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


				Weighted average
	Number of shares			Exercise price		Remaining contractual term (years)
Balance at December 31, 2020		1,760		$	10.07
Granted		431			23.35
Exercised		(144	)		11.45
Expired/Forfeited		(11	)		11.94
Balance at March 31, 2021		2,036			12.77		7.8

Vested and exercisable at March 31, 2021		1,028			9.35		6.8
Vested and expected to vest at March 31, 2021		2,036			12.77		7.8

The weighted average grant date fair value of options granted during the three months ended March 31, 2021 and for the year ended December 31, 2020, was $11.23 and $4.30, respectively. For fully vested stock options, the aggregate intrinsic value as of March 31, 2021 and December 31, 2020 was $12.2 million and $8.2 million, respectively. For stock options expected to vest, the aggregate intrinsic value as of March 31, 2021, and December 31, 2020, was $6.2 million and $5.3 million, respectively.

At March 31, 2021 and December 31, 2020, there was $7.0 million and $3.0 million, respectively, of total unrecognized compensation cost related to unvested stock options granted under the Company’s equity plans. That cost is expected to be recognized over the next 2.8 years.

The following table summarizes the Company's Restricted Stock Unit (RSU) activity during the period indicated (shares in thousands):


	Restricted stock units			Weighted average grant date fair value
Non-vested balance at December 31, 2020		202		$	10.51
Grants		158			24.23
Vested and released		(61	)		10.43
Non-vested balance at March 31, 2021		299			17.75

As of March 31, 2021 and December 31, 2020 there was $5.0 million and $1.5 million, respectively, of total unrecognized compensation cost related to unvested RSUs having a weighted average remaining contractual term of 3.2 years.

The Company maintains the 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.

Under the ESPP the Company received proceeds of $0.1 million from the issuances of approximately 10,000 shares in February 2021.

Stock based compensation expense was $0.9 million and $0.7 million for the three months ended March 31, 2021 and 2020, respectively. The stock-based compensation is recorded in the consolidated statements of operations as follows (in thousands):


	Three months ended March 31,
	2021		2020
Cost of goods sold	$	1	$	—
Research and Development		204		152
Sales and marketing		215		90
General and administrative		508		426
Total stock-based compensation expense	$	928	$	668