The Company maintains a stock incentive plan that permits the granting of incentive stock options or nonqualified stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance awards, and other stock-based awards. The equity-based awards for employees will generally vest over a four-year period, pursuant to two different vesting schedules. For initial equity-based awards granted to employees, the first vest is generally a one-year cliff vest, followed by monthly vesting for the final three years. Thereafter, annual equity-based awards granted to employees typically vest monthly over the four-year vest term. The initial equity-based awards granted to the Company’s non-employee, independent directors upon appointment to the board of directors will vest over a three-year period, with the first vest being a one-year cliff, followed by monthly vesting over the remaining two years. Thereafter, annual equity-based awards granted to the Company’s non-employee, independent directors will cliff vest after one year from the date of grant.

The following table summarizes the stock option activity for the six months ended June 30, 2024 (in thousands, except share and per share data):


	Number of Shares			Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at January 1, 2024		4,041,807		$	6.41		7.53	$	5,159
Granted		727,500		$	2.85
Exercised		—		$	—
Forfeited		(425,900	)	$	9.57
Expired		(315,750	)	$	11.94
Outstanding at June 30, 2024		4,027,657		$	5.00		7.37	$	1,005
Exercisable at June 30, 2024		2,177,898		$	5.10		6.71	$	708
Vested and expected to vest at June 30, 2024		3,742,975		$	5.35		7.58	$	744

The weighted average assumptions used in the Black-Scholes pricing model for stock options granted during the three and six months ended June 30, 2024, were as follows:


	For the Three Months Ended June 30,				For the Six Months Ended June 30,
	2024	2023			2024			2023
Estimated dividend yield	n/a		-	%		-	%		-	%
Weighted-average expected stock price volatility	n/a		35.35	%		35.91	%		35.04	%
Weighted-average risk-free interest rate	n/a		3.47	%		4.33	%		4.11	%
Expected average term of options (in years)	n/a		6.25			6.25			6.25
Weighted-average fair value of common stock	n/a	$	1.97		$	2.85		$	5.37
Weighted-average fair value per option	n/a	$	0.81		$	1.24		$	2.28

The following table summarizes the restricted stock unit activity for the six months ended June 30, 2024 (in thousands, except share and per share data):


	Number of Shares			Weighted Average Grant Date Fair Value per Share		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in thousands)
Outstanding at January 1, 2024		155,780		$	5.05		1.36	$	581
Granted		60,000		$	1.29
Vested		(67,169	)	$	4.58
Forfeited		(21,000	)	$	5.41
Outstanding at June 30, 2024		127,611		$	3.47		1.34	$	175
Vested and expected to vest at June 30, 2024		127,611		$	3.47		1.34	$	175

The Company also maintains an employee stock purchase plan (ESPP) that authorizes the issuance of shares of common stock pursuant to purchase rights granted to eligible employees. Unless otherwise determined by the Company’s board of directors, shares of the Company’s common stock will be purchased for the accounts of employees participating in the Company’s ESPP at a price per share equal to the lesser of (i) 85% of the fair market value of a share of the Company’s common stock on the first day of an offering; or (ii) 85% of the fair market value of a share of the Company’s common stock on the date of purchase. Offering periods are generally six months long; offering periods begin on June 1 and December 1 of each year. The Company issued 54,314 shares of common stock under the ESPP during the three and six months ended June 30, 2024 and 82,034 shares of common stock under the ESPP during the three and six months ended June 30, 2023.

In January 2024, the Company’s board of directors approved a one-time repricing of certain previously granted and still outstanding vested and unvested stock option awards held by eligible employees, executive officers, and non-employee directors. As a result, the exercise price for these awards will be lowered to $2.97 per share, which was the closing price of the Company’s common stock as reported on the Nasdaq Global Stock Market on March 14, 2024, so long as the holder remains employed by the Company or continues to serve as a member of the board of directors through September 14, 2025 absent earlier trigger events defined in the option repricing plan. No other terms of the stock options were modified, and the stock options will continue to vest according to their original vesting schedules and will retain their original expiration dates. As a result of the repricing, 1,631,016 vested and unvested stock options outstanding as of March 14, 2024, with original exercise prices ranging from $3.02 to $27.49, were repriced.

The repricing on March 14, 2024 resulted in incremental stock-based compensation expense of $0.9 million, of which $0.5 million related to vested stock option awards and was expensed on the repricing date. The remaining $0.4 million related to unvested stock option awards and is being amortized on a straight-line basis over the weighted-average vesting period of those awards of approximately 2.38 years as of March 14, 2024.

Stock-based compensation expense included in the accompanying condensed financial statements was as follows (in thousands):


	For the Three Months Ended June 30,				For the Six Months Ended June 30,
	2024		2023		2024		2023
Cost of sales	$	30	$	40	$	79	$	76
Research and development		17		40		47		77
Sales and marketing		33		172		129		324
General and administrative		753		818		1,885		1,603
Total stock-based compensation expense	$	833	$	1,070	$	2,140	$	2,080

Stock-based compensation expense related to stock options was $0.8 million and $2.0 million for the three and six months ended June 30, 2024, respectively, and $1.0 million and $1.9 million for the three and six months ended June 30, 2023, respectively. Unrecognized compensation expense related to stock options was $4.6 million at June 30, 2024, which is expected to be recognized as expense over the weighted-average period of 3.05 years.

Stock-based compensation expense related to restricted stock units was $0.1 million for each of the three and six months ended June 30, 2024, and $0.1 million and $0.2 million for the three and six months ended June 30, 2023, respectively. Unrecognized compensation expense related to restricted stock units was $0.4 million at June 30, 2024, which is expected to be recognized as expense over the weighted-average period of 1.87 years.

Stock-based compensation expense related to the ESPP was not significant in either of the three or six months ended June 30, 2024 and 2023. Total compensation cost related to the ESPP not yet recognized was not significant at June 30, 2024. As of June 30, 2024, an insignificant amount has been withheld on behalf of employees for future purchases under the ESPP.