Income taxes - Schedule of deferred tax assets and liabilities (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
U.S. and state net operating loss carryforwards	$ 24,322,172	$ 11,118,289
Accruals and other temporary differences	529,462	510,897
Amortization	32,171	33,824
Stock-based compensation	1,847,441	473,801
Tax credit carryforward	1,423,292	671,012
Total deferred tax assets	28,154,538	12,807,823
Less valuation allowance	(28,127,611)	(12,688,401)
Deferred tax assets	26,927	119,422
Deferred tax liabilities:
Stock-based compensation	(23,316)	(110,366)
Depreciation	(3,611)	(9,056)
Accruals and other temporary differences	0	0
Deferred tax liabilities	(26,927)	(119,422)
Net deferred tax assets	$ 0	$ 0