Goodwill and intangible assets - Intangible assets (Details) - USD ($)	3 Months Ended		15 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Dec. 31, 2016
Intangible assets
Gross book value	$ 3,548,087		$ 3,548,087	$ 3,508,116
Accumulated amortization	(2,849,402)		(2,849,402)	(2,680,642)
Net book value	698,685		698,685	827,474
Amortization expense	136,427	$ 142,138
Trademarks and patents
Intangible assets
Gross book value	1,208,510		1,208,510	1,203,097
Accumulated amortization	(577,748)		(577,748)	(557,794)
Net book value	630,762		$ 630,762	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	2,339,577		$ 2,339,577	2,305,019
Accumulated amortization	(2,271,654)		(2,271,654)	(2,122,848)
Net book value	$ 67,923		$ 67,923	$ 182,171
Amortization period			5 years