Warrants (Tables)

12 Months Ended

Dec. 31, 2019

Warrants

Schedule of warrant activity

		Weighted
		average
	Warrants	exercise price
Outstanding at December 31, 2016	337,746	$	3.06
Issued	1,890,602		5.33
Exercised	(129,149)		1.30
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

Summary of warrant fair value inputs

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