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Equity-based Compensation - Fair value of options (Details) - $ / shares
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Fair value of common stock   $ 1.20
Expected volatility, minimum 100.82% 104.60%
Expected volatility, maximum 101.83% 109.34%
Risk-free interest rate, minimum 0.67% 0.38%
Risk-free interest rate, maximum 1.06% 1.60%
Dividend yield 0.00% 0.00%
Minimum    
Fair value of common stock $ 1.75  
Expected term (years) 5 years 5 years
Maximum    
Fair value of common stock $ 6.15  
Expected term (years) 6 years 2 months 19 days 6 years 3 months