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Stock-based Compensation Expense (Tables)
6 Months Ended
Jun. 30, 2024
Stock-based Compensation Expense [Abstract]  
Stock-Based Compensation Expense

The following table summarizes stock-based compensation expense for the three and six months ended June 30, 2024 and 2023 (in thousands):

 
Three Months Ended
June 30,
   
Six Months Ended
June 30,
 
 
2024
   
2023
   
2024
   
2023
 
Cost of sales
 
$
246
   
$
118
   
$
450
   
$
237
 
Research, development and patent expense
   
22,766
     
19,249
     
44,991
     
38,816
 
Selling, general and administrative expense
   
7,716
     
7,194
     
16,627
     
14,457
 
Total stock-based compensation expense
 
$
30,728
   
$
26,561
   
$
62,068
   
$
53,510
 
Weighted-Average Assumptions for Stock Options

Employee Stock Options:
 
Six Months Ended
June 30,
 
   
2024
   
2023
 
Risk-free interest rate
   
4.1
%
   
3.6
%
Dividend yield
   
0.0
%
   
0.0
%
Volatility
   
43.9
%
   
47.3
%
Expected life
 
6.3 years
   
6.3 years
 
Weighted-Average Assumptions for ESPP

ESPP:
 
Six Months Ended
June 30,
 
   
2024
   
2023
 
Risk-free interest rate
   
5.3
%
   
5.2
%
Dividend yield
   
0.0
%
   
0.0
%
Volatility
   
38.4
%
   
36.7
%
Expected life
 
6 months
   
6 months