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

	Three Months Ended March 31,
	2020		2019
Cost of goods sold	$	71	$	111
Research and development		201		265
Sales and marketing		440		513
General and administrative		735		1,014
Total	$	1,447	$	1,903

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 Company granted options to purchase 752,346 shares of common stock at exercise prices ranging from $16.34 to $17.15 per share, with a weighted average exercise price of $17.11 per share, during the three months ended March 31, 2019. The weighted average fair value of stock options granted during the three months ended March 31, 2020 and 2019 was $8.93 and $11.59, respectively.

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

	Three Months Ended March 31,
	2020			2019
Expected dividend yield		0.0	%		0.0	%
Risk free interest rate		1.8	%		2.5	%
Expected stock price volatility		87.6	%		75.5	%
Expected term (years)		6.1			6.1

A summary of restricted stock activity for the three months ended March 31, 2020 is as follows:

				Weighted
				Average
				Grant Date
	Shares			Fair Value
Unvested at December 31, 2019		229,913		$	3.76
Granted/purchased		36,367			10.40
Vested		(40,931	)		3.30
Canceled		-			-
Unvested at March 31, 2020		225,349		$	4.65

In connection with our initial public offering in November 2018, the Company’s board of directors adopted the ESPP and a total of 166,500 shares of common stock were initially reserved for issuance under the ESPP. The number of shares of common stock available for issuance under the ESPP is increased on the first day of each calendar year beginning January 1, 2019 and each year thereafter until 2028 by the lessor of (i) 1% of the number of shares of common stock issued and outstanding on the immediately preceding December 31, and (ii) the number of shares of common stock determined by the Company’s board of directors up to such an initial maximum of 1,741,300 shares of common stock. The number of shares of common stock reserved under the plan at March 31, 2020 totals 548,437.

The ESPP provides for successive discreet offering periods of approximately six months or as determined by the plan administrator. The first offering period began on January 2, 2020 and has not yet concluded as of March 31, 2020. As a result, the Company has not yet issued any shares of common stock pursuant to the ESPP in any period.

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.

The fair value of the purchase right for the ESPP option is estimated on the date of grant using the Black-Scholes model with the following assumptions for the initial offering period:

Expected dividend yield	0.0	%
Risk free interest rate	1.6	%
Expected stock price volatility	107.9	%
Expected term (years)	0.4