Stockholder's equity (deficit) - Restricted stock activity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Number of Shares
Beginning balance of unvested restricted common stock	155,149	80,190
Early exercise of unvested stock options		491,207
Shares vesting	(155,149)	(416,248)
Ending balance of unvested restricted common stock		155,149
Grant-Date Fair Value
Beginning balance of unvested restricted common stock	$ 0.52
Early exercise of unvested stock options		$ 0.65
Shares vesting	$ 0.52	0.65
Ending balance of unvested restricted common stock		$ 0.52