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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

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