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Note 4 - Stockholders' Equity, Stock Options and Warrants - Summary of Transactions for Stock Options and Warrants (Details) - $ / shares
6 Months Ended 12 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Warrant [Member]    
Number of Shares Outstanding (in shares) 3,626,643 1,951,257
Average Exercise Price Outstanding (in dollars per share) $ 4.17 $ 23.74
Number of Shares Issued (in shares) 17,212,545 2,336,154
Average Exercise Price Issued (in dollars per share) $ 0.95 $ 1.07
Number of Shares Expired (in shares) (919) (10,706)
Average Exercise Price Expired (in dollars per share) $ 603.89 $ 199.55
Number of Shares Exercised (in shares) (578,560) (650,062)
Average Exercise Price Exercised (in dollars per share) $ 0.01 $ 1
Number of Shares Outstanding (in shares) 20,259,709 3,626,643
Average Exercise Price Outstanding (in dollars per share) $ 1.67 $ 4.17
Share-based Payment Arrangement, Option [Member]    
Number of Shares Outstanding (in shares) 3,669,277 2,764,983
Average Exercise Price Outstanding (in dollars per share) $ 1.70 $ 2
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Gross 3,731,868 1,098,858
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value $ 0.72 $ 1.01
Number of Shares Expired (in shares) (101,397) (194,564)
Average Exercise Price Expired (in dollars per share) $ 2 $ 2
Number of Shares Exercised (in shares)
Average Exercise Price Exercised (in dollars per share)
Average Exercise Price Exercised (in dollars per share)
Number of Shares Outstanding (in shares) 7,299,748 3,669,277
Average Exercise Price Outstanding (in dollars per share) $ 1.20 $ 1.70