The Company's intangible assets at June 30, 2016 include patents with estimated remaining economic useful lives ranging from 4 to 5.25 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 June 30, 2016 and December 31, 2015 are as follows:

Amortization expense for the three months ended June 30, 2016 and June 30, 2015 was $297,000 and $413,000, respectively.  Amortization expense for the six months ended June 30, 2016 and June 30, 2015 was $711,000 and $826,000.  Future amortization of current intangible assets 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 September 2016 to February 2020. The expiration dates of the patents within the Cox Patent Portfolio range from September 2021 to November 2023 and the expiration date of the QoS Patents is June 2019.