Inspire Medical Systems (INSP) 10-Q 7 Jun 18 2018 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	Jun. 07, 2018
Document and Entity Information [Abstract]
Entity Registrant Name	Inspire Medical Systems, Inc.
Entity Central Index Key	1,609,550
Document Type	10-Q
Document Period End Date	Mar. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		21,298,809
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1

CONDENSED BALANCE SHEETS

CONDENSED BALANCE SHEETS - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 11,147	$ 8,955
Short-term investments	6,756	7,188
Accounts receivable, net of allowances of $47	3,459	3,858
Inventories	3,394	3,670
Prepaid expenses and other assets	2,187	426
Total current assets	26,943	24,097
Property and equipment, cost	1,794	1,804
Less: accumulated depreciation	(883)	(810)
Property and equipment, net	911	994
Total assets	27,854	25,091
Current liabilities:
Accounts payable	3,916	2,998
Accrued expenses	3,143	4,032
Accrued interest	181	117
Current portion of notes payable	681
Total current liabilities	7,921	7,147
Notes payable	23,890	16,460
Preferred stock warrants	978	157
Total long-term liabilities	24,868	16,617
Stockholders' (deficit) equity
Preferred Stock, $0.001 par value per share, 76,894,620 shares authorized at March 31, 2018 and December 31, 2017, and 76,235,050 shares issued and outstanding at March 31, 2018 and December 31, 2017	119,106	119,106
Common Stock, $0.001 par value per share; 110,000,000 shares authorized at March 31, 2018 and December 31, 2017; 1,379,099 and 1,272,360 issued and outstanding at March 31, 2018 and December 31, 2017, respectively	1	1
Additional paid in capital	7,546	7,305
Accumulated deficit	(131,586)	(125,085)
Total stockholders' (deficit) equity	(4,935)	1,327
Total liabilities and stockholders' (deficit) equity	$ 27,854	$ 25,091

CONDENSED BALANCE SHEETS (Paren

CONDENSED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
CONDENSED BALANCE SHEETS
Accounts receivable, allowances (in dollars)	$ 47
Preferred shares, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred shares, authorized shares	76,894,620	76,894,620
Preferred shares, issued shares	76,235,050	76,235,050
Preferred shares, outstanding shares	76,235,050	76,235,050
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Common stock, authorized shares	110,000,000	110,000,000
Common stock, issued shares	1,379,099	1,272,360
Common stock, outstanding shares	1,379,099	1,272,360

CONDENSED STATEMENTS OF OPERATI

CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS
Revenues	$ 10,042	$ 5,297
Cost of goods sold	2,294	1,196
Gross profit	7,748	4,101
Operating expenses:
Selling and marketing	9,868	5,745
Research and development	1,730	1,593
General and administrative	1,345	887
Total operating expenses	12,943	8,225
Operating loss	(5,195)	(4,124)
Other expense (income):
Interest income	(60)	(18)
Interest expense	1,385	374
Other expense (income), net	19	1
Loss before income taxes	(6,501)	(4,481)
Income taxes	0
Net loss	(6,501)	(4,481)
Total comprehensive loss	$ (6,501)	$ (4,481)
Net loss per share, basic and diluted	$ (5.05)	$ (3.90)
Weighted average common shares used to compute net loss per share, basic and diluted	1,286,633	1,148,565

CONDENSED STATEMENTS OF CASH FL

CONDENSED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating activities
Net loss	$ (6,501)	$ (4,481)
Adjustments to reconcile net loss:
Depreciation and amortization	90	43
Accretion of debt discount	213	61
Stock-based compensation expense	55	54
Change in the fair value of preferred stock warrants	718
Changes in operating assets and liabilities:
Accounts receivable	399	(188)
Inventories	276	162
Prepaid expenses and other assets	(1,761)	(47)
Accounts payable	918	127
Accrued expenses	(825)	(1,091)
Net cash used in operating activities	(6,418)	(5,360)
Investing activities
Purchases of property and equipment	(7)	(206)
Purchases of short-term investments	(4,642)
Proceeds from sales or maturities of short-term investments	5,074
Net cash provided by (used in) investing activities	425	(206)
Financing activities
Proceeds from issuance of notes payable	8,000	458
Proceeds from the exercise of stock options	185	6
Proceeds from sale of preferred stock		24,968
Net cash provided by financing activities	8,185	25,432
Increase in cash and cash equivalents	2,192	19,866
Cash and cash equivalents at beginning of period	8,955	6,685
Cash and cash equivalents at end of period	11,147	26,551
Supplemental cash flow information
Cash paid for interest	347	308
Issuance of preferred stock warrants	$ 103	$ 4

