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Stock-Based Compensation | 9 Months Ended
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Sep. 30, 2021
Stock-Based Compensation [Abstract] |
Stock-Based Compensation |
7. | Stock-Based compensation
Summary of plans and activity In June 2001, the Company’s Board of Directors and stockholders established the 2001 Stock Incentive Award Plan (“2001 Plan”). Under the 2001 Plan, as amended, 2,674,749 shares of common stock had been reserved for the issuance of incentive stock options granted to employees, nonemployee directors, consultants or independent contractors. Options granted under the 2001 Plan have vesting terms that range from the day of grant to four years and expire within a maximum term of 10 years from the grant date. In connection with the IPO in 2021, the Company’s Board of Directors and stockholders established the 2021 Equity Incentive Plan (“2021 Plan”). The number of shares of common stock initially reserved for issuance under the 2021 Plan was 1,854,490 newly reserved shares in addition to the 600,373 shares that remained available for issuance under the 2001 Plan. The shares available for issuance under the 2021 Plan will automatically increase on the first day of each year, commencing January 1, 2022 and ending on (and including) January 1, 2031, in an amount equal to 5% of the total number of shares of the Company’s common stock outstanding on the last day of the calendar month before the date of each automatic increase, or such lesser number of shares as determined by the Board of Directors. The 2021 Plan provides for the issuance of stock options, stock appreciation rights, restricted stock awards, stock unit awards and other stock-based awards and cash incentive awards to employees, consultants and non-employee directors of the Company and its subsidiaries. Awards granted under the 2021 Plan will have such vesting schedules and other terms as determined by the Compensation Committee and stock options and stock appreciation rights have a maximum term of 10 years from the grant date. No further awards can be made under the 2001 Plan following the adoption of the 2021 Plan. As of September 30, 2021, there were 1,791,888 shares available for future issuance under the 2021 Plan. Options are granted at exercise prices not less than the fair market value (as determined by the Board of Directors) of the Company’s common stock on the date of grant. During the years 2008 through September 30, 2021, the Board of Directors authorized the grant of stock options for the purchase of shares of common stock to the employers of certain nonemployee directors. The options were not granted under the 2001 Plan or the 2021 Plan, but terms are substantially the same as the Company’s standard form of option agreement for nonemployee directors as they have an exercise price not less than the fair market value on the grant date and vest over 48 months from the date of grant. The following is a summary of stock option activity:
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Weighted |  | ​ | ​
​ | ​ | Number | ​ | Average | ​ | Aggregate
​ | ​ | of | ​ | Exercise | ​ | Intrinsic
​ | ​ | Options | ​ | Price | ​ | Value
​ | ​ | ​ | ​ | ​ | ​ |  | (in thousands)
Balance as of December 31, 2020 |  | 1,473,359 | ​ | $ | 2.64 |  | ​ | ​ |
Granted |  | 1,238,426 | ​ |  | 13.00 |  | ​ | ​ |
Cancelled / Forfeited |  | (42,669) | ​ |  | 10.31 |  | ​ | ​ |
Exercised |  | (11,783) | ​ |  | 0.88 |  | ​ | ​ |
Balance as of September 30, 2021 |  | 2,657,333 | ​ | $ | 7.35 | ​ | ​ | $ | 25,475
Options exercisable as of September 30, 2021 |  | 1,009,785 | ​ | $ | 2.36 | ​ | ​ | $ | 14,330
​ As of September 30, 2021, stock options outstanding included 9,993 options that were not granted under the 2001 Plan or the 2021 Plan. For options outstanding as of September 30, 2021, the weighted average remaining contractual life was 8.4 years. For options exercisable as of September 30, 2021, the weighted average remaining contractual life was 7.3 years. In connection with the IPO, the Company’s Board of Directors and stockholders also established an Employee Stock Purchase Plan (the “ESPP”). The number of shares of common stock initially reserved for issuance under the ESPP was 278,170. The shares available for issuance under the ESPP will automatically increase on the first day of each year, commencing January 1, 2022 and ending on (and including) January 1, 2031, in an amount equal to 1% of the total number of shares of the Company’s common stock outstanding on the last day of the calendar month before the date of each automatic increase, or such lesser number of shares as determined by the Board of Directors. The ESPP will permit certain of the Company’s U.S. employees to purchase shares of the Company’s common stock at a price per share not less than 85% of the lower of (i) the closing market price per share of the Company’s common stock on the first day of the applicable purchase period or (ii) the closing market price per share of the Company’s common stock on the purchase date at the end of the applicable six-month purchase period. The Company expects the first purchase period to commence in 2022. Accordingly, as of September 30, 2021, no shares of common stock have been purchased under the ESPP. Stock-based compensation expense The Company uses the Black-Scholes option pricing model to determine the fair value of stock options on the grant date. The Company measures stock-based compensation expense based on the grant date fair value of the award and recognizes compensation expense over the requisite service period, which is generally the vesting period. The amount of stock-based compensation expense recognized during a period is based on the portion of the awards that are ultimately expected to vest. The Company estimates pre-vesting forfeitures at the time of grant by analyzing historical data and revises those estimates in subsequent periods if actual forfeitures differ from those estimates. The following table provides the weighted average fair value of options granted to employees and the related assumptions used in the Black-Scholes option pricing model for the nine months ended September 30, 2021. No options were issued during the nine months ended September 30, 2020.
