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Income taxes - Reconciliation of Tax Rate (Details)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Income taxes    
U.S. Statutory Tax Rate 21.00% 21.00%
Change in Valuation Allowance (10.20%) (21.50%)
State tax expense, net 0.90% 1.80%
Research and Development Tax Credit 0.90% 0.50%
Uncertain Tax Position - Research and Development Tax Credit (3.40%) 0.00%
Permanent Difference - Interest Expense (7.80%) 0.00%
Provision to Return, Permanent Differences and Other Adjustments (1.40%) (1.80%)
Tax Expense / (Benefit) 0.00% 0.00%