing debt discount	(1,037		)
Total minimum payments	$	38,963

The New Term Loan required the Company to maintain a minimum cash balance of $5.0 million and to achieve certain revenue targets, which we were in compliance with through December 31, 2018.

Beginning with the three months ended March 31, 2019, the Company is required to meet either minimum net sales or trailing 12-month consolidated EBITDA targets. The Company needs to meet one or the other, but not both. If the Company does not meet either the minimum net sales or trailing 12-month consolidated EBITDA targets, the debt will immediately become due. The minimum net sales and trailing 12-month consolidated EBITDA targets are as follows (in thousands):



Twelve Months Ending	Minimum Net Sales			Trailing 12-Month Consolidated EBITDA
March 31, 2019	$	52,000	or	$	(5,000	)
June 30, 2019	$	53,500	or	$	(3,500	)
September 30, 2019	$	54,500	or	$	(2,000	)
December 31, 2019	$	56,000	or	$	—
March 31, 2020	$	57,500	or	$	1,000
June 30, 2020	$	58,500	or	$	2,000
thereafter, as applicable	$	60,000	or	$	3,000

The New Term Loan is collateralized by all of the Company's assets, including intellectual property.

Through December 31, 2018, the Company was in compliance with all of its debt obligations and covenants.

Line of Credit

In October 2015, the Company entered into an agreement with its existing lender SVB and Oxford. The amount of the revolving line of credit was $4.0 million (or 80% of the amount of certain customer accounts receivable). It carried an interest rate equal to the WSJ Prime rate plus 3% with a maturity of December 1, 2019. In October 2017, this line of credit was cancelled in conjunction with the SVB and Oxford debt extinguishment discussed above. No draws were made on this facility through its cancellation.

Redeemable Convertible Preferred Stock

12 Months Ended

Dec. 31, 2018

Temporary Equity Disclosure [Abstract]

Redeemable Convertible Preferred Stock

In connection with the IPO, the Company's outstanding shares of redeemable convertible preferred stock were automatically converted into an aggregate of 12,066,654 shares of common stock. There was no preferred stock balance outstanding at December 31, 2018.

Preferred stock at December 31, 2017 consisted of the following:



	Shares
Series	Authorized	Issued and Outstanding	Carrying Value		Liquidation Value
			(in thousands)
Series 1	245,096	245,096	$	154	$	154
Series 2	709,617	709,608	1,489		1,520
Series 3	498,958	498,938	2,862		2,874
Series 4	2,509,047	2,509,032	15,656		15,807
Series 5	2,086,138	2,009,226	18,127		18,275
Series 6	3,389,227	3,319,274	54,508		54,674
Series 7	2,666,666	2,580,404	25,752		25,890
Total	12,104,749	11,871,578	$	118,548	$	119,194

Prior to the conversion of redeemable preferred stock to common stock during the IPO, the holders of preferred stock had various rights and preferences as follows:

Voting Rights

The holders of Series 1, Series 2, Series 3, Series 4, Series 5, Series 6, and Series 7 preferred stock shares were entitled to vote on all matters on which the common stockholders were entitled to vote. The holders of Series 1, Series 2, Series 3 had the right to 0.352941 votes for each share of Series 2 common stock into which such preferred stock would had converted and the holders of Series 4, Series 5, Series 6 and Series 7 had the right to one vote for each share of Series 2 common stock into which such preferred stock would had converted. As long as there were any shares of Series 4, Series 5, Series 6, and Series 7 shares that were outstanding, the holders of such Series 4, Series 5, Series 6 and Series 7, at each respective series, were entitled to elect one member of the Board of Directors each; the holders of Series 2 common stock were entitled to elect two members of the Board of Directors; and the holders of the preferred stock and Series 2 common stock, voting together as a single class were entitled to elect the remaining members of the Board of Directors, as determined at each annual meeting of the Board of Directors.

As long as at least 277,778 preferred stock shares remained outstanding, the Company must obtain approval from a majority of the holders of the then outstanding shares of preferred stockholders and a majority of the voting power of all outstanding shares of Series 5 preferred stock in order to (i) consummate or agree to consummate a Liquidation Event (as defined in the Company’s certificate of incorporation); (ii) amend, alter, restate or repeal any provision of the Company’s certificate of incorporation or bylaws so as to adversely alter, affect or change the powers, preferences, rights or privileges of the shares of preferred stock; (iii) increase or decrease (other than by redemption or conversion) the total number of authorized shares of common stock or preferred stock or designated shares of any series of preferred stock; (iv) authorize or issue, or obligate itself to issue, any equity security (including any other security convertible into or exercisable for any such equity security) having a preference over, or being on a parity with, any series of preferred stock with respect to dividends, liquidation or redemption, other than the issuance of any authorized but unissued shares of Series 6 preferred stock designated in the Company’s certificate of incorporation (including any security convertible into or exercisable for such shares of preferred stock); (v) redeem, purchase or otherwise acquire (or pay into or set aside for a sinking fund for such purpose) any share or shares of preferred stock or common stock; provided, however, that this restriction shall not apply to the repurchase of shares of common stock from employees, officers, directors, consultants or other persons performing services for the Company or any subsidiary pursuant to agreements under which the Company has the option to repurchase such shares upon the occurrence of certain events, such as the termination of employment or service, or pursuant to an agreement providing for a right of first refusal in favor of the Company, in each case, provided that such agreement has been approved by the Company’s board of directors; or (vi) pay or declare any dividend on any shares of capital stock of the Company.

Dividends

The holders of preferred stock were entitled to receive noncumulative dividends, when and if declared by the Board of Directors, out of any assets legally available, prior to and in preference to any declaration or payment of dividends on the common stock of the Company. Dividend rates, on a per annum basis, for Series 1, Series 2, Series 3, Series 4, Series 5, Series 6, and Series 7 preferred stock are $0.050112, $0.171360, $0.4608, $0.504, $0.72774, $1.317744, and $0.802656, respectively, (adjusted to reflect subsequent stock dividends, stock splits or recapitalization). No dividends on preferred stock or common stock have been declared as of December 31, 2018 and 2017.

Liquidation

In the event of any liquidation, dissolution or winding-up of the Company or Liquidating Event, the holders of the preferred stock were entitled to receive prior to and in preference to any distribution to holders of the common stock, an amount equal to their respective original issuance price per share (original issuance price per share for Series 1, Series 2, Series 3, Series 4, Series 5, Series 6, and Series 7 preferred stock are $0.6264, $2.142, $5.76, $6.30, $9.0954, $16.4718, and $10.0332, respectively), plus any declared but unpaid dividends on such shares. Should the Company’s legally available assets be insufficient to satisfy the full liquidation preference, the funds will be distributed ratably among the holders of the preferred stock in proportion to the preferential amount each holder is otherwise entitled to receive.

Upon the closing of the distribution as above, the remaining proceeds shall be distributed among the holders of Series 4, Series 5, Series 6, Series 7 preferred stock and common stock pro rata based on the number of shares of common stock held by each until the holders of the preferred stock have received the “participation cap.” Thereafter, if proceeds remain, the holders of Series 7 preferred stock and common stock of this corporation shall receive all of the remaining proceeds pro rata based on the number of shares of common stock held by each (assuming full conversion of all such Series 7 preferred stock). The Company has a per share “Participation Cap” of $32.9436 for the Series 6 preferred stock, $18.1908 for the Series 5 preferred stock, and $12.60 for the Series 4 preferred stock (each as adjusted for any stock splits, stock dividends, combinations, subdivisions, recapitalizations or the like with respect to such series of preferred stock).

Conversion

Each share of Series 1, Series 2, Series 3, Series 4, Series 5, Series 6, and Series 7 preferred stock was convertible at the option of the holder, into the number of shares of Series 2 common stock into which such shares are at the then effective conversion ratio or one to one ratio. The conversion price per share for Series 1, Series 2, Series 3 and Series 4, Series 5, Series 6, and Series 7 preferred stock shall be the respective issuance price per share, respectively. The initial conversion price was subject to adjustment from time to time. In March 2017, the conversion price per share for the Series 6 preferred stock was amended from $15.714 per share to $15.5574 per share which resulted in the conversion ratio increasing from 1.05 to 1.06 per share.

Each share of preferred stock shall be converted into common stock shares upon the earlier of (i) immediately before the closing of a firm commitment underwritten public offering in which the aggregate gross proceeds of not less than $50.0 million and a per share public offering of not less than $30.0996, or (ii) the Company’s receipt of a written request for such conversion from the holders of at least the voting majority of all outstanding preferred stock (voting as a single class and on an as-converted basis).

Other Matters

Prior to the conversion of redeemable convertible preferred stock to common stock during the IPO, the Company has classified the preferred stock as temporary equity on the consolidated balance sheets as the shares can be redeemed upon the occurrence of certain change in control events that are outside the Company’s control, including deemed liquidation, sale or transfer of the Company. The Company has not adjusted the carrying values of the preferred stock to the liquidation preferences of such shares because it was uncertain whether or when an event would occur that would obligate the Company to pay the liquidation preferences to holders of shares of preferred stock.

Warrants

12 Months Ended

Dec. 31, 2018

Other Liabilities Disclosure [Abstract]

Warrants

In connection with the IPO, the Company's outstanding warrants to purchase 156,550 shares of redeemable convertible preferred stock were automatically converted into warrants to purchase an aggregate of 160,657 shares of common stock, resulting in reclassification of the related redeemable convertible preferred stock warrant liability of $1.2 million in additional paid-in-capital. In addition, warrants to purchase Series 6 Preferred Stock were converted to warrants to purchase common stock at a conversion ratio of 1:1.06.

Warrants issued and outstanding at December 31, 2018 are as follows(in thousands, except share and per share data):

Date

Number of
Shares
Underlying
Warrants

Price per
Share

Fair Value

Warrants to purchase

Issuance

Expiration

Common stock

3/1/2017

3/1/2027

[a]

1,388

5.94

[b]

Common stock

7/19/2013

7/22/2023

[a]

32,983

9.10

122

[b]

Common stock

11/26/2014

11/26/2024

[a]

6,680

16.47

[b]

Common stock

10/20/2015

10/20/2025

[a]

41,650

16.47

396

[c]

Common stock

11/9/2015

11/9/2025

[a]

25,709

16.47

244

[c]

Common stock

12/22/2016

12/22/2026

[a]

9,712

10.03

[c]

Total outstanding common stock warrants

118,122


[a]	Common stock warrants will remain outstanding until exercised by the holder.


[b]	Fair value at the date of issuance.


[c]	Fair value at the date of conversion from redeemable convertible preferred stock to common stock warrants in conjunction with the IPO on October 16, 2018.

