Warrants (Tables)

12 Months Ended

Dec. 31, 2021

Warrants and Rights Note Disclosure [Abstract]

Summary of Assumptions using Black- Scholes Option Pricing Model to estimate fair value

The aggregate fair value of the 2018 Offering Warrants of $7.4 million was estimated using the Black-Scholes option-pricing model with the following assumptions:

	Upon Issuance
Common Stock Warrants:
Expected term (in years)	7.0
Expected volatility (%)	62.5%
Risk-free interest rate (%)	2.8%
Expected dividend yield (%)	0%

The fair value upon issuance of $3.4 million and $2.7 million were estimated using the Black-Scholes option-pricing model using the following assumptions:

	Upon Issuance
	2017 Placement Warrants	2016 Placement Warrants
Expected term (in years)	7.0	7.0
Expected volatility (%)	62.9%	61.6%
Risk-free interest rate (%)	2.2%	1.4%
Expected dividend yield (%)	0%	0%

The fair value of the 2017 and 2016 Placement Warrants at December 31, 2021 and 2020 was estimated using the Black-Scholes option-pricing model and the following weighted-average assumptions:

	2017 Placement Warrants		2016 Placement Warrants
	December 31, 2021	December 31, 2020	December 31, 2021	December 31, 2020
Expected term (in years)	2.0	3.0	1.6	2.6
Expected volatility	86.0%	86.9%	85.5%	86.3%
Risk-free interest rate	0.4%	0.2%	0.3%	0.2%
Expected dividend yield	0%	0%	0%	0%

Schedule of Key Terms of Placement Warrants

The key terms and activity of the 2017 and 2016 Placement Warrants are summarized as follows:

	Issuance Date	Term	Exercise Price Per Share		Warrants Exercised during the Year Ended December 31, 2020	Warrants Outstanding at December 31, 2020	Warrants Exercised during the Year Ended December 31, 2021	Warrants Outstanding at December 31, 2021
2017 Placement Warrants	January 2017	7 years	$	3.17	—	1,618,890	118,868	1,500,022
2016 Placement Warrants	August and September 2016	7 years	$	2.95	—	537,263	552	536,711
Total					—	2,156,153	119,420	2,036,733

The following table summarizes the change in fair value the Company recognized related to its 2017 and 2016 Placement Warrants in the consolidated statements of operations and comprehensive loss for the years ended December 31, 2021, 2020, and 2019:

	December 31, 2021		December 31, 2020		December 31, 2019
Gain (loss) on 2017 Placement Warrants	$	(1,705)	$	367	$	2,554
Gain (loss) on 2016 Placement Warrants	(579)		142		(59)
	$	(2,284)	$	509	$	2,496