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Debt (Tables)
 6 Months Ended
Jun. 30, 2020
Debt Disclosure [Abstract]  
Schedule of debt valuation assumptions
The assumptions used in the Black-Scholes option-pricing model remain unchanged and are determined as follows:

June 30, 2020
Volatility98.01 %
Risk-free interest rate1.58 %
Expected life (in years)7
Dividend yield0 %
The assumptions used in the Black-Scholes option-pricing model are determined as follows:
June 30, 2020December 31, 2019
Volatility
97.96% - 98.04%
97.96% - 98.04%
Risk-free interest rate0.49 %1.81 %
Weighted average expected life (in years)5.756.25
Dividend yield— %— %
Schedule of carrying amounts of debt
The net carrying amounts of the liability components consists of the following (in thousands):
June 30, 2020December 31, 2019
Principal$39,466  $36,502  
Less: debt discount(4,644) (4,905) 
Net carrying amount
$34,822  $31,597  
Schedule of interest expense recognized
The following table presents the interest expense recognized related to the 2024 Notes and 2022 Loan (in thousands):

Three Months Ended June 30, Six Months Ended June 30,
2020201920202019
Contractual interest expense
$1,511  $313  $2,964  $534  
Accretion of debt discount173  158  373  258  
Total interest expense$1,684  $471  $3,337  $792