Goodwill and intangible assets - Intangible assets (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Intangible assets
Gross book value	$ 3,573,464	$ 3,820,736
Accumulated amortization	(2,195,200)	(1,774,036)
Net book value	1,378,264	2,046,700
Amortization expense	573,116	707,657	$ 735,265
Trademarks and patents
Intangible assets
Gross book value	1,184,612	1,158,572
Accumulated amortization	(472,914)	(387,134)
Net book value	$ 711,698	771,438
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	$ 2,388,852	2,662,164
Accumulated amortization	(1,722,286)	(1,386,902)
Net book value	$ 666,566	$ 1,275,262
Amortization period	5 years