| Summary of Significant Accounting Policies | Summary of Significant Accounting Policies Basis of Presentation The financial statements have been prepared in accordance with U.S. generally accepted accounting principles ("U.S. GAAP") and pursuant to the rules and regulations of the Securities and Exchange Commission ("SEC"). 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 have been made. The results of operations for the year ended December 31, 2018 are not necessarily indicative of the operating results for any future periods. Certain prior period amounts have been reclassified to conform to the current presentation. These reclassifications had no effect on the reported results of operations. Reverse Stock Split In connection with our initial public offering of common stock ("IPO"), our board of directors and stockholders approved a 1-for-6.650 reverse stock split of our 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 converted 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, we completed our IPO by issuing 7,762,500 shares of common stock, at an offering price of $16.00 per share, for net proceeds of approximately $112.0 million after deducting underwriting discounts and commissions and offering expenses payable by us. In connection with the IPO, our outstanding shares of convertible preferred stock were automatically converted into an aggregate of 12,111,710 shares of common stock, and our outstanding warrants to purchase shares of convertible preferred stock were automatically converted into warrants to purchase up to an aggregate of 100,558 shares of common stock, resulting in the reclassification of the related redeemable convertible preferred stock warrant liability of $0.9 million to additional paid-in capital ("APIC"). Follow-On Public Offering On December 11, 2018, we completed a follow-on offering that included our offer and sale of 1,875,000 shares of common stock and the selling stockholders’ offer and sale of 1,000,000 shares of common stock, at a public offering price of $40.00 per share. We received net proceeds of approximately $69.8 million after deducting underwriting discounts and commissions and offering expenses. We received no proceeds from the sale of our common stock by the selling stockholders. 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. We use significant judgment when making estimates related to the allowance for doubtful accounts, inventory reserves, warranty reserves, and the valuations of our common stock prior to our IPO, share-based awards, and certain of our previously outstanding preferred stock warrants. We base our 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 (the "JOBS Act"), we are eligible to take advantage of certain exemptions from various reporting requirements that are applicable to other public companies that are not emerging growth companies. We have 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 We consider 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. Foreign Currency Sales and expenses denominated in foreign currencies are translated at average exchange rates in effect throughout the year. Foreign currency transaction gains and losses are included in other expense (income) in the statements of operations and comprehensive loss. Assets and liabilities of foreign operations are remeasured at period-end exchange rates with the impacts of foreign currency remeasurement recognized in other expense (income) in the statements of operations and comprehensive loss. Short-Term Investments At December 31, 2018 and 2017, our short-term investments consisted of commercial paper, corporate bonds, asset-backed securities, and U.S. government securities 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 as a separate component of accumulated other comprehensive loss. We had $0.1 million and $0 of unrecognized losses in accumulated other comprehensive loss balance at December 31, 2018 and 2017, respectively. Any realized gains and losses are calculated on the specific identification method and reported net in interest income or interest expense. For the years ended December 31, 2018 and 2017, we recognized less than $0.1 million and no gains, net, respectively. We review our investment portfolio periodically to assess for other-than-temporary impairment. Should we determine that any unrealized losses on the investments are other-than-temporary, the amount of that impairment to be recognized in earnings will depend on whether we intend to sell the security or more likely than not will be required to sell the security before recovery of its amortized cost basis less any current period credit loss. Fair Value of Financial Instruments We measure certain financial assets and liabilities at fair value on a recurring basis, including cash equivalents, short-term investments, and our previously 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 us to develop our own assumptions. We use the methods and assumptions described below in determining the fair value of our 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 notes, asset-backed securities 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. U.S. government securities: Consists of U.S. Government Treasury bills with original maturities of less than one year. These are included as a Level 1 measurement in the table below. The following tables sets forth by level within the fair value hierarchy our assets and liabilities that are reported at fair value as of December 31, 2018 and 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 December 31, 2018 Estimated Fair Value Level 1 Level 2 Level 3 Assets Cash equivalents: Money market funds $ 94,700 $ 94,700 $ — $ — Total cash equivalents 94,700 94,700 — — Short-term investments: Commercial paper $ 27,898 $ — $ 27,898 $ — Corporate bonds 28,012 — 28,012 — Asset-backed securities 17,055 — 17,055 — U.S. government securities 17,957 17,957 — — Total short-term investments 90,922 17,957 72,965 — Total assets $ 185,622 $ 112,657 $ 72,965 $ — Fair Value Measurements as of December 31, 2017 Estimated Fair Value Level 1 Level 2 Level 3 Assets Cash equivalents: Money market funds $ 6,446 $ 6,446 $ — $ — Commercial paper 1,099 — 1,099 — Total cash equivalents 7,545 6,446 1,099 — Short-term investments: Commercial paper 5,384 — 5,384 — Corporate bonds 1,804 — 1,804 — Total short-term investments 7,188 — 7,188 — Total assets $ 14,733 $ 6,446 $ 8,287 $ — Liabilities Preferred stock warrants $ 157 $ — $ — $ 157 There were no transfers between Level 1, Level 2, or Level 3 during the years ended December 31, 2018 and 2017. The recurring Level 3 fair value measurements of our preferred stock warrant liabilities used the Black-Scholes option pricing model and value of the respective class of our convertible preferred stock (see Note 8), which was 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 our preferred stock warrant liabilities: Year Ended December 31, 2018 2017 2016 Balance at beginning of period $ 157 $ 53 $ 248 Initial fair value of preferred stock warrants issued 103 4 — Reclassified to equity (855) — — Change in fair value of preferred stock warrants 595 100 (195) Balance at end of period $ — $ 157 $ 53 Changes in the fair value of the preferred stock warrant liability were recorded in other expenses on the statements of operations and comprehensive loss. In connection with the closing of the IPO in May 2018, warrants to purchase shares of preferred stock automatically converted into warrants to purchase shares of common stock, resulting in the reclassification of the related convertible preferred stock warrant liability to APIC. Concentration of Credit Risk Financial instruments, which potentially subject us to concentrations of credit risk, consist principally of cash equivalents, short-term investments, and accounts receivable. Our 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. We place restrictions on maturities and concentration by type and issuer. We are 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 December 31, 2018 and 2017, we limited our credit risk associated with cash equivalents by placing investments with banks we believe are highly creditworthy. We believe that the credit risk in our accounts receivable is mitigated by our credit evaluation process, relatively short collection terms, and dispersion of our customer base. We generally do not require collateral, and losses on accounts receivable have historically been within management's expectations. Accounts Receivable and Allowance for Doubtful Accounts Trade accounts receivable are recorded at the invoiced amount and do not bear interest. Customer credit terms are established prior to shipment with the general standard being net 30 days. Collateral or any other security to support payment of these receivables generally is not required. We record an allowance for doubtful accounts for accounts receivable deemed uncollectible. We evaluate the collectability of our accounts receivable based on known collection risks and historical experience. In circumstances where we are aware of a specific customer's inability to meet its financial obligations to us (e.g., bankruptcy filings, substantial downgrading of credit ratings), we record a specific allowance for bad debts against amounts due to reduce the carrying amount of accounts receivable to the amount we reasonably believe will be collected. Specific accounts receivable are written-off once a determination is made that the account is uncollectible. Inventories Inventories are valued at the lower of cost or net realizable value, computed on a first-in, first out basis. We regularly review inventory quantities on-hand for excess and obsolete inventory and, when circumstances indicate, incur charges to write down inventories to their net realizable value. Our 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 $0.8 million and $0.5 million as of December 31, 2018 and 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 3 years to 5 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, we compare 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, we determine the fair value of the asset and recognize an impairment loss to the extent the carrying amount of the asset exceeds its fair value. We determine fair value using the income approach based on the present value of expected future cash flows or other appropriate measures of estimated fair value. Our cash flow assumptions consider historical and forecasted revenue and operating costs and other relevant factors. We did not record any material impairment charges on long-lived assets during the years ended December 31, 2018 and 2017. Revenue Recognition We recognize 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. We make 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, our 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. Shipping and Handling We expense shipping and handling costs as incurred and include them in the cost of goods sold. In those cases where shipping and handling costs are billed to customers, we classify 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, warranty replacement costs, 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 We maintain 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 directors. We recognize equity-based compensation expense for awards of equity instruments to employees and directors 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 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. We estimate the fair value of stock options using the Black-Scholes option pricing model. We have not granted any restricted shares. We have not granted any share-based awards to our 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 our common stock and a lack of company-specific historical and implied volatility data, we have based our 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 us, including stage of product development and focus on the life science industry. We use 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 and directors as we do not have sufficient historical exercise data to provide a reasonable basis upon which to estimate the expected term. The risk-free interest rate is based on a Treasury instrument whose term is consistent with the expected term of the stock options. We use