Organization

Organization	3 Months Ended
	Mar. 31, 2018
Organization
Organization	1. Organization Description of Business Inspire Medical Systems, Inc. (the Company) is a medical technology company focused on the development and commercialization of innovative and minimally invasive solutions for patients with obstructive sleep apnea. The Company's proprietary Inspire system is the first and only FDA-approved neurostimulation technology that provides a safe and effective treatment for moderate to severe obstructive sleep apnea. The Company has developed a novel, closed-loop solution that continuously monitors a patient's breathing and delivers mild hypoglossal nerve stimulation to maintain an open airway. Inspire therapy received premarket approval (PMA) from the U.S. Food and Drug Administration, or FDA, in April 2014 and has been commercially available in certain European markets since November 2011.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2018
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying condensed financial statements have been prepared by the Company, without audit, pursuant to the rules and regulations of the SEC. The condensed financial statements may not include all disclosures required by U.S. GAAP; however, the Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited condensed financial statements should be read in conjunction with the audited financial statements and the notes thereto for the fiscal year ended December 31, 2017 included in the Company's Registration Statement on Form S-1 and Prospectus dated May 2, 2018. The condensed balance sheet at December 31, 2017 was derived from the audited financial statements. In the opinion of management, all adjustments, consisting of only normal recurring adjustments that are necessary to present fairly the financial position, results of operations, and cash flows for the interim periods, have been made. The results of operations for the interim periods are not necessarily indicative of the operating results for the full fiscal year or any future periods. Reverse Stock Split In connection with its initial public offering of common stock (the IPO) the Company's board of directors and stockholders approved a 1-for-6.650 reverse stock split of the Company's common stock. The reverse stock split became effective on April 20, 2018. The par value of the common stock was not adjusted as a result of the reverse stock split. Adjustments corresponding to the reverse stock split were made to the ratio at which the convertible preferred stock will convert into common stock immediately prior to the closing of the IPO. Accordingly, all share and per-share amounts for all periods presented in these financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect the reverse stock split and adjustment of the conversion ratio of the convertible preferred stock. Initial Public Offering On May 7, 2018, the Company completed its IPO by issuing 7,762,500 shares of common stock, at an offering price of $16.00 per share, for net proceeds of approximately $112.2 million after deducting underwriting discounts and commissions and offering expenses payable by the Company. In connection with the IPO, the Company's outstanding shares of convertible preferred stock were automatically converted into an aggregate of 12,111,706 shares of common stock, and the Company's outstanding warrants to purchase shares of convertible preferred stock were automatically converted into warrants to purchase up to an aggregate of 98,846 shares of common stock, resulting in the reclassification of the related redeemable convertible preferred stock warrant liability of $978 to additional paid-in-capital. Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts and disclosures reported in the financial statements. Management uses significant judgment when making estimates related to its allowance for doubtful accounts, inventory reserves and the valuations of its common stock, share-based awards, and certain of its outstanding preferred stock warrants. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. 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. Cash and Cash Equivalents The Company considers all highly liquid securities, readily convertible to cash, that mature within 90 days or less from the date of purchase to be cash equivalents. The carrying amount reported in the balance sheets for cash is cost, which approximates fair value. Short-term Investments At March 31, 2018 and December 31, 2017, the Company's short-term investments consisted of commercial paper and corporate bonds which are classified as available-for-sale and had maturities less than one year. Short-term investments are reported at their estimated fair market value which approximates cost. Any unrealized gains and losses are reported in accumulated other comprehensive loss. Fair Value of Financial Instruments The Company measures certain financial assets and liabilities at fair value on a recurring basis, including cash equivalents and certain of its outstanding preferred stock warrants. 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 a liability. A three-tier fair value hierarchy is established as a basis for considering such assumptions and for inputs used in the valuation methodologies in measuring fair value: Level 1—Observable inputs, such as quoted prices (unadjusted) for identical assets or liabilities in active markets. Level 2—Other inputs that are based upon quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-based valuation techniques for which all significant inputs are observable in the market or can be derived from observable market data. Where applicable, these models project future cash flows and discount the future amounts to a present value using market-based observable inputs, including interest rate curves, foreign exchange rates, and credit ratings. Level 3—Unobservable inputs that are supported by little or no market activities, which would require the Company to develop its own assumptions. The Company uses the methods and assumptions described below in determining the fair value of its financial instruments. Money market funds: Fair values of money market funds are based on quoted market prices in active markets. Commercial paper: Short-term, highly liquid investments are included as a Level 2 measurement in the tables below. Corporate bonds: Consists of U.S. Government treasury bills, notes, and bonds with original maturities of less than one year and various yields. These are included as a Level 2 measurement in the tables below. The following tables sets forth by level within the fair value hierarchy the Company's assets and liabilities that are reported at fair value as of March 31, 2018 and December 31, 2017. As required by Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) Topic 820, Fair Value Measurement, assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The following tables summarize certain information for assets and liabilities measured at fair value on a recurring basis: Fair Value Measurements as of Estimated Level 1 Level 2 Level 3 Assets Cash equivalents: Money market funds $ $ $ — $ — Commercial paper — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total cash equivalents — Short-term investments: Commercial paper $ $ — $ — Corporate bonds — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total short-term investments — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total assets $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Liabilities Preferred stock warrants $ $ — $ — $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements as of Estimated Level 1 Level 2 Level 3 Assets Cash equivalents: Money market funds $ $ $ — $ — Commercial paper — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total cash equivalents — Short-term investments: Commercial paper $ $ — $ — Corporate bonds — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total short-term investments — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total assets $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Liabilities Preferred stock warrants $ $ — $ — $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ There were no transfers in and out of Level 1 and Level 2 fair value measurements during the periods ended March 31, 2018 and December 31, 2017. The recurring Level 3 fair value measurements of the Company's preferred stock warrant liabilities use the Black-Scholes option pricing model and value of the respective class of the Company's convertible preferred stock (see Note 7), which is unobservable. All other assumptions included in the model are observable Level 1 inputs. The following table provides a reconciliation of the beginning and ending balances of the Company's preferred stock warrant liabilities: Three Months 2018 2017 Balance at beginning of period $ $ Initial fair value of preferred stock warrants issued Change in fair value of preferred stock warrants — ​ ​ ​ ​ ​ ​ ​ ​ Balance at end of period $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Changes in the fair value of the preferred stock warrant liability are recorded in other expenses on the condensed statements of operations and comprehensive loss. Concentration of Credit Risk Financial instruments, which potentially subject the Company to concentrations of credit risk, consist principally of cash equivalents and accounts receivable. The Company believes that the credit risk in its accounts receivable is mitigated by its credit evaluation process, relatively short collection terms, and dispersion of its customer base. The Company generally does not require collateral, and losses on accounts receivable have historically been within management's expectations. The Company's investment policy limits investments to certain types of debt securities issued by the U.S. government and its agencies, corporations with investment-grade credit ratings, or commercial paper and money market funds issued by the highest quality financial and non-financial companies. The Company places restrictions on maturities and concentration by type and issuer. The Company is exposed to credit risk in the event of a default by the issuers of these securities to the extent recorded on the balance sheets. However, as of March 31, 2018 and December 31, 2017, the Company has limited its credit risk associated with its cash equivalents by placing its investments with banks it believes are highly creditworthy. Allowance for Doubtful Accounts The Company records an allowance for doubtful accounts for accounts receivable deemed uncollectible. The Company evaluates the collectability of its accounts receivable based on known collection risks and historical experience. In circumstances where the Company is aware of a specific customer's inability to meet its financial obligations to the Company (e.g., bankruptcy filings, substantial downgrading of credit ratings), the Company records a specific allowance for bad debts against amounts due to reduce the carrying amount of accounts receivable to the amount it reasonably believes will be collected. Inventories Inventories are valued at the lower of cost or net realizable value, computed on a first-in, first out basis. The Company regularly reviews inventory quantities on-hand for excess and obsolete inventory and, when circumstances indicate, incurs charges to write down inventories to their net realizable value. The Company's review of inventory for excess and obsolete quantities is based primarily on the estimated forecast of future product demand, product life cycles, including expiration of inventory prior to sale, and introduction of new products. The reserve for excess and obsolete inventory was $792 and $518 as of March 31, 2018 and December 31, 2017, respectively. Property and Equipment Property and equipment are stated at cost, less accumulated depreciation and amortization. Depreciation is determined using the straight-line method over the estimated useful lives of the respective assets, generally three to seven years. Leasehold improvements are amortized on a straight-line basis over the shorter of their estimated useful lives or the term of the lease. Impairment of Long-lived Assets Long-lived assets consist primarily of property and equipment and are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. If circumstances require that an asset be tested for possible impairment, the Company compares the undiscounted cash flows expected to be generated by the asset to the carrying amount of the asset. If the carrying amount of the asset is not recoverable on an undiscounted cash flow basis, the Company determines the fair value of the asset and recognizes an impairment loss to the extent the carrying amount of the asset exceeds its fair value. The Company determines fair value using the income approach based on the present value of expected future cash flows or other appropriate measures of estimated fair value. The Company's cash flow assumptions consider historical and forecasted revenue and operating costs and other relevant factors. The Company did not record any impairment charges on long-lived assets during the three months ended March 31, 2018 and 2017. Revenue Recognition The Company recognizes revenue when persuasive evidence of an arrangement exists, product shipment has occurred, or there are no further obligations yet to be performed, pricing is fixed or determinable, and collection is reasonably assured. The Company makes reasonable assumptions regarding the future collectability of amounts receivable from customers to determine whether the revenue recognition criteria have been met. Taxes assessed by a governmental authority that are directly imposed on revenue-producing transactions between a seller and a customer are not recorded as revenue. In general, the Company's standard terms and conditions of sale do not allow for product returns. Sales returns have been limited to damaged product and have not been material. The Company expenses shipping and handling costs as incurred and includes them in the cost of goods sold. In those cases where shipping and handling costs are billed to customers, the Company classifies the amounts billed as a component of cost of goods sold. Cost of Goods Sold Cost of goods sold consists primarily of manufacturing overhead costs, material costs, and direct labor. Overhead costs include the cost of material procurement, inventory control, facilities, equipment, and operations supervision and management. Cost of goods sold also includes depreciation expense for production equipment and certain direct costs such as shipping costs. Research and Development Research and development expenses consist primarily of product development, clinical and regulatory affairs, consulting services, and other costs associated with products and technologies in development. These expenses include employee compensation, stock-based compensation, supplies, travel, and facility costs. Clinical expenses include clinical trial design, clinical site reimbursement, data management, travel expenses, and the cost of manufacturing products for clinical trials. Common Stock Valuation and Stock-Based Compensation The Company maintains an equity incentive plan to provide long-term incentives for employees, consultants, and members of the board of directors. The plan allows for the issuance of non-statutory and incentive stock options to employees and non-statutory stock options to consultants and non-employee directors. The Company recognizes equity-based compensation expense for awards of equity instruments to employees and non-employees based on the grant date fair value of those awards in accordance with FASB ASC Topic 718, Stock Compensation (ASC 718). ASC 718 requires all equity-based compensation awards to employees and nonemployee directors, including grants of restricted shares and stock options, to be recognized as expense in the statements of operations and comprehensive loss based on their grant date fair values. The Company estimates the fair value of stock options using the Black-Scholes option pricing model. The Company uses the value of its common stock to determine the fair value of restricted shares. The Company accounts for restricted stock and common stock options issued to nonemployees under FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees (ASC 505-50). As such, the value of such options is periodically remeasured and income or expense is recognized over their vesting terms. Compensation cost related to awards with service-based vesting schedules is recognized using the straight-line method. The Company determines the fair value of the restricted stock and common stock granted to nonemployees as either the fair value of the consideration received or the fair value of the equity instruments issued. The Company has not granted any share-based awards to its consultants. The Black-Scholes option pricing model requires the input of certain subjective assumptions, including (i) the expected share price volatility, (ii) the expected term of the award, (iii) the risk-free interest rate and (iv) the expected dividend yield. Due to the lack of a public market for the trading of the Company's common stock and a lack of company-specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The group of representative companies have characteristics similar to the Company, including stage of product development and focus on the life science industry. The Company uses the simplified method, which is the average of the final vesting tranche date and the contractual term, to calculate the expected term for options granted to employees as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. For options granted to non-employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected term of the stock options. The Company uses an assumed dividend yield of zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock. The Company expenses the fair value of its equity-based compensation awards granted to employees on a straight-line basis over the associated service period, which is generally the period in which the related services are received. The Company measures equity-based compensation awards granted to nonemployees at fair value as the awards vest and recognizes the resulting value as compensation expense at each financial reporting period. The Company accounts for award forfeitures as they occur. Advertising Expenses The Company expenses the costs of advertising, including promotional expenses, as incurred. Advertising expenses were $1.9 million and $1.0 million during the periods ended March 31, 2018 and 2017, respectively. Income Taxes The Company accounts for income taxes using the liability method. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates that will be in effect when the differences are expected to reverse. Valuation allowances against deferred tax assets are established, when necessary, to reduce deferred tax assets to the amounts expected to be realized. As the Company has historically incurred operating losses, the Company has recorded a full valuation allowance against its net deferred tax assets, and there is no provision for income taxes. The Company's policy is to record interest and penalties expense related to uncertain tax positions as other expense in the statements of operations and comprehensive loss. Comprehensive Loss Comprehensive loss consists of net loss and changes in unrealized gains and losses on short-term investments classified as available-for-sale, if any. Accumulated other comprehensive loss is presented in the accompanying balance sheets as a component of stockholders' (deficit) equity. Loss Per Share Basic net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock outstanding during the period. Diluted net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock and dilutive potential shares of common stock outstanding during the period. Because the Company has reported a net loss for all periods presented, diluted net loss per share is the same as basic net loss per share for those periods as all potentially dilutive shares consisting of convertible preferred stock, stock options and warrants were antidilutive in those periods. Recent Accounting Pronouncements In May 2014, the FASB issued Accounting Standards Update (ASU) No. 2014-09, Revenue From Contracts With Customers (ASU 2014-09), which requires 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. In doing so, companies will need to use more judgment and make more estimates than under current guidance. These may include identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price, and allocating the transaction price to each separate performance obligation. Under the original pronouncement, ASU 2014-09 would have been effective for the Company's annual reporting periods beginning January 1, 2018. In August 2015, the FASB issued ASU No. 2015-14, which formally defers the effective date of the new revenue standard by one year. As a result, the updated revenue guidance will be effective for the Company's annual reporting periods beginning January 1, 2019, and early adoption is permitted as of the original effective date contained within the original standard. The standard permits the use of either the retrospective or cumulative effect transition method. The Company is evaluating the effect that ASU 2014-09 will have on its financial statements and related disclosures. In November 2015, the FASB issued ASU No. 2015-17, Balance Sheet Classification of Deferred Taxes (ASU 2015-17). ASU 2015-17 is intended to reduce complexity surrounding the presentation of deferred taxes within the balance sheet. Specifically, ASU 2015-17 requires that all deferred tax assets and liabilities, along with any related valuation allowance, be classified as non-current on the balance sheet, effectively eliminating the requirement to allocate deferred taxes between current and non-current amounts. The new guidance does not permit companies to offset deferred tax liabilities from one jurisdiction against deferred tax assets of another jurisdiction. This guidance was effective January 1, 2018, and did not significantly impact its financial statements and related disclosures. In February 2016, the FASB issued ASU No. 2016-02, Leases (ASU 2016-02). ASU 2016-02 will require entities that lease assets to recognize the rights and obligations associated with those leases on their balance sheets. The guidance will be effective for the Company's annual reporting period beginning January 1, 2020, with early adoption permitted. The Company is evaluating the impact this standard will have on its financial statements and related disclosures. In March 2016, the FASB issued No. 2016-09, Improvements to Employee Share-Based Payment Accounting (ASU 2016-09), which changes how companies will account for certain aspects of share-based payments to employees. As part of the new guidance, entities will be required to record the impact of income taxes arising from share-based compensation when awards vest or are settled within earnings as part of income tax expense rather than recorded as part of additional paid-in capital (APIC) and will eliminate the requirement that excess tax benefits be realized prior to recognition. Additionally, the guidance requires entities to present excess tax benefits as an operating activity on the statement of cash flows rather than as a financing activity. Additionally, companies will be required to make an accounting policy election at the time of adoption of the new guidance to either account for forfeitures of share-based awards in a manner similar to today's requirements (i.e., estimating the number of awards expected to be forfeited at the grant date and adjusting the estimate when awards are actually forfeited), or recognizing forfeitures as they occur with no estimate of forfeitures determined at the grant date. Companies will also be able to set a maximum statutory tax rate as it pertains to share-based awards it net settles on behalf of its employees. This will provide companies a greater ability to retain equity-award accounting treatment. Entities will apply the provisions of ASU 2016-09 using a modified retrospective transition approach, with a cumulative-effect adjustment booked to retained earnings as of the beginning of the period of adoption. This guidance was effective January 1, 2018, and did not significantly impact its financial statements and related disclosures. The Company has reviewed and considered all other recent accounting pronouncements and believes there are none that could potentially have a material impact on its business practices, financial condition, results of operations, or disclosures.

