Note 11 - Stock-based Compensation

6 Months Ended

Jun. 30, 2022

Notes to Financial Statements

Share-Based Payment Arrangement [Text Block]

11. Stock-Based Compensation

Stock Options

The following table summarizes the Company’s stock option activity, including grants to non-employees, for the six months ended June 30, 2022:

	Number of Shares			Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)
Balance at December 31, 2021		4,640,660		$	6.64		5.66
Granted		2,555,396			0.77
Forfeited		(23,047	)		3.22
Cancelled		(16,983	)		33.27
Exercised		(43,453	)		0.41
Balance at June 30, 2022		7,112,573		$	4.52		5.71

The following table summarizes information about stock options outstanding at June 30, 2022:

	Number of Shares		Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (Millions)
Exercisable at June 30, 2022		3,119,355	$	8.02		5.08	$	-
Vested or expected to vest at June 30, 2022		6,777,170	$	4.67		5.67	$	-

Restricted Stock Units

The following is a summary of the restricted stock units activity, including performance restricted stock units, for the six months ended June 30, 2022:

	Number of Restricted Stock Units Outstanding			Weighted- Average Grant Date Fair Value
Unvested December 31, 2021		3,839,030		$	2.36
Granted		6,040,915			0.75
Vested		(1,794,623	)		2.47
Forfeited		(100,047	)		1.36
Unvested Jun 30, 2022		7,985,275		$	1.13

Performance Restricted Stock Units

In 2022 and 2021, the Company granted performance-based restricted stock units with vesting terms based on our attainment of certain operational targets by October 1, 2023 and October 1, 2022, respectively. The number of shares earnable under the 2022 and 2021 awards are based on achieving designated corporate goals.

Stock-based Compensation Expense

The following table summarizes non-cash stock-based compensation expense by award type for the three and six months ended June 30, 2022, and 2021:

	Three Months Ended June 30,				Six Months Ended June 30,
	2022		2021		2022		2021
	(in thousands)				(in thousands)
Stock options	$	924	$	1,087	$	1,901	$	2,207
Restricted stock units		822		690		1,707		1,293
Performance restricted stock units		337		33		720		70
	$	2,083	$	1,810	$	4,328	$	3,570

As of June 30, 2022, the Company had future employee stock-based compensation expense of approximately $3.6 million related to unvested stock options, which is expected to be recognized over an estimated weighted-average period of 2.0 years. As of June 30, 2022, the unrecognized stock-based compensation expense related to unvested restricted stock units was approximately $6.0 million, which is expected to be recognized over a weighted average period of approximately 1.8 years.

The fair value of options granted were estimated using the Black-Scholes-Merton option pricing model based on the assumptions in the table below:

	Six Months Ended June 30,
	2022				2021
Expected dividend yield			0%			0%
Expected volatility		128%	-	133%	118%	-	136%
Risk-free interest rate		1.25%	-	2.98%	0.33%	-	0.58%
Expected life (in years)		4.3	-	4.5	3.8	-	4.5