Warrants issued and outstanding at December 31, 2017 are as follows (in thousands, except share and per share data):



	Series	Date			Number of Shares Underlying Warrants	Price per Share		Fair Value
Warrants to purchase	Series	Issuance	Expiration		Number of Shares Underlying Warrants	Price per Share		Fair Value
Common stock		7/19/2013	7/22/2023	[a]	101,010	$	3.96	$	244	[b]
Common stock		11/26/2014	11/26/2024	[a]	21,928	$	3.42	$	47	[b]
Common stock		3/1/2017	3/1/2027	[a]	1,388	$	5.94	$	5	[b]
Total common stock warrants					124,326

Redeemable convertible preferred stock	Series 5	7/1/2012	7/25/2019	[d]	54,917	$	9.10	$	255	[c]
Redeemable convertible preferred stock	Series 5	7/19/2013	7/22/2023	[e]	21,989	$	9.10	$	122	[c]
Redeemable convertible preferred stock	Series 6	11/26/2014	11/26/2024	[e]	6,310	$	16.47	$	49	[c]
Redeemable convertible preferred stock	Series 6	10/20/2015	10/20/2025	[e]	39,339	$	16.47	$	396	[c]
Redeemable convertible preferred stock	Series 6	11/9/2015	11/9/2025	[e]	24,283	$	16.47	$	244	[c]
Redeemable convertible preferred stock	Series 7	12/22/2016	12/22/2026	[e]	9,712	$	10.03	$	45	[c]
Total redeemable convertible preferred stock warrants					156,550
Total outstanding common and redeemable convertible preferred stock warrants					280,876


[a]	Common stock warrants will remain outstanding until exercised by the holder.


[b]	Fair value at the date of issuance.


[c]	Fair value as of December 31, 2017.


[d]	These warrants will be net exercised immediately upon the closing of the Company’s IPO, or upon a corporate transaction as defined in the Note and Warrant Purchase Agreement dated July 25, 2012.


[e]	Convertible preferred stock warrants will remain outstanding until exercised by the holder and will convert to common stock warrants upon an IPO. The warrants will be exercisable for 10 years from the date of issuance.

In connection with previously issued debt, the Company issued 101,010 warrants to purchase common shares of the Company at an exercise price of $3.96 per share in July 2013. Additionally, the Company issued warrants to purchase an additional 21,928 shares of common stock at an exercise price of $3.42 per share in November 2014. The Company determined that its warrants to purchase shares of common stock meet the requirements for equity classification.

In conjunction with debt issued in July 2012, the Company issued warrants which became exercisable for an aggregate of 54,917 shares of Series 5 preferred stock of the Company at an exercise price of $9.10 per share.

In conjunction with debt issued in 2013 and 2014, the Company issued 32,983 warrants to purchase Series 5 redeemable convertible preferred stock of the Company at an exercise price of $9.10 per share. Subsequently, the Company issued additional warrants to purchase 6,310 shares of Series 6 redeemable convertible preferred stock at an exercise price of $16.47 per share.

In conjunction with the debt agreement with SVB and Oxford, or Term Loan agreement (refer to Note 7), the Company issued warrants to purchase 39,339 shares of Series 6 redeemable convertible preferred stock at an exercise price of $16.47 per share in October 2015 and additional 24,283 shares of Series 6 redeemable convertible preferred stock at an exercise price of $16.47 per share in November 2015.

In conjunction with the Term Loan agreement and its modification (refer to Note 7), the Company issued additional warrants for the purchase of 9,712 shares of Series 7 redeemable convertible preferred stock at an exercise price of $10.03 per share in December 2016.

In March 2017, the Company issued a warrant to purchase 1,388 shares of common stock at an exercise price of $5.94 to a consultant. The Company determined that such warrant meets the requirements for equity classification.

In October 2017, the Company extinguished its debt with SVB and Oxford. All related debt discounts were written off upon repayment of the loan.

The Company estimates the fair value of warrants using the Black-Scholes option valuation model. The fair value is estimated using certain assumptions. Refer to Note 11. Stock-Based Incentive Compensation Plans, for further discussions on how the Company determines these input assumptions. Each of these inputs is subjective and its determination generally requires significant judgment. The fair value of warrants to purchase preferred stock were recorded at the date of issuance as a discount to debt and amortized to interest expense over the term of the note. The changes in the fair value of the redeemable convertible preferred stock warrants are recorded in other income and expense. Weighted-average assumptions used in computation of the fair value of the redeemable convertible preferred stock warrants are summarized in the table below:



	Year Ended December 31,
	2018		2017
Remaining contractual term (in years)	4.9		5.3
Expected volatility	53.89	%	59.06	%
Risk-free interest rate	2.62	%	2.16	%
Dividend yield	—	%	—	%

Common Stock

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Common Stock

In March 2017, the Board of Directors approved an increase of 555,555 Series 2 common stock. As a result, the Company’s restated certificate of incorporation, as amended, authorizes the Company to issue 19,333,333 shares of $0.0001 par value common stock, of which 6,000,000 has been designated as Series 1 common stock and 13,333,333 has been designated as Series 2 common stock. The holders of Series 1 common stock shall have no voting rights; the holders of Series 2 common stock shall have the right to one vote for each such share.

In October 2018, the Company amended and restated its certification of incorporation, which authorizes the Company to issue 100,000,000 shares of common stock and 5,000,000 shares of preferred stock, each having a par value of $0.0001. At December 31, 2018, there was no preferred stock issued and outstanding.

The holders of common stock are entitled to receive dividends whenever funds are legally available, as, when, and if declared by the Board of Directors. There have been no dividends declared to date.

The Company has reserved shares of common stock, on an issued and as-converted basis, for future issuance as follows:



	December 31, 2018	December 31, 2017
	Issued and Outstanding	Issued and Outstanding	Common Stock Equivalent Shares
Common stock	24,450,757	—	—
Series 1 common stock	—	3,112,955	3,112,955
Series 2 common stock	—	490,185	490,185
Redeemable convertible preferred stock	—	11,871,578	12,066,654
Restricted stock units outstanding	53,436	—	—
Stock options outstanding	2,641,198	3,001,929	3,001,929
Shares available for grant	2,497,082	29,654	29,654
Common stock warrants	118,122	124,326	124,326
Redeemable convertible preferred stock warrants	—	156,550	160,657
Total	29,760,595	18,787,177	18,986,360

Stock-Based Incentive Compensation Plans

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Incentive Compensation Plans

In April 2008, the Company adopted the 2008 Stock Option Plan, as amended, under which the Board of Directors may issue incentive and nonqualified stock options to employees, directors and consultants. In October 2018, the Company adopted the 2018 Equity Incentive Plan, which serves as the successor to the 2008 Stock Option Plan, under which the Board of Directors may issue incentive and nonqualified stock options and restricted stock units (RSUs) to employees, directors and consultants. No new options have been granted under the 2008 Stock Option Plan since August 2018. Outstanding options under the 2008 Stock Option Plan continue to be subject to the terms and conditions of that plan.

The Board of Directors has the authority to determine to whom options will be granted, the number of shares, the term and the exercise price. If an individual owns stock representing more than 10% of the outstanding shares, the price of each share shall be at least 110% of the fair market value, as determined by the Board of Directors. The exercise price of an incentive stock option and a nonqualified stock option shall not be less than 100% and 85%, respectively, of the fair market value on the date of grant. As of December 31, 2017, a total of 5,350,080 shares of common stock had been reserved for issuance under the 2008 Stock Option Plan. As of December 31, 2018, a total of 2,576,538 shares of common stock had been reserved for issuance under the 2018 Equity Incentive Plan.

Options granted have a term of 10 years, except, options granted to individuals holding more than 10% of the outstanding shares have a term of five years. Options generally vest over a four-year period. Certain shares issued under the Plan are exercisable immediately, but subject to a right of repurchase by the Company of any unvested shares. RSUs granted under the 2018 Equity Incentive Plan generally vest over two to four years based upon continued services and are settled at vesting in shares of the Company's common stock.

The following table summarizes stock option activity for the year ended December 31, 2018 for all stock plans:



	Options Outstanding
	Number of Shares		Weighted-Average Exercise Price		Weighted-Average Contractual Remaining Life	Aggregate Intrinsic Value
					(Years)	(in thousands)
Balances at December 31, 2017	3,001,929		$	4.15		$	3,585
Options granted	100,080		$	8.88
Options exercised	(395,117	)	$	4.08
Options canceled	(65,694	)	$	4.81
Balances at December 31, 2018	2,641,198		$	4.27	6.6	$	43,905

Options vested and exercisable - December 31, 2018	1,762,687		$	3.73	5.9	$	41,882

Options vested and expected to vest - December 31, 2018	2,366,723		$	4.14	6.5	$	40,154

The weighted-average grant date fair value of all options granted were $3.98 and $1.80 for years ended December 31, 2018 and 2017, respectively. The aggregate intrinsic values of options outstanding, options vested and exercisable, and options vested and expected to vest were calculated as the difference between the exercise price of the options and the close price of the Company’s common stock on the last trading day of the year, as of December 31, 2018 and 2017, respectively.

Employee Stock Purchase Plan

The Company allows eligible employees to purchase shares of the Company's common stock through payroll deductions at the price equal to 85% of the lesser of the fair market value of the stock as of the first date or the ending date of each six month offering period.

Early Exercise of Unvested Stock Options

Early exercises of stock options are subject to a right of repurchase by the Company of any unvested shares. The repurchase rights lapse over the original vesting period of the options. The Company accounts for the cash received in consideration for the early exercised options as a liability included in accrued liabilities, which is then reclassified to stockholders’ equity (deficit) as the options vest. At December 31, 2018 and December 31, 2017, the Company had a total of 74,019 and 16,117 shares of common stock, respectively, subject to repurchase under the Plan and $0.3 million and $0.1 million, respectively, of associated liabilities for the repurchase.

Stock-Based Compensation

The following table sets forth stock-based compensation expense recognized for the periods presented (in thousands):



	Year ended December 31,
	2018		2017
Cost of goods sold	$	34	$	23
Research and development	156		143
Sales and marketing	651		438
General and administrative	1,471		1,271
	$	2,312	$	1,875

Amounts above do not include $0.4 million of stock-based compensation expense related to forgiveness of notes receivable for the year ended December 31, 2017. No amounts of stock-based compensation expense related to forgiveness of notes receivable were recognized for the year ended December 31, 2018. Refer to Note 13 for details.

The Company estimates the fair value of stock options using the Black-Scholes option valuation model. The fair value of employee stock options is being amortized on a straight-line basis over the requisite service period of the awards. The fair value of employee stock options was estimated using the assumptions below. Each of these inputs is subjective and its determination generally requires significant judgment.

Fair Value of Common Stock

The fair value of the shares of the Company’s common stock underlying the stock options has historically been determined by the Company’s Board of Directors. Because there had been no public market for the Company’s common stock prior to its IPO, its Board of Directors has determined the fair value of the Company’s common stock at the time of grant of the option by considering a number of objective and subjective factors, including independent third-party valuations, the Company’s stage of development, sales of the Company’s redeemable convertible preferred stock, the Company’s operating and financial performance, equity market conditions affecting comparable public companies, the lack of liquidity of the Company’s capital stock, and the general and industry-specific economic outlooks.

Subsequent to its IPO, the Company uses the market closing price for its common stock as reported on the NASDAQ Global Market on the date of grant.

Expected Term

The expected term represents the period that the share-based awards are expected to be outstanding. The Company used the simplified method to determine the expected term, which is calculated as the average of the time to vesting and the contractual life of the options.