​ | ​ | ​ | ​ | ​
​ |  | ​ | September 30, | ​
​ | ​ | ​ | 2021 |
Weighted average fair value of options granted |  | $ | 5.25 | ​
Expected term (in years) — non-officer employees |  | ​ | 2.7 to 6.1 | ​
Expected term (in years) — officer employees |  | ​ | 3.0 | ​
Expected volatility |  | ​ | 56.1% to 63.4 | %
Expected dividend yield |  | ​ | — | %
Risk-free interest rate |  | ​ | 0.17% to 0.47 | %
​ The Company reviews these assumptions on a periodic basis and adjusts them, as necessary. The expected term of an award was determined based on the Company’s analysis of historical exercise behavior while taking into consideration various participant demographics and option characteristics. We utilize the simplified method to develop the estimate of the expected term. The expected volatility is based upon observed volatility of comparable public companies. The expected dividend yield is assumed to be zero, as the Company has never paid dividends and has no current plans to do so. The risk-free interest rate is based on the yield on U.S. Treasury securities for a period approximating the expected term of the options being valued. For the three and nine months ended September 30, 2021 and 2020, the Company recognized stock-based compensation expense as follows:
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Three Months Ended September 30, |  | Nine Months Ended September 30,
(in thousands) | ​ | 2021 | ​ | 2020 | ​ | 2021 | ​ | 2020
Selling, general & administrative | ​ | $ | 353 | ​ | $ | 20 | ​ | $ | 830 | ​ | $ | 62
Research & development | ​ | ​ | 115 |  | ​ | 11 | ​ | ​ | 319 |  | ​ | 32
Cost of goods sold | ​ | ​ | 6 |  | ​ | — | ​ | ​ | 10 |  | ​ | 1
​ | ​ | $ | 474 | ​ | $ | 31 | ​ | $ | 1,159 | ​ | $ | 95
​ As of September 30, 2021, unrecognized compensation expense related to unvested stock-based compensation arrangements was $6.3 million. As of September 30, 2021, the related weighted average period over which it is expected to be recognized is approximately 3.5 years. Performance-Based options As of September 30, 2021, the Company had 10,493 stock options outstanding that contained vesting conditions contingent on the achievement of certain milestones. Assuming continued service by the employees, the options would start vesting over a 48-month period upon achievement of the performance criteria. As of September 30, 2021, the Company determined that the likelihood of achieving the milestones was not probable and therefore no stock-based compensation expense was recorded. Early exercise of stock options Under the 2001 Plan, the Company has issued options to certain executive officers with early-exercise provisions. The options may be exercised by the holder any time after they are granted. The Company has the right to repurchase, at the original option exercise price, shares issued pursuant to such early-exercise provisions, upon the termination of employment or death of the stockholder. This repurchase right expires based upon the original option vesting schedule. As of September 30, 2021 and 2020, there have been no early exercises and therefore there is no liability recorded for the early exercise of stock options.

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