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Note 14 - Income Taxes - Effective Income Tax Reconciliation (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Tax at federal statutory rate $ (2,937,000) $ (4,185,000) $ (3,505,000)
State, net of federal benefit (455,000) (1,238,000) (315,000)
Research and development credit (236,000) (135,000) (89,000)
Stock-based compensation 440,000 344,000 136,000
Nondeductible interest 590,000
Warrant and derivative revaluation 328,000
Change in Federal tax rate 8,172,000
Other 22,000 7,000 94,000
Change in valuation allowance 3,166,000 (2,965,000) 2,761,000
Total provision for income taxes $ 0 $ 0 $ 0