Expected Volatility

As the Company’s common stock has never been publicly traded, the expected volatility is derived from the average historical volatilities of publicly traded companies within its industry that the Company considers to be comparable to its business over a period approximately equal to the expected term for its stock options.

Risk-Free Interest Rate

The risk-free interest rate is based on the U.S. Treasury zero coupon issues in effect at the time of grant for periods corresponding with the expected term of the option.

Dividend Yield

The Company has never paid dividends on its common stock and has no plans to pay dividends on its common stock. Therefore, the Company used an expected dividend yield of zero.

Expected Forfeiture Rate

The Company is required to estimate forfeitures at the time of grant and revise those estimates in subsequent periods if actual forfeitures differ from those estimates. The Company uses historical data to estimate pre-vesting option forfeitures and record stock-based compensation expense only for those awards that are expected to vest. To the extent actual forfeitures differ from the estimates, the difference will be recorded as a cumulative adjustment in the period that the estimates are revised.

The grant date fair value of the stock option awards granted to employees was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:



	Year ended
	December 31,
	2018	2017
Expected term	6.17	5.71
Expected volatility	42%-47%	42%-55%
Risk-free interest rate	2.35%-2.96%	1.73%-2.31%
Dividend yield	—%	—%

Performance Stock Options

In March 2017, the Company granted 564,098 performance stock options at an exercise price of $5.94, of which 408,544 performance options will vest monthly over four years and 155,554 performance options will vest monthly over three years. The vesting period will begin on the date of the closing of an IPO, the performance condition, subject to the optionee’s continuous service. Stock-based compensation expense for performance stock options is based on the probability of achieving certain performance criteria, as defined in the individual option grant agreement. Periodically, the Company estimates the number of performance options ultimately expected to vest and recognizes stock-based compensation expense for those options when it becomes probable that the performance criteria will be met. In October 2018, the Company completed its IPO and recognized $0.6 million expense related to the performance stock options for the year ended December 31, 2018.

In December 2017, the Company modified the terms of 394,652 unvested stock option awards granted in March 2017, by reducing their exercise price from $5.94 to $4.68 per share. In addition, the vesting performance conditions for these options were removed and the vesting commencement date changed from the IPO date to September 2017. There were no other changes in any of the other terms of the option awards. Due to these options previously subject to performance conditions that were not deemed probable of occurring, the Company had not recognized any expense related to these grants. The modification resulted in total expense of $0.8 million that is recognized over the amended vesting period of forty-eight months

Restricted Stock Units

The following table summarizes restricted stock units activity for the year ended December 31, 2018:



	Number of Shares		Weighted- Average Grant Date Fair Value
Unvested as of December 31, 2017	—		$	—
Granted	54,036		$	11.79
Forfeited	(600	)	20.60
Vested	—		$	—
Unvested as of December 31, 2018	53,436		$	11.69

As of December 31, 2018, there was a total unrecognized compensation cost of $4.5 million. These costs are expected to be recognized over a period of approximately 2.29 years.

Employee Stock Purchase Plan

In October 2018, the Company adopted the 2018 Employee Stock Purchase Plan (the "ESPP"). 515,307 shares of common stock are reserved for issuance under the ESPP. The Company recognized $0.1 million compensation expense for the ESPP. As of December 31, 2018, the unrecognized compensation cost for the ESPP was approximately $0.3 million.

The assumptions used to determine the grant date fair value of employee stock options and ESPP purchase rights for the periods presented are as follows:



	Year ended
	December 31,
	2018
Expected term	0.5
Expected volatility	43.96%
Risk-free interest rate	2.49%
Dividend yield	—%

Related Party Transactions	12 Months Ended
Related Party Transactions	Dec. 31, 2018
Related Party Transactions [Abstract]
Related Party Transactions	Related Party Transactions In March 2013, the Company granted a loan to its then current Chief Financial Officer to assist with the exercise of his stock option grants in the form of a full recourse promissory note with an aggregate principal amount of $0.2 million. The note is collateralized by the common stock issued upon the exercise of the stock options, as well as personal assets of the borrower. Interest under this note will accrue at the rate of 1.09% per annum. In November 2016, the loan amount was partially repaid in the amount of $0.1 million (including principal and interest). The remainder of the principal balance of this note, together with all accrued and unpaid interest to date, was fully paid in December 2017. In February 2014, the Company granted a loan to its Chief Executive Officer to assist with the exercise of his stock option grants in the form of a full recourse promissory note with an aggregate principal amount of $0.4 million. The note is collateralized by the common stock issued upon the exercise of the stock options, as well as personal assets of the borrower. At the time of issuance, the Company accounted for the note as a full recourse promissory note based on historical pattern of collecting payment on notes in full and no other notes had been forgiven, nor had any recourse notes been substantively converted to nonrecourse. Interest under this note will accrue at the rate of 1.97% per annum. The principal balance of this note, together with all accrued and unpaid interest to date, was due in February 2019. In March 2017, the Company forgave $0.2 million of principal and interest due on this promissory note from its Chief Executive Officer. In addition, the Board of Directors approved the forgiveness of the remaining 50% of the principal balance of the note in January 2018. At the time of the forgiveness, all of the related stock options were fully vested. As a result, the Company expensed the principal note balance of $0.4 million to stock-based compensation expense and accrued interest of $0.1 million to general and administrative expenses in the consolidated statement of operations in the year ended December 31, 2017.

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Income Taxes

The components of the Company’s loss before income taxes are as follows (in thousands):



	Year Ended December 31,
	2018		2017
Domestic	(16,835	)	(22,717	)
Foreign	(618	)	(322	)
Loss before income taxes	(17,453	)	(23,039	)

The components of income tax expense are as follows (in thousands):



	Year Ended December 31,
	2018			2017
Current tax expense:
Federal	$	—		$	—
State	—			—
Foreign	—			—
Total current tax expense	—			—

Deferred tax expense:
Federal	3,555			(9,574		)
State	822			2,061
Foreign	200			—
Total deferred tax expense	4,577			(7,513		)
Change in deferred tax valuation allowance	(4,577		)	7,513

Net deferred tax expense	—			—
Provision for income taxes	$	—		$	—

Income tax expense differs from the amount computed by applying the statutory federal income tax rate due to the following:



	Year Ended December 31,
	2018		2017
Tax at statutory federal rate	(21.0	)%	(34.0	)%
State tax, net of federal benefit	(5.3	)%	(4.3	)%
Measurement of deferred taxes as a result of tax reform	—	%	68.7	%
Tax credits	(0.7	)%	(0.3	)%
Change in deferred tax valuation allowance	26.2	%	(32.6	)%
Other	0.8	%	2.5	%
Total income tax expense	—	%	—	%

The tax effects of temporary differences and carryforwards that give rise to significant portions of the deferred tax assets are presented below (in thousands):



	December 31,
	2018			2017
Net operating loss carryforwards	$	35,067		$	32,210
Research and development credits	2,255			2,070
Depreciation and amortization	132			179
Accruals and reserves	2,958			1,376

	40,412			35,835
Less: Valuation allowance	(40,412		)	(35,835		)
Total deferred tax asset	$	—		$	—

The Company has established a full valuation allowance against its deferred tax assets due to the uncertainty surrounding realization of such assets. The following table summarizes changes in the valuation allowance for the year ended December 31, 2018 and 2017 (in thousands):



	December 31,
	2018		2017
Beginning balance	$	35,835	$	43,348
Additions during the period	4,577		—
Deductions during the period	—		(7,513		)
Ending balance	$	40,412	$	35,835

As of December 31, 2018, the Company had net operating loss (“NOL”) carryforwards of approximately $135.5 million and $109.9 million available to reduce future taxable income, if any, for federal and state income tax purposes, respectively. If not utilized, the Company’s federal net operating loss carryforward begins to expire in 2029, and the state net operating loss carryforward begins to expire in 2019.

As of December 31, 2018, the Company had credit carryforwards of approximately $1.7 million and $1.9 million available to reduce future taxable income, if any, for both federal and state income tax purposes, respectively. The federal credits begin to expire in 2030, and the state credits have no expiration date.

The Tax Reform Act of 1986 limits the use of net operating loss and tax credit carryforwards in certain situations where changes occur in the stock ownership of a company. In general, if the Company experiences a greater than 50 percentage point aggregate change in ownership over a three-year period (a Section 382 ownership change), utilization of its pre-change NOL carryforwards are subject to an annual limitation under Section 382 of the Internal Revenue Code (California has similar laws). The annual limitation generally is determined by multiplying the value of the Company’s stock at the time of such ownership change (subject to certain adjustments) by the applicable long-term tax-exempt rate. Such limitations may result in expiration of a portion of the NOL carryforwards before utilization. The Company has not utilized any NOL carryovers through December 31, 2018. In addition, the Company’s deferred tax assets are subject to full valuation allowance, and thus no benefit for deferred tax assets have been recorded. The Company has determined that it experienced Section 382 ownership changes in 2010 and $1.4 million of its NOLs are limited. The Company is currently performing an analysis to determine whether any additional NOL carryforwards are limited due to a change in ownership as a result of its recent IPO.

The Company accounts for the uncertainty in income taxes by utilizing a comprehensive model for the recognition, measurement, presentation and disclosure in financial statements of any uncertain tax positions that have been taken or are expected to be taken on an income tax return. The changes in the Company’s uncertain income tax positions for the years ended December 31, 2018 and 2017 consisted of the following (in thousands):



Beginning balance as of January 1, 2017	$	950
Increases in balances related to tax positions taken during 2017	43
Ending balance as of December 31, 2017	993
Increases in balances related to tax positions taken during 2018	91
Ending balance as of December 31, 2018	$	1,084

The Company has elected to recognize interest and penalties related to uncertain tax positions as a component of income tax expense. The Company has accrued zero at December 31, 2018 and 2017 for payment of interest related to unrecognized tax benefits. None of the Company’s unrecognized tax benefits that, if recognized, would affect its effective tax rate at December 31, 2018.

The Company currently has no federal or state tax examinations in progress nor has it had any federal or state examinations since inception. As a result of the Company’s net operating loss carry forwards, all of its tax years are subject to federal and state tax examinations.

On December 22, 2017, the United States enacted a law commonly known as the Tax Cuts and Jobs Act (“TCJA”) which makes widespread changes to the Internal Revenue Code, including a reduction in the federal corporate tax rate to 21%, effective January 1, 2018.

The Company is subject to the provisions of the Financial Accounting Standards Board (“FASB”) ASC 740-10, Income Taxes, which requires that the effect on deferred tax assets and liabilities of a change in tax rates be recognized in the period the tax rate change was enacted. The carrying value of U.S. deferred taxes is determined by the enacted U.S. corporate income tax rate. Consequently, the reduction in the U.S. corporate income tax rate impacts the carrying value of deferred tax assets. Under the new corporate income tax rate of 21%, the U.S. net deferred tax asset position decreased by approximately $15.8 million. Uncertainty regarding the impact of tax reform remains, as a result of factors including future regulatory and rulemaking processes, the prospects of additional corrective or supplemental legislation, potential trade or other litigation, and other factors.

