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Note 5 - Convertible Preferred Stock, Common Stock and Common Stock Warrants (Tables)
 6 Months Ended
Jun. 30, 2018
Notes Tables  
Schedule of Warrants Outstanding to Acquire Shares of Common Stock [Table Text Block]
   
Outstanding
    
Range of
exercise price
per share
  
Expiration dates
Common stock warrants issued prior to 2016    
36,000
      
$37.50
-
$49.00
  
2018 through 2019
Common stock warrants issued related to Series A convertible preferred stock offering
    
13,555,887
      
 
$2.22
 
  
March 2022
Common stock warrants issued in 2018 related to the common stock offering
    
17,720,625
      
$0.80
-
$1.00
  
January 2023
     
31,312,512