Composition of Certain Financia

Composition of Certain Financial Statement Items	3 Months Ended
	Mar. 31, 2018
Composition of Certain Financial Statement Items
Composition of Certain Financial Statement Items	3. Composition of Certain Financial Statement Items Inventories Inventory balances, net of reserves, consist of the following: March 31, December 31, Raw materials $ $ Finished goods ​ ​ ​ ​ ​ ​ ​ ​ $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Property and Equipment March 31, December 31, Computer equipment and software $ $ Furniture and office equipment Manufacturing equipment Research and development equipment Leasehold improvements ​ ​ ​ ​ ​ ​ ​ ​ Less: accumulated depreciation and amortization ) ) ​ ​ ​ ​ ​ ​ ​ ​ $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Depreciation and amortization expense was $90 and $43 during the three months ended March 31, 2018 and 2017, respectively. Accrued Expenses March 31, December 31, Payroll and commissions payable $ $ Vacation Other accrued expenses ​ ​ ​ ​ ​ ​ ​ ​ $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Long-Term Debt

Long-Term Debt	3 Months Ended
	Mar. 31, 2018
Long-Term Debt
Long-Term Debt	4. Long-Term Debt Credit Facility In August 2015, the Company entered into a loan and security agreement, which provided for a term A loan facility in the amount of $15.5 million, the proceeds of which were used to refinance the $12.0 million of borrowings outstanding under the Company's original credit facility, and a term B loan facility in an amount between $3.5 million and $10.0 million, subject to the Company's achievement of certain revenue milestones. Amounts outstanding under the credit facility bore interest at a fixed rate of 7.95% per annum. In February 2017, the Company amended the loan and security agreement. Under the loan and security agreement, as amended, and subject to the limitation noted below, amounts outstanding under the credit facility bear interest at a floating interest rate equal to the greater of 7.95% or LIBOR plus 6.9% per annum. Upon execution of the amendment, the Company borrowed an additional $1.0 million under the term A loan portion of the credit facility, receiving net proceeds of $0.5 million, net of expenses, for a total of $16.5 million outstanding under the credit facility and reduced borrowings available under the term B loan facility to $9.0 million. In connection with the execution of the amendment to the loan and security agreement, the Company issued 29,197 ten-year warrants to purchase Series F preferred shares of stock at an exercise price of $1.37 per share. On February 7, 2018, the Company borrowed an additional $8.0 million under the term B loan facility portion of its credit facility. After receipt of the $8.0 million, Company had a total of $24.5 million outstanding under the credit facility, which bears interest at a floating interest rate equal to the greater of 7.95% or LIBOR plus 6.9% per annum. All amounts borrowed under the credit facility are interest-only through March 1, 2019, after which the Company will make monthly payments of principal and interest through March 1, 2022; provided that the interest-only period will be extended will be extended to March 1, 2020 if the Company has revenue, measured on a trailing 12-month basis as of December 31, 2018, of at least $25.0 million. In connection with this borrowing, the Company issued 233,577 ten-year warrants to purchase Series F preferred shares of stock at an exercise price of $1.37 per share. In addition to the principal and interest payments, under the credit facility, the Company is required to pay a final payment fee of 5.0% on all amounts outstanding, which is being accreted using the effective interest rate method over the term of the loan and security agreement and shall be due at the earlier of maturity or prepayment. If the Company repays all the amounts borrowed under the term A loan facility on or prior to maturity, the Company will also be required to pay a prepayment fee equal to 1.5% if such borrowings are prepaid after February 24, 2018 but prior to February 24, 2019, and 1.00% if such borrowings are prepaid on or after February 24, 2019. Borrowings under the term B loan facility are prepayable at the Company's option in whole, but not in part, together with all accrued and unpaid interest thereon and, if not previously made, the Final Payment, subject to a prepayment fee of 2.5% if the such borrowings are prepaid prior to February 7, 2019, 1.5% if such borrowings are prepaid on or after February 7, 2019 but prior to February 7, 2020 and 1.00% if such borrowings are on or after February 7, 2020. The credit facility includes affirmative and restrictive covenants and events of default, including the following events of default: payment defaults, breaches of covenants, judgment defaults, cross defaults to certain other contracts, certain events with respect to governmental approvals if such events could cause a material adverse change, a material impairment in the perfection or priority of the lender's security interest or in the value of the collateral, a material adverse change in the business, operations, or condition of the Company or any of its subsidiaries, and a material impairment of the prospect of repayment of the loans. Upon the occurrence of an event of default, a default increase in the interest rate of an additional 5.00% could be applied to the outstanding loan balance and the lender could declare all outstanding obligations immediately due and payable and take such other actions as set forth in the loan and security agreement. The Company's obligations under the credit facility are secured by a first priority security interest in substantially all of its assets, other than its intellectual property. There are no financial covenants contained in the loan and security agreement. The Company was in compliance with the affirmative and restrictive covenants as of March 31, 2018. The Company paid debt issuance costs of $72 in connection with its entry into the loan and security agreement in August 2015. The costs are being amortized over the term of the loan using the effective interest rate method. The Company also issued preferred stock warrants in connection with its borrowings under its credit facilities (see Note 7). Expected future principal payments for the credit facility are as follows: Year ending December 31 : 2018 (remaining) $ — 2019 2020 2021 2022 ​ ​ ​ ​ ​ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Commitments

Commitments	3 Months Ended
	Mar. 31, 2018
Commitments
Commitments	5. Commitments Operating Lease The Company rents office space under an operating lease that expires on March 31, 2019. The lease allows the Company to terminate the lease any time after March 31, 2017 without a penalty. Future minimum annual operating lease payments are as follows: Year ending December 31 : 2018 (remaining) $ 2019 ​ ​ ​ ​ ​ Total $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Rent expense was $52 and $39 for the three months ended March 31, 2018 and 2017, respectively. Purchase Commitments As of March 31, 2018 and December 31, 2017, the Company had commitments to suppliers for inventory purchases totaling $7.1 million and $9.0 million, respectively, of which $0.0 million and $0.4 million, respectively, was committed to Medtronic, a related party.

Employee Retirement Plan

Employee Retirement Plan	3 Months Ended
	Mar. 31, 2018
Employee Retirement Plan
Employee Retirement Plan	6. Employee Retirement Plan The Company sponsors an employee retirement plan covering all full-time employees of the Company. The plan allows for eligible employees to defer a portion of their eligible compensation up to the maximum allowed by IRS Regulations. The Company may elect to make a voluntary contribution to the plan. The Company has not made contributions since inception.

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity	3 Months Ended
	Mar. 31, 2018
Stockholders' (Deficit) Equity
Stockholders' (Deficit) Equity	7. Stockholders' (Deficit) Equity Authorized Shares The Company has authorized 186,894,620 shares of stock, of which 110,000,000 shares are designated as common stock and 76,894,620 shares are designated as Series A, B, C, D, E, and F preferred stock. All stock has a par value of $0.001. Preferred Stock A summary of the Company's preferred stock as of March 31, 2018 and December 31, 2017, is as follows: Series Shares Shares Carrying A $ B C D E F ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ A summary of the Company's convertible preferred stock terms is as follows: Series Liquidation 8% Conversion A $ $ $ B C D E F The dividend per share on the convertible preferred stock is only payable when, as and if declared by the Board of Directors. The Company has issued 5,375,507 shares of Preferred Stock, Series A, with an original issue price of $1.00 per share. Each share of Series A Preferred Stock may be converted into 0.1504 shares of common stock. The conversion price is subject to change for certain subdivisions, combinations of common stock, or other dilutive issuances of common stock. Each share of Series A Preferred Stock entitles the holder to vote on all matters submitted to holders of common stock, and each share of Series A Preferred Stock has the number of votes equal to the number of shares of common stock into which it may be converted. The Company has issued 8,706,909 shares of Preferred Stock, Series B, with an original issue price of $1.84 per share. Each share of Series B Preferred Stock may be converted into 0.1855 shares of common stock. The conversion price is subject to change for certain subdivisions, combinations of common stock, or other dilutive issuances of common stock. Ownership of Series B Preferred Stock entitles the holder to vote on all matters submitted to holders of common stock. Each share of Series B Preferred Stock has the number of votes equal to the number of shares of common stock into which it may be converted. The Company has issued 13,829,906 shares of Preferred Stock, Series C, with an original issue price of $1.07 per share. Each share of Series C Preferred Stock may be converted into 0.1504 shares of common stock. The conversion price is subject to change for certain subdivisions, combinations of common stock, or other dilutive issuances of common stock. Each share of Series C Preferred Stock entitles the holder to vote on all matters submitted to holders of common stock, and each share of Series C Preferred Stock has the number of votes equal to the number of shares of common stock into which it may be converted. The Company has issued 5,683,292 shares of Preferred Stock, Series D, with an original issue price of $1.07 per share. Each share of Series D Preferred Stock may be converted into 0.1504 shares of common stock. The conversion price is subject to change for certain subdivisions, combinations of common stock, or other dilutive issuances of common stock. Each share of Series D Preferred Stock entitles the holder to vote on all matters submitted to holders of common stock, and each share of Series D Preferred Stock has the number of votes equal to the number of shares of common stock into which it may be converted. In March 2014, the Company raised $39.8 million, net of stock issuance costs, through the issuance of 15,267,175 shares of Series E Preferred Stock, with an original issue price of $2.62 per share. Each share of Series E Preferred Stock may be converted into 0.1728 shares of common stock. The conversion price is subject to change for certain subdivisions, combinations of common stock, or other dilutive issuances of common stock. Each share of Series E Preferred Stock entitles the holder to vote on all matters submitted to holders of common stock, and each share of Series E Preferred Stock has the number of votes equal to the number of shares of common stock into which it may be converted. In October 2016, the Company raised $12.3 million, net of stock issuance costs, through the issuance of 9,124,084 shares of Series F Preferred Stock, with an original issue price of $1.37 per share (Tranche 1). In February 2017, the Company raised $25.0 million, net of stock issuance costs, through the issuance of 18,248,177 shares of Series F Preferred Stock, with an original issue price of $1.37 per share (Tranche 2). Each share of Series F Preferred Stock may be converted into 0.1504 shares of common stock. The conversion price is subject to change for certain subdivisions, combinations of common stock, or other dilutive issuances of common stock. Each share of Series F Preferred Stock entitles the holder to vote on all matters submitted to holders of common stock, and each share of Series F Preferred Stock has the number of votes equal to the number of shares of common stock into which it may be converted. Upon a liquidation, dissolution, or winding up of the Company, the holders of the Series F Preferred Stock shall be entitled to receive prior and in preference to any distribution of any of the assets of the Company to holders of Series A Preferred, Series B Preferred, Series C Preferred, Series D Preferred, Series E Preferred and common stock. If, upon a liquidation, the assets to be distributed to the holders of the Series F Preferred are insufficient to permit the payment to such holders of the full amount payable, then the entire assets of the Company legally available for distribution shall be distributed pro rata to the holders of the Series F Preferred Stock in proportion to the full preferential amounts which each such holder would otherwise be entitled to receive. Preferred Stock Warrants In connection with certain convertible promissory notes issued by the Company during 2011 and 2012 and subsequently paid, the Company issued 278,506 five-year warrants to purchase Series C preferred shares of stock at an exercise price of $1.07 per share. Based on the Black-Scholes option pricing model, the value of each warrant was determined to be $0.34, for a total value of $94, and was fully expensed during the year ended December 31, 2012. As of December 31, 2015, the Company had a total of 258,880 outstanding warrants. In 2016, a total of 258,880 Series C preferred shares were issued upon exercise of the entire remaining warrants for proceeds of $277. In connection with the execution of the Company's original credit facility entered into during 2012, the Company issued 186,916 ten-year warrants to purchase Series C preferred shares of stock at an exercise price of $1.07 per share. Based on the Black-Scholes option pricing model, the value of each warrant was determined to be $0.515, for a total value of $96 at the date of issuance and was fully expensed during the year ended December 31, 2012. In connection with the added borrowings drawn upon the Company's original credit facility in 2013, the Company issued 74,768 ten-year warrants to purchase Series C preferred shares of stock at an exercise price of $1.07 per share. Based on the Black-Scholes option pricing model, the value of each warrant was determined to be $0.525, for a total value of $39 at the date of issuance and was fully expensed during the year ended December 31, 2013. In connection with the execution of the Company's amended credit facility completed in June 2014, the Company also issued 76,334 ten-year warrants to purchase Series E preferred shares of stock at an exercise price of $2.62 per share. Based on the Black-Scholes option pricing model, the value of each warrant was determined to be $1.11, for a total value of $85 at the date of issuance and was fully expensed during the year ended December 31, 2014. In connection with the execution of the Company's current credit facility completed in August 2015 (see Note 4), the Company issued 29,580 ten-year warrants to purchase Series E preferred shares of stock at an exercise price of $2.62 per share. Based on the Black-Scholes option pricing model, the value of each warrant was determined to be $1.13, for a total value of $33 at the date of issuance and was fully expensed during the year ended December 31, 2015. In connection with the execution of the amendment to the Company's current credit facility completed in February 2017 (see Note 4), the Company issued 29,197 ten-year warrants to purchase Series F preferred shares of stock at an exercise price of $1.37 per share. Based on the Black-Scholes option pricing model, the value of each warrant was determined to be $0.13, for a total value of $4 at the date of issuance and was fully expensed during the year ended December 31, 2017. In connection with the Company's borrowing completed in February 2018 (see Note 4), the Company issued 233,577 ten-year warrants to purchase Series F preferred shares of stock at an exercise price of $1.37 per share. Based on the Black-Scholes option pricing model, the value of each warrant was determined to be $0.44, for a total value of $103 at the date of issuance and was fully expensed during the three months ended March 31, 2018. The preferred stock warrants issued in connection with the execution of the original credit facility and its subsequent amendments require re-measurement of the value of the preferred stock warrants each period, with changes in fair value recognized within other expenses on the statements of operations and comprehensive loss. The fair value of the preferred stock warrants was determined using the Black-Scholes option pricing model. As of March 31, 2018 and December 31, 2017, the following preferred stock warrants issued under the Company's original credit facility and subsequent amendments were outstanding and exercisable: Dates Exercise Warrants Initial Fair Value at Issuance Expiration Series February 8, 2018 February 8, 2028 F $ $ $ February 24, 2017 February 24, 2027 F August 7, 2015 August 7, 2025 E June 27, 2014 June 27, 2024 E August 5, 2013 August 5, 2023 C November 16, 2012 November 16, 2022 C ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Dates Exercise Warrants Initial Fair Value at Issuance Expiration Series February 24, 2017 February 24, 2027 F $ $ August 7, 2015 August 7, 2025 E June 27, 2014 June 27, 2024 E August 5, 2013 August 5, 2023 C November 16, 2012 November 16, 2022 C ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ The warrants issued on August 7, 2015 to purchase Series E preferred stock are subject to the conversion price of the underlying preferred stock. The warrants to purchase 29,580 shares of Series E preferred stock at $2.62 per share has been adjusted to purchase 56,569 shares of Series F preferred stock at $1.37 per share due to the dilutive issuance of the Series F preferred stock.