In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118 (“SAB 118”), which provides guidance for the tax effect of the 2017 Tax Act. SAB 118 provides a measurement period that should not extend beyond one year from the 2017 Tax Act’s enactment date for companies to complete the accounting under Accounting Standards Codification Topic 740, Income Taxes (“ASC 740”). In accordance with SAB 118, the Company must reflect the income tax effects of those aspects of the 2017 Tax Act for which the accounting under ASC 740 is complete. To the extent that its accounting for certain income tax effects of the 2017 Tax Act is incomplete, but the Company is able to determine a reasonable estimate, the Company must record a provisional estimate in its consolidated financial statements. If the Company cannot determine a provisional estimate to be included in its consolidated financial statements, the Company should continue to apply ASC 740 on the basis of the provisions of the tax laws that were in effect immediately before the enactment of the 2017 Tax Act. It is expected that the U.S. Treasury will issue regulations and other guidance on the application of certain provisions of the 2017 Tax Act. The Company has analyzed the guidance and other necessary information related to the tax effects of the 2017 Tax Act and considers the accounting of its net deferred tax assets complete in accordance with SAB 118.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Basis of Presentation

The accompanying consolidated financial statements have been prepared in accordance with generally accepted accounting principles in the United States of America (“U.S. GAAP”).

Principles of Consolidation

The consolidated financial statements include the Company's accounts, as well as those of the Company's wholly-owned international subsidiaries. All inter-company accounts and transactions have been eliminated.

Use of Estimates

Segments

Foreign Currency

Concentration of Credit Risk

Fair Value of Financial Instruments

Level 1: Quoted prices (unadjusted) in active market that are accessible at measurement date for assets or liabilities. The fair value hierarchy gives the highest priority to Level 1 inputs.

Level 2: Observable prices that are based on inputs not quoted on active markets, but corroborated by market data.

Level 3: Unobservable inputs are used when little or no market data is available. The fair value hierarchy gives the lowest priority to Level 3 inputs.

Cash and Cash Equivalents

The Company considers all highly liquid investments with remaining maturities at the date of purchase of three months or less to be cash equivalents.

Marketable Securities

Accounts Receivable and Allowance for Doubtful Accounts

Inventory

Property and Equipment



Computer and office equipment	3 – 5 years
Machinery and equipment	3 – 5 years
Furniture and fixtures	7 years

Impairment of Long-Lived Assets

Public Offering Costs

Common Stock Warrants

Redeemable Convertible Preferred Stock Warrants

Revenue Recognition

The Company’s revenue is derived from the sale of its products to medical groups and hospitals through its direct sales force and distributors throughout the United States and Europe.

Warranty Program

Medical Device Excise Tax

Shipping and Handling Costs

Shipping and handling costs are expensed as incurred and are included in cost of goods sold.

Research and Development

Advertising Expenditures

The cost of advertising is included in Sales and Marketing expense in the consolidated financial statements and expensed as incurred.

Stock-Based Compensation

Income Taxes

Net Loss per Share of Common Stock

Comprehensive Loss

Recent Accounting Pronouncements

Recently Adopted Accounting Standards

Recently Issued Accounting Standards Not Yet Effective

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Accounting Policies [Abstract]

Schedule of Revenue by Geography

Following table summarizes the Company's revenue by geography (in thousands):



	Year ended December 31,
	2018		2017
Domestic	$	50,137	$	43,351
International	5,243		4,632
	$	55,380	$	47,983

Schedule of Property and Equipment Useful Lives

All property and equipment is depreciated on a straight-line basis over the estimated useful lives of the assets, which are as follows:



Computer and office equipment	3 – 5 years
Machinery and equipment	3 – 5 years
Furniture and fixtures	7 years

Property and Equipment, net (in thousands):



	December 31,			December 31,
	2018			2017
Machinery and equipment	$	3,785		$	3,428
Construction in progress	730			879
Computer and office equipment	407			310
Leasehold improvements	448			272
Furniture and fixtures	148			29
	5,518			4,918
Less: Accumulated depreciation and amortization	(3,364		)	(3,022		)
	$	2,154		$	1,896

Fair Value Measurement (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value Disclosures [Abstract]

Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis



	December 31, 2018
	Level 1		Level 2		Level 3		Total
Assets
Money market funds^[1]	$	15,223	$	—	$	—	$	15,223
U.S. treasury securities	66,489		—		—		66,489
Corporate bonds	—		19,720		—		19,720
Commercial paper	—		18,529		—		18,529

Liabilities
Redeemable convertible preferred stock warrants	$	—	$	—	$	—	$	—

	December 31, 2017
	Level 1		Level 2		Level 3		Total
Assets
Money market funds^[1]	$	22,115	$	—	$	—	$	22,115

Liabilities
Redeemable convertible preferred stock warrants	$	—	$	—	$	422	$	422


[1]	Included in cash and cash equivalents on the consolidated balance sheet

Summary of Changes in Fair Value of the Redeemable Convertible Preferred Stock Warrants




Balances at December 31, 2017	$	422
Change in fair value recorded in other (income) expense, net	826
Conversion of preferred stock warrants to common stock warrants	(1,248		)
Balances at December 31, 2018	$	—

Balance Sheet Components (Tables)

12 Months Ended

Dec. 31, 2018

Organization, Consolidation and Presentation of Financial Statements [Abstract]

Schedule of Property and Equipment, Net

All property and equipment is depreciated on a straight-line basis over the estimated useful lives of the assets, which are as follows:



Computer and office equipment	3 – 5 years
Machinery and equipment	3 – 5 years
Furniture and fixtures	7 years

Property and Equipment, net (in thousands):



	December 31,			December 31,
	2018			2017
Machinery and equipment	$	3,785		$	3,428
Construction in progress	730			879
Computer and office equipment	407			310
Leasehold improvements	448			272
Furniture and fixtures	148			29
	5,518			4,918
Less: Accumulated depreciation and amortization	(3,364		)	(3,022		)
	$	2,154		$	1,896

Schedule of Accrued Liabilities and Other

Accrued Liabilities and Other (in thousands):



	December 31,		December 31,
	2018		2017
Accrued compensation and related expenses	$	5,425	$	3,732
Accrued interest	—		831
Accrued professional services	583		341
Sales tax payable	388		466
Liability for early exercise of unvested stock options	331		65
Sales and warranty reserves	35		149
Other	98		140
	$	6,860	$	5,724

Borrowings (Tables)

12 Months Ended

Dec. 31, 2018

Debt Disclosure [Abstract]

Schedule of Annual Future Minimum Principal Payments Under Loan Agreements

Approximate annual future minimum principal payments under the loan agreements as of December 31, 2018 are as follows (in thousands):



Year Ending at, December 31,
2019	$	—
2020	4,444
2021	17,778
2022	17,778
Total future minimum payments	40,000
Less:
Amount representing debt discount	(1,037		)
Total minimum payments	$	38,963

Debt Covenant EBITDA Targets

The minimum net sales and trailing 12-month consolidated EBITDA targets are as follows (in thousands):



Twelve Months Ending	Minimum Net Sales			Trailing 12-Month Consolidated EBITDA
March 31, 2019	$	52,000	or	$	(5,000	)
June 30, 2019	$	53,500	or	$	(3,500	)
September 30, 2019	$	54,500	or	$	(2,000	)
December 31, 2019	$	56,000	or	$	—
March 31, 2020	$	57,500	or	$	1,000
June 30, 2020	$	58,500	or	$	2,000
thereafter, as applicable	$	60,000	or	$	3,000

Redeemable Convertible Preferred Stock (Tables)

12 Months Ended

Dec. 31, 2018

Temporary Equity Disclosure [Abstract]

Schedule of Redeemable Convertible Preferred Stock

Preferred stock at December 31, 2017 consisted of the following:



	Shares
Series	Authorized	Issued and Outstanding	Carrying Value		Liquidation Value
			(in thousands)
Series 1	245,096	245,096	$	154	$	154
Series 2	709,617	709,608	1,489		1,520
Series 3	498,958	498,938	2,862		2,874
Series 4	2,509,047	2,509,032	15,656		15,807
Series 5	2,086,138	2,009,226	18,127		18,275
Series 6	3,389,227	3,319,274	54,508		54,674
Series 7	2,666,666	2,580,404	25,752		25,890
Total	12,104,749	11,871,578	$	118,548	$	119,194

Warrants (Tables)

12 Months Ended

Dec. 31, 2018

Other Liabilities Disclosure [Abstract]

Schedule of Warrants Issued and Outstanding

Warrants issued and outstanding at December 31, 2018 are as follows(in thousands, except share and per share data):

Date

Number of
Shares
Underlying
Warrants

Price per
Share

Fair Value

Warrants to purchase

Issuance

Expiration

Common stock

3/1/2017

3/1/2027

[a]

1,388

5.94

[b]

Common stock

7/19/2013

7/22/2023

[a]

32,983

9.10

122

[b]

Common stock

11/26/2014

11/26/2024

[a]

6,680

16.47

[b]

Common stock

10/20/2015

10/20/2025

[a]

41,650

16.47

396

[c]

Common stock

11/9/2015

11/9/2025

[a]

25,709

16.47

244

[c]

Common stock

12/22/2016

12/22/2026

[a]

9,712

10.03

[c]

Total outstanding common stock warrants

118,122


[a]	Common stock warrants will remain outstanding until exercised by the holder.


[b]	Fair value at the date of issuance.


[c]	Fair value at the date of conversion from redeemable convertible preferred stock to common stock warrants in conjunction with the IPO on October 16, 2018.

Warrants issued and outstanding at December 31, 2017 are as follows (in thousands, except share and per share data):



	Series	Date			Number of Shares Underlying Warrants	Price per Share		Fair Value
Warrants to purchase	Series	Issuance	Expiration		Number of Shares Underlying Warrants	Price per Share		Fair Value
Common stock		7/19/2013	7/22/2023	[a]	101,010	$	3.96	$	244	[b]
Common stock		11/26/2014	11/26/2024	[a]	21,928	$	3.42	$	47	[b]
Common stock		3/1/2017	3/1/2027	[a]	1,388	$	5.94	$	5	[b]
Total common stock warrants					124,326

Redeemable convertible preferred stock	Series 5	7/1/2012	7/25/2019	[d]	54,917	$	9.10	$	255	[c]
Redeemable convertible preferred stock	Series 5	7/19/2013	7/22/2023	[e]	21,989	$	9.10	$	122	[c]
Redeemable convertible preferred stock	Series 6	11/26/2014	11/26/2024	[e]	6,310	$	16.47	$	49	[c]
Redeemable convertible preferred stock	Series 6	10/20/2015	10/20/2025	[e]	39,339	$	16.47	$	396	[c]
Redeemable convertible preferred stock	Series 6	11/9/2015	11/9/2025	[e]	24,283	$	16.47	$	244	[c]
Redeemable convertible preferred stock	Series 7	12/22/2016	12/22/2026	[e]	9,712	$	10.03	$	45	[c]
Total redeemable convertible preferred stock warrants					156,550
Total outstanding common and redeemable convertible preferred stock warrants					280,876


[a]	Common stock warrants will remain outstanding until exercised by the holder.


