Goodwill and intangible assets - Intangible assets (Details) - USD ($)	3 Months Ended		6 Months Ended		18 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Dec. 31, 2016
Intangible assets
Gross book value	$ 3,723,173		$ 3,723,173		$ 3,723,173	$ 3,508,116
Accumulated amortization	(3,100,457)		(3,100,457)		(3,100,457)	(2,680,642)
Net book value	622,716		622,716		622,716	827,474
Amortization expense	88,764	$ 145,880	225,191	$ 288,018
Trademarks and patents
Intangible assets
Gross book value	1,221,339		1,221,339		1,221,339	1,203,097
Accumulated amortization	(598,623)		(598,623)		(598,623)	(557,794)
Net book value	622,716		622,716		$ 622,716	645,303
Trademarks and patents | Minimum
Intangible assets
Amortization period					15 years
Trademarks and patents | Maximum
Intangible assets
Amortization period					21 years
Technology
Intangible assets
Gross book value	2,501,834		2,501,834		$ 2,501,834	2,305,019
Accumulated amortization	$ (2,501,834)		$ (2,501,834)		$ (2,501,834)	(2,122,848)
Net book value						$ 182,171
Amortization period					5 years