Stock-Based Compensation | Note 10. Stock-Based Compensation Shares of common stock reserved for issuance under the Company’s equity incentive plans were as follows: June 30, 2024 Outstanding stock awards 9,155,525 Reserved for future award grants 5,386,630 Reserved for future ESPP 192,341 Total common stock reserved for stock awards 14,734,496 Service-Based Stock Option Activity A summary of the Company’s service-based stock option activity (excluding performance-based stock option activity, which is presented separately below) for the six months ended June 30, 2024 is as follows: Outstanding Service-Based Options (in thousands, except share and per share data) Number of Weighted- Weighted- Aggregate Balance—December 31, 2023 5,805,586 $ 7.40 6.90 $ 64 Options granted 2,924,500 1.56 Options exercised — — Options forfeited or expired ( 617,967 ) 10.36 Balance—June 30, 2024 8,112,119 $ 5.07 8.01 $ 6 Options vested and exercisable as of June 30, 2024 3,663,128 $ 8.19 6.49 $ — The weighted-average grant date fair value of options granted was $ 0.92 and $ 1.51 per share for the three months ended June 30, 2024 and 2023, and $ 1.05 and $ 1.84 per share for the six months ended June 30, 2024 and 2023, respectively. As of June 30, 2024, the unrecognized stock-based compensation cost of unvested options was $ 6.7 million, which is expected to be recognized over a weighted-average period of 2.2 years. Valuation of Service-Based Stock Options The Company estimated the fair value of service-based stock options using the Black-Scholes option-pricing model. Fair value of stock options is recognized as compensation expense on a straight-line basis over the requisite service periods of the awards. Fair value of stock options was estimated using the following range of assumptions: Three Months Ended June 30, Six Months Ended June 30, 2024 2023 2024 2023 Expected term (in years) 5.50 - 6.08 5.50 - 6.02 5.50 - 6.08 5.50 - 6.02 Volatility 72.61 - 73.12 % 78.47 - 79.31 % 72.61 - 80.28 % 78.47 - 79.31 % Risk-free interest rate 4.35 - 4.65 % 3.47 - 3.98 % 3.88 - 4.65 % 3.47 - 3.98 % Dividend yield – % –% –% –% Performance-Based Stock Option Activity During 2024, the Company granted performance-based stock options ("PSOs") to the executive leadership team. Vesting of the PSOs is based upon attainment of certain Medicare reimbursement coverages by the end of 2025 and subject to continuous service by the executives. Fair value was estimated using the Black-Scholes option-pricing model. Total grant-date fair value of the PSOs was $ 0.3 million. A summary of the Company's performance-based stock option activity for the six months ended June 30, 2024 is as follows: Outstanding Performance-Based Options (in thousands, except share and per share data) Number of Weighted- Weighted- Aggregate Balance—December 31, 2023 — $ — — $ — Options granted 271,500 1.61 Balance—June 30, 2024 271,500 $ 1.61 9.71 $ — Options vested and exercisable as of June 30, 2024 — Restricted Stock Units ("RSU") Activity and Valuation A summary of the Company’s RSU activity for the six months ended June 30, 2024 is as follows: Unvested Restricted Stock Units (in thousands, except share and per share data) Number of Weighted- Aggregate Balance—December 31, 2023 1,253,826 $ 8.99 $ 2,633 RSUs granted — — RSUs vested ( 280,595 ) 12.48 394 RSUs forfeited ( 201,325 ) 10.25 Balance—June 30, 2024 771,906 $ 7.39 $ 903 As of June 30, 2024, the unrecognized stock-based compensation cost of unvested RSUs was $ 4.7 million, which is expected to be recognized over a weighted-average period of 1.6 years. ESPP Activity and Valuation During the three and six months ended June 30, 2024 and 2023, 395,552 and 468,643 shares of common stock were purchased under the Employee Stock Purchase Plan ("ESPP"). The fair value of stock purchase rights granted under the ESPP was estimated using the following assumptions: Three and Six Months Ended June 30, 2024 2023 Expected term (in years) 0.50 0.50 Volatility 61.35 % 84.88 % Risk-free interest rate 5.43 % 5.14 % Dividend yield –% –% Grant-date fair value per share $ 0.51 $ 0.91 Stock-Based Compensation Expense The following is a summary of stock-based compensation expense by award type (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2024 2023 2024 2023 Service-based stock options $ 1,237 $ 1,505 $ 2,452 $ 2,848 Performance-based stock options 35 — 41 — RSUs 1,324 1,757 2,732 3,880 ESPP 57 223 115 452 Total stock-based compensation expense $ 2,653 $ 3,485 $ 5,340 $ 7,180 The following is a summary of stock-based compensation expense by function (in thousands): Three Months Ended June 30, Six Months Ended June 30, 2024 2023 2024 2023 Cost of revenue $ 153 $ 436 $ 307 $ 926 Research and development 1,046 1,241 1,993 2,486 Selling, general and administrative 1,454 1,808 3,040 3,768 Total stock-based compensation expense $ 2,653 $ 3,485 $ 5,340 $ 7,180 |