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Income taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Income Tax Disclosure [Abstract]    
Net operating loss carryforwards $ 14,120,607 $ 12,411,425
Foreign research and development tax credit carryforwards 2,628,190 2,832,340
Property and equipment 463,343 482,161
Other 19,695 13,627
Total deferred tax assets 17,231,835 15,739,553
Valuation allowance (17,231,835) (15,739,553)
Net deferred tax assets 0 0
Increases in valuation allowance $ 1,500,000 $ 131,147