The Company's intangible assets at March 31, 2019 include patents with estimated remaining economic useful lives ranging from 1.25 to 14.75 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 March 31, 2019 and December 31, 2018 were as follows:

Amortization expense for the three months ended March 31, 2019 and March 31, 2018 was $54,000 and $70,000, respectively.  Future amortization of 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 nine 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 the QoS Patents is June 2019.