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Income taxes - Federal income tax (Details)
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Income taxes    
Federal tax benefit at statutory rate 21.00% 21.00%
State tax, net of federal benefit 4.30% 8.00%
Effects of state tax legislation, net of federal benefit (7.10%)  
Research and development credits 1.90% 2.40%
Permanent differences (0.20%) 0.30%
Change in valuation allowance (19.90%) (31.70%)
Total