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


	Three months ended September 30,				Nine months ended September 30,
	2021		2020		2021		2020
Cost of goods sold	$	1	$	—	$	3	$	—
Research and development		211		124		591		425
Sales and marketing		230		101		658		291
General and administrative		626		409		1,752		1,240
Total stock-based compensation expense	$	1,068	$	634	$	3,004	$	1,956

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, 2020		1,760		$	10.07		7.6
Granted		516			22.43
Exercised		(226	)		11.11
Expired/Forfeited		(97	)		14.26
Balance at September 30, 2021		1,953			13.00		7.5

Vested and exercisable at September 30, 2021		1,079			9.47		6.5
Vested and expected to vest at September 30, 2021		1,953			13.00		7.5

The weighted average grant date fair value of options granted during the nine months ended September 30, 2021 and for the year ended December 31, 2020, was $10.66 and $4.30, respectively. For stock options vested and expected to vest, the aggregate intrinsic value as of September 30, 2021, and December 31, 2020, was $4.3 million and $13.6 million, respectively.

At September 30, 2021, there was $6.0 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 2.6 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, 2020		202		$	10.51
Grants		223			22.28
Vested and released		(61	)		10.43
Forfeited		(61	)		14.15
Balance at September 30, 2021		303			18.44

As of September 30, 2021, there was $4.6 million of total unrecognized compensation cost related to unvested restricted stock units having a weighted average remaining contractual term of 3.0 years.

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 first offering period under the ESPP commenced on March 1, 2019. 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 three months ended September 30, 2021, under the ESPP, the Company received $0.2 million from the issuance of approximately 17,000 shares. During the nine months ended September 30, 2021, under the ESPP, the Company received $0.3 million from the issuance of approximately 27,000 shares.