Stock Options

Stock Options	3 Months Ended
	Mar. 31, 2018
Stock Options
Stock Options	8. Stock Options The Company adopted the 2007 Stock Incentive Plan, or the 2007 Plan, in November 2007, which terminated in accordance with its terms on November 28, 2017; however, the outstanding stock options may continue to be exercised in accordance with their terms. Immediately following the termination of the 2007 Plan, the Company adopted the 2017 Stock Incentive Plan, or the 2017 Plan, which contains substantially similar terms and conditions as the 2007 Plan. Upon the IPO, no further grants were made under the 2017 Plan and the Company adopted the 2018 Stock Incentive Plan, or the 2018 Plan. The purpose of the 2018 Plan is to promote the interest of the Company and its stockholders by aiding the Company in attracting and retaining employees, officers, consultants, independent contractors, and directors capable of assuring the future success of the Company's business and to afford such persons an opportunity to acquire a proprietary interest in the Company. The Board may amend, alter, suspend, discontinue, or terminate the 2018 Plan at any time with the approval of the stockholders of the Company. As of March 31, 2018, the number of shares reserved for issuance under the 2017 Plan was 2,945,384 shares. The exercise price of stock options represent fair value of the common stock at the time of issuance and is determined by the Board of Directors. The options granted contain fixed exercise prices ranging from $0.94 to $6.65 per share with varying expiration and exercise dates. The stock options granted by the Company to employees during the three months ended March 31, 2018 have a weighted average exercise price of $2.80 per share. The stock options granted include a four year service period and 25% vest after the first year of service and then pro rata over the next 36 months of service. The stock options have a contractual life of ten years. A summary of the Company's stock option activity and related information is as follows: Options Weighted Average Outstanding at December 31, 2017 $ Granted Exercised ) Forfeited ) ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at March 31, 2018 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Exercisable at March 31, 2018 At March 31, 2018, the Company had 1,975,700 common stock options outstanding, with an average remaining contractual life of 5.2 years at a weighted average exercise price of $1.48 per share. Total stock-based compensation recognized, before taxes, during the three months ended March 31, 2018 and 2017, is as follows: Three 2018 2017 General and administrative $ $ Sales and marketing Research and development ​ ​ ​ ​ ​ ​ ​ ​ $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of March 31, 2018, the amount of unearned stock-based compensation currently estimated to be expensed from now through the year 2022 related to unvested employee and non-employee director share-based awards is $295 and the weighted average period over which the unearned stock-based compensation is expected to be recognized is 3 years. If there are any modifications or cancellations of the underlying unvested securities, the Company may be required to accelerate, increase, or cancel any remaining unearned stock compensation expense. Future stock-based compensation expense and unearned stock-based compensation will increase to the extent that the Company grants additional share-based awards. The Company estimates the fair value of share-based awards on the date of grant using the Black-Scholes option pricing model using the fair market value of the Company's common stock on the date of grant and a number of other complex and subjective assumptions. These assumptions include, but are not limited to, estimates regarding the expected term of the Company's outstanding awards, estimates of the stock volatility over a duration that approximates the expected life of the Company's outstanding awards, estimates of the risk-free rate, and estimates of expected dividend rates. Due to the Company's limited amount of historical exercise, forfeiture, and expiration activity, the Company has opted to use the "simplified method" for estimating the expected term of options, whereby the expected term equals the arithmetic average of the vesting terms and the original contractual term of the option. The Company will continue to analyze its expected term assumption as more historical data becomes available. Due to the Company's limited operating history and a lack of company specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. When selecting these public companies on which it has based its expected stock price volatility, the Company generally selected companies with comparable characteristics to it, including enterprise value, stages of clinical development, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected life of the stock-based awards. The historical volatility data was computed using the daily closing prices for the selected companies' shares over historical periods that approximate calculated expected term of the Company's share-based awards. The Company will continue to analyze the historical stock price volatility assumption as more historical data for the Company's common stock becomes available. The risk-free rate assumption is based on the U.S. Treasury instruments with maturities similar to the expected term of the Company's stock options. The expected dividend assumption is based on the Company's history of not paying dividends and its expectation that it will not declare dividends for the foreseeable future. The amount of stock-based compensation expense recognized is based on awards ultimately expected to vest. The amount of expense recognized by the Company has been reduced by actual forfeitures as they occur. The fair value of options granted to employees or non-employee directors during the three months ended March 31, 2018 and 2017 was estimated as of the grant date using the Black-Scholes option pricing model, assuming the weighted average assumptions listed in the following table: Three Months Ended March 31, 2018 2017 Expected life (years) 6.2 6.2 Expected volatility 37.5% 41.5% Risk-free interest rate 2.38 - 2.84% 1.94 - 2.32% Dividend yield 0.0% 0.0% Weighted average fair value $1.15 $0.40

Related-Party Transactions

Related-Party Transactions	3 Months Ended
	Mar. 31, 2018
Related-Party Transactions
Related-Party Transactions	9. Related-Party Transactions Board of Directors' Appointment The Company has entered into various agreements and contracts with Medtronic, one of the Company's stockholders. Under these various agreements and contracts, Medtronic is allowed to name one person to be a member of the Company's Board of Directors. In connection with the Series F Preferred Stock purchase agreement in 2016, Medtronic agreed to move from one voting member of the Company's Board of Directors to two non-voting members. This right terminated in connection with the completion of the Company's IPO. Supply Agreement The Company contracts with Medtronic to supply all of the Company's commercial and clinical requirements of certain components used to manufacture the Inspire system. The current Supply Agreement expired on June 5, 2017, but was extended to allow Medtronic to complete a final build of the pressure sensor used in the Company's original pressure sensing lead, which is expected to be completed in 2018. Upon a change of control event whereby the Company is owned or controlled by a competitor of Medtronic, Medtronic would have the right to terminate the Supply Agreement, provided that, upon any such termination the Company would be entitled to exercise a one-time buy right of inventory using current product pricing and upon terms to be agreed upon in the definitive agreements. The Company purchased inventory at arms-length with Medtronic, a related party, of $197 and $223 for the three months ended March 31, 2018 and 2017, respectively. Right-of-First-Offer of the Company Under a Right-of-First-Offer with Medtronic that expired on May 16, 2017, had the Company decided to initiate a possible sale of the Company prior to the expiration date, it would have been required to negotiate exclusively with Medtronic for such transaction for a period of 90 days prior to negotiating with a third party.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 2018
Income Taxes
Income Taxes	10. Income Taxes During the three months ended March 31, 2018 and 2017, the Company did not record an income tax benefit related to its loss before income taxes in the statement of operations and comprehensive loss because a valuation allowance has been required to be established for all deferred tax assets due to its cumulative net loss position. As of December 31, 2017, the Company's gross federal net operating loss carryforwards of $110.9 million will expire at various dates beginning in 2028. In addition, net operating loss carryforwards for state income tax purposes of $65.9 million that include net operating losses that will begin to expire in 2028. The Company also has R&D credit carryforwards of $1.4 million as of December 31, 2017 of which will expire at various dates beginning in 2032. Utilization of the net operating loss carryforwards may be subject to an annual limitation due to the ownership change limitations provided by Section 382 of the Internal Revenue Code of 1986 and similar state provisions. The annual limitation may result in the expiration of the net operating loss before utilization. Realization of the deferred tax assets is dependent upon the generation of future taxable income, if any, the amount and timing of which are uncertain. Based on available objective evidence and cumulative losses, management believes it is more likely than not that the deferred tax assets are not recognizable and will not be recognizable until the Company has sufficient taxable income. Accordingly, the net deferred tax assets have been fully offset by a valuation allowance. The Company had no unrecognized tax benefits as of March 31, 2018 and December 31, 2017. The Company files income tax returns in the U.S. federal and various state jurisdictions. The 2014 to 2017 tax years remain open to examination by the major taxing authorities to which the Company is subject. The Company does not expect a significant change to its unrecognized tax benefits over the next 12 months. The Tax and Jobs Act (the Act) was enacted on December 22, 2017. The Act reduces the U.S. federal corporate tax rate from 35% to 21%, requires companies to pay a one-time transition tax on earnings of certain foreign subsidiaries that were previously tax deferred and creates new taxes on certain foreign-sourced earnings. The Company has applied the guidance in ASU 2018-05, Income Taxes (Topic 740): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118, when accounting for the enactment-date effects of the Act. At March 31, 2018, the Company has not completed its accounting for the tax effects of the Act, as the Company is in the process of analyzing certain aspects of the Act, obtaining information, and refining its calculations of the Act's impact. There have been no material measurement period adjustments made during the quarter ended March 31, 2018 related to the provisional amounts recorded and disclosed in the Company's Registration Statement Form S-1 and Prospectus dated May 2, 2018. The Company expects to complete the accounting for the tax effects of the Act during 2018.

