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Note 6 - Stockholders' Equity and Stock-based Compensation (Tables)
12 Months Ended
Dec. 31, 2022
Notes Tables  
Schedule of Stockholders' Equity Note, Warrants or Rights [Table Text Block]
          

Weighted

 
          

Average

 
      

Weighted

  

Remaining

 
  

Number of

  

Average

  

Contractual

 
  

Shares

  

Exercise Price

  

Life (in Years)

 

Warrants outstanding at December 31, 2021

    $    

Issued

  7,317,072   2.05     

Exercised

  (13,240)  2.05     

Expired/Redeemed

          

Warrants outstanding and exercisable at December 31, 2022

  7,303,832  $2.05   4.43 
Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block]
  

Year Ended December 31,

 
  

2022

  

2021

  

2020

 

Cost of revenues

 $217  $129  $ 

Research and development

  1,563   5,211   4,013 

Sales and marketing

  733   2,749   1,187 

General and administrative

  2,678   6,512   4,875 

Total stock-based compensation expense

 $5,191  $14,601  $10,075 
Share-Based Payment Arrangement, Cost by Plan [Table Text Block]
  

Year Ended December 31,

 
  

2022

  

2021

  

2020

 

Time-based options

 $4,467  $9,235  $8,739 

Performance options

  233   4,840   133 

RSU

        86 

ESPP

  491   526   1,117 

Total stock-based compensation expense

 $5,191  $14,601  $10,075 
Share-Based Payment Arrangement, Option [Member]  
Notes Tables  
Schedule of Share-Based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
  

Year Ended December 31,

 
  

2022

 

2021

 

2020

 

Expected term in years

 5.3 - 6.8 5.3 - 6.1 5.3 - 6.1 

Expected volatility

 

83 - 88%

 

78%

 

70%

 

Risk-free interest rate

 

1.9 - 3.2%

 

0.9 - 1.4%

 

0.3 - 0.5%

 

Dividend yield

 

 

 

 
Performance Options [Member]  
Notes Tables  
Share-Based Payment Arrangement, Option, Activity [Table Text Block]
  

Stock Options Outstanding

 
  

Number of shares

  

Weighted average exercise price

  

Weighted average remaining life (in years)

 

Balances — December 31, 2021

  1,200,097  $13.11   8.44 

Options granted

  200,000   5.32     

Options exercised

          

Options canceled

  (797,920)  13.32     

Options expired

  (81,875)  11.28     

Balances — December 31, 2022

  520,302  $10.08   7.02 

Exercisable — December 31, 2022

  248,861  $12.25   4.92 
Time-based Options [Member]  
Notes Tables  
Share-Based Payment Arrangement, Option, Activity [Table Text Block]
  

Stock Options Outstanding

 
  

Number of shares

  

Weighted average exercise price

  

Weighted average remaining life (in years)

 

Balances — December 31, 2021

  4,796,716  $16.44   7.05 

Options granted

  1,240,100   2.59     

Options exercised

          

Options canceled

  (674,465)  16.73     

Options expired

  (631,957)  15.05     

Balances — December 31, 2022

  4,730,394  $12.95   6.16 

Exercisable — December 31, 2022

  3,230,670  $15.39   5.07 
The 2017 Employee Stock Purchase Plan [Member]  
Notes Tables  
Schedule of Share-Based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
  

Year Ended December 31,

 
  

2022

 

2021

 

2020

 

Expected term in years

 0.5 - 1.0 0.5 - 1.0 0.5 - 1.0 

Expected volatility

 

83%

 

78%

 

70

%

Risk-free interest rate

 

0.6% - 3.5%

 

0.06% - 0.1%

 

0.1% - 1.0%

 

Dividend yield

 

 

 

 
The 2015 Plan, 2017 Plan and Inducement Plan [Member]  
Notes Tables  
Share-Based Payment Arrangement, Option, Activity [Table Text Block]
  

Stock Options Outstanding

 
  

Number of shares

  

Weighted average exercise price

  

Weighted average remaining life (in years)

 

Balances — December 31, 2021

  5,996,813  $15.77   7.33 

Options granted

  1,440,100   2.97     

Options exercised

          

Options canceled

  (1,472,385)  14.88     

Options expired

  (713,832)  14.62     

Balances — December 31, 2022

  5,250,696  $12.67   6.24 

Exercisable — December 31, 2022

  3,479,531  $15.17   5.06