Income taxes - Deferred taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 13,693	$ 10,202
Research and development credits	1,560	1,355
Share-based compensation	82	194
Accrued bonus	201	120
Amortization	2	3
Gross deferred tax assets	15,538	11,874
Less: valuation allowance	(15,435)	(11,711)
Net deferred tax asset	103	163
Deferred tax liability
Depreciation	(103)	(163)
Total deferred tax liabilities