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Share-based compensation - Assumptions (Details) - $ / shares
6 Months Ended
Jun. 30, 2023
Jun. 30, 2022
Share-based compensation    
Expected volatility 88.50% 85.60%
Risk-free interest rate 3.80% 2.70%
Expected term (in years) 6 years 10 days 5 years 11 months 19 days
Expected dividend yield 0.00%  
Fair value of common stock $ 4.87 $ 3.64