Goodwill and intangible assets - Intangible assets (Details) - USD ($)	3 Months Ended		15 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Dec. 31, 2015
Intangible assets
Gross book value	$ 3,674,281		$ 3,674,281	$ 3,573,464
Accumulated amortization	(2,412,201)		(2,412,201)	(2,195,200)
Net book value	1,262,080		1,262,080	1,378,264
Amortization expense	142,138	$ 145,000
Trademarks and patents
Intangible assets
Gross book value	1,187,142		1,187,142	1,184,612
Accumulated amortization	(494,697)		(494,697)	(472,914)
Net book value	692,445		$ 692,445	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	2,487,139		$ 2,487,139	2,388,852
Accumulated amortization	(1,917,504)		(1,917,504)	(1,722,286)
Net book value	$ 569,635		$ 569,635	$ 666,566
Amortization period			5 years