19. STOCK BASED COMPENSATION

Stock Incentive Plan—In 2015, the Board of Directors approved the adoption of the 2015 stock incentive plan (the “2015 Plan”). The 2015 Plan allowed for the award of incentive and nonqualified stock options, restricted stock, and other stock-based awards to employees, officers, directors, consultants, and advisers of the Company. Awards could be made under the 2015 Plan for up to 26,283,789 shares of Common Stock. Option awards expire 10 years from the grant date and generally vest over four years; however, vesting conditions can vary at the discretion of our Board of Directors.

As part of the acquisition of Make Composites, Inc. (“Make”) in 2019, the Company assumed the 2018 equity incentive plan of Make (the “Make Plan”). The Make Plan allows for the award of incentive and nonqualified stock options and warrants for those employees and contractors that were hired as part of the acquisition. The Make Plan allowed for 232,304 options and warrants to be issued, which were issued in 2019, with no additional options to be issued in the future. Option awards expire 10 years from the grant date and generally vest over four years; however, vesting conditions can vary at the discretion of our Board of Directors.

In December 2020, the Board of Directors and stockholders of the Company approved the adoption of the 2020 Incentive Award Plan (the “2020 Plan” and together with the 2015 Plan and the Make Plan, the “Plans”), which became effective on the date of the Business Combination. Upon effectiveness of the 2020 Plan, the Company ceased granting new awards under the 2015 Plan.

The 2020 Plan allows for the award of incentive and nonqualified stock options, restricted stock, and other stock-based awards to employees, officers, directors, consultants, and advisers of the Company. The number of shares of common stock initially available for issuance under the 2020 Plan was 12,400,813 shares of common stock plus the number of shares subject to awards outstanding under the 2015 Plan that expire, lapse, terminate, or are exchanged for cash, surrendered, repurchased, or canceled without having been fully exercised or forfeited. In addition, the number of shares of common stock available for issuance under the 2020 Plan is subject to an annual increase on the first day of each calendar year beginning on January 1, 2021 and ending on and including January 1, 2030 equal to the lesser of (i) 5% of the aggregate number of shares of common stock outstanding on the final day of the immediately preceding calendar year and (ii) such smaller number of shares as is determined by the Board. On January 1, 2021, 11,337,837 shares were added to the plan.

The Company grants stock options at exercise prices deemed by the Board of Directors to be equal to the fair value of the Common Stock at the time of grant. The fair value of Common Stock has been determined by the Board of Directors of the Company at each stock option measurement date based on a variety of different factors, including the results obtained from independent third-party appraisals, the Company’s consolidated financial position and historical financial performance, the status of technological development within the Company, the composition and ability of the current engineering and management team, an evaluation and benchmark of the Company’s competition, the current climate in the marketplace, the illiquid nature of the Common Stock, arm’s-length sales of the Company’s capital stock, and the prospects of a liquidity event, among others.

During the three and nine months ended September 30, 2021, the Company did not grant any options to purchase shares of Common Stock to employees. During the three and nine months ended September 30, 2020, the Company granted options to purchase 2,269,131 and 6,925,144 shares of Common Stock to employees with fair values of $26.1 million and $29.8 million, respectively, calculated using the Black-Scholes option-pricing model with the following assumptions:

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During the three and nine months ended September 30, 2021, the Company did not grant any options to purchase shares of Common Stock to non-employees. During the three months ended September 30, 2020, the Company did not grant any options to purchase shares of Common stock to non-employees. During the nine months ended September 30, 2020, the Company granted options to purchase 12,212 shares of Common Stock to non-employees with a fair value of $0.1 million, calculated using the Black-Scholes option-pricing model with the following assumptions:

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The risk-free interest rate assumption is based upon observed interest rates appropriate for the term of the related stock options. The expected life of stock options was calculated using the average of the contractual term of the option and the weighted-average vesting period of the option, as the Company does not have sufficient history to use an alternative method to the simplified method to calculate an expected life for employees. The Company has not paid a dividend and is not expected to pay a dividend in the foreseeable future. Expected volatility for the Common Stock was determined based on an average of the historical volatility of a peer group of similar public companies.

At September 30, 2021, the total unrecognized stock-based compensation expense related to unvested stock options aggregated $8.8 million. The costs are expected to be recognized over a weighted-average period of 2.7 years.

There were 17,933,802 shares available for award under the 2020 Plan at September 30, 2021. The option activity of the Plans for the nine months ended September 30, 2021, is as follows (shares in thousands):

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The weighted-average grant-date fair value for options granted during the nine months ended September 30, 2020 was $0.98. The aggregate intrinsic value of options exercised during the nine months ended September 30, 2021 and 2020, was $49.6 million and $1.7 million, respectively.

Restricted Stock Units—RSUs awarded to employees and non-employees generally vest over four years from the anniversary date of the grant, with 1-year cliff vesting and monthly vesting thereafter, provided service with the Company is not terminated. The fair value of RSUs is equal to the fair market value of the Company’s Common Stock on the date of grant. Total unrecognized compensation costs related to unvested RSUs at September 30, 2021 was approximately $90.1 million and is expected to be recognized over a period of 3.5 years. The total expense recognized during the three and nine months ended September 30, 2021 was $6.7 million and $10.1 million, respectively.

RSU activity under the 2020 Plan for the nine months ended September 30, 2021 is as follows (shares in thousands):

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Restricted Stock Awards—In connection with acquisitions, the Company has issued shares of restricted stock that are considered post-combination expense and accounted for as stock-based compensation as the shares vest.

The activity for stock subject to vesting as of September 30, 2021 is as follows (shares in thousands):

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At September 30, 2021, the remaining weighted-average vesting period for the stock subject to vesting was 1.8 years.

Total stock-based compensation expense related to all of the Company’s stock-based awards granted is reported in the condensed consolidated statements of operations as follows (in thousands):

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