Warrants (Tables)

12 Months Ended

Dec. 31, 2018

Equity [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, 2018 and 2017 was estimated using the Black-Scholes option-pricing model and the following weighted-average assumptions:

	2017 Placement Warrants				2016 Placement Warrants
	December 31, 2018		December 31, 2017		December 31, 2018	December 31, 2017
Expected term (in years)		5.1		6.1	4.7		5.7
Expected volatility	60.8%		62.3%		60.9%	62.1%
Risk-free interest rate	2.5%		2.3%		2.5%	2.2%
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, 2017		Warrants Outstanding at December 31, 2017		Warrants Exercised during the Year Ended December 31, 2018		Warrants Outstanding at December 31, 2018
2017 Placement Warrants	January 2017	7 years	$	3.17		9,389		1,711,123		1,591		1,709,532
2016 Placement Warrants	August and September 2016	7 years	$	2.95		25,104		1,355,641		225,026		1,130,615
Total						34,493		3,066,764		226,617		2,840,147