Note 9 — Equity Incentive Plan

In May 2013, the Company adopted the 2013 Equity Incentive Plan (as amended and restated, the “Plan”) and reserved shares of common stock for issuance under the Plan, which was amended in June 2020. As a result of the amendment, the number of shares authorized for issuance under the Plan increased by 350,000 shares and the Plan will now terminate in June 2030, unless sooner terminated or extended by the Board. The Plan is administered by the Compensation Committee of the Board.At December 31, 2020, there were 148,961 shares of common stock available for issuance under the Plan.

During the year ended December 31, 2020, the Company granted 52,791 stock options to Board members, 168,400 stock options to executives and 5,500 stock options to employees under the Plan. The estimated fair value of these stock options, calculated using the Black-Scholes option valuation model, was $758,214, of which $754,369 was recognized during the year ended December 31, 2020.

In April 2020, the Board approved a modification of a stock option grant to Dr. Lon E. Bell in connection with his retirement as Chief Executive Officer and President. The modification accelerated the vesting of Dr. Bell’s October 2019 stock option grant with full vesting effective immediately prior to the end of Dr. Bell’s term on the Board in June 2020. During the year ended December 31, 2020, the Company recognized $79,444 of expense related to this grant subsequent to the modification.

During the year ended December 31, 2019, the Company granted 23,400 stock options to Board members, 94,000 stock options to executives and 1,000 stock options to an employee under the Plan. The estimated fair value of these stock options, calculated using the Black-Scholes option valuation model, was $253,074, of which $76,808 was recognized during the year ended December 31, 2019.

As permitted by SAB 107, management utilizes the simplified approach to estimate the expected term of stock options, which represents the period of time that options granted are expected to be outstanding. The risk-free interest rate for periods within the contractual life of the option is based on the U.S. treasury yield in effect at the time of grant. The volatility is estimated based on the historical volatilities of comparable companies. The Company has never declared or paid dividends and has no plans to do so in the foreseeable future.

The assumptions used in the Black-Scholes model are as follows:

A summary of the Company’s stock option activity and related information is as follows:

The following table sets forth additional information about stock options outstanding at December 31, 2020:

Stock options granted under the Plan have ten-year terms and generally vest immediately or annually over a three-year or four-year vesting period except for option grants to independent directors that generally vest quarterly over a one-year vesting period.

The estimated aggregate pretax intrinsic value (the difference between the Company’s stock price on the last day of the year ended December 31, 2020 and the exercise prices, multiplied by the number of vested in-the-money options) is $1.2 million. This amount changes based on the fair value of the Company’s stock.

As of December 31, 2020, there was $47,372 of unrecognized compensation cost related to non-vested share-based compensation arrangements. That cost is expected to be recognized over a weighted average period of 1 year.