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Note 4 - Stockholders' Equity, Stock Options and Warrants - Summary of Transactions for Stock Options and Warrants (Details) - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2019
Dec. 31, 2018
Number of Shares Outstanding (in shares) 4,071,579  
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) 2,325,406 2,336,154
Average Exercise Price Issued (in dollars per share) $ 0.92 $ 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)   (650,062)
Average Exercise Price Exercised (in dollars per share)   $ 1
Number of Shares Outstanding (in shares) 5,951,130 3,626,643
Average Exercise Price Outstanding (in dollars per share) $ 3.29 $ 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
Number of Shares Issued (in shares) 459,169 1,098,858
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value $ 0.73 $ 1.01
Number of Shares Expired (in shares) (56,867) (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)  
Number of Shares Outstanding (in shares) 4,071,579 3,669,277
Average Exercise Price Outstanding (in dollars per share) $ 1.59 $ 1.70