We typically issue warrants to purchase shares of our common stock to investors as part of a financing transaction or in connection with services rendered by placement agents and consultants. A summary of warrant activity is as follows:

During the year ended December 31, 2020, the Company issued 963,149 shares of common stock in connection with the exercise of warrants for proceeds of $3.7 million.

During the year ended December 31, 2019, the Company issued 985,626 shares of common stock in connection with the exercise of warrants for proceeds of $3.4 million. In addition, during the year ended December 31, 2019, the Company issued 117,663 shares of common stock in connection with the cashless exercise of warrants to purchase 159,583 shares of common stock.

The total intrinsic value of warrants exercised during the years ended December 31, 2021, 2020 and 2019 was $0, $17.6 million and $12.4 million, respectively.

We have five stock incentive plans: the 2002 Stock Incentive Plan (the “2002 Plan”), the 2009 Stock Incentive Plan (the “2009 Plan”), the 2011 Stock Incentive Plan (the “2011 Plan”), the 2015 Omnibus Equity Incentive Plan (the “2015 Plan”), and the 2018 Omnibus Equity Incentive Plan (the “2018 Plan”) (collectively, the “Plans”). The 2002 Plan, the 2009 Plan, the 2011 Plan and the 2015 Plan (the “Prior Plans”) have been superseded by the 2018 Plan. In May 2018, the stockholders approved the 2018 Plan for issuances up to an aggregate of 3,730,179 shares plus 1,269,821 shares that were authorized but unissued under the Prior Plans as of the effective date of the 2018 Plan and in April 2021, the stockholders approved an increase of 2,850,000 shares authorized under the 2018 Plan. The Prior Plans will remain in effect until all awards granted under such Prior Plans have been exercised, forfeited, cancelled, or have otherwise expired or terminated in accordance with the terms of such awards, but no awards will be made pursuant to the Prior Plans after the effectiveness of the 2018 Plan. As of December 31, 2021, the Company had 7,012,279 shares available for future awards under the 2018 Plan.

During the years ended December 31, 2021, 2020 and 2019, we granted stock options at exercise prices equal to or greater than the quoted market price of our common stock on the grant date. The fair value of each option grant was estimated on the date of grant using Black-Scholes with the following assumptions:

The expected option life assumption is estimated based on the simplified method. Accordingly, the Company has utilized the average of the contractual term of the options and the weighted average vesting period for all options to calculate the expected option term. The risk-free interest rate assumption is based upon observed interest rates appropriate for the expected term of our employee stock options. The expected volatility is based on the average of the historical volatility and the implied volatility of our stock commensurate with the expected life of the stock-based award. We do not anticipate paying dividends on the common stock in the foreseeable future.

We recognize stock-based compensation cost on a straight-line basis over the vesting period. Stock-based compensation expense is recognized only for those awards that ultimately vest.

Total stock-based compensation expense related to our share-based payment awards is comprised of the following (in thousands):

For the year ended December 31, 2019, we recognized expense of $9.6 million due to the accelerated vesting under the terms of certain outstanding stock option grants as a result of the Company meeting certain financial performance criteria defined in such

grants. Of this amount, $0.4 million, $8.4 million, and $0.9 million are included in cost of revenues, selling, general and administrative and engineering and development, respectively.

(1)	Aggregate intrinsic value represents the difference between the exercise price of the option and the closing market price of the common stock on December 31, 2021, which was $59.17 per share.

The following table summarizes information with respect to stock options outstanding and exercisable at December 31, 2021:


		Weighted-
		Average	Weighted-			Weighted-
		Remaining	Average			Average
	Number	Contractual	Exercise		Number	Exercise
Exercise Price	Outstanding	Life (Years)	Price		Exercisable	Price
$1.87 – 3.07	705,996	4.1	$	2.30	705,996	$	2.30
$3.19 – 3.44	730,738	3.8	$	3.31	730,738	$	3.31
$4.80 – 4.92	587,200	3.4	$	4.80	587,200	$	4.80
$5.00 – 7.67	691,907	3.6	$	5.04	691,907	$	5.04
$7.80 – 8.65	765,615	5.2	$	8.34	765,615	$	8.34
$9.29 – 12.79	994,310	7.1	$	11.99	992,921	$	11.99
$13.37 – 16.95	1,424,960	8.1	$	16.52	680,786	$	16.30
$17.36 – 73.15	1,127,215	7.5	$	40.77	396,005	$	31.20
	7,027,941				5,551,168

As of December 31, 2021, there was unrecognized compensation expense of $26.9 million related to unvested stock options, which we expect to recognize over a weighted average period of 2.7 years.

The total intrinsic value of options exercised during the years ended December 31, 2021, 2020 and 2019 was $57.5 million, $24.1 million and $6.5 million, respectively.

Restricted stock units

A summary of our restricted stock unit activity is as follows:


		Weighted Average
	Number of Restricted	Fair Value per
	Stock Units	Share
Outstanding – December 31, 2020	—	$	—
Granted	392,940		55.51
Forfeited	(19,091)		55.23
Share issuance	—		—
Outstanding – December 31, 2021	373,849	$	55.53

For the year ended December 31, 2021, we recorded stock-based compensation expense on our issued restricted stock units of $4.2 million. There was no stock-based compensation expense on restricted stock units in 2020. As of December 31, 2021, there was unrecognized compensation expense of $16.6 million related to unvested restricted stock units, which we expect to recognize over a weighted average period of 3.2 years.