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Commitments and contingencies - Components of operating lease expense (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Operating lease expense:    
Total operating lease cost $ 81 $ 94
Other information:    
Cash paid for amounts included in the measurement of operating lease liability $ 75 $ 59
Weighted average remaining lease term, in years 6 years 3 months 18 days 1 year
Weighted average discount rate 7.60% 8.00%
General and administrative expense    
Operating lease expense:    
Operating lease cost $ 81 $ 59
Variable operating lease cost   27
Research and development expense    
Operating lease expense:    
Short-term lease cost   $ 8