Stock-based compensation is recorded in the consolidated statements of operations as follows (in thousands):


	Three months ended June 30,				Six months ended June 30,
	2022		2021		2022		2021
Cost of goods sold	$	22	$	1	$	26	$	2
Research and development		278		176		554		380
Sales and marketing		296		213		583		428
General and administrative		618		618		1,292		1,126
Total stock-based compensation expense	$	1,214	$	1,008	$	2,455	$	1,936

The following table summarizes the outstanding stock option activity during the period indicated (shares in thousands):


				Weighted average
	Number of stock options			Exercise price		Remaining contractual term (in years)
Balance at December 31, 2021		2,000		$	12.79		7.3
Granted		308			9.24
Exercised		(8	)		2.33
Expired/Forfeited		(241	)		13.47
Balance at June 30, 2022		2,059			12.22		7.0

Vested and exercisable at June 30, 2022		1,288			11.12		6.0
Vested and expected to vest at June 30, 2022		2,059			12.22		7.0

The weighted average grant date fair value of options granted during the six months ended June 30, 2022 was $4.87. For stock options vested and expected to vest, the aggregate intrinsic value as of June 30, 2022 and December 31, 2021, was $1.0 million and $2.2 million, respectively.

At June 30, 2022, there was $4.8 million of unrecognized compensation cost related to unvested stock options granted under the Company’s equity plans that is expected to be recognized over the next 2.5 years.

The following table summarizes the Company's restricted stock unit activity during the period indicated (shares in thousands):


	Restricted stock units			Weighted average grant date fair value
Balance at December 31, 2021		333		$	17.55
Grants		145		$	9.02
Vested and released		(78	)	$	17.90
Forfeited		(69	)	$	15.54
Balance at June 30, 2022		331		$	14.13

As of June 30, 2022, there was $3.9 million of total unrecognized compensation cost related to unvested restricted stock units having a weighted average remaining contractual term of 2.9 years.

The following table summarizes the Company's performance stock unit (PSU) activity during the period indicated (shares in thousands):


	Performance stock units		Weighted average grant date fair value
Balance at December 31, 2021		—	$	—
Grants		174	$	2.20
Vested and released		—	$	—
Forfeited		—	$	—
Balance at June 30, 2022		174	$	—

Service as well as market and performance conditions determine the number of PSUs that the holder will earn from 0% to 150% of the target number of shares. The percentage received is based on the Company common stock price targets over a three-year service period. Additionally, the Company must achieve or exceed 75% of the year to date revenue target measured at the end of the quarter in which the price target is achieved. As of June 30, 2022, there was $0.3 million of total unrecognized compensation cost related to unvested PSUs having a weighted average remaining contractual term of 1.8 years.

We estimate the fair value of PSUs with a market condition using a Monte Carlo simulation model as of the date of grant using historical volatility.

Share-based compensation expense for the three and six months ended June 30, 2022 was $0.2 million and $0.4 million, respectively for the liability classified restricted stock unit payout obligation related to the 2022 executive bonus accrual. The bonus accrual is based on probable achievement on financial and other performance targets.

The Company maintains the 2016 Employee Stock Purchase Plan (ESPP) that provides employees an opportunity to purchase common stock through payroll deductions. The ESPP is implemented through consecutive 6-month offering periods commencing on March 1 and September 1 of each year. The purchase price is set at 85% of the fair market value of the Company's common stock on either the first or last trading day of the offering period, whichever is lower. Annual contributions are limited to the lower of 20% of an employee's eligible compensation or such other limits as apply under Section 423 of the Internal Revenue Code. The ESPP is intended to qualify as an employee stock purchase plan for purposes of Section 423 of the Internal Revenue Code.

Based on the 15% discount and the fair value of the option feature of the ESPP, it is considered compensatory. Compensation expense is calculated using the fair value of the employees’ purchase rights under the Black-Scholes model. The Company currently uses authorized and unissued shares to satisfy share award exercises.

During the six months ended June 30, 2022, the Company received $0.2 million from the issuance of 31,170 shares under the ESPP.