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Stock Based Compensation
 12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Stock Based Compensation Stock Based Compensation
Our 2017 Stock Incentive Plan (the “2017 Plan”) and 2010 Stock Incentive Plan (the “2010 Plan”) provides for the issuance of 5,082,955 shares of our common stock. We have granted stock options to executive officers, employees, members of our board of directors, and certain outside consultants, and restricted stock units ("RSUs") to employees. The terms and conditions upon which options become exercisable vary among grants; however, option rights expire no later than ten years from the date of grant and employee and Board of Director awards generally vest over three to five years on a straight-line basis. The terms and conditions upon which RSUs vest vary among grants; however, RSUs generally vest over four to five years on a straight-line basis. As of December 31, 2021, we had 3,730,019 stock options and RSUs outstanding. We had no shares available for future awards as of December 31, 2021; however, 226,779 evergreen shares were registered on January 3, 2022 and reserved for future awards under the 2017 Plan.
Stock-based compensation expense was approximately $11.9 million and $8.1 million for the years ended December 31, 2021 and 2020, respectively. The Company issued $0.4 million of common stock to one of the Company's executives during the quarter ended March 31, 2020 and it is included in stock-based compensation expense for the year ended December 31, 2020.
The assumptions used in the Black-Scholes option-pricing model were determined as follows:
Year Ended December 31,
20212020
Volatility79.00% - 88.00%78.00 %
Risk-free interest rate0.19% - 1.24%0.00% - 1.55%
Expected life (in years)2.75 - 6.082.63 - 6.08
Dividend yield0 %0 %
The expected volatility assumptions have been based on the historical and expected volatility of our stock, measured over a period generally commensurate with the expected term. The weighted average expected option term for the year ended December 31, 2021, reflects the application of the simplified method prescribed in SEC's Staff Accounting Bulletin (“SAB”) No.
107 (as amended by SAB 110), which defines the life as the average of the contractual term of the options and the weighted average vesting period for all option tranches.
Stock Options – Employees and Directors
A summary of stock option activity for employee and director grants was as follows:
Number of
Shares 		Weighted-
Average
Exercise Price
Outstanding at December 31, 20203,616,314 $14.66 
Granted1,081,478 18.27 
Exercised(395,181)14.13 
Forfeited(684,195)13.91 
Expired(271)48.00 
Outstanding at December 31, 20213,618,145 13.55 
Options vested and exercisable at December 31, 20211,212,049 $13.79 
As of December 31, 2021, there was $18.7 million of unrecognized compensation costs related to non-vested share-based compensation arrangements granted to employees and directors under the Plan. These costs are expected to be recognized over a weighted-average period of 2.08 years.
Restricted Stock Units - Employees
We estimate the fair value of RSUs based on the closing price of our common stock on the date of grant. The following table summarizes our RSU award activity during the year ended December 31, 2021 issued under the 2017 Plan:

Restricted Stock UnitsWeighted-
Average
Grant Date Fair Value
Non-vested at December 31, 202030,000 $51.98 
Granted119,000 28.64 
Vested(8,126)51.98 
Forfeited(29,000)28.39 
Non-vested at December 31, 2021111,874 33.27 


As of December 31, 2021, there was $3.4 million of unrecognized compensation cost related to unvested outstanding RSUs. These costs are expected to be recognized over a weighted average period of 3.70 years.
Stock Options and Warrants – Non-employees
The Company did not issue any stock options to consultants during the years ended December 31, 2021 and 2020. Stock compensation expense related to consultants was $0.03 million for each of the years ended December 31, 2021 and 2020. As of December 31, 2021, there was $0.02 million of total unrecognized compensation cost related to non-vested stock compensation arrangements granted to non-employees under the 2017 Amended Plan, which is expected to be recognized over a weighted-average period of approximately 0.17 years.
In addition to stock options granted under the Plan, we have also granted warrants to purchase common stock to certain non-employees that have been approved by our Board of Directors. A summary of warrants activity for non-employees was as follows:
Number of WarrantsWeighted Average
Exercise Price
Outstanding as of December 31, 20201,465,927 $6.02 
Issued (1)1,181 16.75 
Exercised (2)
(1,402,866)5.77 
Expired(28,410)4.80 
Outstanding as of December 31, 202135,832 16.75 
Warrants exercisable as of December 31, 202135,832 16.75 
_______________________
(1) Represents shares increased per warrant's anti-dilution requirement as ATM common shares were issued as discussed in Note 9 above.
(2) Included in total number of warrants exercised are 218,225 shares that were net settled at the election of the warrant holders during the year ended December 31, 2021.
There were no new warrants issued to non-employees during the year ended December 31, 2021 and 2020, except the anti-dilutive shares issued as described in the table above.
Performance-Based and Market-Based Awards
The Company’s Compensation Committee designed a compensation structure to align the compensation levels of certain executives to the performance of the Company through the issuance of performance-based and market-based stock options. The performance-based options vest upon the Company meeting certain revenue targets and the total amount of compensation expense recognized is based on the number of shares that the Company determines are probable of vesting. The market-based options vest upon the Company’s stock price reaching a certain price at a specific performance period and the total amount of compensation expense recognized is based on a Monte Carlo simulation that factors in the probability of the award vesting. The following table summarizes the Company's outstanding awards under this structure:

Grant DatePerformance MeasuresVesting TermPerformance Period# of SharesExercise Price
December 2017
Weighted Average Price of our common stock is $15.00 for at least twenty trading days within a period of thirty consecutive trading days ending on the trading day prior to January 1, 2023.
Fully vest on January 1, 2023January 1, 2023642,307 $7.50 
August 2018
Weighted Average Price of our common stock is $15.00 for at least twenty trading days within a period of thirty consecutive trading days ending on the trading day prior to January 1, 2023.
Fully vest on January 1, 2023January 1, 2023397,693 $7.50 
During the quarter ended March 31, 2020, the Company amended the option agreement of one of the Company's former executives to vest additional options previously forfeited and extend the period to exercise, resulting in $0.6 million of additional stock-based compensation expense, which is included in stock-based compensation expense for the year ended December 31, 2020.