The Company’s intangible assets at September 30, 2020 include patents with estimated remaining economic useful lives ranging from 1.00 to 13.00 years. For all periods presented, all of the Company’s patents were subject to amortization. The gross carrying amounts and accumulated amortization related to acquired intangible assets as of September 30, 2020 and December 31, 2019 were as follows:

Amortization expense for the three months ended September 30, 2020 and September 30, 2019 was $72,000 and $71,000, respectively. Amortization expense for the nine months ended September 30, 2020 and September 30, 2019, was $216,000 and $212,000, respectively. Future amortization of intangible assets, net is as follows:

The Company’s Remote Power Patent expired on March 7, 2020. All of the patents within the Company’s Mirror Worlds Patent Portfolio have expired. The expiration dates of the patents within the Cox Patent Portfolio range from September 2021 to November 2023. The expiration dates of patents within the Company’s M2M/IoT Patent Portfolio range from September 2033 to May 2034.