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Note 5 - Equity (Tables)
3 Months Ended
Mar. 31, 2020
Notes Tables  
Schedule of Stockholders' Equity Note, Warrants or Rights [Table Text Block]
   
Number of
   
Weighted Avg.
 
   
Warrants
   
Exercise Price
 
Outstanding at January 1, 2019
   
3,864,552
    $
2.36
 
Issued
   
3,611,504
     
0.46
 
Cancelled or expired
   
     
 
Exercised
   
     
 
Outstanding at December 31, 2019
   
7,476,056
    $
1.45
 
Issued
   
     
 
Cancelled or expired
   
     
 
Exercised
   
     
 
Outstanding at March 31, 2020
   
7,476,056
    $
1.45
 
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
Expected term:
 
10 years
Risk-free rate:
 
0.21%
2.63%
Volatility:
 
75.98%
79.89%
Dividend yield:
 
0%
Share-based Payment Arrangement, Option, Activity [Table Text Block]
   
Number of
   
Weighted Avg.
 
   
Shares
   
Exercise Price
 
Outstanding at January 1, 2019
   
4,535,681
    $
3.31
 
Options granted
   
1,744,300
     
1.48
 
Options exercised
   
     
 
Options expired
   
(297,058
)    
 
Outstanding at December 31, 2019
   
5,982,923
    $
2.66
 
Options granted
   
1,165,325
     
2.00
 
Options exercised
   
     
 
Options expired or cancelled
   
     
 
Outstanding at March 31, 2020
   
7,148,248
    $
2.55