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Stock-Based Compensation | 12 Months Ended
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Dec. 31, 2023
Stock-Based Compensation [Abstract] |
Stock-Based Compensation |
7. | Stock-based compensation
Summary of plans and activity In June 2001, our 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, non-employee directors, consultants or independent contractors. Options granted under the 2001 Plan have vesting terms that range from the date of grant to four years and expire within a maximum term of 10 years from the grant date. In 2021, our 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 600,737 shares that remained available for issuance under the 2001 Plan. The shares available for issuance under the 2021 Plan 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 our 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 annual increase resulted in an additional 1,033,186 shares being reserved for issuance under the 2021 Plan as of January 1, 2023. 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 December 31, 2023, there were 1,671,202 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 our common stock on the date of grant. During the years 2008 through the IPO, the Board of Directors authorized the grant of stock options for the purchase of shares of common stock to the employers of certain non-employee directors. The options were not granted under the 2001 Plan or the 2021 Plan, but terms are substantially the same as our standard form of option agreement for non-employee 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, 2021 | ​ | 2,749,441 | ​ | $ | 7.93 | ​ | ​ | $ | 16,262
Granted | ​ | 1,486,805 | ​ | $ | 8.14 | ​ | ​ | ​ | ​
Cancelled / Forfeited | ​ | (328,902) | ​ | $ | 7.96 | ​ | ​ | ​ | ​
Exercised | ​ | (150,509) | ​ | $ | 1.22 | ​ | ​ | ​ | ​
Balance as of December 31, 2022 |  | 3,756,835 | ​ | $ | 8.28 |  | ​ | $ | 36,616
Granted |  | 1,161,926 | ​ | $ | 14.20 |  | ​ | ​ |
Cancelled / Forfeited |  | (285,729) | ​ | $ | 10.52 |  | ​ | ​ |
Exercised |  | (144,187) | ​ | $ | 5.05 |  | ​ | ​ |
Balance as of December 31, 2023 |  | 4,488,845 | ​ | $ | 9.77 | ​ | ​ | $ | 97,266
Options exercisable as of December 31, 2023 |  | 2,450,327 | ​ | $ | 7.27 | ​ | ​ | $ | 59,295
​ As of December 31, 2023, stock options outstanding included 8,559 options that were not granted under the 2001 Plan or the 2021 Plan. For options outstanding as of December 31, 2023, the weighted average remaining contractual life was 7.3 years. For options exercisable as of December 31, 2023, the weighted average remaining contractual life was 6.3 years. Our 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 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 our 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 annual increase resulted in an additional 206,637 shares being reserved for issuance under the ESPP as of January 1, 2023. The ESPP permits certain of our U.S. employees to purchase shares of our common stock at a price per share not less than 85% of the lower of (i) the closing market price per share of our common stock on the first day of the applicable purchase period or (ii) the closing market price per share of our common stock on the purchase date at the end of the applicable six-month purchase period. The first purchase date under the ESPP was June 30, 2022. For the year ended December 31, 2023, 71,276 shares of common stock were purchased under the ESPP for $0.9 million of employee contributions. As of December 31, 2023, there were 503,634 shares available for issuance under the ESPP. Stock-based compensation expense We use the Black-Scholes option pricing model to determine the fair value of stock options and ESPP purchase rights on the grant date. We measure 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 for stock options and the offering period for ESPP purchase rights. The amount of stock-based compensation expense recognized for stock option awards during a period is based on the portion of the awards that are ultimately expected to vest. The amount of stock-based compensation expense recognized for ESPP purchase rights during a period is based on the estimated purchase rights as of the grant date. We account for forfeitures as they occur. 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 years ended December 31, 2023 and 2022.
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ | December 31,
​ | ​ | ​ | 2023 |  | ​ | 2022 | ​
Weighted average fair value of options granted |  | $ | 10.65 | ​ | ​ | $ | 4.72 | ​
Expected term (in years) — non-officer employees |  | ​ | 5.5 to 6.1 | ​ | ​ | ​ | 5.3 to 6.1 | ​
Expected term (in years) — officer employees |  | ​ | 2.5 to 6.1 | ​ | ​ | ​ | 3.2 to 6.1 | ​
Expected volatility |  | ​ | 77.2% to 79.6 | % | ​ | ​ | 56.3% to 60.4 | %
Expected dividend yield |  | ​ | — | % | ​ | ​ | — | %
Risk-free interest rate |  | ​ | 3.40% to 4.89 | % | ​ | ​ | 1.75% to 4.18 | %
​ The following table provides the weighted average fair value of ESPP purchase rights and the related assumptions used in the Black-Scholes option pricing model for the years ended December 31, 2023 and 2022.
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | December 31, | ​
​ | ​ | ​ | 2023 |  | ​ | ​ | 2022 | ​
Weighted average fair value per ESPP purchase right |  | $ | 5.64 | ​ | ​ | $ | 1.95 | ​
Expected term (in years) |  | ​ | 0.5 | ​ | ​ | ​ | 0.5 | ​
Expected volatility |  | ​ | 76.2% to 84.6 | % | ​ | ​ | 51.3% to 62.9 | %
Expected dividend yield |  | ​ | — | % | ​ | ​ | — | %
Risk-free interest rate |  | ​ | 4.77% to 5.53 | % | ​ | ​ | 0.22% to 2.52 | %
​ We review these assumptions on a periodic basis and adjust them, as necessary. We utilize the simplified method to develop the estimate of the expected term for stock option awards and ESPP purchase rights. The expected volatility is based upon observed volatility of comparable public companies. The expected dividend yield is assumed to be zero, as we have never paid dividends and have 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. The following table presents the components and classification of stock-based compensation expense for the periods indicated:
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Year Ended
​ | ​ | December 31,
(in thousands) | ​ | 2023 | ​ | 2022
Stock options | ​ | $ | 5,901 | ​ | $ | 3,664
Employee Stock Purchase Plan | ​ | ​ | 402 | ​ | ​ | 223
Total stock-based compensation expense | ​ | ​ | 6,303 | ​ | ​ | 3,887
​ | ​ | ​ | ​ | ​ | ​ | ​
Selling, general & administrative | ​ | $ | 5,073 | ​ | $ | 3,304
Research & development | ​ | ​ | 1,152 |  | ​ | 519
Cost of goods sold | ​ | ​ | 78 |  | ​ | 64
​ | ​ | $ | 6,303 | ​ | $ | 3,887
​ As of December 31, 2023, unrecognized compensation expense related to unvested stock-based compensation arrangements was $13.4 million. As of December 31, 2023, the related weighted average period over which the expense is expected to be recognized is approximately 2.4 years.

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