Stock-based compensation expense was allocated based on the employees’ and non-employees’ functions as follows:

	Three Months Ended March 31,
	2021		2020
Cost of goods sold	$	176	$	71
Research and development		341		201
Sales and marketing		921		440
General and administrative		1,247		735
Total	$	2,685	$	1,447

The Company granted options to purchase 357,140 shares of common stock at exercise prices ranging from $23.01 to $35.51 per share, with a weighted average exercise price of $27.06 per share, during the three months ended March 31, 2021. The Company granted options to purchase 909,960 shares of common stock at exercise prices ranging from $10.60 to $12.28 per share, with a weighted average exercise price of $12.13 per share, during the three months ended March 31, 2020. The weighted average fair value of stock options granted during the three months ended March 31, 2021 and 2020 was $19.73 and $8.93, respectively.

The weighted average assumptions used in the Black-Scholes options pricing model are as follows:

	Three Months Ended March 31,
	2021			2020
Expected dividend yield		0.0	%		0.0	%
Risk free interest rate		0.5	%		1.8	%
Expected stock price volatility		88.3	%		87.6	%
Expected term (years)		6.1			6.1

The Company has granted both restricted stock units and restricted stock awards.

A summary of restricted stock unit activity for the three months ended March 31, 2021 is as follows:

				Weighted
				Average
				Grant Date
	Shares			Fair Value
Unvested at December 31, 2020		228,489		$	25.60
Granted		169,096			27.53
Vested		(1,921	)		22.35
Canceled		(2,050	)		12.74
Unvested at March 31, 2021		393,614		$	26.52

A summary of restricted stock award activity for the three months ended March 31, 2021 is as follows:

				Weighted
				Average
				Grant Date
	Shares			Fair Value
Unvested at December 31, 2020		103,650		$	1.68
Granted/purchased		-			-
Vested		(27,778	)		1.68
Canceled		-			-
Unvested at March 31, 2021		75,872		$	1.68

The ESPP provides for successive discrete offering periods of approximately six months or as determined by the plan administrator. Offering periods begin on each November 15^th and May 15^th or the first trading day thereafter.

The ESPP permits eligible employees to elect to purchase shares of common stock through fixed whole percentage contributions from eligible compensation during each offering period, not to exceed 10% of the eligible compensation a participant receives during an offering period and not to accrue at a rate which exceeds $25,000 of the fair value of the stock (determined on the grant date(s)) for each calendar year. A participant may purchase the lower of (a) a number of shares of common stock determined by dividing such participant’s accumulated payroll deductions on the exercise date by the option price, (b) 5,000 shares, or (c) such other lesser maximum number of shares as shall have been established by the plan administrator.

Amounts deducted and accumulated by the participant will be used to purchase shares of common stock at the end of each offering period. The purchase price of the shares will be 85% of the lower of the fair value of common stock on the first trading day of each offering period or on the purchase date. Participants may end their participation during an offering period up to ten days in advance of the exercise date and will be paid their accumulated contributions that have not been used to purchase shares of common stock. Participation ends automatically upon termination of employment.