| Intangible Assets, Net  Intangible assets consist of the following: 
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
|  | March 31, 2023 |  | December 31, 2022 |  
| (in thousands) | Gross Carrying Amount |  | Accumulated Amortization |  | Net Carrying Amount |  | Gross Carrying Amount |  | Accumulated Amortization |  | Net Carrying Amount |  
| Amortizable intangible assets: |  |  |  |  |  |  |  |  |  |  |  |  
| Patents | $ | 4,061 |  |  | $ | (665) |  |  | $ | 3,396 |  |  | $ | 3,792 |  |  | $ | (621) |  |  | $ | 3,170 |  |  
| License agreements | 1,101 |  |  | (1,041) |  |  | 60 |  |  | 1,101 |  |  | (1,014) |  |  | 87 |  |  
| Total amortizable intangible assets | 5,162 |  |  | (1,706) |  |  | 3,456 |  |  | 4,893 |  |  | (1,635) |  |  | 3,258 |  |  
|  |  |  |  |  |  |  |  |  |  |  |  |  
| Unamortized intangible assets: |  |  |  |  |  |  |  |  |  |  |  |  
| Trademarks | 402 |  |  | — |  |  | 402 |  |  | 391 |  |  | — |  |  | 391 |  |  
| Total intangible assets | $ | 5,564 |  |  | $ | (1,706) |  |  | $ | 3,858 |  |  | $ | 5,284 |  |  | $ | (1,635) |  |  | $ | 3,649 |  | Amortization expense is as follows: 
|  |  |  |  |  |  |  |  |  |  |  |  |  
|  | Three Months Ended March 31, |  
| (in thousands) | 2023 |  | 2022 |  
| Amortization expense | $ | 71 |  |  | $ | 69 |  | As of March 31, 2023, future amortization of patents and license agreements is as follows: 
|  |  |  |  |  |  |  
| Year Ending December 31, | (in thousands) |  
| 2023 (excluding the three months ended March 31, 2023) | $ | 189 |  |  
| 2024 | 195 |  |  
| 2025 | 195 |  |  
| 2026 | 194 |  |  
| 2027 | 190 |  |  
| Thereafter | 2,493 |  |  
| Total | $ | 3,456 |  | 
 License Agreements The Company has various license agreements that require the payment of royalty fees. Royalty fee expense included in sales and marketing expense is as follows: 
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  
|  | Three Months Ended March 31, |  |  |  
| (in thousands) | 2023 |  | 2022 |  |  |  |  |  
| Royalty fee expense | $ | 831 |  |  | $ | 673 |  |  |  |  |  |  |