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Stock-Based Compensation (Tables)
12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Stock-based compensation recognized
Stock-based compensation has been recognized in operating results as follows:
(in thousands)Years ended December 31,
 20212020
Research and development expenses491 355 
Selling, general and administrative expenses2,743 1,410 
Total stock-based compensation$3,234 $1,765 
Estimated Fair Value of Stock Options - Assumptions
The estimated fair value of stock option awards was determined on the date of grant using the Black-Scholes option valuation model with the following assumptions during the years indicated below:
 Years ended December 31,
 20212020
Risk-free interest rate
0.83% - 1.3%
0.39% - 0.93%
Dividend yield0%0%
Expected volatility (range)
107% - 110%
102% - 106%
Expected volatility (weighted-average)108%105%
Expected term (in years)6.0 years5.9 years
Summary of Stock Option Activity
A summary of stock option activity and of changes in stock options outstanding is presented below:
Number of OptionsWeighted-Average Exercise Price Per ShareIntrinsic
Value
Weighted-Average
Remaining
Contractual Life
Balance outstanding, December 31, 20201,860,507 $7.43 $27,963,363 8.9 years
Granted1,926,466 $7.27  
Forfeited(10,770)$2.55  
Expired(4,219)$216.00 
Balance outstanding, December 31, 20213,771,984 $7.13 $6,405,258 8.7 years
Vested and exercisable, December 31, 20211,281,604 $8.96 $4,303,588 7.6 years
Vested and expected to vest, December 31, 20213,671,664 $7.18 $6,253,298 8.7 years
Summary of Restricted Stock Unit Activity
A summary of the RSU's activity is presented below:
Number of SharesWeighted Average
Grant Date Fair Value
Per Share
Intrinsic
Value
Non-vested RSU's outstanding, December 31, 201911,301 $15.38 $14,013 
Vested(10,810)$9.37 
Non-vested RSU's outstanding, December 31, 2020491 $147.60 $7,641 
Vested(491)$147.60 
Non-vested RSU's outstanding, December 31, 2021— $— $—