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Stock option plan - Summary (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Oct. 31, 2016
Stock option plan          
Stock based compensation expense $ 4,983,023 $ 1,345,722 $ 329,814    
Options to purchase common stock          
Stock option plan          
Expiration period after termination of employment 6 months        
Vesting period 4 years        
2016 Amended and Restated Stock Option Program          
Stock option plan          
Options outstanding   0      
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 3,771,459 3,499,934 1,844,188 1,897,206  
Shares available for grant 1,580,804        
Unrecognized compensation cost $ 16,359,450        
2019 Equity Incentive Plan | Weighted average          
Stock option plan          
Recognition period for unrecognized compensation cost 2 years 10 months 24 days