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Stock-based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2023
Share-Based Payment Arrangement [Abstract] |
Schedule of Stock-based Compensation Expense | Stock-based compensation expense is included in the consolidated statements of operations as follows (in thousands):
| Year Ended December 31,
| 2023 |  | 2022 |  | 2021
Costs of product revenue | $ | 354 |  |  | $ | 452 |  |  | $ | 224 |
Research and development | 2,631 |  |  | 3,907 |  |  | 2,663 |
Selling, general and administrative | 6,986 |  |  | 10,172 |  |  | 8,706 |
Total | $ | 9,971 |  |  | $ | 14,531 |  |  | $ | 11,593 |
The following table presents total stock-based compensation expense by security type included in the consolidated statements of operations (in thousands):
| Year Ended December 31,
| 2023 |  | 2022 |  | 2021
Stock options | $ | 3,962 |  |  | $ | 4,167 |  |  | $ | 2,764 |
RSUs and RSAs | 4,447 |  |  | 4,807 |  | 2,768
PSUs | 1,649 |  |  | 3,268 |  |  | 2,333 |
PBOs | (112) |  |  | 2,289 |  | 3,728
ESPP | 25 |  |  | — |  |  | — |
Total | $ | 9,971 |  |  | $ | 14,531 |  |  | $ | 11,593 |
Assumptions Used to Estimate the Fair Value of Option Grants | The following summarizes the weighted-average assumptions used to estimate the fair value of employee stock options granted:
| Year Ended December 31,
| 2023 |  | 2022 |  | 2021
Expected life (years) | 5.8 |  | 5.7 |  | 5.6
Volatility | 66.2 | % |  | 62.1 | % |  | 52.5 | %
Risk-free interest rate | 4.0 | % |  | 3.1 | % |  | 0.8 | %
Expected dividend yield | 0.0 | % |  | 0.0 | % |  | 0.0 | %
The following summarizes the weighted-average assumptions used to estimate the fair value of 50,000, nil, and 9,000 shares of stock options granted to non-employees for services valued at $0.1 million, nil, and $0.1 million during the years ended December 31, 2023, 2022, and 2021 respectively:
| Year Ended December 31,
| 2023 |  | 2022 |  | 2021
Expected life (years) | 5.8 |  | 0.0 |  | 5.6
Volatility | 70.1 | % |  | — | % |  | 54.1 | %
Risk-free interest rate | 4.7 | % |  | — | % |  | 0.9 | %
Expected dividend yield | 0.0 | % |  | 0.0 | % |  | 0.0 | %
The following summarize the weighted-average assumptions used to estimate the fair value of PBOs granted:
| Year Ended December 31,
| 2022 |  | 2021
Expected life (years) | 5.6 |  | 5.5
Volatility | 54.9 | % |  | 51.9 | %
Risk-free interest rate | 1.8 | % |  | 0.7 | %
Expected dividend yield | 0.0 | % |  | 0.0 | %
Schedule of Share-based Compensation, Stock Options, Activity | The following tables summarize s stock option activities:
| Number of Shares |  | Weighted Average Exercise Price Per Share
| (In Thousands) |  |
Outstanding at December 31, 2020 | 3,385 |  |  | $ | 7.19 |
Granted | 286 |  |  | $ | 26.85 |
Exercised | (664) |  |  | $ | 6.96 |
Forfeited/Expired | (72) |  |  | $ | 17.99 |
Outstanding at December 31, 2021 | 2,935 |  |  | $ | 8.90 |
Granted | 2,000 |  |  | $ | 8.90 |
Exercised | (410) |  |  | $ | 2.33 |
Forfeited/Expired | (275) |  |  | $ | 19.01 |
Outstanding at December 31, 2022 | 4,250 |  |  | $ | 8.88 |
Granted | 2,046 |  |  | $ | 5.23 |
Exercised | (283) |  |  | $ | 1.97 |
Forfeited/Expired | (839) |  |  | $ | 12.08 |
Outstanding at December 31, 2023 | 5,174 |  |  | $ | 7.31 |
| Number of Shares |  | Weighted Average Exercise Price Per Share |  | Weighted Average Remaining Contractual Term |  | Aggregate Intrinsic Value
| (In Thousands) |  |  |  | (In Years) |  | (In Thousands)
Outstanding at December 31, 2023 | 5,174 |  |  | $ | 7.31 |  |  | 6.7 |  | $ | 198 |
Exercisable at December 31, 2023 | 2,231 |  |  | $ | 8.61 |  |  | 3.6 |  | $ | 79 |
Vested and expected to vest at December 31, 2023 | 4,748 |  |  | $ | 7.42 |  |  | 6.4 |  | $ | 170 |
Schedule of Share-based Compensation, RSA Activity | The following table summarizes RSA activities:
| Number of Shares |  | Weighted Average Grant Date Fair Value Per Share
| (In Thousands) |  |
Non-vested balance at December 31, 2020 | 96 |  |  | $ | 11.44 |
Granted | 46 |  |  | $ | 21.91 |
Vested | (62) |  |  | $ | 11.31 |
Non-vested balance at December 31, 2021 | 80 |  |  | $ | 17.53 |
Granted | 159 |  |  | $ | 7.53 |
Vested | (58) |  |  | $ | 18.42 |
Non-vested balance at December 31, 2022 | 181 |  |  | $ | 8.45 |
Granted | 277 |  |  | $ | 2.89 |
Vested | (133) |  |  | $ | 9.04 |
Non-vested balance at December 31, 2023 | 325 |  |  | $ | 3.48 |
Schedule of Share-based Compensation, RSU Activity | The following table summarizes RSU activities:
| Number of Shares |  | Weighted Average Grant Date Fair Value Per Share
| (In Thousands) |  |
Non-vested balance at December 31, 2020 | 176 |  |  | $ | 14.17 |
Granted | 163 |  |  | $ | 26.59 |
Vested | (70) |  |  | $ | 13.57 |
Forfeited/Expired | (37) |  |  | $ | 21.89 |
Non-vested balance at December 31, 2021 | 232 |  |  | $ | 21.83 |
Granted | 518 |  |  | $ | 17.46 |
Vested | (106) |  |  | $ | 21.21 |
Forfeited/Expired | (126) |  |  | $ | 19.55 |
Non-vested balance at December 31, 2022 | 518 |  |  | $ | 18.15 |
Granted | 1,049 |  |  | $ | 5.24 |
Vested | (204) |  |  | $ | 18.42 |
Forfeited/Expired | (343) |  |  | $ | 9.71 |
Non-vested balance at December 31, 2023 | 1,020 |  |  | $ | 7.66 |
Share-based Compensation, Performance Shares Award Outstanding Activity | The following table summarizes PSU activities:
| Number of Shares |  | Weighted Average Grant Date Fair Value Per Share
| (In Thousands) |  |
Non-vested balance at December 31, 2020 | 131 |  |  | $ | 15.34 |
Granted | 82 |  |  | $ | 26.16 |
Vested | (66) |  |  | $ | 16.14 |
Forfeited/Expired | (19) |  |  | $ | 19.38 |
Non-vested balance at December 31, 2021 | 128 |  |  | $ | 21.24 |
Granted | 686 |  |  | $ | 9.55 |
Vested | (107) |  |  | $ | 20.52 |
Forfeited/Expired | (40) |  |  | $ | 19.93 |
Non-vested balance at December 31, 2022 | 667 |  |  | $ | 9.41 |
Vested | (315) |  |  | $ | 11.02 |
Forfeited/Expired | (75) |  |  | $ | 12.49 |
Other | 15 |  |  | $ | 25.05 |
Non-vested balance at December 31, 2023 | 292 |  |  | $ | 7.69 |
The following tables summarizes PBOs activities:
| Number of Shares |  | Weighted Average Grant Date Fair Value Per Share
| (In Thousands) |  |
Outstanding at December 31, 2020 | 1,560 |  |  | $ | 5.05 |
Granted | 433 |  |  | $ | 12.23 |
Exercised | (35) |  |  | $ | 9.02 |
Forfeited/Expired | (118) |  |  | $ | 12.23 |
Outstanding at December 31, 2021 | 1,840 |  |  | $ | 4.11 |
Granted | 733 |  |  | $ | 9.89 |
Forfeited/Expired | (747) |  |  | $ | 8.29 |
Outstanding at December 31, 2022 | 1,826 |  |  | $ | 4.70 |
Forfeited/Expired | (178) |  |  | $ | 9.73 |
Outstanding at December 31, 2023 | 1,648 |  |  | $ | 5.64 |
| Number of Shares |  | Weighted Average Exercise Price Per Share |  | Weighted Average Remaining Contractual Term |  | Aggregate Intrinsic Value
| (In Thousands) |  |  |  | (In Years) |  | (In Thousands)
Exercisable at December 31, 2023 | 1,627 |  |  | $ | 10.96 |  |  | 4.4 |  | $ | — |
Vested and expected to vest at December 31, 2023 | 1,646 |  |  | $ | 11.06 |  |  | 4.5 |  | $ | — |
Schedule of Share-Based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions | The fair value of shares to be issued under the ESPP is computed using the Black-Scholes-Merton option pricing model at the commencement of the offering period. The following summarizes the weighted-average assumptions used to estimate the fair value of ESPP for the initial offering period:
| Year Ended December 31,
| 2023
Expected life (years) | 0.4
Volatility | 89.6 | %
Risk-free interest rate | 5.3 | %
Expected dividend yield | 0.0 | %

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