an assumed dividend yield of zero as we have never paid dividends and have no current plans to pay any dividends on our common stock. We expense the fair value of our equity-based compensation awards granted to employees and directors on a straight-line basis over the associated service period, which is generally the period in which the related services are received. We account for award forfeitures as they occur. Advertising Expenses We expense the costs of advertising, including promotional expenses, as incurred. Advertising expenses were $7.8 million, $5.5 million and $3.4 million during the years ended December 31, 2018, 2017 and 2016, respectively Income Taxes We account 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 we have historically incurred operating losses, we have recorded a full valuation allowance against our net deferred tax assets, and there is no provision for income taxes. Our 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. Accumulated other comprehensive loss is presented in the accompanying balance sheets as a component of stockholders' 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 we have 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 We are an “emerging growth company” as defined by the JOBS Act. The JOBS Act provides that an emerging growth company can take advantage of the extended transition period provided in Section 7(a)(2)(B) of the Securities Act of 1933, as amended, (the "Securities Act"), for complying with new or revised accounting standards. Accordingly, an emerging growth company can selectively delay the adoption of certain accounting standards until those standards would otherwise apply to private companies. We have elected to avail ourselves of this exemption and, as a result, our financial statements may not be comparable to the financial statements of issuers that are required to comply with the effective dates for new or revised accounting standards that are applicable to public companies. Section 107 of the JOBS Act provides that we can elect to opt out of the extended transition period at any time, which election is irrevocable. In May 2014, the FASB issued guidance creating ASC Section 606, Revenue from Contracts with Customers ("ASC 606"), which established a comprehensive new model for the recognition of revenue from contracts with customers. This guidance requires expanded disclosures relating to the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. Additionally, qualitative and quantitative disclosures are required for customer contracts, significant judgments and changes in judgments, and assets recognized from the costs to obtain or fulfill a contract. The updated guidance is effective for interim and annual reporting periods beginning on or after December 15, 2018 for private companies and, therefore, us due to the JOBS Act exemption described above. We have completed the process of evaluating the effect of the adoption and determined there were no changes required to our reported revenues as a result of the adoption. The majority of our revenue arrangements generally consist of a single performance obligation to transfer promised goods or services. Based on our evaluation process and review of our contracts with customers, the timing and amount of revenue recognized based on ASC 606 is consistent with our revenue recognition policy under previous guidance. We will adopt the new standard effective January 1, 2019, using the modified retrospective approach, and will expand our financial statement disclosures in order to comply with the guidance. We have determined the adoption of ASC 606 will not have a material impact on our results of operations, cash flows, or financial position. We have assessed and do not anticipate any material changes to information technology systems, processes, and internal controls to support recognition and disclosure of ASC 606. In November 2015, the FASB issued ASU 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 our financial statements and related disclosures. In February 2016, the FASB issued ASU 2016-02, Leases (Topic 842), which supersedes the existing guidance for lease accounting, Leases (Topic 840). ASU 2016-02 requires lessees to recognize a lease liability and a right-of-use asset for all leases with lease terms greater than 12 months. Lessor accounting remains largely unchanged. The amendments in this ASU are effective for interim and annual periods beginning after December 15, 2019 for private companies; and, therefore, us due to the JOBS Act exemption described above. Early adoption is permitted for all entities. ASU 2016-02 requires a modified retrospective approach for all leases existing at, or entered into after, the date of initial adoption, with an option to elect to use certain transition relief. We plan to further evaluate the anticipated impact of the adoption of this ASU on our financial statements beginning in the second quarter of 2019. In March 2016, the FASB issued ASU 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 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. Companies will also 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 for us on January 1, 2018 and did not significantly impact our financial statements and related disclosures. In August 2018, the FASB issued ASU 2018-13, Fair Value Measurement (Topic 820) Disclosure Framework-Changes to the Disclosure Requirements for Fair Value Measurement. The amendments in the standard apply to all entities that are required, under existing GAAP, to make disclosures about recurring or nonrecurring fair value measurements. ASU 2018-13 removes, modifies, and adds certain disclosure requirements in ASC 820, Fair Value Measurement . The standard is effective for all entities for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. We are currently evaluating the impact of this ASU on our financial statements and related disclosures. We have reviewed and considered all other recent accounting pronouncements and believe there are none that could potentially have a material impact on our business practices, financial condition, results of operations, or disclosures. |
