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Note 6 - Stockholders' Equity (Tables)
 12 Months Ended
Dec. 31, 2020
Notes Tables  
Share-based Payment Arrangement, Option, Activity [Table Text Block]
   
Number of
Options
    
Weighted
Average
Exercise
Price
    
Weighted
Average
Remaining
Contractual
Term
(in years)
    
Aggregate
Intrinsic
Value
  
Outstanding, December 31, 2018
    
1,206,709
     $
17.93
      
6.97
     $
-
  
Granted
    
714,350
     $
5.76
      
 
      
 
  
Cancelled
    
(254,262
)
   $
9.78
      
 
      
 
  
Outstanding, December 31, 2019
    
1,666,797
     $
13.96
      
7.22
     $
91,475
  
Granted
    
1,511,000
     $
4.03
      
 
      
 
  
Cancelled
    
(198,750
)
   $
7.52
      
 
      
 
  
Outstanding, December 31, 2020
    
2,979,047
     $
9.35
      
7.84
     $
2,074,150
  
                                 
Exercisable, December 31, 2020
    
1,131,314
     $
17.34
      
5.38
     $
15,360