Net loss per share - Basic and diluted net loss per share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Numerator:
Net loss	$ (14,678)	$ (10,138)	$ (48,782)	$ (33,577)
Denominator:
Weighted average ordinary shares outstanding, basic	25,039,990	19,426,833	23,719,124	18,504,013
Weighted average ordinary shares outstanding, diluted	25,039,990	19,426,833	23,719,124	18,504,013
Net loss per share, basic	$ (0.59)	$ (0.52)	$ (2.06)	$ (1.81)
Net loss per share, diluted	$ (0.59)	$ (0.52)	$ (2.06)	$ (1.81)