Warrants to Acquire Shares of Common Stock (Tables)

12 Months Ended

Dec. 31, 2021

Warrants and Rights Note Disclosure [Abstract]

The following is a summary of the Company's warrants to acquire shares of common stock activity for the year ended December 31, 2021 (in thousands, except per share data):


Warrant Issuance	Outstanding, December 31, 2020	Exercised	Outstanding, December 31, 2021	Exercise Price Per Share		Expiration
Warrants classified as equity:
January 2020 Offering	719	(410)	309	$	3.93	July 2025
July 2020 PIPE Offering	445	(420)	25	$	3.30	August 2025
July 2018 Offering	141	(9)	132	$	7.50	July 2023
March 2019 Exercise Agreement	63	(33)	30	$	7.50	March 2024
Other	10	(1)	9	$	306.66	December 2022 - June 2024
	1,378	(873)	505
Warrants classified as liability:	14	—	14	$	729.94	January 2022 - November 2023
	1,392	(873)	519

Schedule of Fair Value of Warrants is Estimated Using Black-Scholes Option Pricing Model

The fair value of the warrants accounted for as liabilities is estimated using a Black-Scholes pricing model with the following inputs:


	As of December 31,
	2021			2020
Risk free interest rate	0.65		%	0.16		%
Volatility	131.04%			150.38%
Expected term (years)	1.75			2.75
Expected dividend yield	—		%	—		%
Strike price	$	7.50		$	7.50

Schedule of Changes in Fair Value of Warrant Liability

The changes in fair value of the warrant liability for the year ended December 31, 2021 and 2020 were as follows (in thousands):


	Warrant Liability
Warrant liability, December 31, 2019	$	52

Fair value of warrants exercised	(94)
Change in fair value of warrants	97
Warrant liability, December 31, 2020	55


Change in fair value of warrants	(15)
Warrant liability, December 31, 2021	$	40