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Income taxes (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Income taxes      
Income tax benefits for the net operating losses incurred $ 0 $ 0 $ 0
U.S. federal statutory income tax rate 21.00% 21.00% 21.00%
Increase in tax rate differences 4.00% 4.00% 4.00%
Increase in non-taxable research subsidies received 3.80% 3.80% 4.70%
Increase in non-taxable stock-based compensation (0.50%) 1.50% 1.00%
Reduction in change in deferred tax asset valuation allowance 27.10% 30.20% 30.20%