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Income Taxes - Net deferred tax assets and liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Deferred tax assets:    
Net operating loss carryforwards $ 44,142 $ 41,989
Charitable contribution carryforwards 433 439
Research and experimentation credits 1,860 1,668
Capitalized intangibles 416 387
Stock-based compensation 116 119
Depreciation and amortization (5)  
Accrued compensation 22 16
Lease liabilities 111  
Other   9
Total deferred tax assets 47,095 44,627
Valuation allowance (46,990) (44,627)
Deferred tax assets, net of valuation allowance 105  
Deferred tax liabilities:    
Right-of-use asset (105)  
Net deferred tax liability $ 0 $ 0