[b]	Fair value at the date of issuance.


[c]	Fair value as of December 31, 2017.


[d]	These warrants will be net exercised immediately upon the closing of the Company’s IPO, or upon a corporate transaction as defined in the Note and Warrant Purchase Agreement dated July 25, 2012.


[e]	Convertible preferred stock warrants will remain outstanding until exercised by the holder and will convert to common stock warrants upon an IPO. The warrants will be exercisable for 10 years from the date of issuance.

Schedule of Weighted-Average Assumptions Used In Computation of Fair Value of Warrants

Weighted-average assumptions used in computation of the fair value of the redeemable convertible preferred stock warrants are summarized in the table below:



	Year Ended December 31,
	2018		2017
Remaining contractual term (in years)	4.9		5.3
Expected volatility	53.89	%	59.06	%
Risk-free interest rate	2.62	%	2.16	%
Dividend yield	—	%	—	%

- Definition

Tabular disclosure of warrants or rights issued. Warrants and rights outstanding are derivative securities that give the holder the right to purchase securities (usually equity) from the issuer at a specific price within a certain time frame. Warrants are often included in a new debt issue to entice investors by a higher return potential. The main difference between warrants and call options is that warrants are issued and guaranteed by the company, whereas options are exchange instruments and are not issued by the company. Also, the lifetime of a warrant is often measured in years, while the lifetime of a typical option is measured in months. Disclose the title of issue of securities called for by warrants and rights outstanding, the aggregate amount of securities called for by warrants and rights outstanding, the date from which the warrants or rights are exercisable, and the price at which the warrant or right is exercisable.

+ References + Details

Common Stock (Tables)

12 Months Ended

Dec. 31, 2018

Equity [Abstract]

Schedule of Common Stock Reserved For Future Issuance

The Company has reserved shares of common stock, on an issued and as-converted basis, for future issuance as follows:



	December 31, 2018	December 31, 2017
	Issued and Outstanding	Issued and Outstanding	Common Stock Equivalent Shares
Common stock	24,450,757	—	—
Series 1 common stock	—	3,112,955	3,112,955
Series 2 common stock	—	490,185	490,185
Redeemable convertible preferred stock	—	11,871,578	12,066,654
Restricted stock units outstanding	53,436	—	—
Stock options outstanding	2,641,198	3,001,929	3,001,929
Shares available for grant	2,497,082	29,654	29,654
Common stock warrants	118,122	124,326	124,326
Redeemable convertible preferred stock warrants	—	156,550	160,657
Total	29,760,595	18,787,177	18,986,360

- Definition

Tabular disclosure of an entity's stock, including par or stated value per share, number and dollar amount of share subscriptions, shares authorized, shares issued, shares outstanding, number and dollar amount of shares held in an employee trust, dividend per share, total dividends, share conversion features, par value plus additional paid in capital, the value of treasury stock and other information necessary to a fair presentation, and EPS information. Stock by class includes common, convertible, and preferred stocks which are not redeemable or redeemable solely at the option of the issuer. Includes preferred stock with redemption features that are solely within the control of the issuer and mandatorily redeemable stock if redemption is required to occur only upon liquidation or termination of the reporting entity. If more than one issue is outstanding, state the title of each issue and the corresponding dollar amount; dollar amount of any shares subscribed but unissued and the deduction of subscriptions receivable there from; number of shares authorized, issued, and outstanding.

+ References

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+ Details

Stock-Based Incentive Compensation Plans (Tables)

12 Months Ended

Dec. 31, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Schedule of Stock Option Activity

The following table summarizes stock option activity for the year ended December 31, 2018 for all stock plans:



	Options Outstanding
	Number of Shares		Weighted-Average Exercise Price		Weighted-Average Contractual Remaining Life	Aggregate Intrinsic Value
					(Years)	(in thousands)
Balances at December 31, 2017	3,001,929		$	4.15		$	3,585
Options granted	100,080		$	8.88
Options exercised	(395,117	)	$	4.08
Options canceled	(65,694	)	$	4.81
Balances at December 31, 2018	2,641,198		$	4.27	6.6	$	43,905

Options vested and exercisable - December 31, 2018	1,762,687		$	3.73	5.9	$	41,882

Options vested and expected to vest - December 31, 2018	2,366,723		$	4.14	6.5	$	40,154

Schedule of Stock-Based Compensation

The following table sets forth stock-based compensation expense recognized for the periods presented (in thousands):



	Year ended December 31,
	2018		2017
Cost of goods sold	$	34	$	23
Research and development	156		143
Sales and marketing	651		438
General and administrative	1,471		1,271
	$	2,312	$	1,875

Schedule of Valuation Assumptions Related to Stock Option Awards

The grant date fair value of the stock option awards granted to employees was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:



	Year ended
	December 31,
	2018	2017
Expected term	6.17	5.71
Expected volatility	42%-47%	42%-55%
Risk-free interest rate	2.35%-2.96%	1.73%-2.31%
Dividend yield	—%	—%

The assumptions used to determine the grant date fair value of employee stock options and ESPP purchase rights for the periods presented are as follows:



	Year ended
	December 31,
	2018
Expected term	0.5
Expected volatility	43.96%
Risk-free interest rate	2.49%
Dividend yield	—%

Schedule of Restricted Stock Unit Activity

The following table summarizes restricted stock units activity for the year ended December 31, 2018:



	Number of Shares		Weighted- Average Grant Date Fair Value
Unvested as of December 31, 2017	—		$	—
Granted	54,036		$	11.79
Forfeited	(600	)	20.60
Vested	—		$	—
Unvested as of December 31, 2018	53,436		$	11.69

Net Loss Per Share of Common Stock (Tables)

12 Months Ended

Dec. 31, 2018

Earnings Per Share [Abstract]

Schedule of Basic and Diluted Net Loss per Share

The following table summarizes the computation of basic and diluted net loss per share (in thousands, except share and per share data):



	Year ended December 31,
	2018			2017

Net loss	$	(17,453	)	$	(23,039	)
Weighted-average shares used to compute basic and diluted net loss per share*	7,950,284			3,467,096
Net loss per share, basic and diluted*	$	(2.20	)	$	(6.65	)

_________________________________________

* Calculated based on the 1-for-18 reverse stock split effected October 4, 2018.

Schedule of Antidilutive Securities Excluded from Computation of Diluted Net Loss per Share

The following common stock equivalents were excluded from the computation of diluted net loss per share for the periods presented because including them would have been antidilutive:



	December 31,
	2018	2017
Stock options	2,641,198	3,001,929
Shares subject to repurchase	74,019	16,117
Unvested restricted stock units	53,436	—
Estimated ESPP shares	89,606	—
Redeemable convertible preferred stock	—	12,066,654
Redeemable convertible preferred stock warrants	—	160,657
Common stock warrants	118,122	124,326

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax Disclosure [Abstract]

Schedule of Components of Loss Before Income Taxes

The components of the Company’s loss before income taxes are as follows (in thousands):



	Year Ended December 31,
	2018		2017
Domestic	(16,835	)	(22,717	)
Foreign	(618	)	(322	)
Loss before income taxes	(17,453	)	(23,039	)

Schedule of Components of Income Tax Expense

The components of income tax expense are as follows (in thousands):



	Year Ended December 31,
	2018			2017
Current tax expense:
Federal	$	—		$	—
State	—			—
Foreign	—			—
Total current tax expense	—			—

Deferred tax expense:
Federal	3,555			(9,574		)
State	822			2,061
Foreign	200			—
Total deferred tax expense	4,577			(7,513		)
Change in deferred tax valuation allowance	(4,577		)	7,513

Net deferred tax expense	—			—
Provision for income taxes	$	—		$	—

Schedule of Effective Income Tax Rate Reconciliation

Income tax expense differs from the amount computed by applying the statutory federal income tax rate due to the following:



	Year Ended December 31,
	2018		2017
Tax at statutory federal rate	(21.0	)%	(34.0	)%
State tax, net of federal benefit	(5.3	)%	(4.3	)%
Measurement of deferred taxes as a result of tax reform	—	%	68.7	%
Tax credits	(0.7	)%	(0.3	)%
Change in deferred tax valuation allowance	26.2	%	(32.6	)%
Other	0.8	%	2.5	%
Total income tax expense	—	%	—	%

Schedule of Deferred Tax Assets

The tax effects of temporary differences and carryforwards that give rise to significant portions of the deferred tax assets are presented below (in thousands):



	December 31,
	2018			2017
Net operating loss carryforwards	$	35,067		$	32,210
Research and development credits	2,255			2,070
Depreciation and amortization	132			179
Accruals and reserves	2,958			1,376

	40,412			35,835
Less: Valuation allowance	(40,412		)	(35,835		)
Total deferred tax asset	$	—		$	—

Summary of Changes in Valuation Allowance

The following table summarizes changes in the valuation allowance for the year ended December 31, 2018 and 2017 (in thousands):



	December 31,
	2018		2017
Beginning balance	$	35,835	$	43,348
Additions during the period	4,577		—
Deductions during the period	—		(7,513		)
Ending balance	$	40,412	$	35,835

Schedule of Uncertain Income Tax Positions

The changes in the Company’s uncertain income tax positions for the years ended December 31, 2018 and 2017 consisted of the following (in thousands):



Beginning balance as of January 1, 2017	$	950
Increases in balances related to tax positions taken during 2017	43
Ending balance as of December 31, 2017	993
Increases in balances related to tax positions taken during 2018	91
Ending balance as of December 31, 2018	$	1,084

The Company and Nature of Business (Details) $ / shares in Units, $ in Millions			1 Months Ended
	Oct. 16, 2018 USD ($) $ / shares shares	Oct. 04, 2018	Oct. 31, 2018	Dec. 31, 2018 shares	Dec. 31, 2017 shares
Subsidiary, Sale of Stock [Line Items]
Reverse stock split ratio		0.0556	0.0556
Common stock issued upon conversion of redeemable convertible preferred stock (in shares)	12,066,654
Warrants to purchase shares of stock (in shares)					280,876
Adjustments to additional paid-in capital related to warrants \| $	$ 1.2
IPO
Subsidiary, Sale of Stock [Line Items]
Number of shares issued in IPO (in shares)	8,280,000
IPO share price (in dollars per share) \| $ / shares	$ 15.00
Net proceeds from IPO \| $	$ 113.6
Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Warrants to purchase shares of stock (in shares)				0	156,550
Common Stock
Subsidiary, Sale of Stock [Line Items]
Warrants to purchase shares of stock (in shares)	160,657			118,122	124,326
Preferred Stock Warrants \| Redeemable Convertible Preferred Stock
Subsidiary, Sale of Stock [Line Items]
Number of warrants converted (in shares)	156,550
Warrants to purchase shares of stock (in shares)					156,550

Summary of Significant Accounting Policies - Revenues (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)
Accounting Policies [Abstract]
Number of reportable segments \| segment	1
Number of operating segments \| segment	1
Disaggregation of Revenue [Line Items]
Revenue	$ 55,380	$ 47,983
Domestic
Disaggregation of Revenue [Line Items]
Revenue	50,137	43,351
International
Disaggregation of Revenue [Line Items]
Revenue	$ 5,243	$ 4,632

