The following table presents a summary of activity related to stock options during the three months ended March 31, 2021:

	Number of Options		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (Years)			Aggregate Intrinsic Value
Outstanding, January 1, 2021		3,815,585	$	5.56		8.9	$	342,551
Granted		1,090,000		2.05				218,950
Exercised		—		—				—
Cancelled		—		—				—
Outstanding, March 31, 2021		4,905,585	$	4.78		8.7	$	1,396,195
Exercisable, March 31, 2021		1,597,409	$	8.38		8.0	$	223,339

The intrinsic value is calculated as the difference between the fair market value at March 31, 2021 and the exercise price per share of the stock options. The options granted to employees generally vest over a four-year period.

The following table presents a summary of activity related to restricted stock awards (“RSAs”) granted during the three months ended March 31, 2021:

	Number of Shares			Weighted Average Grant Date Fair Value
Nonvested, January 1, 2021		25,645		$	10.07
Granted		165,413			2.42
Vested		(120,051	)		3.33
Cancelled		—			—
Nonvested, March 31, 2021		71,007		$	4.74

The following table presents a summary of activity related to restricted and deferred stock units (“Stock Units”) granted during the three months ended March 31, 2021:

	Number of Shares			Weighted Average Grant Date Fair Value
Nonvested, January 1, 2021		1,014,123		$	1.50
Granted		147,000			2.83
Vested		(15,675	)		1.83
Cancelled		—			—
Nonvested, March 31, 2021		1,145,448		$	1.67

In general, Stock Units granted to employees vest over two to four-year periods.

Immediately following the Company’s annual meeting of stockholders, the Company generally grants each non-employee director an equity award that vests over a 12-month period. Upon a non-employee director’s initial appointment or election to the board of directors, the Company grants such non-employee director an equity award subject to vesting as determined by the board of directors.

The Company recognized stock compensation expense for its equity awards as follows:

	Three months ended March 31,
	2021		2020
Selling, general and administrative	$	809,547	$	547,090
Research and development		112,986		186,599
Cost of goods sold		19,650		11,176
Total expense	$	942,183	$	744,865

Total unrecognized compensation cost related to unvested awards as of March 31, 2021 was $6.0 million and is expected to be recognized over the next 2.6 years.

The fair value of each stock option award during the three months ended March 31, 2021 and 2020 was estimated on the date of grant using the Black-Scholes model. For the three months ended March 31, 2021, expected volatility was based on historical common stock volatility of the Company’s peers. Expected volatility for the three months ended March 31, 2020, was based on historical volatility of the Company’s common stock. The risk-free interest rate was based on the average U.S. Treasury rate that most closely resembled the expected life of the related award. The expected term of the award was calculated using the simplified method. No dividend was assumed as the Company does not pay regular dividends on its common stock and does not anticipate paying any dividends in the foreseeable future.

The weighted average assumptions used in the Black-Scholes option pricing model in valuing stock options granted in the three months ended March 31, 2021 and 2020 are summarized in the table below.

	Three months ended March 31, 2021				Three months ended March 31, 2020
Fair value at grant date	$	1.39		$	1.27
Expected volatility	79.9		%		113.4	%
Risk-free interest rate	0.6		%		1.4	%
Expected holding period, in years	6.1				6.1
Dividend yield		—	%		—	%