Warrants (Tables)

12 Months Ended

Dec. 31, 2020

Warrants And Rights Note Disclosure [Abstract]

Schedule of Warrant Activity

As of December 31, 2020, the Company had outstanding warrants to purchase 15,541 shares of common stock. The following table summarizes warrant activity as of December 31, 2020, 2019, and 2018:

	Warrants			Weighted average exercise price
Outstanding at December 31, 2017		2,099,199		$	5.21
Issued		15,750			3.06
Exercised		(101,310	)		1.30
Expired		(1,890,602	)		5.33
Outstanding at December 31, 2018		123,037			6.35
Issued		58,334			8.10
Exercised		(111,715	)		6.73
Outstanding at December 31, 2019		69,656			7.21
Issued		15,541			80.43
Exercised		(69,656	)		7.21
Outstanding at December 31, 2020		15,541		$	80.43

Summary of Initial Fair Value of Warrants Estimated using Black - Scholes Option Pricing Model

The initial fair value of the warrants were estimated using the Black‑Scholes option pricing model with the following assumptions:

Black-Scholes option valuation assumptions	2020 warrants			2019 warrants			2018 warrants			2017 warrants			2016 warrants			2014 warrants			2013 warrants
Risk-free interest rate		0.5	%		2.8	%		3.0	%		1.7	%		1.8	%		1.6	%		1.4	%
Dividend yield		—			—			—			—			—			—			—
Volatility		88	%		97	%		85	%		61	%		73	%		70	%		64	%
Weighted average contractual term	7 years			7 years			7 years			1 year			7 years			5 years			5 years