Summary of Significant Accounting Policies - Liquidity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Accumulated deficit	$ 157,177	$ 139,724
Net cash used in operations	14,519	17,530
Cash and cash equivalents	25,120	22,408	$ 27,900
Short-term investments	97,103	$ 0
Debt covenant minimum cash balance	$ 5,000
Minimum
Debt Covenant [Line Items]
Quarterly increase in revenue target (percent)	2.00%
Maximum
Debt Covenant [Line Items]
Quarterly increase in revenue target (percent)	4.00%

Summary of Significant Accounting Policies - Property and Equipment (Details)	12 Months Ended
	Dec. 31, 2018
Computer and office equipment \| Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Computer and office equipment \| Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Machinery and equipment \| Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Machinery and equipment \| Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Estimated useful life	7 years

Summary of Significant Accounting Policies - Public Offering Costs (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Abstract]
Offering costs capitalized	$ 0	$ 0	$ 0
Offering costs transferred to additional paid-in capital		$ 1,900,000
Offering costs expensed	$ 1,300,000		$ 1,300,000

Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Short-term investments
Amortized Cost	$ 97,093
Unrealized Gains	17
Unrealized Losses	(7)
Aggregate Fair Value	97,103	$ 0
Cash equivalents
Carrying Value	22,858
Total marketable securities, Amortized Cost	119,951
Total marketable securities, Unrealized Gains	17
Total marketable securities, Unrealized Losses	(7)
Total marketable securities, Aggregate Fair Value	119,961
U.S. treasury securities
Short-term investments
Amortized Cost	65,491
Unrealized Gains	2
Unrealized Losses	(4)
Aggregate Fair Value	65,489
Cash equivalents
Carrying Value	1,000
Corporate bonds
Short-term investments
Amortized Cost	19,708
Unrealized Gains	15
Unrealized Losses	(3)
Aggregate Fair Value	19,720
Commercial paper
Short-term investments
Amortized Cost	11,894
Unrealized Gains	0
Unrealized Losses	0
Aggregate Fair Value	11,894
Cash equivalents
Carrying Value	6,635
Money market funds
Cash equivalents
Carrying Value	$ 15,223	$ 22,100

Fair Value Measurement - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Liabilities
Redeemable convertible preferred stock warrants	$ 0	$ 422
Money market funds
Assets
Investments	15,223	22,115
U.S. treasury securities
Assets
Investments	66,489
Corporate bonds
Assets
Investments	19,720
Commercial paper
Assets
Investments	18,529
Level 1
Liabilities
Redeemable convertible preferred stock warrants	0	0
Level 1 \| Money market funds
Assets
Investments	15,223	22,115
Level 1 \| U.S. treasury securities
Assets
Investments	66,489
Level 1 \| Corporate bonds
Assets
Investments	0
Level 1 \| Commercial paper
Assets
Investments	0
Level 2
Liabilities
Redeemable convertible preferred stock warrants	0	0
Level 2 \| Money market funds
Assets
Investments	0	0
Level 2 \| U.S. treasury securities
Assets
Investments	0
Level 2 \| Corporate bonds
Assets
Investments	19,720
Level 2 \| Commercial paper
Assets
Investments	18,529
Level 3
Liabilities
Redeemable convertible preferred stock warrants	0	422
Level 3 \| Money market funds
Assets
Investments	0	$ 0
Level 3 \| U.S. treasury securities
Assets
Investments	0
Level 3 \| Corporate bonds
Assets
Investments	0
Level 3 \| Commercial paper
Assets
Investments	$ 0

Fair Value Measurement - Changes in Redeemable Convertible Preferred Stock Warrants (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)
Changes in fair value of redeemable convertible preferred stock warrants
Beginning balance	$ 422
Change in fair value recorded in other (income) expense, net	826
Conversion of preferred stock warrants to common stock warrants	(1,248)
Ending balance	$ 0

Balance Sheet Components - Property and Equipment, Net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property and Equipment, Net
Property and equipment, gross	$ 5,518	$ 4,918
Less: Accumulated depreciation and amortization	(3,364)	(3,022)
Property and equipment, net	2,154	1,896
Depreciation expense	700	1,000
Machinery and equipment
Property and Equipment, Net
Property and equipment, gross	3,785	3,428
Construction in progress
Property and Equipment, Net
Property and equipment, gross	730	879
Computer and office equipment
Property and Equipment, Net
Property and equipment, gross	407	310
Leasehold improvements
Property and Equipment, Net
Property and equipment, gross	448	272
Furniture and fixtures
Property and Equipment, Net
Property and equipment, gross	$ 148	$ 29

Balance Sheet Components - Accrued Liabilities and Other (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Liabilities and Other
Accrued compensation and related expenses	$ 5,425	$ 3,732
Accrued interest	0	831
Accrued professional services	583	341
Sales tax payable	388	466
Liability for early exercise of unvested stock options	331	65
Sales and warranty reserves	35	149
Other	98	140
Accrued liabilities and other	$ 6,860	$ 5,724

Commitments and Contingencies - Narrative (Details) - USD ($)	1 Months Ended	12 Months Ended
	May 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Feb. 28, 2018	Aug. 31, 2012
Operating Leased Assets [Line Items]
Rent expense		$ 1,200,000	$ 1,000,000
Purchase commitments related to inventory		200,000	$ 100,000
San Jose, California
Operating Leased Assets [Line Items]
Non-cancelable operating lease term					4 years
Early termination fees	$ 0
Santa Clara, California
Operating Leased Assets [Line Items]
Non-cancelable operating lease term				7 years
Indemnification Agreement
Operating Leased Assets [Line Items]
Costs to defend lawsuits or settle claims		0
Costs accrued to defend lawsuits		$ 0

Commitments and Contingencies - Aggregate Future Minimum Lease Payments (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Aggregate Future Minimum Lease Payments
2019	$ 1,035
2020	1,033
2021	914
2022	842
2023	844
Thereafter	1,184
Total	$ 5,852

Borrowings - Narrative (Details) - USD ($)	1 Months Ended			12 Months Ended
Borrowings - Narrative (Details) - USD ($)	Oct. 31, 2017	Aug. 31, 2016	Oct. 31, 2015	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long term borrowings				$ 38,963,000	$ 38,700,000
Repayment of debt financing				0	1,119,000
Repayment of debt extinguishment				0	29,081,000
Amortization of debt discount				259,000	285,000
Debt covenant minimum cash balance				5,000,000
SVB and Oxford Term Loan
Debt Instrument [Line Items]
Repayment of debt extinguishment	$ 1,500,000
Loss on extinguishment of debt	2,200,000
Term Loan \| SVB and Oxford Term Loan
Debt Instrument [Line Items]
Debt face amount		$ 35,200,000.0
Debt period payment, interest only period		6 months
Debt period payment, principal and interest period		25 months
Debt interest stated percentage		11.00%
Repayment of debt financing	29,100,000
Amortization of debt discount	700,000
Term Loan \| SVB and Oxford Term Loan \| Prime Rate
Debt Instrument [Line Items]
Basis spread on variable rate		7.75%
Term Loan \| Pharmakon Term Loan
Debt Instrument [Line Items]
Long term borrowings				39,000,000	$ 38,700,000
Debt face amount	$ 40,000,000.0
Debt period payment, interest only period	35 months
Debt interest stated percentage	11.50%
Debt issuance costs	$ 1,300,000
Debt closing fee percentage	1.50%
Debt instrument fee amount	$ 600,000
Debt covenant minimum cash balance	$ 5,000,000
Debt borrowing capacity				$ 10,000,000
Debt effective percentage				12.30%	12.00%
Term Loan \| Pharmakon Term Loan \| Prepayment Penalty, 31-48 Months
Debt Instrument [Line Items]
Debt prepayment penalty percentage	2.00%
Term Loan \| Pharmakon Term Loan \| Prepayment Penalty, 49-60 Months
Debt Instrument [Line Items]
Debt prepayment penalty percentage	1.00%
Line of Credit \| Revolving Credit Facility \| SVB and Oxford Term Loan
Debt Instrument [Line Items]
Line of credit maximum borrowing capacity			$ 4,000,000.0
Line of credit percent of customer accounts receivable			80.00%
Line of Credit \| Revolving Credit Facility \| SVB and Oxford Term Loan \| Prime Rate
Debt Instrument [Line Items]
Basis spread on variable rate			3.00%

Borrowings - Annual Future Minimum Principal Payments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Maturities of Long-term Debt [Abstract]
2019	$ 0
2020	4,444
2021	17,778
2022	17,778
Total future minimum payments	40,000
Amount representing debt discount	(1,037)
Total minimum payments	$ 38,963	$ 38,700

Borrowings - EBITDA Targets (Details) $ in Thousands	12 Months Ended
Borrowings - EBITDA Targets (Details) $ in Thousands	Dec. 31, 2018 USD ($)
Period One
Debt Instrument [Line Items]
Debt Covenant, Minimum Net Sales	$ 52,000
Debt Covenant, Trailing 12-Month Consolidated EBITDA	(5,000)
Period Two
Debt Instrument [Line Items]
Debt Covenant, Minimum Net Sales	53,500
Debt Covenant, Trailing 12-Month Consolidated EBITDA	(3,500)
Period Three
Debt Instrument [Line Items]
Debt Covenant, Minimum Net Sales	54,500
Debt Covenant, Trailing 12-Month Consolidated EBITDA	(2,000)
Period Four
Debt Instrument [Line Items]
Debt Covenant, Minimum Net Sales	56,000
Debt Covenant, Trailing 12-Month Consolidated EBITDA	0
Period Five
Debt Instrument [Line Items]
Debt Covenant, Minimum Net Sales	57,500
Debt Covenant, Trailing 12-Month Consolidated EBITDA	1,000
Period Six
Debt Instrument [Line Items]
Debt Covenant, Minimum Net Sales	58,500
Debt Covenant, Trailing 12-Month Consolidated EBITDA	2,000
Period Seven
Debt Instrument [Line Items]
Debt Covenant, Minimum Net Sales	60,000
Debt Covenant, Trailing 12-Month Consolidated EBITDA	$ 3,000

Redeemable Convertible Preferred Stock - IPO Narrative (Details) - shares	Oct. 16, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Temporary Equity Disclosure [Abstract]
Common stock issued upon conversion of redeemable convertible preferred stock (in shares)	12,066,654
Redeemable convertible preferred stock outstanding (in shares)		0	11,871,578	11,330,704

