Note 5. Patents

The majority of the patents were purchased from another commercial corporation, but we also capitalize patent defense and application costs. Patent costs are amortized on a straight-line basis over the estimated remaining useful lives at acquisition of the applicable patents which range from 13 to 16 years. At March 31, 2021 and December 31, 2020, the gross carrying value of patents subject to amortization was approximately $7.8 million and $7.8 million, respectively. Accumulated amortization was approximately $6.6 million and $6.5 million at March 31, 2021 and December 31, 2020, respectively. Amortization expense was $0.1 million for each of the quarters ended March 31, 2021 and 2020 and is included in research and development costs in the Condensed Consolidated Statements of Operations. At March 31, 2021 and December 31, 2020, capitalized patent acquisition and defense costs not yet subject to amortization were $0.6 million and $0.5 million, respectively.