Warrants to Acquire Shares of Common Stock (Tables)

6 Months Ended

Jun. 30, 2021

Warrants and Rights Note Disclosure [Abstract]

The following is a summary of the activity of the Company's warrants to acquire shares of common stock for the six months ended June 30, 2021 (in thousands):


Warrant Issuance	Outstanding, December 31, 2020	Granted	Exercised	Canceled/Expired	Outstanding, June 30, 2021	Expiration
July 2020 PIPE Offering	445	—	(420)	—	25	August 2025
January 2020 Offering	719	—	(410)	—	309	July 2025

June 2019 Offering	2	—	(1)	—	1	June 2024

March 2019 Exercise Agreement	63	—	—	—	63	March 2024
July 2018 Offering	141	—	(1)	—	140	July 2023




Other	22	—	—	(1)	21	November 2021 - November 2023






	1,392	—	(832)	(1)	559

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

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


As of June 30, 2021

Warrant Issuance	Outstanding (in thousands)	Strike price (per share)		Expected term (years)	Volatility %		Risk-free rate %
Other warrants (liability-classified)	13	$	7.50	2.26	151.45	%	0.30	%






As of December 31, 2020
Warrant Issuance	Outstanding (in thousands)	Strike price (per share)		Expected term (years)	Volatility %		Risk-free rate %
Other warrants (liability-classified)	13	$	7.50	2.75	150.38	%	0.16	%

Schedule of Changes in Fair Value of Warrant Liability

The changes in fair value of the warrant liability for the six months ended June 30, 2021 were as follows (in thousands):


Warrant Issuance	Warrant liability, December 31, 2020		Change in fair value of warrants		Warrant liability, June 30, 2021
Other warrants (liability-classified)	$	55	$	59	$	114






	$	55	$	59	$	114