Redeemable Convertible Preferred Stock - Preferred Stock (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) vote director $ / shares shares	Mar. 31, 2017 $ / shares shares	Feb. 28, 2017 $ / shares shares	Dec. 31, 2016 USD ($) shares
Temporary Equity [Line Items]
Shares Authorized (in shares)	0	12,104,749
Redeemable convertible preferred stock issued (in shares)	0	11,871,578
Redeemable convertible preferred stock outstanding (in shares)	0	11,871,578			11,330,704
Carrying Value \| $	$ 0	$ 118,548			$ 113,121
Liquidation Value \| $	$ 0	$ 119,194
Series 2 common stock, number of directors entitled to elect \| director		2
Shareholder approval, minimum shares outstanding (in shares)		277,778
Preferred stock dividends declared (in dollars per share) \| $ / shares	$ 0	$ 0
Common stock dividends declared (in dollars per share) \| $ / shares	$ 0	$ 0
Minimum public offering for conversion \| $		$ 50,000
Minimum public offering per share for conversion (in dollars per share) \| $ / shares		$ 30.0996
Series 1 Preferred Stock
Temporary Equity [Line Items]
Shares Authorized (in shares)		245,096
Redeemable convertible preferred stock issued (in shares)		245,096
Redeemable convertible preferred stock outstanding (in shares)		245,096
Carrying Value \| $		$ 154
Liquidation Value \| $		$ 154
Preferred stock votes per share \| vote		0.352941
Dividend rate (in dollars per share) \| $ / shares		$ 0.050112
Liquidation preference per share (in dollars per share) \| $ / shares		$ 0.6264
Series 2 Preferred Stock
Temporary Equity [Line Items]
Shares Authorized (in shares)		709,617
Redeemable convertible preferred stock issued (in shares)		709,608
Redeemable convertible preferred stock outstanding (in shares)		709,608
Carrying Value \| $		$ 1,489
Liquidation Value \| $		$ 1,520
Preferred stock votes per share \| vote		0.352941
Dividend rate (in dollars per share) \| $ / shares		$ 0.171360
Liquidation preference per share (in dollars per share) \| $ / shares		$ 2.142
Series 3 Preferred Stock
Temporary Equity [Line Items]
Shares Authorized (in shares)		498,958
Redeemable convertible preferred stock issued (in shares)		498,938
Redeemable convertible preferred stock outstanding (in shares)		498,938
Carrying Value \| $		$ 2,862
Liquidation Value \| $		$ 2,874
Preferred stock votes per share \| vote		0.352941
Dividend rate (in dollars per share) \| $ / shares		$ 0.4608
Liquidation preference per share (in dollars per share) \| $ / shares		$ 5.76
Series 4 Preferred Stock
Temporary Equity [Line Items]
Shares Authorized (in shares)		2,509,047
Redeemable convertible preferred stock issued (in shares)		2,509,032
Redeemable convertible preferred stock outstanding (in shares)		2,509,032
Carrying Value \| $		$ 15,656
Liquidation Value \| $		$ 15,807
Preferred stock votes per share \| vote		1
Preferred stock, number of directors entitled to elect \| director		1
Dividend rate (in dollars per share) \| $ / shares		$ 0.504
Liquidation preference per share (in dollars per share) \| $ / shares		6.30
Liquidation participation cap per share (in dollars per share) \| $ / shares		$ 12.60
Series 5 Preferred Stock
Temporary Equity [Line Items]
Shares Authorized (in shares)		2,086,138
Redeemable convertible preferred stock issued (in shares)		2,009,226
Redeemable convertible preferred stock outstanding (in shares)		2,009,226
Carrying Value \| $		$ 18,127
Liquidation Value \| $		$ 18,275
Preferred stock votes per share \| vote		1
Preferred stock, number of directors entitled to elect \| director		1
Dividend rate (in dollars per share) \| $ / shares		$ 0.72774
Liquidation preference per share (in dollars per share) \| $ / shares		9.0954
Liquidation participation cap per share (in dollars per share) \| $ / shares		$ 18.1908
Series 6 Preferred Stock
Temporary Equity [Line Items]
Shares Authorized (in shares)		3,389,227
Redeemable convertible preferred stock issued (in shares)		3,319,274
Redeemable convertible preferred stock outstanding (in shares)		3,319,274
Carrying Value \| $		$ 54,508
Liquidation Value \| $		$ 54,674
Preferred stock votes per share \| vote		1
Preferred stock, number of directors entitled to elect \| director		1
Dividend rate (in dollars per share) \| $ / shares		$ 1.317744
Liquidation preference per share (in dollars per share) \| $ / shares		16.4718
Liquidation participation cap per share (in dollars per share) \| $ / shares		$ 32.9436
Conversion price (in dollars per share) \| $ / shares			$ 15.5574	$ 15.714
Conversion ratio (in shares)			1.06	1.05
Series 7 Preferred Stock
Temporary Equity [Line Items]
Shares Authorized (in shares)		2,666,666
Redeemable convertible preferred stock issued (in shares)		2,580,404
Redeemable convertible preferred stock outstanding (in shares)		2,580,404
Carrying Value \| $		$ 25,752
Liquidation Value \| $		$ 25,890
Preferred stock votes per share \| vote		1
Preferred stock, number of directors entitled to elect \| director		1
Dividend rate (in dollars per share) \| $ / shares		$ 0.802656
Liquidation preference per share (in dollars per share) \| $ / shares		$ 10.0332

Warrants - IPO Narrative (Details) $ in Millions		12 Months Ended
Warrants - IPO Narrative (Details) $ in Millions	Oct. 16, 2018 USD ($) shares	Dec. 31, 2018 shares	Dec. 31, 2017 shares
Class of Warrant or Right [Line Items]
Warrants to purchase shares of stock (in shares)			280,876
Adjustments to additional paid-in capital related to warrants \| $	$ 1.2
Redeemable Convertible Preferred Stock
Class of Warrant or Right [Line Items]
Warrants to purchase shares of stock (in shares)		0	156,550
Redeemable Convertible Preferred Stock \| Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Number of warrants converted (in shares)	156,550
Warrants to purchase shares of stock (in shares)			156,550
Common Stock
Class of Warrant or Right [Line Items]
Warrants to purchase shares of stock (in shares)	160,657	118,122	124,326
Series 6 Preferred Stock \| Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Warrant conversion ratio		0.9433

Warrants - Issued and Outstanding (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2018	Oct. 16, 2018	Dec. 31, 2017	Mar. 01, 2017	Dec. 22, 2016	Nov. 09, 2015	Oct. 20, 2015	Nov. 26, 2014	Jul. 19, 2013	Jul. 01, 2012
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			280,876
Common Stock
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	118,122	160,657	124,326
Common Stock \| Warrants Issued 7/19/2013
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	32,983		101,010						101,010
Price per Share (in dollars per share)	$ 9.1		$ 3.96						$ 3.96
Fair Value	$ 122		$ 244
Common Stock \| Warrants Issued 11/26/2014
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	6,680		21,928					21,928
Price per Share (in dollars per share)	$ 16.47		$ 3.42					$ 3.42
Fair Value	$ 49		$ 47
Common Stock \| Warrants Issued 3/1/2017
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	1,388		1,388	1,388
Price per Share (in dollars per share)	$ 5.94		$ 5.94	$ 5.94
Fair Value	$ 5		$ 5
Common Stock \| Warrants Issued 10/20/2015
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	41,650
Price per Share (in dollars per share)	$ 16.47
Fair Value	$ 396
Common Stock \| Warrants Issued 11/9/2015
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	25,709
Price per Share (in dollars per share)	$ 16.47
Fair Value	$ 244
Common Stock \| Warrants Issued 12/22/2016
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	9,712
Price per Share (in dollars per share)	$ 10.03
Fair Value	$ 45
Common Stock \| Common Stock Warrants
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	118,122		124,326
Redeemable Convertible Preferred Stock
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)	0		156,550
Redeemable Convertible Preferred Stock \| Preferred Stock Warrants
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			156,550
Warrants exercise period from date of issuance			10 years
Series 5 Preferred Stock \| Warrants Issued 7/1/2012
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			54,917							54,917
Price per Share (in dollars per share)			$ 9.1							$ 9.10
Fair Value			$ 255
Series 5 Preferred Stock \| Warrants Issued 7/19/2013
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			21,989						32,983
Price per Share (in dollars per share)			$ 9.1						$ 9.10
Fair Value			$ 122
Series 6 Preferred Stock \| Warrants Issued 11/26/2014
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			6,310					6,310
Price per Share (in dollars per share)			$ 16.47					$ 16.47
Fair Value			$ 49
Series 6 Preferred Stock \| Warrants Issued 10/20/2015
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			39,339				39,339
Price per Share (in dollars per share)			$ 16.47				$ 16.47
Fair Value			$ 396
Series 6 Preferred Stock \| Warrants Issued 11/9/2015
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			24,283			24,283
Price per Share (in dollars per share)			$ 16.47			$ 16.47
Fair Value			$ 244
Series 7 Preferred Stock \| Warrants Issued 12/22/2016
Class of Warrant or Right [Line Items]
Number of Shares Underlying Warrants (in shares)			9,712		9,712
Price per Share (in dollars per share)			$ 10.03		$ 10.03
Fair Value			$ 45

Warrants - Weighted-Average Assumptions in Computation of Fair Value (Details) - Warrants	Dec. 31, 2018	Dec. 31, 2017
Remaining contractual term (in years)
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants, term	4 years 10 months 24 days	5 years 3 months 18 days
Expected volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants, measurement input	0.5389	0.5906
Risk-free interest rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants, measurement input	0.0262	0.0216
Dividend yield
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants, measurement input	0	0

Common Stock - Narrative (Details) - $ / shares	1 Months Ended	12 Months Ended
Common Stock - Narrative (Details) - $ / shares	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Oct. 31, 2018
Class of Stock [Line Items]
Common stock authorized (in shares)	19,333,333	100,000,000	19,333,333	100,000,000
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001	$ 0.0001	$ 0.0001
Preferred stock authorized (in shares)		5,000,000		5,000,000
Preferred stock, par value (in dollars per share)		$ 0.0001		$ 0.0001
Preferred stock issued (in shares)		0
Preferred stock outstanding (in shares)		0
Common stock, dividends declared (in dollars per share)		$ 0	$ 0
Series 1 Common Stock
Class of Stock [Line Items]
Common stock authorized (in shares)	6,000,000
Series 2 Common Stock
Class of Stock [Line Items]
Increase in common stock authorized (in shares)	555,555
Common stock authorized (in shares)	13,333,333

Common Stock - Shares of Stock Issued and Reserved (Details) - shares	Dec. 31, 2018	Oct. 16, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Common stock outstanding (in shares)	24,450,757		3,603,140
Redeemable convertible preferred stock outstanding (in shares)	0		11,871,578	11,330,704
Stock options outstanding (in shares)	2,641,198		3,001,929
Stock options outstanding, common stock equivalents (in shares)			3,001,929
Stock options available for grant (in shares)	2,497,082		29,654
Warrants to purchase shares of stock (in shares)			280,876
Total issued and reserved for issuance (in shares)	29,760,595		18,787,177
Total issued and reserved for issuance, common stock equivalents (in shares)			18,986,360
Series 1 Common Stock
Class of Stock [Line Items]
Common stock outstanding (in shares)			3,112,955
Series 2 Common Stock
Class of Stock [Line Items]
Common stock outstanding (in shares)			490,185
Warrants to purchase shares of stock (in shares)	118,122	160,657	124,326
Warrants to purchase shares of stock, common stock equivalents (in shares)			124,326
Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Redeemable convertible preferred stock outstanding (in shares)	0		11,871,578
Redeemable convertible preferred stock outstanding, common stock equivalents (in shares)			12,066,654
Warrants to purchase shares of stock (in shares)	0		156,550
Warrants to purchase shares of stock, common stock equivalents (in shares)			160,657
Restricted Stock Units
Class of Stock [Line Items]
Restricted stock units outstanding (in shares)	53,436		0

