As of March 31, 2021, 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.

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 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, 2021 and 2020, 587,141 and 57,581 options under the Plans were exercised for total proceeds of $3,656 and $177, 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, 2021 and 2020 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.

As of March 31, 2021 and December 31, 2020, there were 2,534,234 and 2,168,706 shares available for grant under the Plans. Following are the weighted average valuation assumptions used for options:

	Three Months Ended March 31,
	2021			2020
Valuation assumptions
Expected dividend yield		—	%		—	%
Expected volatility		59	%		60	%
Expected term (years)		4.67			4.49
Risk-free interest rate		0.67	%		1.44	%

The following table presents classification of stock-based compensation expense within the condensed consolidated statements of operations:

	Three Months Ended March 31,
	2021		2020
Cost of sales	$	657	$	253
Research and development		1,228		508
Sales and marketing		218		93
General and administrative		2,263		921
Total stock-based compensation	$	4,366	$	1,775

The weighted average grant date fair value per share of options granted during the three months ended March 31, 2021 and 2020 was $49.30 and $7.66, respectively. The intrinsic value of options exercised during the three months ended March 31, 2021 and 2020 was $43,077 and $1,231, respectively.

As of March 31, 2021, there was $91,330 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 3.6 years. The fair value of shares vested during the three months ended March 31, 2021 and 2020 was $9,462 and $1,213, respectively.