Goodwill and intangible assets - Intangible assets (Details) - USD ($)	3 Months Ended		6 Months Ended		18 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Dec. 31, 2015
Intangible assets
Gross book value	$ 3,630,480		$ 3,630,480		$ 3,630,480	$ 3,573,464
Accumulated amortization	(2,513,245)		(2,513,245)		(2,513,245)	(2,195,200)
Net book value	1,117,235		1,117,235		1,117,235	1,378,264
Amortization expense	145,880	$ 142,789	288,018	$ 287,789
Trademarks and patents
Intangible assets
Gross book value	1,198,419		1,198,419		1,198,419	1,184,612
Accumulated amortization	(516,602)		(516,602)		(516,602)	(472,914)
Net book value	681,817		681,817		$ 681,817	711,698
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	2,432,061		2,432,061		$ 2,432,061	2,388,852
Accumulated amortization	(1,996,643)		(1,996,643)		(1,996,643)	(1,722,286)
Net book value	$ 435,418		$ 435,418		$ 435,418	$ 666,566
Amortization period					5 years