Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Stockholders? Equity

Schedule of stock option activity

				Weighted
		Weighted		average	Aggregate
	Number	average		contractual	intrinsic
	of shares	exercise price		term	value
Outstanding at December 31, 2016	1,772,050	$	5.74
Granted	847,510		5.24
Exercised	(88,922)		3.56
Forfeited	(101,497)		6.69
Expired	(113,503)		6.30
Outstanding at December 31, 2017	2,315,638	$	5.57	8.2	$	2,385,761
Vested and expected to vest at December 31, 2017	2,309,469	$	5.58	8.2	$	2,359,235
Exercisable at December 31, 2017	1,190,135	$	4.78	7.5	$	2,042,324

Summary of Black-Scholes option valuation assumptions

Black-Scholes option valuation assumptions	2017				2016				2015
Risk-free interest rates	1.6	-	2.2	%	0.9	-	2.0	%	0.7	-	1.9	%
Dividend yield		—				—				—
Volatility	73	-	77	%	69	-	75	%	67	-	71	%
Weighted average expected term	3.50	-	6.12	years	3.25	-	6.25	years	2.5	-	6	years

Schedule of stock-based compensation expense

	2017		2016		2015
Research and development	$	613,629	$	1,115,626	$	622,901
General and administrative		1,458,581		915,792		180,378
	$	2,072,210	$	2,031,418	$	803,279