Note 12.  Stock Options, Warrants and Restricted Common Stock

During the year ended December 31, 2019, the Company’s shareholders approved its 2019 Equity Incentive Plan (the “2019 Plan”), which reserves a total of 3,600,000 shares of the Company’s common stock plus the remaining shares reserved under the Company’s 2017 Equity Incentive Plan. On December 5, 2019 the Company registered 3,930,603 shares of common stock under the 2019 Plan. The Company currently provides stock-based compensation to employees, directors and consultants, under the Company's 2019 Plan, as approved by the Company's shareholders and non-qualified options and warrants issued outside of the Plan. 

Stock-based Compensation

The Company’s stock-based compensation expenses recognized during the years ended December 31, 2019 and 2018, were attributable to selling, general and administrative expenses, which are included in the accompanying consolidated statements of operations.

The Company recognized total stock-based compensation expense during the years ended December 31, 2019 and 2018, from the following categories (in thousands):

Restricted common stock awards

A summary of the Company's restricted stock activity in the years ended December 31, 2019 and 2018 is as follows:

The value of restricted common stock grants are measured based on their fair market value on the date of grant and amortized over their respective vesting periods. As of December 31, 2019, there was approximately $1.8 million of unrecognized compensation cost related to unvested restricted common stock rights, which is expected to be recognized over a remaining weighted-average vesting period of approximately three months.

Stock Incentive Plan Options

The Company estimates the fair value of the share-based option awards on the date of grant using the Black-Scholes option-pricing model (the “Black-Scholes model”).  Using the Black-Scholes model, the value of the award that is ultimately expected to vest is recognized over the requisite service period in the statement of operations. The Company attributes compensation to expense using the straight-line single option method for all options granted. 

The Company's determination of the estimated fair value of share-based payment awards on the date of grant under the Plan is affected by the following variables and assumptions:

The Company currently provides stock-based compensation to employees, directors and consultants under the Plan.  There were no stock options issued during the year ended December 31, 2019. The Company utilized assumptions in the estimation of fair value of stock-based compensation for the year ended December 31, 2018, as follows: 

A summary of stock option activity under the Plan for options to employees, officers, directors and consultants, for the years ended December 31, 2019 and 2018, is presented below:

The aggregate intrinsic value in the table above represents the total intrinsic value (the difference between the Company's closing stock price on December 31, 2019 and 2018, and the exercise price, multiplied by the number of in-the-money options) that would have been received by the option holders, had all option holders been able to, and in fact had, exercised their options on December 31, 2019 and 2018, respectively.

As of December 31, 2018, total stock-based compensation expense related to unvested options not yet recognized totaled approximately $58,000, which was fully amortized in the first quarter of 2019.

Other common stock purchase warrants

As of December 31, 2019, the Company had outstanding 3,574,257 warrants issued in connection with offerings.  The following is a summary of the change in outstanding warrants during the years ended December 31, 2019 and 2018:

The Company issued Warrants to purchase 1,908,144 shares of its common stock with an exercise price of $1.94, in connection with the Notes issued on January 28, 2019. During the year ended December 31, 2018, 13,009 of the warrants issued in the May 2013 private offering were exercised on a cashless basis for the issuance of 3,215 shares of common stock, 100,000 warrants issued in March 2018, were exercised for cash proceeds of approximately $0.4 million and 160,773 warrants were forfeited.

The aggregate intrinsic value in the table above represents the total intrinsic value (the difference between the Company's closing stock price on December 31, 2019 and the exercise price, multiplied by the number of in-the-money options) that would have been received by the option holders, had all option holders been able to, and in fact had, exercised their options on December 31, 2019.