Segment Reporting and Significa

Segment Reporting and Significant Customers	3 Months Ended
	Mar. 31, 2018
Segment Reporting and Significant Customers
Segment Reporting and Significant Customers	11. Segment Reporting and Significant Customers Operating segments are defined as components of an enterprise for which separate discrete financial information is available and evaluated regularly by the chief operating decision maker, or decision-making group, in deciding how to allocate resources and in assessing performance. The Company globally manages the business within one reporting segment. Segment information is consistent with how management reviews the business, makes investing and resource allocation decisions and assesses operating performance. Revenue by geographic region are as follows: Three Months 2018 2017 United States $ $ Europe ​ ​ ​ ​ ​ ​ ​ ​ Total revenue $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ All the Company's long-lived assets are located in the United States.

Loss Per Share

Loss Per Share	3 Months Ended
	Mar. 31, 2018
Loss Per Share
Loss Per Share	12. Loss Per Share Under the two-class method, for periods with net income, basic net income per common share is computed by dividing the net income attributable to common stockholders by the weighted average number of shares of common stock outstanding during the period. Net income attributable to common stockholders is computed by subtracting from net income the portion of current year earnings that participating securities would have been entitled to receive pursuant to their dividend rights had all of the year's earnings been distributed. No such adjustment to earnings is made during periods with a net loss as the holders of the participating securities have no obligation to fund losses. Diluted net loss per common share is computed under the two-class method by using the weighted average number of shares of common stock outstanding plus, for periods with net income attributable to common stockholders, the potential dilutive effects of stock options and warrants. In addition, the Company analyzes the potential dilutive effect of the outstanding participating securities under the if-converted method when calculating diluted earnings per share in which it is assumed that the outstanding participating securities convert into common stock at the beginning of the period. The Company reports the more dilutive of the approaches (two-class or if-converted) as its diluted net income per share during the period. Due to the existence of net losses for the three months ended March 31, 2018 and 2017, basic and diluted loss per share were the same, as the effect of potentially dilutive securities would have been anti-dilutive. The following potentially dilutive securities outstanding have been excluded from the computations of diluted weighted average shares outstanding because such securities have an antidilutive impact due to losses reported: Three Months Ended 2018 2017 Convertible preferred stock outstanding Convertible preferred stock warrants Common stock options outstanding ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Summary of Significant Accoun18

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2018
Summary of Significant Accounting Policies
Basis of Presentation	Basis of Presentation The accompanying condensed financial statements have been prepared by the Company, without audit, pursuant to the rules and regulations of the SEC. The condensed financial statements may not include all disclosures required by U.S. GAAP; however, the Company believes that the disclosures are adequate to make the information presented not misleading. These unaudited condensed financial statements should be read in conjunction with the audited financial statements and the notes thereto for the fiscal year ended December 31, 2017 included in the Company's Registration Statement on Form S-1 and Prospectus dated May 2, 2018. The condensed balance sheet at December 31, 2017 was derived from the audited financial statements. In the opinion of management, all adjustments, consisting of only normal recurring adjustments that are necessary to present fairly the financial position, results of operations, and cash flows for the interim periods, have been made. The results of operations for the interim periods are not necessarily indicative of the operating results for the full fiscal year or any future periods. Reverse Stock Split In connection with its initial public offering of common stock (the IPO) the Company's board of directors and stockholders approved a 1-for-6.650 reverse stock split of the Company's common stock. The reverse stock split became effective on April 20, 2018. The par value of the common stock was not adjusted as a result of the reverse stock split. Adjustments corresponding to the reverse stock split were made to the ratio at which the convertible preferred stock will convert into common stock immediately prior to the closing of the IPO. Accordingly, all share and per-share amounts for all periods presented in these financial statements and notes thereto have been adjusted retroactively, where applicable, to reflect the reverse stock split and adjustment of the conversion ratio of the convertible preferred stock. Initial Public Offering On May 7, 2018, the Company completed its IPO by issuing 7,762,500 shares of common stock, at an offering price of $16.00 per share, for net proceeds of approximately $112.2 million after deducting underwriting discounts and commissions and offering expenses payable by the Company. In connection with the IPO, the Company's outstanding shares of convertible preferred stock were automatically converted into an aggregate of 12,111,706 shares of common stock, and the Company's outstanding warrants to purchase shares of convertible preferred stock were automatically converted into warrants to purchase up to an aggregate of 98,846 shares of common stock, resulting in the reclassification of the related redeemable convertible preferred stock warrant liability of $978 to additional paid-in-capital.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the amounts and disclosures reported in the financial statements. Management uses significant judgment when making estimates related to its allowance for doubtful accounts, inventory reserves and the valuations of its common stock, share-based awards, and certain of its outstanding preferred stock warrants. Management bases its estimates on historical experience and on various other assumptions that are believed to be reasonable under the circumstances, the results of which form the basis for making judgments about the carrying values of assets and liabilities that are not readily apparent from other sources. Actual results could differ from those estimates.
JOBS Act Accounting Election	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.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid securities, readily convertible to cash, that mature within 90 days or less from the date of purchase to be cash equivalents. The carrying amount reported in the balance sheets for cash is cost, which approximates fair value.
Short-term Investments	Short-term Investments At March 31, 2018 and December 31, 2017, the Company's short-term investments consisted of commercial paper and corporate bonds which are classified as available-for-sale and had maturities less than one year. Short-term investments are reported at their estimated fair market value which approximates cost. Any unrealized gains and losses are reported in accumulated other comprehensive loss.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The Company measures certain financial assets and liabilities at fair value on a recurring basis, including cash equivalents and certain of its outstanding preferred stock warrants. 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 a liability. A three-tier fair value hierarchy is established as a basis for considering such assumptions and for inputs used in the valuation methodologies in measuring fair value: Level 1—Observable inputs, such as quoted prices (unadjusted) for identical assets or liabilities in active markets. Level 2—Other inputs that are based upon quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-based valuation techniques for which all significant inputs are observable in the market or can be derived from observable market data. Where applicable, these models project future cash flows and discount the future amounts to a present value using market-based observable inputs, including interest rate curves, foreign exchange rates, and credit ratings. Level 3—Unobservable inputs that are supported by little or no market activities, which would require the Company to develop its own assumptions. The Company uses the methods and assumptions described below in determining the fair value of its financial instruments. Money market funds: Fair values of money market funds are based on quoted market prices in active markets. Commercial paper: Short-term, highly liquid investments are included as a Level 2 measurement in the tables below. Corporate bonds: Consists of U.S. Government treasury bills, notes, and bonds with original maturities of less than one year and various yields. These are included as a Level 2 measurement in the tables below. The following tables sets forth by level within the fair value hierarchy the Company's assets and liabilities that are reported at fair value as of March 31, 2018 and December 31, 2017. As required by Financial Accounting Standards Board (FASB) Accounting Standards Codification (ASC) Topic 820 , Fair Value Measurement, assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The following tables summarize certain information for assets and liabilities measured at fair value on a recurring basis: Fair Value Measurements as of Estimated Level 1 Level 2 Level 3 Assets Cash equivalents: Money market funds $ $ $ — $ — Commercial paper — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total cash equivalents — Short-term investments: Commercial paper $ $ — $ — Corporate bonds — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total short-term investments — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total assets $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Liabilities Preferred stock warrants $ $ — $ — $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements as of Estimated Level 1 Level 2 Level 3 Assets Cash equivalents: Money market funds $ $ $ — $ — Commercial paper — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total cash equivalents — Short-term investments: Commercial paper $ $ — $ — Corporate bonds — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total short-term investments — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total assets $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Liabilities Preferred stock warrants $ $ — $ — $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ There were no transfers in and out of Level 1 and Level 2 fair value measurements during the periods ended March 31, 2018 and December 31, 2017. The recurring Level 3 fair value measurements of the Company's preferred stock warrant liabilities use the Black-Scholes option pricing model and value of the respective class of the Company's convertible preferred stock (see Note 7), which is unobservable. All other assumptions included in the model are observable Level 1 inputs. The following table provides a reconciliation of the beginning and ending balances of the Company's preferred stock warrant liabilities: Three Months 2018 2017 Balance at beginning of period $ $ Initial fair value of preferred stock warrants issued Change in fair value of preferred stock warrants — ​ ​ ​ ​ ​ ​ ​ ​ Balance at end of period $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Changes in the fair value of the preferred stock warrant liability are recorded in other expenses on the condensed statements of operations and comprehensive loss.
Concentration of Credit Risk	Concentration of Credit Risk Financial instruments, which potentially subject the Company to concentrations of credit risk, consist principally of cash equivalents and accounts receivable. The Company believes that the credit risk in its accounts receivable is mitigated by its credit evaluation process, relatively short collection terms, and dispersion of its customer base. The Company generally does not require collateral, and losses on accounts receivable have historically been within management's expectations. The Company's investment policy limits investments to certain types of debt securities issued by the U.S. government and its agencies, corporations with investment-grade credit ratings, or commercial paper and money market funds issued by the highest quality financial and non-financial companies. The Company places restrictions on maturities and concentration by type and issuer. The Company is exposed to credit risk in the event of a default by the issuers of these securities to the extent recorded on the balance sheets. However, as of March 31, 2018 and December 31, 2017, the Company has limited its credit risk associated with its cash equivalents by placing its investments with banks it believes are highly creditworthy.
Allowance for Doubtful Accounts	Allowance for Doubtful Accounts The Company records an allowance for doubtful accounts for accounts receivable deemed uncollectible. The Company evaluates the collectability of its accounts receivable based on known collection risks and historical experience. In circumstances where the Company is aware of a specific customer's inability to meet its financial obligations to the Company (e.g., bankruptcy filings, substantial downgrading of credit ratings), the Company records a specific allowance for bad debts against amounts due to reduce the carrying amount of accounts receivable to the amount it reasonably believes will be collected.
Inventories	Inventories Inventories are valued at the lower of cost or net realizable value, computed on a first-in, first out basis. The Company regularly reviews inventory quantities on-hand for excess and obsolete inventory and, when circumstances indicate, incurs charges to write down inventories to their net realizable value. The Company's review of inventory for excess and obsolete quantities is based primarily on the estimated forecast of future product demand, product life cycles, including expiration of inventory prior to sale, and introduction of new products. The reserve for excess and obsolete inventory was $792 and $518 as of March 31, 2018 and December 31, 2017, respectively.
Property and Equipment	Property and Equipment Property and equipment are stated at cost, less accumulated depreciation and amortization. Depreciation is determined using the straight-line method over the estimated useful lives of the respective assets, generally three to seven years. Leasehold improvements are amortized on a straight-line basis over the shorter of their estimated useful lives or the term of the lease.
Impairment of Long-lived Assets	Impairment of Long-lived Assets Long-lived assets consist primarily of property and equipment and are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. If circumstances require that an asset be tested for possible impairment, the Company compares the undiscounted cash flows expected to be generated by the asset to the carrying amount of the asset. If the carrying amount of the asset is not recoverable on an undiscounted cash flow basis, the Company determines the fair value of the asset and recognizes an impairment loss to the extent the carrying amount of the asset exceeds its fair value. The Company determines fair value using the income approach based on the present value of expected future cash flows or other appropriate measures of estimated fair value. The Company's cash flow assumptions consider historical and forecasted revenue and operating costs and other relevant factors. The Company did not record any impairment charges on long-lived assets during the three months ended March 31, 2018 and 2017.
Revenue Recognition	Revenue Recognition The Company recognizes revenue when persuasive evidence of an arrangement exists, product shipment has occurred, or there are no further obligations yet to be performed, pricing is fixed or determinable, and collection is reasonably assured. The Company makes reasonable assumptions regarding the future collectability of amounts receivable from customers to determine whether the revenue recognition criteria have been met. Taxes assessed by a governmental authority that are directly imposed on revenue-producing transactions between a seller and a customer are not recorded as revenue. In general, the Company's standard terms and conditions of sale do not allow for product returns. Sales returns have been limited to damaged product and have not been material. The Company expenses shipping and handling costs as incurred and includes them in the cost of goods sold. In those cases where shipping and handling costs are billed to customers, the Company classifies the amounts billed as a component of cost of goods sold.
Cost of Goods Sold	Cost of Goods Sold Cost of goods sold consists primarily of manufacturing overhead costs, material costs, and direct labor. Overhead costs include the cost of material procurement, inventory control, facilities, equipment, and operations supervision and management. Cost of goods sold also includes depreciation expense for production equipment and certain direct costs such as shipping costs.
Research and Development	Research and Development Research and development expenses consist primarily of product development, clinical and regulatory affairs, consulting services, and other costs associated with products and technologies in development. These expenses include employee compensation, stock-based compensation, supplies, travel, and facility costs. Clinical expenses include clinical trial design, clinical site reimbursement, data management, travel expenses, and the cost of manufacturing products for clinical trials.
Common Stock Valuation and Stock-Based Compensation	Common Stock Valuation and Stock-Based Compensation The Company maintains an equity incentive plan to provide long-term incentives for employees, consultants, and members of the board of directors. The plan allows for the issuance of non-statutory and incentive stock options to employees and non-statutory stock options to consultants and non-employee directors. The Company recognizes equity-based compensation expense for awards of equity instruments to employees and non-employees based on the grant date fair value of those awards in accordance with FASB ASC Topic 718, Stock Compensation (ASC 718). ASC 718 requires all equity-based compensation awards to employees and nonemployee directors, including grants of restricted shares and stock options, to be recognized as expense in the statements of operations and comprehensive loss based on their grant date fair values. The Company estimates the fair value of stock options using the Black-Scholes option pricing model. The Company uses the value of its common stock to determine the fair value of restricted shares. The Company accounts for restricted stock and common stock options issued to nonemployees under FASB ASC Topic 505-50, Equity-Based Payments to Non-Employees (ASC 505-50). As such, the value of such options is periodically remeasured and income or expense is recognized over their vesting terms. Compensation cost related to awards with service-based vesting schedules is recognized using the straight-line method. The Company determines the fair value of the restricted stock and common stock granted to nonemployees as either the fair value of the consideration received or the fair value of the equity instruments issued. The Company has not granted any share-based awards to its consultants. The Black-Scholes option pricing model requires the input of certain subjective assumptions, including (i) the expected share price volatility, (ii) the expected term of the award, (iii) the risk-free interest rate and (iv) the expected dividend yield. Due to the lack of a public market for the trading of the Company's common stock and a lack of company-specific historical and implied volatility data, the Company has based its estimate of expected volatility on the historical volatility of a group of similar companies that are publicly traded. The historical volatility is calculated based on a period of time commensurate with the expected term assumption. The group of representative companies have characteristics similar to the Company, including stage of product development and focus on the life science industry. The Company uses the simplified method, which is the average of the final vesting tranche date and the contractual term, to calculate the expected term for options granted to employees as it does not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. For options granted to non-employees, the Company utilizes the contractual term of the arrangement as the basis for the expected term assumption. The risk-free interest rate is based on a treasury instrument whose term is consistent with the expected term of the stock options. The Company uses an assumed dividend yield of zero as the Company has never paid dividends and has no current plans to pay any dividends on its common stock. The Company expenses the fair value of its equity-based compensation awards granted to employees on a straight-line basis over the associated service period, which is generally the period in which the related services are received. The Company measures equity-based compensation awards granted to nonemployees at fair value as the awards vest and recognizes the resulting value as compensation expense at each financial reporting period. The Company accounts for award forfeitures as they occur.
Advertising Expenses	Advertising Expenses The Company expenses the costs of advertising, including promotional expenses, as incurred. Advertising expenses were $1.9 million and $1.0 million during the periods ended March 31, 2018 and 2017, respectively.
Income Taxes	Income Taxes The Company accounts for income taxes using the liability method. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and tax bases of assets and liabilities and are measured using the enacted tax rates that will be in effect when the differences are expected to reverse. Valuation allowances against deferred tax assets are established, when necessary, to reduce deferred tax assets to the amounts expected to be realized. As the Company has historically incurred operating losses, the Company has recorded a full valuation allowance against its net deferred tax assets, and there is no provision for income taxes. The Company's policy is to record interest and penalties expense related to uncertain tax positions as other expense in the statements of operations and comprehensive loss.
Comprehensive Loss	Comprehensive Loss Comprehensive loss consists of net loss and changes in unrealized gains and losses on short-term investments classified as available-for-sale, if any. Accumulated other comprehensive loss is presented in the accompanying balance sheets as a component of stockholders' (deficit) equity.
Loss Per Share	Loss Per Share Basic net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock outstanding during the period. Diluted net loss per share is computed by dividing the net loss by the weighted average number of shares of common stock and dilutive potential shares of common stock outstanding during the period. Because the Company has reported a net loss for all periods presented, diluted net loss per share is the same as basic net loss per share for those periods as all potentially dilutive shares consisting of convertible preferred stock, stock options and warrants were antidilutive in those periods.
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the FASB issued Accounting Standards Update (ASU) No. 2014-09, Revenue From Contracts With Customers (ASU 2014-09), which requires 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. In doing so, companies will need to use more judgment and make more estimates than under current guidance. These may include identifying performance obligations in the contract, estimating the amount of variable consideration to include in the transaction price, and allocating the transaction price to each separate performance obligation. Under the original pronouncement, ASU 2014-09 would have been effective for the Company's annual reporting periods beginning January 1, 2018. In August 2015, the FASB issued ASU No. 2015-14, which formally defers the effective date of the new revenue standard by one year. As a result, the updated revenue guidance will be effective for the Company's annual reporting periods beginning January 1, 2019, and early adoption is permitted as of the original effective date contained within the original standard. The standard permits the use of either the retrospective or cumulative effect transition method. The Company is evaluating the effect that ASU 2014-09 will have on its financial statements and related disclosures. In November 2015, the FASB issued ASU No. 2015-17, Balance Sheet Classification of Deferred Taxes (ASU 2015-17). ASU 2015-17 is intended to reduce complexity surrounding the presentation of deferred taxes within the balance sheet. Specifically, ASU 2015-17 requires that all deferred tax assets and liabilities, along with any related valuation allowance, be classified as non-current on the balance sheet, effectively eliminating the requirement to allocate deferred taxes between current and non-current amounts. The new guidance does not permit companies to offset deferred tax liabilities from one jurisdiction against deferred tax assets of another jurisdiction. This guidance was effective January 1, 2018, and did not significantly impact its financial statements and related disclosures. In February 2016, the FASB issued ASU No. 2016-02 , Leases (ASU 2016-02). ASU 2016-02 will require entities that lease assets to recognize the rights and obligations associated with those leases on their balance sheets. The guidance will be effective for the Company's annual reporting period beginning January 1, 2020, with early adoption permitted. The Company is evaluating the impact this standard will have on its financial statements and related disclosures. In March 2016, the FASB issued No. 2016-09, Improvements to Employee Share-Based Payment Accounting (ASU 2016-09), which changes how companies will account for certain aspects of share-based payments to employees. As part of the new guidance, entities will be required to record the impact of income taxes arising from share-based compensation when awards vest or are settled within earnings as part of income tax expense rather than recorded as part of additional paid-in capital (APIC) and will eliminate the requirement that excess tax benefits be realized prior to recognition. Additionally, the guidance requires entities to present excess tax benefits as an operating activity on the statement of cash flows rather than as a financing activity. Additionally, companies will be required to make an accounting policy election at the time of adoption of the new guidance to either account for forfeitures of share-based awards in a manner similar to today's requirements (i.e., estimating the number of awards expected to be forfeited at the grant date and adjusting the estimate when awards are actually forfeited), or recognizing forfeitures as they occur with no estimate of forfeitures determined at the grant date. Companies will also be able to set a maximum statutory tax rate as it pertains to share-based awards it net settles on behalf of its employees. This will provide companies a greater ability to retain equity-award accounting treatment. Entities will apply the provisions of ASU 2016-09 using a modified retrospective transition approach, with a cumulative-effect adjustment booked to retained earnings as of the beginning of the period of adoption. This guidance was effective January 1, 2018, and did not significantly impact its financial statements and related disclosures. The Company has reviewed and considered all other recent accounting pronouncements and believes there are none that could potentially have a material impact on its business practices, financial condition, results of operations, or disclosures.

