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Stock option plan - Summary (Details) - USD ($)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Oct. 31, 2016
Stock option plan          
Options outstanding 3,499,934 1,844,188 1,897,206 1,237,292  
Stock based compensation expense $ 1,345,722 $ 329,814 $ 395,440    
Options to purchase common stock          
Stock option plan          
Expiration period after termination 6 months        
Vesting period 4 years        
2016 Amended and Restated Stock Option Program | Options to purchase common stock          
Stock option plan          
Number of shares authorized         1,224,824
2017 Amended and Restated Stock Option Program | Options to purchase common stock          
Stock option plan          
Number of shares authorized     1,946,849    
2018 Amended and Restated Stock Option Program | Options to purchase common stock          
Stock option plan          
Number of shares authorized   3,069,999      
2019 Equity Incentive Plan          
Stock option plan          
Options outstanding 0        
Shares available for grant 1,175,494        
Unrecognized compensation cost $ 10,811,939        
2019 Equity Incentive Plan | Weighted average          
Stock option plan          
Recognition period for unrecognized compensation cost 3 years 7 months 6 days