The 2015 Equity Incentive Award Plan (2015 Plan) functions as the primary equity plan for the Company. The 2015 Plan includes an “evergreen provision” that allows for an annual increase in the number of shares of common stock available for issuance under the 2015 Plan, which annual increase will be added on the first day of each fiscal year from 2016 through 2025, inclusive, and will be equal to the lesser of (i) five percent of the shares outstanding on the last day of the immediately preceding fiscal year and (ii) such smaller number of shares as determined by the Board of Directors. On January 1, 2021, the Company added 1,909,163 shares to the 2015 Plan pursuant to the “evergreen provision”. The 2015 Plan provides for the granting of a variety of stock‑based compensation awards, including stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, deferred stock awards, dividend equivalent awards, stock payment awards, performance awards and other stock‑based awards.

The 2017 Stock Incentive Plan (the 2017 Plan) provides for the grant of incentive stock options, non-qualified stock options, restricted and unrestricted stock awards and other stock-based awards.

The 2015 Employee Stock Purchase Plan (ESPP) allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 15% of their eligible compensation, subject to any plan limitations. The ESPP generally provides for set offering periods, and at the end of each offering period, employees are able to purchase shares at 85% of the lower of the fair market value of the Company’s common stock on the first trading day of the offering period or on the last trading day of the offering period. The Company suspended the ESPP in 2017. In December 2019, the Board reactivated the 2015 ESPP and approved an amendment to the ESPP to (i) reduce the permitted payroll deduction and number of shares of the Company’s common stock that a participant may purchase per calendar year and offering period under the ESPP and (ii) establish a period for enrollment for eligible participants. The reactivation of the 2015 ESPP was effective immediately. The Company’s executive officers are eligible to participate in the 2015 ESPP. The ESPP includes an “evergreen provision,” allowing for an annual increase in the number of shares of common stock available for issuance. On January 1, 2021, the Company added 381,832 shares to the ESPP pursuant to the “evergreen provision”. There were 42,653 shares of common stock purchased under the ESPP during the year ended December 31, 2021.

The weighted average assumptions used in the Black-Scholes option-pricing model for stock options issued to employees and non-employees under the 2015 Plan and the 2017 Plan, during the years ended December 31, 2021 and 2020 were:

	Year ended December 31,
Employees:	2021		2020
Expected term	6.2 years		6.2 years
Weighted-average, risk-free interest rate	1.0%		0.6%
Expected volatility	84.9%		77.2%
Dividend yield		—		—

The following table summarizes stock option activity under the 2015 and 2017 Plans.

	Stock options outstanding
						Weighted
						average
				Weighted		remaining		Aggregate
				average		contractual		intrinsic
	Number of			exercise		term		value^(a)
	options			price		(in years)		(in thousands)
Outstanding at December 31, 2020		3,343,836		$	6.00		8.4	$	399
Granted		2,215,796		$	3.37				—
Exercised		(139,772	)	$	2.24				160
Cancelled/Forfeited		(918,947	)	$	4.30				—
Outstanding at December 31, 2021		4,500,913		$	5.17		8.2	$	588

Vested or expected to vest at December 31, 2021		4,500,913		$	5.17		8.2	$	588
Exercisable at December 31, 2021		2,032,515		$	7.33		7.2	$	258

The weighted average grant date fair value per share of options granted during the years ended December 31, 2021 and 2020 was approximately $2.42 and $1.27, respectively. The total fair value of awards that vested during the years ended December 31, 2021 and 2020 was $2.8 million and $3.4 million, respectively.

As of December 31, 2021, there was approximately $5.2 million of unrecognized share-based compensation for unvested stock option grants which is expected to be recognized over a weighted average period of 2.6 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur.

During the years ended December 31, 2021 and 2020, 242,454 and 226,335 shares of restricted common stock were granted, respectively.

	Restricted stock awards
				Weighted average grant date
	Number of			fair value
	shares			(per share)
Unvested at December 31, 2020		478,207		$	2.19
Granted		242,454			3.50
Vested		(343,619	)		2.39
Forfeited		(77,794	)		2.96
Unvested at December 31, 2021		299,248		$	2.83

The total fair value of shares that vested during the years ended December 31, 2021 and 2020 was $0.8 million and $0.3 million, respectively.

As of December 31, 2021, there was approximately $0.7 million of unrecognized share-based compensation related to restricted stock awards granted, which is expected to be recognized over a weighted average period of 3.0 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur.

The ESPP is considered a compensatory plan with the related compensation expense recognized over the six-month offering periods. The compensation expense for the years ended December 31, 2021 and December 31, 2020 was $58,000 and $27,000, respectively.

The Company has recorded total equity‑based compensation expense of approximately $3.4 million and $3.9 million, during the years ended December 31, 2021 and 2020, respectively. Equity compensation during the years ended December 31, 2021 and 2020 is derived from stock options, restricted stock awards, and the ESPP.

The following table summarizes equity‑based compensation expense within the Company’s consolidated statements of operations and comprehensive loss for the years ended December 31, 2021 and 2020 (in thousands):

The following table summarizes equity‑based compensation expense by type of award for the years ended December 31, 2021 and 2020 (in thousands):

	Years ended December 31,
	2021		2020
Stock options	$	2,969	$	3,200
Restricted stock awards		334		680
ESPP		58		27
	$	3,361	$	3,907