As of March 31, 2022, the Company’s stock incentive plans included the 2010 Stock Plan (the “2010 Plan”), the 2020 Equity Incentive Plan (the “2020 Plan”), and the 2021 Inducement Equity Incentive Plan (the "2021 Plan”) (together, the “Plans”). The 2020 Plan provides for the award of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units, and other stock-based awards to employees, directors, consultants or advisors.

The 2021 Plan provides for the award of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock units, and other stock-based awards to persons who were not previously an employee or director of the Company or who are commencing employment with the Company following a bona fide period of non-employment, in either case, as an inducement material to such person’s entry into employment with the Company and in accordance with the requirements of the Nasdaq Stock Market Rule 5635(c)(4). Neither consultants nor advisors are eligible to participate in the 2021 Plan.

The 2010 Plan provided for the granting of incentive stock options and nonstatutory stock options to employees, directors, consultants or advisors. As of the effective date of the 2020 Plan, no further awards will be made under the 2010 Plan. Any options or awards outstanding under the 2010 Plan remain outstanding and effective. Shares of common stock subject to outstanding awards granted under the 2010 Plan that expire, terminate, or are otherwise surrendered, cancelled, forfeited, or repurchased by the Company are available for issuance under the 2020 Plan.

As of March 31, 2022 and December 31, 2021, there were 354,595 and 2,283,037 shares available for grant under the Plans. The following table presents classification of stock-based compensation expense within the unaudited condensed consolidated statements of operations:

	Three Months Ended March 31,
	2022		2021
Cost of sales	$	1,288	$	657
Research and development		2,582		1,228
Sales and marketing		524		218
General and administrative		4,740		2,263
Total stock-based compensation	$	9,134	$	4,366

Each restricted stock unit (“RSU”) represents the right to receive one share of the Company’s common stock upon vesting. The fair value of RSUs granted by the Company was calculated based upon the Company’s closing stock price on the date of the grant, and the stock-based compensation expense is recognized over the vesting period. RSUs generally vest over four years with 25% of the grants vesting at the end of the first year and the remaining vesting annually over the following three years.

The weighted average grant date fair value for each RSU granted during the three months ended March 31, 2022 was $27.76. There was no intrinsic value of RSUs settled during the three months ended March 31, 2022.

As of March 31, 2022, there was $1,299 of unrecognized compensation cost related to RSUs granted under the Plans, which is expected to be recognized over a weighted average period of 3.87 years. No RSUs vested during the three months ended March 31, 2022.

Stock options must be granted at an exercise price not less than 100% of the fair market value per share at the grant date. The board of directors or compensation committee determines the exercise price of the Company’s stock options based on the closing price of the common stock as reported on the Nasdaq Global Select Market on the day of the grant. The maximum contractual term of options granted under the Plans is typically 10 years, options generally vest over four years with 25% of the shares underlying the option vesting at the end of the first year and the remaining vesting monthly over the following three years.

During the three months ended March 31, 2022 and 2021, 137,885 and 587,141 options under the Plans were exercised for total proceeds of $908 and $3,656, respectively.

The fair value of each option award is determined on the date of grant using the Black Scholes Merton option-pricing model. The calculation of fair value includes several assumptions that require management’s judgment. The expected terms of options granted to employees during 2021 and 2020 were calculated using an average of historical exercises. Estimated volatility for the three months ended March 31, 2022 and 2021 incorporates a calculated volatility derived from the historical closing prices of shares of common stock of similar entities whose share prices were publicly available for the expected term of the option. The risk-free interest rate is based on the U.S. Treasury constant maturities in effect at the time of grant for the expected term of the option. The Company accounts for forfeitures as they occur, as such, the Company does not estimate forfeitures at the time of grant.

Following are the weighted average valuation assumptions used for option awards during the periods presented:

	Three Months Ended March 31,
	2022			2021
Valuation assumptions
Expected dividend yield		—	%		—	%
Expected volatility		56	%		59	%
Expected term (years)		4.74			4.67
Risk-free interest rate		1.77	%		0.67	%

The weighted average grant date fair value per share of options granted during the three months ended March 31, 2022 and 2021 was $13.52 and $49.30, respectively. The intrinsic value of options exercised during the three months ended March 31, 2022 and 2021 was $3,309 and $43,077, respectively.

As of March 31, 2022, there was $109,764 of unrecognized compensation cost related to unvested stock options granted under the Plans, which is expected to be recognized over a weighted average period of 3.09 years. The fair value of shares vested during the three months ended March 31, 2022 and 2021 was $19,545 and $9,462, respectively.