Summary of Significant Accoun19

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2018
Summary of Significant Accounting Policies
Schedule of assets and liabilities measured at fair value on a recurring basis	Fair Value Measurements as of Estimated Level 1 Level 2 Level 3 Assets Cash equivalents: Money market funds $ $ $ — $ — Commercial paper — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total cash equivalents — Short-term investments: Commercial paper $ $ — $ — Corporate bonds — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total short-term investments — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total assets $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Liabilities Preferred stock warrants $ $ — $ — $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements as of Estimated Level 1 Level 2 Level 3 Assets Cash equivalents: Money market funds $ $ $ — $ — Commercial paper — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total cash equivalents — Short-term investments: Commercial paper $ $ — $ — Corporate bonds — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total short-term investments — — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Total assets $ $ $ $ — ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Liabilities Preferred stock warrants $ $ — $ — $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of reconciliation of preferred stock warrant liabilities	Three Months 2018 2017 Balance at beginning of period $ $ Initial fair value of preferred stock warrants issued Change in fair value of preferred stock warrants — ​ ​ ​ ​ ​ ​ ​ ​ Balance at end of period $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Composition of Certain Financ20

Composition of Certain Financial Statement Items (Tables)	3 Months Ended
	Mar. 31, 2018
Composition of Certain Financial Statement Items
Schedule of inventory	March 31, December 31, Raw materials $ $ Finished goods ​ ​ ​ ​ ​ ​ ​ ​ $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of property and equipment	March 31, December 31, Computer equipment and software $ $ Furniture and office equipment Manufacturing equipment Research and development equipment Leasehold improvements ​ ​ ​ ​ ​ ​ ​ ​ Less: accumulated depreciation and amortization ) ) ​ ​ ​ ​ ​ ​ ​ ​ $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of accrued expenses	March 31, December 31, Payroll and commissions payable $ $ Vacation Other accrued expenses ​ ​ ​ ​ ​ ​ ​ ​ $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Long-Term Debt (Tables)

Long-Term Debt (Tables)	3 Months Ended
	Mar. 31, 2018
Long-Term Debt
Schedule of expected future principal payments for the credit facility	Year ending December 31 : 2018 (remaining) $ — 2019 2020 2021 2022 ​ ​ ​ ​ ​ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Commitments (Tables)

Commitments (Tables)	3 Months Ended
	Mar. 31, 2018
Commitments
Schedule of future minimum annual operating lease payments	Year ending December 31 : 2018 (remaining) $ 2019 ​ ​ ​ ​ ​ Total $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Stockholders' (Deficit) Equity

