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Stockholder's equity (deficit) - Restricted stock activity (Details) - Restricted stock
9 Months Ended
Sep. 30, 2021
USD ($)
shares
Number of Shares  
Beginning balance of unvested restricted common stock | shares 33,614
Shares vesting | shares (33,614)
Grant-Date Fair Value  
Beginning balance of unvested restricted common stock | $ $ 0.62
Shares vesting | $ $ 0.62