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Stockholder's equity (deficit) - Restricted stock activity (Details)
3 Months Ended
Mar. 31, 2021
USD ($)
shares
Number of Shares  
Beginning balance of unvested restricted common stock | shares 33,614
Shares vesting | shares (14,889)
Ending balance of unvested restricted common stock | shares 18,725
Grant-Date Fair Value  
Beginning balance of unvested restricted common stock | $ $ 0.62
Shares vesting | $ 0.62
Ending balance of unvested restricted common stock | $ $ 0.62