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Stockholders' Equity - Weighted-Average assumptions (Details) - $ / shares
3 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Weighted-average assumptions    
Expected term 6 years 3 months 6 years 3 months
Expected volatility 84.51% 83.72%
Risk-free interest rate 1.66% 0.63%
Expected dividend yield 0.00% 0.00%
2013 Equity Incentive and Inducement Plan    
Share-based Compensation Arrangement by Share-based Payment Award    
Weighted-average fair value of the options granted $ 0.80 $ 2.70