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Note 5 - Fair Value Measurements (Details Textual)
3 Months Ended 12 Months Ended
Jun. 30, 2021
$ / shares
shares
Dec. 31, 2021
USD ($)
$ / shares
shares
Dec. 31, 2020
USD ($)
Oct. 01, 2021
shares
Sep. 30, 2021
USD ($)
Jul. 31, 2021
USD ($)
Jul. 21, 2021
USD ($)
$ / shares
shares
May 20, 2021
USD ($)
$ / shares
shares
Feb. 01, 2021
USD ($)
Dec. 16, 2020
USD ($)
Derivative liability, noncurrent   $ 2,651,000 $ 1,830,000              
Warrants and rights outstanding, measurement input             3,900,000      
Loss on Issuance of warrants   $ (3,247,000) 0              
Maximum [Member]                    
Class of warrant or right, exercise price of warrants or rights (in dollars per share) | $ / shares   $ 4,695.60                
Minimum [Member]                    
Class of warrant or right, exercise price of warrants or rights (in dollars per share) | $ / shares   3.45                
Senior Secured Promissory Note Warrants [Member]                    
Derivative liability, noncurrent     $ 1,800,000           $ 1,800,000  
Class of warrant or right, exercise price of warrants or rights (in dollars per share) | $ / shares   $ 3.88                
Class of warrant or right, number of securities called by warrants or rights (in shares) | shares   858,892                
Senior Secured Promissory Note Warrants [Member] | Measurement Input, Share Price [Member]                    
Warrants and rights outstanding, measurement input     17.71           17.71  
Senior Secured Promissory Note Warrants [Member] | Measurement Input, Price Volatility [Member]                    
Warrants and rights outstanding, measurement input     0.87   0.727         88
Senior Secured Promissory Note Warrants [Member] | Measurement Input, Risk Free Interest Rate [Member]                    
Warrants and rights outstanding, measurement input     0.50             1
Senior Secured Promissory Note Warrants [Member] | Measurement Input, Expected Term [Member]                    
Warrants and rights outstanding, measurement input     6   4.6         5.8
Senior Secured Promissory Note Warrants [Member] | Measurement Input, Expected Dividend Rate [Member]                    
Warrants and rights outstanding, measurement input     0   0         0
Senior Secured Promissory Note Warrants [Member] | Measurement Input, Exercise Price [Member]                    
Warrants and rights outstanding, measurement input         3.88          
Senior Secured Promissory Note Warrants [Member] | Altium Growth Fund, LP [Member]                    
Securities purchase agreement, maximum borrowing capacity                   $ 5,000,000.0
Securities purchase agreement, aggregate purchase price                   3,750,000
Securities purchase agreement, aggregate purchase price, original issue discount                   $ 1,250,000
The Equity Warrant [Member]                    
Derivative liability, noncurrent             $ 1,700,000 $ 21,900,000    
The Equity Warrant [Member] | Measurement Input, Share Price [Member]                    
Warrants and rights outstanding, measurement input             3.58      
The Equity Warrant [Member] | Measurement Input, Price Volatility [Member]                    
Warrants and rights outstanding, measurement input             99.1      
The Equity Warrant [Member] | Measurement Input, Risk Free Interest Rate [Member]                    
Warrants and rights outstanding, measurement input             0.82      
The Equity Warrant [Member] | Measurement Input, Expected Term [Member]                    
Warrants and rights outstanding, measurement input             5.5      
The Equity Warrant [Member] | Measurement Input, Expected Dividend Rate [Member]                    
Warrants and rights outstanding, measurement input             0      
The Equity Warrant [Member] | Altium Growth Fund, LP [Member]                    
Class of warrant or right, exercise price of warrants or rights (in dollars per share) | $ / shares   $ 3.88           $ 4.70    
Class of warrant or right, number of securities called by warrants or rights (in shares) | shares   3,994,302   1,309,266       4,995,893    
May 2021 Warrants [Member]                    
Derivative liability, noncurrent   $ 1,700,000                
May 2021 Warrants [Member] | Measurement Input, Share Price [Member]                    
Warrants and rights outstanding, measurement input   1,300,000                
May 2021 Warrants [Member] | Measurement Input, Price Volatility [Member]                    
Warrants and rights outstanding, measurement input   0.727                
May 2021 Warrants [Member] | Measurement Input, Risk Free Interest Rate [Member]                    
Warrants and rights outstanding, measurement input   1.20                
May 2021 Warrants [Member] | Measurement Input, Expected Term [Member]                    
Warrants and rights outstanding, measurement input   4.6                
May 2021 Warrants [Member] | Measurement Input, Expected Dividend Rate [Member]                    
Warrants and rights outstanding, measurement input   0                
May 2021 Warrants [Member] | Measurement Input, Exercise Price [Member]                    
Warrants and rights outstanding, measurement input   3.88                
May 2021 Warrants [Member] | Altium Growth Fund, LP [Member]                    
Class of warrant or right, exercise price of warrants or rights (in dollars per share) | $ / shares $ 3.88                  
Class of warrant or right, unissued | shares 5,303,568                  
July 2021 Warrants [Member]                    
Derivative liability, noncurrent           $ 500,000        
Class of warrant or right, exercise price of warrants or rights (in dollars per share) | $ / shares             $ 3.63      
July 2021 Warrants [Member] | Measurement Input, Price Volatility [Member]                    
Warrants and rights outstanding, measurement input         92.6          
July 2021 Warrants [Member] | Measurement Input, Risk Free Interest Rate [Member]                    
Warrants and rights outstanding, measurement input         1.27          
July 2021 Warrants [Member] | Measurement Input, Expected Term [Member]                    
Warrants and rights outstanding, measurement input         5.1          
July 2021 Warrants [Member] | Measurement Input, Expected Dividend Rate [Member]                    
Warrants and rights outstanding, measurement input         0          
July 2021 Warrants [Member] | Maximum [Member]                    
Class of warrant or right, number of securities called by each warrant or right (in shares) | shares             1,100,000      
Warrants Issued in Connection with May 2016 and August 2017 Offerings [Member]                    
Class of warrant or right, put feature settled (in shares) | shares 7,813                  
Warrants Associated with the October 2018 Offering [Member]                    
Class of warrant or right, put feature settled (in shares) | shares 12,500