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Income taxes - Components of Loss Before Income Taxes (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Abstract]      
Domestic $ (9,702,869) $ (6,526,706) $ (5,950,862)
Foreign (679,190) (2,731,941) (2,519,799)
Loss before income taxes $ (10,382,059) $ (9,258,647) $ (8,470,661)