6. Intangible Assets

The gross carrying amount and accumulated amortization of the Company’s acquired identifiable intangible assets as of September 30, 2018 are as follows (in thousands):


	Weighted Average	Gross Carrying		Accumulated
	Amortization Period	Amount		Amortization		Total
Acquired technology	9 years	$	5,941		(2,032)		3,909
Customer relationships	10 years		260		(117)		143
Trademark	5 years		60		(54)		6
		$	6,261	$	(2,203)	$	4,058

The gross carrying amount and accumulated amortization of the Company’s acquired identifiable intangible assets as of December 31, 2017 are as follows (in thousands):


	Weighted Average	Gross Carrying		Accumulated
	Amortization Period	Amount		Amortization		Total
Acquired technology	9 years	$	5,200	$	(1,593)	$	3,607
Customer relationships	10 years		260		(97)		163
Trademark	5 years		60		(45)		15
		$	5,520	$	(1,735)	$	3,785

The change in the gross carrying amount and accumulated amortization of the acquired technology from December 31, 2017 to September 30, 2018 is due to the acquisition of intellectual property from American Fuel Cell LLC in April 2018, as well as changes attributed to foreign currency translation. As part of the agreement to acquire the intellectual property, the Company shall pay American Fuel Cell LLC milestone payments not to exceed $2.9 million in total, if certain milestones are met by April 2021.

Amortization expense for acquired identifiable intangible assets was $0.2 million and $0.1 million for the three months ended September 30, 2018 and 2017, respectively. Amortization expense for acquired identifiable intangible assets was $0.5 million for each of the nine month periods ended September 2018 and 2017. Estimated amortization expense for subsequent years is as follows (in thousands):


Remainder of 2018	$	175
2019		590
2020		554
2021		554
2022		554
2023 and thereafter		1,631
Total	$	4,058