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Share-based compensation - Assumptions (Details) - $ / shares
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Share-based compensation    
Expected volatility 83.00% 83.00%
Risk-free interest rate 1.00% 0.60%
Expected term (in years) 6 years 6 years 1 month 6 days
Expected dividend yield 0.00%  
Fair value of common stock $ 23.04 $ 5.30