Summary of significant accounting policies - Net loss per share (Details) - shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Potentially dilutive securities
Total shares of common stock equivalents	4,986,334	3,771,459	3,499,934
Options to purchase common stock
Potentially dilutive securities
Total shares of common stock equivalents	4,586,615	3,771,459	3,499,934
Restricted stock units
Potentially dilutive securities
Total shares of common stock equivalents	399,719