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INTANGIBLE ASSETS - Note 9
12 Months Ended
Dec. 31, 2020
Notes To Financial Statements Abstract  
Intangible assets - Note 9

9. INTANGIBLE ASSETS

Our intangible assets consist exclusively of technology-based purchased patents. The gross book value of our intangible assets was $951,000 in the years ended December 31, 2020 and 2019, respectively. Amortization expense was $57,000 in 2020 and $105,000 in 2019. In 2019, we recorded an impairment amounting to $160,000 on 52 patents that we elected not to renew, and one patent abandoned in prosecution. The following table outlines our estimated future amortization expense related to intangible assets held at December 31, 2020 (in thousands):

Years Ended December 31,     Amount
2021   $ 49 
2022     40 
2023     32 
2024     22 
2025     14 
Thereafter    
    $ 164