Stock options vest over a prescribed service period and are approved by the board of directors on an award-by-award basis. Options have a prescribed service period generally lasting up to four years, with certain options vesting immediately upon issuance. Upon the exercise of any stock options, the Company issues shares to the award holder from the pool of authorized but unissued common shares.

The fair values of options granted during the period were determined using a Black-Scholes model. The following principal inputs were used in the valuation of awards issued for the three months ended March 31, 2022 and 2021:


	Three Months Ended March 31,
	2022	2021
Expected volatility	83.4%	86.5%
Expected term (years)	5.5-6.5	5.5-7.0
Risk-free interest rate	1.9%	1.7%
Expected dividend yield	0%	0%
Value of underlying share	$1.11	$4.70

Detail of the number of stock options outstanding for the three months ended March 31, 2022 under the Company's 2015 legacy option plan and the Company's amended 2018 long term incentive plan (collectively, the "Plans") is as follows:


	Number of Options	Weighted- Average Exercise Price per Option		Weighted- Average Remaining Contract Term (in years)	Aggregate Intrinsic Value
Outstanding as of December 31, 2021	3,343,883	$	3.16	7.54	$	1,039,229
Granted	2,148,832	1.56
Exercised	—	—
Forfeited (and expired)	(625,251)	6.36
Outstanding as of March 31, 2022	4,867,464	$	2.04	8.60	$	687,476

Exercisable/vested as of March 31, 2022	2,093,641	$	2.13	5.87	$	673,414

The weighted average grant-date fair value of options granted during the three months ended March 31, 2022 was $1.56. The weighted average grant-date fair value of options granted during the three months ended March 31, 2021 was $4.70.

The weighted average share price at the date of exercise of options exercised during the three months ended March 31, 2022 and 2021 was $— and $4.85, respectively.

The Company has issued time-based restricted share units to certain employees as permitted under the 2018 Plan. The restricted share units granted vest in accordance with the board-approved agreement, typically over equal

installments over up to four years. Upon vesting, one share of the Company’s common shares is issued for each restricted share unit awarded. The fair value of each restricted share unit granted is equal to the market price of the Company’s shares at the date of the grant. The fair value of shares vested during the three months ended March 31, 2022 and March 31, 2021 was $295 and $317, respectively.

Details of the number of restricted share units outstanding under the 2018 Plan is as follows:


	Number of Shares	Weighted- Average Grant Date Fair Value
Outstanding as of December 31, 2021	1,816,851	$	2.28
Granted	1,351,114	$	1.35
Forfeited	(482,104)	$	2.68
Vested	(77,193)	$	3.82
Shares withheld upon vesting	(38,979)	$	3.82
Outstanding as of March 31, 2022	2,569,689	$	1.69

Share-based compensation expense for all equity arrangements for the three months ended March 31, 2022 and March 31, 2021 was $1,214 and $832, respectively, included in selling, general and administrative expense in the condensed consolidated statements of operations and comprehensive loss.

As of March 31, 2022, $7,204 of total unrecognized share-based compensation expense related to unvested options granted to employees is expected to be recognized over a weighted-average period of 2.43 years.