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Stock option plans - Summary (Details) - 2019 Equity Incentive Plan - USD ($)
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Stock option plan        
Options outstanding 4,586,615 3,771,459 3,499,934 1,844,188
Shares available for grant 907,589      
Stock based compensation expense $ 7,813,743 $ 4,983,023 $ 1,345,722  
Unrecognized compensation cost $ 13,596,148      
Weighted average        
Stock option plan        
Recognition period for unrecognized compensation cost 2 years 3 months 18 days