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Stockholders' equity - Additional Information (Details) - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation $ 138,498 $ 48,546  
Weighted average fair value of stock options granted (in dollars per share) $ 1.48    
Intrinsic value of options outstanding $ 600   $ 0
Intrinsic value of options exercisable 43   $ 0
Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unearned share-based compensation expense $ 316,699    
Period of recognition 3 years    
Employee Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Period of recognition 1 year    
Share-based compensation $ 47,864 41,579  
Unearned share-based compensation expense $ 74,780    
Expiration period 10 years    
Expected volatility rate, stock options granted in period 89.00%    
Risk free interest rate, stock options granted in period 2.53%    
Expected dividend yield, stock options granted in period 0.00%    
Expected life (in years), stock options granted in period 5 years    
Employee Stock Option | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 1 year    
Employee Stock Option | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Award vesting period 3 years    
Chairman | Restricted Stock      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Allocated share-based compensation expense $ 90,634 $ 6,967