The Company has four equity compensation plans, adopted in 2017, 2013, 2007 and 2004 (the “Plans”) which, taken together, authorize the grant of up to 16,000,000 shares of common stock to employees, directors, and consultants. On June 5, 2020, the amendment and restatement of the 2017 Omnibus Incentive Plan (the “2017 Plan”), which increased the number of shares available for issuance under the plan by up to 500,000 shares, became effective, and was further amended as of June 18, 2021 to increase the number of shares available for issuance under the plan by up to an additional 5 million shares. Additionally, on June 14, 2017, the Company adopted its 2017 Employee Stock Purchase Plan (the “ESPP”), which authorizes the issuance of up to 1,000,000 shares of common stock for future issuances to eligible employees.

The total fair value of stock options vested and recorded as compensation expense during the years ended December 31, 2021, 2020, and 2019 was $8.4 million, $9.6 million, and $7.9 million, respectively. As of December 31, 2021, $11.1 million of unrecognized cost related to non-vested stock options was expected to be recognized over a weighted-average period of approximately 1.71 years. The total intrinsic value of options exercised was approximately $0.2 million and $1.5 million for the years ended December 31, 2021 and 2020, respectively.

The following table summarizes the status of the Company’s stock option activity granted under the Plans and 2017 Plan to employees, directors, and consultants (in thousands, except stock option amounts and exercise price): Stock options granted have a contractual term of 10 years and generally vest over a 2-4 year period. A limited number of stock options vest immediately in certain circumstances. The following table summarizes the status of the Company’s stock option activity granted under the Plans and 2017 Plan to employees, directors, and consultants (in thousands, except stock option amounts):


	Stock Options			Weighted- Average Exercise Price		Weighted- Average Remaining Contractual Term		Aggregate Intrinsic Value
Outstanding at December 31, 2020		12,496,888		$	9.26		5.42	$	22,463
Granted		944,095			3.77		—		—
Exercised		(279,425	)		5.64		—		—
Forfeited and expired		(499,488	)		9.03		—		—
Outstanding at December 31, 2021		12,662,070		$	8.96		4.88	$	—
Vested and exercisable at December 31, 2021		9,934,435		$	8.78		3.91	$	—

The Company determines the fair value of stock option awards on the grant date using the Black-Scholes option pricing model, which is impacted by assumptions regarding a number of highly subjective variables. The following table summarizes the weighted-average assumptions used as inputs to the Black-Scholes option pricing model during the periods indicated:


	Year Ended December 31,
	2021			2020			2019
Weighted average grant date fair value	$	3.77		$	6.93		$	8.06
Expected dividend yield		—	%		—	%		—	%
Expected stock price volatility		68	%		67	%		64	%
Risk-free interest rate		1.29	%		0.91	%		2.60	%
Expected life of options (in years)		6.2			6.2			6.3

The total fair value of restricted stock awards vested and recorded as compensation expense during the years ended December 31, 2021, 2020, and 2019 was $0.6 million, $1.0 million, and $0.6 million, respectively. Restricted stock awards cliff vest on the anniversaries of their grant date. As of December 31, 2021, $3.1 million of unrecognized cost related to non-vested restricted stock awards were expected to be recognized over a weighted-average period of approximately 2.05 years.

The following table summarizes the status of the Company's restricted stock awards.


	Shares of Restricted Stock			Weighted Average Fair Value
Nonvested at December 31, 2020		22,500		$	15.65
Granted		933,595			3.69
Vested		(31,500	)		7.29
Nonvested at December 31, 2021		924,595		$	3.86

The ESPP is available to eligible employees (as defined in the plan document). Under the ESPP, shares of the Company’s common stock may be purchased at a discount (15%) of the lesser of the closing price of the Company’s common stock on the first trading or the last trading day of the offering period. The current offering period extends from December 1, 2021 to May 31, 2022. The Company expects to offer six-month offering periods after the current period. The 2017 Plan reserved 1,000,000 shares of common stock for issuance under the ESPP. Stock-based compensation related to the ESPP amounted to $0.2 million for the year ended December 31, 2021 and $0.3 million for each of the years ended December 31, 2020 and 2019. The Company issued 81,557, 50,827, and 60,825 shares of common stock to participants during the years ended December 31, 2021, 2020, and 2019, respectively.

The components of stock-based compensation and the amounts recorded within research and development expenses and selling, general, and administrative expenses in the Company’s consolidated statements of operations and comprehensive loss consisted of the following for the years ended December 31, 2021, 2020, and 2019 (in thousands):


	Year Ended December 31,
	2021		2020		2019
Stock options	$	8,441	$	9,557	$	7,887
Restricted stock expense		616		1,017		566
Stock awarded to directors and officers		—		—		1,105
Employee stock purchase plan		150		276		327
Total stock-based compensation expense	$	9,207	$	10,850	$	9,885
Cost of sales	$	208	$	249	$	248
Research and development expenses		2,700		3,647		3,251
Selling, general, and administrative expenses		6,299		6,954		6,386
Total stock-based compensation expense	$	9,207	$	10,850	$	9,885