The Company maintains a stock-based compensation plan, the 2011 Incentive Compensation Plan, as amended, (the “2011 Plan”), to attract, motivate, retain and reward employees, directors and consultants by providing its Board or a committee of the Board the discretion to award equity incentives to these persons.

The following is a summary of stock option activity for the six months ended June 30, 2021:

	Number Outstanding			Average Exercise Price per Share		Weighted Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value
Balance at December 31, 2020		550,769		$	5.56		4.88	$	1,875,719
Granted		—			—
Cancelled or Expired		(792	)		20.21
Exercised		(16,651	)		8.19
Balance at June 30, 2021		533,326		$	5.46		4.49	$	6,182,448
Vested or expected to vest at June 30, 2021		533,326		$	5.46		4.49	$	6,182,448
Exercisable at June 30, 2021		533,326		$	5.46		4.49	$	6,182,448

The aggregate intrinsic value in the table above represents the difference between the fair value of the Company’s common stock and the stock option exercise price of in-the-money stock options multiplied by the number of such stock options.

The following table summarizes information about stock options outstanding as of June 30, 2021:

	Stock Options Outstanding						Stock Options Exercisable
Range of Exercise Prices	Number Outstanding		Weighted Average Remaining Contractual Life (Years)		Weighted Average Exercise Price		Number Exercisable		Weighted Average Exercise Price
$4.36 - $7.20		454,010		4.87	$	4.41		454,010	$	4.41
$7.50 - $11.25		61,817		2.73		10.17		61,817		10.17
$11.30 - $16.95		11,302		1.23		13.03		11,302		13.03
$17.60 - $26.40		6,197		0.27		21.38		6,197		21.38
$4.36 - $26.40		533,326		4.49	$	5.46		533,326	$	5.46

As of June 30, 2021, there was no unrecognized stock-based compensation expense related to stock options.

The following is a summary of RSU activity for the six months ended June 30, 2021:

	Number Outstanding			Weighted Average Fair Value
Unvested at December 31, 2020		682,563		$	4.34
Granted		136,356			12.20
Vested		(257,440	)		5.33
Forfeited		(42,973	)		4.32
Unvested at June 30, 2021		518,506		$	5.92
RSUs vested but not released		286,211		$	4.94

The fair value of the Company’s RSUs is calculated based upon the fair market value of the Company’s stock on the date of grant. As of June 30, 2021, there was $2.6 million of unrecognized compensation expense related to unvested RSUs granted, which is expected to be recognized over a weighted average period of 2.4 years.

The Company granted 200,000 PSUs to a certain key employee during the year ended December 31, 2020, with a grant date fair value of $6.38 per share. The PSUs are subject to the attainment of performance goals established by the Company’s Compensation Committee, the periods during which performance is to be measured, and other limitations and conditions. Performance goals are based on pre-established objectives that specify the manner of determining the number of PSUs that will vest if performance goals are attained. If the employee terminates employment, the non-vested portion of the PSUs will not vest and all rights to the non-vested portion will terminate.

The following is a summary of PSU activity for the six months ended June 30, 2021:

	Number Outstanding
Unvested at December 31, 2020		200,000
Granted		—
Vested		(23,500	)
Forfeited		—
Unvested at June 30, 2021		176,500

As of June 30, 2021, there was $1.1 million of unrecognized compensation cost related to unvested PSUs, which is expected to be recognized over a period of 1.5 years. No tax benefit was realized from PSUs for the six months ended June 30, 2021.

The following table summarizes stock-based compensation expense related to stock options, RSUs, and PSUs included in the condensed consolidated statements of comprehensive income (loss) for the three and six months ended June 30, 2021 and 2020 (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Cost of revenue	$	48	$	41	$	81	$	81
Research and development		93		173		255		347
Selling and marketing		124		118		227		244
General and administrative		432		419		892		719
Total	$	697	$	751	$	1,455	$	1,391

During the six months ended June 30, 2021 and 2020, the Company repurchased 48,590 and 94,134 shares, respectively, of common stock surrendered to the Company to satisfy tax withholding obligations in connection with the vesting of RSUs issued to employees.