The Company's intangible assets at December 31, 2018 include patents with estimated remaining economic useful lives ranging from 1.5 to 15 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, 2018 and 2017 are as follows:

Amortization expense for the years ended December 31, 2018 and 2017 was $290,000 and $206,000, respectively. Future amortization of current intangible assets, net is as follows:

The Company's Remote Power Patent expires in March 2020. The expiration date of the patent within the Company's Mirror Worlds Patent Portfolio is February 2020 (eight of the patents in the 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 and the expiration date of patents within the Company's QoS patents is June 2019.