Stockholders' (Deficit) Equity (Tables)	3 Months Ended
	Mar. 31, 2018
Stockholders' (Deficit) Equity
Summary of the Company's preferred stock	A summary of the Company's preferred stock as of March 31, 2018 and December 31, 2017, is as follows: Series Shares Shares Carrying A $ B C D E F ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Summary of the Company's convertible preferred stock terms	Series Liquidation 8% Conversion A $ $ $ B C D E F
Summary of preferred stock warrants issued under the Company's original credit facility and subsequent amendments were outstanding and exercisable	Dates Exercise Warrants Initial Fair Value at Issuance Expiration Series February 8, 2018 February 8, 2028 F $ $ $ February 24, 2017 February 24, 2027 F August 7, 2015 August 7, 2025 E June 27, 2014 June 27, 2024 E August 5, 2013 August 5, 2023 C November 16, 2012 November 16, 2022 C ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Dates Exercise Warrants Initial Fair Value at Issuance Expiration Series February 24, 2017 February 24, 2027 F $ $ August 7, 2015 August 7, 2025 E June 27, 2014 June 27, 2024 E August 5, 2013 August 5, 2023 C November 16, 2012 November 16, 2022 C ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Stock Options (Tables)

Stock Options (Tables)	3 Months Ended
	Mar. 31, 2018
Stock Options
Summary of the Company's stock option activity and related information	Options Weighted Average Outstanding at December 31, 2017 $ Granted Exercised ) Forfeited ) ​ ​ ​ ​ ​ ​ ​ ​ Outstanding at March 31, 2018 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Exercisable at March 31, 2018
Schedule of stock compensation recognized	Three 2018 2017 General and administrative $ $ Sales and marketing Research and development ​ ​ ​ ​ ​ ​ ​ ​ $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
Schedule of weighted average assumptions for fair value of options granted	Three Months Ended March 31, 2018 2017 Expected life (years) 6.2 6.2 Expected volatility 37.5% 41.5% Risk-free interest rate 2.38 - 2.84% 1.94 - 2.32% Dividend yield 0.0% 0.0% Weighted average fair value $1.15 $0.40

Segment Reporting and Signifi25

Segment Reporting and Significant Customers (Tables)	3 Months Ended
	Mar. 31, 2018
Segment Reporting and Significant Customers
Schedule of revenue by geographic region	Three Months 2018 2017 United States $ $ Europe ​ ​ ​ ​ ​ ​ ​ ​ Total revenue $ $ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Loss Per Share (Tables)

Loss Per Share (Tables)	3 Months Ended
	Mar. 31, 2018
Loss Per Share
Schedule of dilutive securities excluded from computations of diluted weighted average shares outstanding	Three Months Ended 2018 2017 Convertible preferred stock outstanding Convertible preferred stock warrants Common stock options outstanding ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

Summary of Significant Accoun27

Summary of Significant Accounting Policies - Reverse Stock Split and Initial Public Offering (Details) - USD ($) $ / shares in Units, $ in Thousands	May 07, 2018	Apr. 20, 2018	Mar. 31, 2018
Reverse stock split ratio		0.150
Convertible preferred stock converted to common chares			12,111,706
Convertible preferred stock converted warrants converted to common shares warrants			98,846
Convertible preferred stock warrants reclassified to additional paid-in capital			$ 978
IPO
Shares issued	7,762,500
Shares issue price (in dollars per share)	$ 16
Net proceeds from initial public offering (IPO)	$ 112,200

Summary of Significant Accoun28

Summary of Significant Accounting Policies - Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Short-term investments:
Short-term Investments, Total	$ 6,756	$ 7,188
Liabilities
Preferred stock warrants	978	157
Fair value of assets transferred from Level 1 to Level 2	0	0
Fair value of assets transferred from Level 2 to Level 1	0	0
Fair value of liabilities transferred from Level 1 to Level 2	0	0
Fair value of liabilities transferred from Level 2 to Level 1	0	0
Recurring basis
Cash equivalents:
Money market funds	7,132	6,446
Commercial paper	3,901	1,099
Total cash equivalents	11,033	7,545
Short-term investments:
Commercial paper	3,995	5,384
Corporate bonds	2,760	1,804
Short-term Investments, Total	6,755	7,188
Total assets	17,788	14,733
Liabilities
Preferred stock warrants	978	157
Level 1 | Recurring basis
Cash equivalents:
Money market funds	7,132	6,446
Total cash equivalents	7,132	6,446
Short-term investments:
Total assets	7,132	6,446
Level 2 | Recurring basis
Cash equivalents:
Commercial paper	3,901	1,099
Total cash equivalents	3,901	1,099
Short-term investments:
Commercial paper	3,995	5,384
Corporate bonds	2,760	1,804
Short-term Investments, Total	6,755	7,188
Total assets	10,656	8,287
Level 3 | Recurring basis
Liabilities
Preferred stock warrants	$ 978	$ 157

Summary of Significant Accoun29

Summary of Significant Accounting Policies - Reconciliation of Preferred Stock Warrant Liabilities (Details) - Preferred stock warrant - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reconciliation of preferred stock warrant liabilities:
Balance at beginning of period	$ 157	$ 53
Initial fair value of preferred stock warrants issued	103	4
Change in fair value of preferred stock warrants	718
Balance at end of period	$ 978	$ 57

Summary of Significant Accoun30

Summary of Significant Accounting Policies - Inventories (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Summary of Significant Accounting Policies
Reserve for excess and obsolete inventory	$ 792	$ 518

Summary of Significant Accoun31

Summary of Significant Accounting Policies - Property and Equipment (Details)	3 Months Ended
	Mar. 31, 2018
Minimum
Property and Equipment
Estimated useful lives	3 years
Maximum
Property and Equipment
Estimated useful lives	7 years

Summary of Significant Accoun32

Summary of Significant Accounting Policies - Stock Based Compensation (Details)	3 Months Ended
	Mar. 31, 2018
Summary of Significant Accounting Policies
Dividend yield	0.00%

Summary of Significant Accoun33

Summary of Significant Accounting Policies - Advertising Expenses (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Advertising Expenses
Advertising expenses	$ 1.9	$ 1

Summary of Significant Accoun34

Summary of Significant Accounting Policies - Income Taxes (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018USD ($)
Income Taxes
Provision for income taxes	$ 0

Composition of Certain Financ35

Composition of Certain Financial Statement Items - Inventories (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Composition of Certain Financial Statement Items
Raw materials	$ 1,098	$ 1,323
Finished goods	2,296	2,347
Inventory, Net, Total	$ 3,394	$ 3,670

Composition of Certain Financ36

Composition of Certain Financial Statement Items - Property and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Property and Equipment
Property and equipment, gross	$ 1,794	$ 1,804
Less: accumulated depreciation	(883)	(810)
Property and equipment, net	911	994
Depreciation and amortization expenses	90	43
Computer equipment and software
Property and Equipment
Property and equipment, gross	333	351
Furniture and office equipment
Property and Equipment
Property and equipment, gross	137	137
Manufacturing equipment
Property and Equipment
Property and equipment, gross	1,108	1,108
Research and development equipment
Property and Equipment
Property and equipment, gross	30	30
Leasehold improvements
Property and Equipment
Property and equipment, gross	$ 186	$ 178

Composition of Certain Financ37

Composition of Certain Financial Statement Items - Accrued Expenses (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Composition of Certain Financial Statement Items
Payroll and commissions payable	$ 1,919	$ 2,871
Vacation	821	723
Other accrued expenses	403	438
Accrued Liabilities, Current, Total	$ 3,143	$ 4,032

Long-Term Debt (Details)

Long-Term Debt (Details) - USD ($) $ / shares in Units, $ in Thousands	Feb. 07, 2018	Feb. 28, 2017	Aug. 31, 2015	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2014
Credit Facility
Outstanding credit facility amount		$ 16,500
Exercise Price						$ 2.62
Debt issuance costs			$ 72
Expected future principal payments
2,019				$ 6,805
2,020				8,167
2,021				8,167
2,022				1,361
Long-term Debt, Total				$ 24,500
Term A loan facility
Credit Facility
Maximum borrowing amount under credit facility			15,500
Fixed interest rate on credit facility (as a percent)		7.95%
Variable interest rate on credit facility (as a percent)		6.90%
Additional borrowing amount under credit facility		$ 1,000
Net proceeds from credit facility		$ 500
Term A loan facility | On or after February 24, 2018 but prior to February 24, 2019/On or after February 7, 2019 but prior to February 7, 2020
Credit Facility
Prepayment fee (in percentage)		1.50%
Term A loan facility | On or after February 24, 2019/On or after February 7, 2020
Credit Facility
Prepayment fee (in percentage)		1.00%
Original credit facility
Credit Facility
Outstanding credit facility amount			$ 12,000
Term B loan facility
Credit Facility
Outstanding credit facility amount	$ 24,500	$ 9,000
Fixed interest rate on credit facility (as a percent)	7.95%		7.95%
Additional borrowing amount under credit facility	$ 8,000
Net proceeds from credit facility	$ 8,000
Trailing period revenue	12 months
Minimum amount of revenue for interest only period	$ 25,000
Final payment fee (in percentage)		5.00%
Increase in interest rate in default (in percentage)					5.00%
Term B loan facility | LIBOR
Credit Facility
Variable interest rate on credit facility (as a percent)	6.90%
Term B loan facility | Minimum
Credit Facility
Outstanding credit facility amount			$ 3,500
Term B loan facility | Maximum
Credit Facility
Outstanding credit facility amount			$ 10,000
Term B loan facility | Prior to February 24, 2018/Prior to February 7, 2019
Credit Facility
Prepayment fee (in percentage)		2.50%
Term B loan facility | On or after February 24, 2018 but prior to February 24, 2019/On or after February 7, 2019 but prior to February 7, 2020
Credit Facility
Prepayment fee (in percentage)		1.50%
Term B loan facility | On or after February 24, 2019/On or after February 7, 2020
Credit Facility
Prepayment fee (in percentage)		1.00%
Series F preferred shares
Credit Facility
Number of shares issued upon exercise of warrants	233,577	29,197
Period of warrants (in years)	10 years	10 years
Exercise Price	$ 1.37	$ 1.37

Commitments - Operating Lease (

Commitments - Operating Lease (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Future minimum annual operating lease payments
2018 (remaining)	$ 145
2,019	48
Total	193
Rent expense
Rent expense	$ 52	$ 39

Commitments - Purchase Commitme

Commitments - Purchase Commitments (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Purchase Commitments
Commitments to suppliers for inventory purchases	$ 7.1	$ 9
Medtronic
Purchase Commitments
Commitments to suppliers for inventory purchases	$ 0	$ 0.4

Stockholders' (Deficit) Equit41

Stockholders' (Deficit) Equity - Authorized Shares (Details) - $ / shares	Mar. 31, 2018	Dec. 31, 2017
Stockholders' (Deficit) Equity
Total shares authorized	186,894,620
Common stock, authorized (in shares)	110,000,000	110,000,000
Preferred shares, authorized shares	76,894,620	76,894,620
Common stock, par value (in dollars per share)	$ 0.001	$ 0.001
Preferred stock, par value (in dollars per share)	$ 0.001	$ 0.001

