The Company s intangible assets at December 31, 2019 include patents with estimated remaining economic useful lives ranging from 0.25 to 14.75 years. For all periods presented, all the Company s patents were subject to amortization. The gross carrying amounts and accumulated amortization related to acquired intangible assets as of December 31, 2019 and 2018 were as follows:

Amortization expense for the years ended December 31, 2019 and 2018 was $285,000 and $290,000, respectively. Future amortization of current intangible assets, net is as follows:

The Company s Remote Power Patent expired on March 7, 2020. All patents within our Mirror Worlds Patent Portfolio and QoS Patents 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 2036.