The Company's intangible assets at December 31, 2017 include patents with estimated remaining economic useful lives ranging from 2.5 to 16 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 December 31, 2017 and 2016 are as follows:

Amortization expense for the years ended December 31, 2017 and 2016 was $206,000 and $813,000, respectively.  Future amortization of current intangible assets, net is as follows:

The Company's Remote Power Patent expires in March 2020. The expiration dates of the patents within the Company's Mirror Worlds Patent Portfolio range from April 2018 to February 2020 (eight of the patents in the Mirror Worlds Patent Portfolio expired as of December 31, 2017).  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 January 2034 to May 2034 and the expiration date of patents within the Company's QoS patents is June 2019.