Stock-Based Incentive Compensation Plans - Narrative (Details) - USD ($)	1 Months Ended			12 Months Ended		49 Months Ended
	Oct. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2021
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted, weighted-average grant date fair value (in dollars per share)				$ 3.98	$ 1.80
Early exercise of stock options, share subject to repurchase (in shares)		16,117		74,019	16,117
Liability for early exercise of unvested stock options		$ 65,000		$ 331,000	$ 65,000
Forgiveness of notes receivable				$ 0	400,000
Stock options granted (in shares)				100,080
Grant price of stock options granted (in dollars per share)				$ 8.88
Unrecognized compensation cost				$ 4,500,000
Unrecognized compensation cost, expected period for recognition (in years)				2 years 3 months 15 days
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of option as percent of fair market value when individual owns more than 10% of shares outstanding (percent)				110.00%
Incentive Stock Options \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of option as percent of fair market value (percent)				100.00%
Nonqualified Stock Options \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price of option as percent of fair market value (percent)				85.00%
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period (in years)				10 years
Expiration period when individual holds more than 10% of shares outstanding (in years)				5 years
Award vesting period (in years)				4 years
Stock-based compensation expense				$ 2,312,000	$ 1,875,000
Restricted Stock Units \| Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)				2 years
Restricted Stock Units \| Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)				4 years
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)			564,098
Grant price of stock options granted (in dollars per share)		$ 4.68	$ 5.94
Stock-based compensation expense				$ 600,000
Number of options with modification of terms (in shares)		394,652
Performance Shares \| Forecast
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense to be recognized related to modification of terms						$ 800,000
Performance Shares \| Vesting Tranche One
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)			4 years
Stock options granted (in shares)			408,544
Performance Shares \| Vesting Tranche Two
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period (in years)			3 years
Stock options granted (in shares)			155,554
2008 Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)		5,350,080			5,350,080
2018 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)				2,576,538
2018 Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)	515,307
Purchase price of common stock as a percent of fair market value (percent)				85.00%
Employees stock purchase program offering period interval (in months)				6 months
Stock-based compensation expense	$ 100,000
Unrecognized compensation cost				$ 300,000

Stock-Based Incentive Compensation Plans - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Number of Shares
Beginning balance (in shares) \| shares	3,001,929
Options granted (in shares) \| shares	100,080
Options exercised (in shares) \| shares	(395,117)
Options canceled (in shares) \| shares	(65,694)
Ending balance (in shares) \| shares	2,641,198	3,001,929
Options vested and exercisable (in shares) \| shares	1,762,687
Options vested and expected to vest (in shares) \| shares	2,366,723
Weighted-Average Exercise Price
Beginning balance (in dollars per share) \| $ / shares	$ 4.15
Options granted (in dollars per share) \| $ / shares	8.88
Options exercised (in dollars per share) \| $ / shares	4.08
Options canceled (in dollars per share) \| $ / shares	4.81
Ending balance (in dollars per share) \| $ / shares	4.27	$ 4.15
Options vested and exercisable (in dollars per share) \| $ / shares	3.73
Options vested and expected to vest (in dollars per share) \| $ / shares	$ 4.14
Weighted-Average Remaining Contractual Life and Aggregate Intrinsic Value
Weighted-average remaining contractual life, options outstanding (in years)	6 years 7 months 6 days
Weighted-average remaining contractual life, options vested and exercisable (in years)	5 years 11 months 8 days
Weighted-average remaining contractual life, options vested and expected to vest (in years)	6 years 5 months 27 days
Aggregate intrinsic value \| $	$ 43,905	$ 3,585
Aggregate intrinsic value, options vested and exercisable \| $	41,882
Aggregate intrinsic value, options vested and expected to vest \| $	$ 40,154

Stock-Based Incentive Compensation Plans - Stock-Based Compensation Expense (Details) - Stock Options - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 2,312	$ 1,875
Cost of goods sold
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	34	23
Research and development
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	156	143
Sales and marketing
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	651	438
General and administrative
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 1,471	$ 1,271

Stock-Based Incentive Compensation Plans - Weighted Average Assumptions (Details) - Stock Options	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Black-Scholes option-pricing model assumptions
Expected term (in years)	6 years 1 month 30 days	5 years 8 months 17 days
Expected volatility, minimum (percent)	42.00%	42.00%
Expected volatility, maximum (percent)	47.00%	55.00%
Risk-free interest rate, minimum (percent)	2.35%	1.73%
Risk-free interest rate, maximum (percent)	2.96%	2.31%
Dividend yield (percent)	0.00%	0.00%

Stock-Based Incentive Compensation Plans - Restricted Stock Unit Activity (Details) - Restricted Stock Units - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Number of Shares
Unvested, beginning balance (in shares)	0
Granted (in shares)	54,036
Forfeited (in shares)	(600)
Vested (in shares)	0	0
Unvested, ending balance (in shares)	53,436	0
Weighted- Average Grant Date Fair Value
Unvested, beginning balance (in dollars per share)	$ 0.00
Granted (in dollars per share)	11.79
Forfeited (in dollars per share)	20.60
Vested (in dollars per share)	0.00
Unvested, ending balance (in dollars per share)	$ 11.69	$ 0.00

Stock-Based Incentive Compensation Plans - Employee Stock Options and ESPP Purchase Rights (Details) - Stock Options and ESPP Purchase Rights	12 Months Ended
	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 months
Expected volatility (percent)	43.96%
Risk-free interest rate (percent)	2.49%
Dividend yield (percent)	0.00%

Related Party Transactions (Details) - USD ($) $ in Thousands	1 Months Ended				12 Months Ended
	Mar. 31, 2017	Nov. 30, 2016	Feb. 28, 2014	Mar. 31, 2013	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Repayment of stockholders’ notes receivable					$ 0	$ 84
Interest expense					$ 5,108	6,204
Chief Financial Officer
Related Party Transaction [Line Items]
Stockholders’ notes receivable				$ 200
Stockholders' note interest rate (percent)				1.09%
Repayment of stockholders’ notes receivable		$ 100
Chief Executive Officer
Related Party Transaction [Line Items]
Stockholders’ notes receivable			$ 400
Stockholders' note interest rate (percent)			1.97%
Chief Executive Officer \| Forgiveness of Related Party Promissory Note
Related Party Transaction [Line Items]
Notes receivable forgiven	$ 200
Notes receivable percentage of principal approved for forgiveness	50.00%
Stock-based compensation expense						400
Interest expense						$ 100

Net Loss Per Share of Common Stock - Schedule of Earnings Per Share (Details) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Oct. 04, 2018	Oct. 31, 2018	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares
Earnings Per Share [Abstract]
Net loss \| $			$ (17,453)	$ (23,039)
Weighted-average number of common shares used to compute basic and diluted net loss per share (in shares) \| shares			7,950,284	3,467,096
Net loss per share, basic and diluted (in dollars per share) \| $ / shares			$ (2.20)	$ (6.65)
Reverse stock split ratio	0.0556	0.0556

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Net Loss Per Share of Common Stock - Antidilutive Securities Excluding from Computation of Earnings Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the computation of diluted net loss per share	2,641,198	3,001,929
Shares subject to repurchase
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the computation of diluted net loss per share	74,019	16,117
Unvested restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the computation of diluted net loss per share	53,436	0
Estimated ESPP shares
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the computation of diluted net loss per share	89,606	0
Redeemable convertible preferred stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the computation of diluted net loss per share	0	12,066,654
Redeemable convertible preferred stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the computation of diluted net loss per share	0	160,657
Common stock warrants
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock equivalents excluded from the computation of diluted net loss per share	118,122	124,326

Income Taxes - Components of Loss Before Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Components of loss before income taxes
Domestic	$ (16,835)	$ (22,717)
Foreign	(618)	(322)
Loss before income taxes	$ (17,453)	$ (23,039)

Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Current tax expense:
Federal	$ 0	$ 0
State	0	0
Foreign	0	0
Total current tax expense	0	0
Deferred tax expense:
Federal	3,555	(9,574)
State	822	2,061
Foreign	200	0
Total deferred tax expense	4,577	(7,513)
Change in deferred tax valuation allowance	(4,577)	7,513
Net deferred tax expense	0	0
Provision for income taxes	$ 0	$ 0

Income Taxes - Reconciliation of Effective Income Tax Rate (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation
Tax at statutory federal rate (percent)	(21.00%)	(34.00%)
State tax, net of federal benefit (percent)	(5.30%)	(4.30%)
Measurement of deferred taxes as a result of tax reform (percent)	(0.00%)	68.70%
Tax credits (percent)	(0.70%)	(0.30%)
Change in deferred tax valuation allowance (percent)	26.20%	(32.60%)
Other (percent)	0.80%	2.50%
Total income tax expense (percent)	(0.00%)	(0.00%)

Income Taxes - Temporary Differences and Carryforwards and Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets
Net operating loss carryforwards	$ 35,067	$ 32,210
Research and development credits	2,255	2,070
Depreciation and amortization	132	179
Accruals and reserves	2,958	1,376
Deferred tax asset, gross	40,412	35,835
Less: Valuation allowance	(40,412)	(35,835)	$ (43,348)
Total deferred tax asset	$ 0	$ 0

Income Taxes - Uncertain Income Tax Positions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Uncertain income tax positions [Roll Forward]
Uncertain income tax positions, beginning balance	$ 993	$ 950
Increases in balances related to tax positions taken during current period	91	43
Uncertain income tax positions, ending balance	$ 1,084	$ 993

Income Taxes - Narrative (Details) - USD ($)	12 Months Ended
Income Taxes - Narrative (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Line Items]
Limited net operating loss carryforwards	$ 1,400,000
Accrued interest related to unrecognized tax benefits	0	$ 0
Decrease in net deferred tax asset position due to change in tax rate	15,800,000
Federal
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	135,500,000
Credit carryforwards	1,700,000
State
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	109,900,000
Credit carryforwards	$ 1,900,000

Employee Benefit Plan	12 Months Ended
Employee Benefit Plan	Dec. 31, 2018
Retirement Benefits [Abstract]
Employee Benefit Plan	Employee Benefit Plan The Company sponsors a 401(k) plan covering all employees. Contributions made by the Company are discretionary and are determined annually by the Board of Directors. The Company has made no contributions to the 401(k) plan since its inception.

Summary of Significant Accounting Policies - Warranty Program (Details)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Warranty term (in years)	1 year
Sales credit reserve term (in years)	1 year

Income Taxes - Changes in Valuation Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Changes in Valuation Allowance
Valuation allowance, beginning balance	$ 35,835	$ 43,348
Additions (Deductions) during the period	4,577	(7,513)
Valuation allowance, ending balance	$ 40,412	$ 35,835