Stockholders' (Deficit) Equit42

Stockholders' (Deficit) Equity - Preferred Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended
	Feb. 28, 2017	Oct. 31, 2016	Mar. 31, 2014	Mar. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Shares Authorized				76,894,620	76,894,620
Shares Issued				76,235,050	76,235,050
Shares Outstanding				76,235,050	76,235,050
Carrying Value				$ 119,106	$ 119,106
Dividend Rate Percentage				8.00%
Series A convertible preferred stock
Class of Stock [Line Items]
Shares Authorized				5,375,507	5,375,507
Shares Issued				5,375,507	5,375,507
Shares Outstanding				5,375,507	5,375,507
Carrying Value				$ 5,037	$ 5,037
Liquidation Preference Per Share				$ 1
8% Dividend Per Share				0.08
Conversion Price Per Share				6.65
Shares issue price (in dollars per share)				$ 1
Convertible preferred stock, conversion shares				0.1504
Series B convertible preferred stock
Class of Stock [Line Items]
Shares Authorized				8,706,909	8,706,909
Shares Issued				8,706,909	8,706,909
Shares Outstanding				8,706,909	8,706,909
Carrying Value				$ 15,913	$ 15,913
Liquidation Preference Per Share				$ 1.84
8% Dividend Per Share				0.12
Conversion Price Per Share				9.91
Shares issue price (in dollars per share)				$ 1.84
Convertible preferred stock, conversion shares				0.1855
Series C convertible preferred stock
Class of Stock [Line Items]
Shares Authorized				14,091,589	14,091,589
Shares Issued				13,829,906	13,829,906
Shares Outstanding				13,829,906	13,829,906
Carrying Value				$ 14,949	$ 14,949
Liquidation Preference Per Share				$ 1.07
8% Dividend Per Share				0.09
Conversion Price Per Share				7.12
Shares issue price (in dollars per share)				$ 1.07
Convertible preferred stock, conversion shares				0.1504
Series D convertible preferred stock
Class of Stock [Line Items]
Shares Authorized				5,683,292	5,683,292
Shares Issued				5,683,292	5,683,292
Shares Outstanding				5,683,292	5,683,292
Carrying Value				$ 6,043	$ 6,043
Liquidation Preference Per Share				$ 1.07
8% Dividend Per Share				0.09
Conversion Price Per Share				7.12
Shares issue price (in dollars per share)				$ 1.07
Convertible preferred stock, conversion shares				0.1504
Series E convertible preferred stock
Class of Stock [Line Items]
Shares Authorized				15,373,091	15,373,091
Shares Issued				15,267,175	15,267,175
Shares Outstanding				15,267,175	15,267,175
Carrying Value				$ 39,848	$ 39,848
Liquidation Preference Per Share				$ 2.62
8% Dividend Per Share				0.21
Conversion Price Per Share				$ 15.16
Shares issued			15,267,175
Shares issue price (in dollars per share)			$ 2.62
Convertible preferred stock, conversion shares			0.1728
Proceeds from issuance of convertible preferred stock			$ 39,800
Series F convertible preferred stock
Class of Stock [Line Items]
Shares Authorized				27,664,232	27,664,232
Shares Issued				27,372,261	27,372,261
Shares Outstanding				27,372,261	27,372,261
Carrying Value				$ 37,316	$ 37,316
Liquidation Preference Per Share				$ 1.37
8% Dividend Per Share				0.11
Conversion Price Per Share				$ 9.11
Shares issued	18,248,177	9,124,084
Shares issue price (in dollars per share)	$ 1.37	$ 1.37
Convertible preferred stock, conversion shares	0.1504
Proceeds from issuance of convertible preferred stock	$ 25,000	$ 12,300

Stockholders' (Deficit) Equit43

Stockholders' (Deficit) Equity - Preferred Stock Warrants (Details) - USD ($) $ / shares in Units, $ in Thousands	Feb. 08, 2018	Feb. 24, 2017	Aug. 07, 2015	Aug. 05, 2013	Nov. 16, 2012	Jun. 30, 2014	Mar. 31, 2018	Dec. 31, 2016	Dec. 31, 2012	Dec. 31, 2017	Dec. 31, 2015	Jun. 27, 2014
Class of Warrant or Right [Line Items]
Warrants issued (in shares)						76,334
Term of warrants						10 years
Value of each warrant (in dollars per warrant)						$ 1.11
Exercise Price						$ 2.62
Warrants Outstanding							630,372			396,795
Initial Value of Warrants						$ 85
Fair Value of Warrants							$ 978			$ 157
Series C convertible preferred stock | Warrants Issued on August 2013
Class of Warrant or Right [Line Items]
Warrants issued (in shares)				74,768
Term of warrants				10 years
Value of each warrant (in dollars per warrant)				$ 0.525
Exercise Price				$ 1.07			$ 1.07			$ 1.07
Warrants Outstanding							74,768			74,768
Initial Value of Warrants				$ 39
Fair Value of Warrants							$ 116			$ 33
Series C convertible preferred stock | Warrants Issued on November 2012
Class of Warrant or Right [Line Items]
Exercise Price							$ 1.07			$ 1.07
Warrants Outstanding							186,916			186,916
Initial Value of Warrants					$ 96					$ 96
Fair Value of Warrants							$ 292			$ 82
Series C convertible preferred stock | Five Year Warrants Issued on November 2012
Class of Warrant or Right [Line Items]
Warrants issued (in shares)									278,506
Term of warrants									5 years
Value of each warrant (in dollars per warrant)									$ 0.34
Exercise Price									$ 1.07
Number of shares issued upon exercise of warrants								258,880
Proceeds from the exercise of preferred stock warrants								$ 277
Warrants Outstanding											258,880
Initial Value of Warrants									$ 94
Series C convertible preferred stock | Ten Year Warrants Issued on November 2012
Class of Warrant or Right [Line Items]
Warrants issued (in shares)					186,916
Term of warrants					10 years
Value of each warrant (in dollars per warrant)					$ 0.515
Exercise Price					$ 1.07
Initial Value of Warrants					$ 96
Series E convertible preferred stock | Warrants Issued on August 2015
Class of Warrant or Right [Line Items]
Warrants issued (in shares)			29,580
Term of warrants			10 years
Value of each warrant (in dollars per warrant)			$ 1.13
Exercise Price			$ 2.62				$ 2.62			$ 2.62
Warrants Outstanding							29,580			29,580
Initial Value of Warrants			$ 33
Fair Value of Warrants							$ 45			$ 8
Series E convertible preferred stock | Warrants Issued on June 2014
Class of Warrant or Right [Line Items]
Exercise Price							$ 2.62			$ 2.62
Warrants Outstanding							76,334			76,334
Initial Value of Warrants												$ 85
Fair Value of Warrants							$ 115			$ 21
Series F convertible preferred stock
Class of Warrant or Right [Line Items]
Exercise Price							$ 1.37
Adjusted warrants due to dilutive issuance							56,569
Series F convertible preferred stock | Warrants Issued on February 2018
Class of Warrant or Right [Line Items]
Warrants issued (in shares)	233,577
Term of warrants	10 years
Value of each warrant (in dollars per warrant)	$ 0.44
Exercise Price	$ 1.37						$ 1.37
Warrants Outstanding							233,577
Initial Value of Warrants	$ 103
Fair Value of Warrants							$ 364
Series F convertible preferred stock | Warrants Issued on February 2017
Class of Warrant or Right [Line Items]
Warrants issued (in shares)		29,197
Term of warrants		10 years
Value of each warrant (in dollars per warrant)		$ 0.13
Exercise Price		$ 1.37					$ 1.37			$ 1.37
Warrants Outstanding							29,197			29,197
Initial Value of Warrants		$ 4
Fair Value of Warrants							$ 46			$ 13

Stock Options (Details)

Stock Options (Details) - Stock Incentive Plan 2017	3 Months Ended
	Mar. 31, 2018$ / sharesshares
Stock Options
Number of shares reserved for issuance | shares	2,945,384
Options granted exercise prices	$ 2.80
Weighted Average Exercise Price for Granted options ( Per share)	$ 2.80
Service period for stock options to grant	4 years
Vesting of stock options granted for each year of service (as a percent)	25.00%
Service period after the first year of service	36 months
Contractual life of stock options (in years)	10 years
Minimum
Stock Options
Options granted exercise prices	$ 0.94
Weighted Average Exercise Price for Granted options ( Per share)	0.94
Maximum
Stock Options
Options granted exercise prices	6.65
Weighted Average Exercise Price for Granted options ( Per share)	$ 6.65

Stock Options - Stock Option Ac

Stock Options - Stock Option Activity (Details) - Stock Incentive Plan 2017	3 Months Ended
	Mar. 31, 2018$ / sharesshares
Options
Outstanding at beginning of the year | shares	2,071,616
Granted (in shares) | shares	34,639
Exercised (in shares) | shares	(106,653)
Forfeited (in shares) | shares	(23,902)
Outstanding at ending of the year | shares	1,975,700
Exercisable (in shares) | shares	1,231,008
Weighted Average Exercise Price
Weighted Average Exercise Price at beginning of the year (per share) | $ / shares	$ 1.47
Weighted Average Exercise Price for Granted options ( Per share) | $ / shares	2.80
Weighted Average Exercise Price for Exercised options (per share) | $ / shares	1.74
Weighted Average Exercise Price for Forfeited options (per share) | $ / shares	1.03
Weighted Average Exercise Price at ending of the year (per share) | $ / shares	1.48
Weighted Average Exercise Price for Exercisable options (per share) | $ / shares	$ 1.68
Average remaining contractual life outstanding stock options (in years)	5 years 2 months 12 days

Stock Options - Stock compensat

Stock Options - Stock compensation expenses (Details) - Stock Incentive Plan 2017 - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Stock Options
Total stock compensation recognized, before taxes	$ 55	$ 54
Amount of unearned stock compensation related to unvested employee and non employee director share based awards	$ 295
Weighted average period over which the unearned stock compensation	3 years
General and administrative
Stock Options
Total stock compensation recognized, before taxes	$ 28	24
Sales and marketing
Stock Options
Total stock compensation recognized, before taxes	21	22
Research and development
Stock Options
Total stock compensation recognized, before taxes	$ 6	$ 8

Stock Options - weighted averag

Stock Options - weighted average assumptions (Details) - Stock Incentive Plan 2017 - $ / shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Weighted average assumptions
Expected life (years)	6 years 2 months 12 days	6 years 2 months 12 days
Expected volatility	37.50%	41.50%
Risk-free interest rate, Minimum	2.38%	1.94%
Risk-free interest rate, Maximum	2.84%	2.32%
Dividend yield	0.00%	0.00%
Weighted average fair value	$ 1.15	$ 0.40

Related-Party Transactions (Det

Related-Party Transactions (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018USD ($)personitem	Mar. 31, 2017USD ($)	Dec. 31, 2016item
Development Agreement
Inventory purchases | $	$ 197	$ 223
Medtronic
Board of Directors' Appointment
Number of person allowed for member of entities Board of Directors | person	1
Number of voting member before the Series F Preferred Stock purchase agreement			1
Number of voting member before the Series F Preferred Stock purchase agreement			2
Supply Agreement
Number of buy right of inventory upon termination of supply agreement	1
Right-of-First-Offer of the company
Period to negotiate exclusively with Medtronic on possible sale of business prior to negotiating with third party	90 days

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Unrecognized tax benefits	$ 0	$ 0
Federal tax at statutory rate (as a percent)	21.00%	35.00%
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		$ 110.9
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards		65.9
R&D credit
Operating Loss Carryforwards [Line Items]
Credit carryforwards		$ 1.4

Segment Reporting and Signifi50

Segment Reporting and Significant Customers (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018USD ($)segment	Mar. 31, 2017USD ($)
Segment Reporting and Significant Customers
Number of reporting segment | segment	1
Total revenue	$ 10,042	$ 5,297
United States
Segment Reporting and Significant Customers
Total revenue	8,744	4,508
Europe
Segment Reporting and Significant Customers
Total revenue	$ 1,298	$ 789

Loss Per Share (Details)

Loss Per Share (Details) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Loss Per Share
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding	14,744,767	14,228,349
Convertible preferred stock
Loss Per Share
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding	12,111,706	12,111,706
Convertible preferred stock warrants
Loss Per Share
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding	657,361	423,784
Common stock options
Loss Per Share
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding	1,975,700	1,692,859