Note 6 - Stockholders' Equity and Stock-based Compensation (Tables)

3 Months Ended

Mar. 31, 2023

Notes Tables

Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block]

	Three Months Ended March 31,
	2023		2022
Cost of revenues	$	—	$	90
Research and development		258		457
Sales and marketing		—		454
General and administrative		638		1,006
Total stock-based compensation expense	$	896	$	2,007

Share-Based Payment Arrangement, Cost by Plan [Table Text Block]

	Three Months Ended March 31,
	2023		2022
Time-based options	$	678	$	1,807
Performance options		117		84
ESPP		101		116
Total stock-based compensation expense	$	896	$	2,007

Share-Based Payment Arrangement, Option [Member]

Notes Tables

Schedule of Share-Based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

	Three Months Ended March 31,
	2023		2022
Expected term in years		6.03		6.3 - 6.8
Expected volatility		94%		83%
Risk-free interest rate		3.80%		1.90%
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, 2022		520,302		$	10.08		7.02
Options granted		782,300			2.92
Options exercised		—			—
Options canceled		—			—
Options expired		(78,172	)		12.84
Balances — March 31, 2023		1,224,430		$	5.33		8.90
Exercisable — March 31, 2023		208,189		$	10.72		6.71

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, 2022		4,730,394		$	12.95		6.16
Options granted		25,000			2.52
Options exercised		—			—
Options canceled		(68,894	)		11.24
Options expired		(302,720	)		13.13
Balances — March 31, 2023		4,383,780		$	12.91		6.34
Exercisable — March 31, 2023		3,226,526		$	14.98		5.59

The 2017 Employee Stock Purchase Plan [Member]

Notes Tables

Schedule of Share-Based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

	Three Months Ended March 31,
	2023		2022
Expected term in years		0.5 - 1.0		0.5 - 1.0
Expected volatility		83%		83%
Risk-free interest rate		5.1% - 5.2%		0.6% - 0.9%
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, 2022		5,250,696		$	12.67		6.24
Options granted		807,300			2.92
Options exercised		—			—
Options canceled		(68,894	)		11.24
Options expired		(380,892	)		13.07
Balances — March 31, 2023		5,608,210		$	11.25		6.90
Exercisable — March 31, 2023		3,434,715		$	14.72		5.66