Note 5. Patents

The majority of our patents were purchased from another commercial corporation, but we have also developed our own patents. We capitalize patent defense and application costs. Patent costs are amortized on a straight-line basis over their estimated useful lives, which range from 13 to 16 years. At June 30, 2021 and December 31, 2020, the gross carrying value of patents subject to amortization was approximately $7.8 million. Accumulated amortization was approximately $6.7 million and $6.5 million at June 30, 2021 and December 31, 2020, respectively. Amortization expense was $0.1 million and $0.2 million for the three and six months ended June 30, 2021 and 2020 and is included in research and development costs. At June 30, 2021 and December 31, 2020, capitalized patent acquisition and defense costs not yet subject to amortization were $0.7 million and $0.5 million, respectively.