6. Intangible Assets

The gross carrying amount and accumulated amortization of the Company’s acquired identifiable intangible assets as of September 30, 2019 were as follows (in thousands):


	Weighted Average	Gross Carrying		Accumulated
	Amortization Period	Amount		Amortization		Total
Acquired technology	10 years	$	7,587	$	(2,591)	$	4,996
Customer relationships	10 years		260		(143)		117
Trademark	5 years		60		(60)		—
		$	7,907	$	(2,794)	$	5,113

The gross carrying amount and accumulated amortization of the Company’s acquired identifiable intangible assets as of December 31, 2018 were as follows (in thousands):


	Weighted Average	Gross Carrying		Accumulated
	Amortization Period	Amount		Amortization		Total
Acquired technology	9 years	$	5,926	$	(2,176)	$	3,750
Customer relationships	10 years		260		(123)		137
Trademark	5 years		60		(57)		3
		$	6,246	$	(2,356)	$	3,890

The change in the gross carrying amount of the acquired technology from December 31, 2018 to September 30, 2019 was due to the acquisition of intellectual property from EnergyOr in May 2019, a milestone payment to American Fuel Cell LLC (AFC), as well as changes attributed to foreign currency translation. The Company acquired intellectual property from EnergyOr for $1.5 million. In addition, the Company agreed to pay the sellers a royalty based on future sales of relevant applications, not to exceed $3.0 million, by May 22, 2025.

As part of the agreement to acquire the intellectual property from AFC, the Company shall pay AFC milestone payments not to exceed $2.9 million in total, if certain milestones associated with the production of components related to the acquired technology are met before April 2021. As of September 30, 2019, the Company paid $0.4 million to AFC in relation to the aforementioned milestones.

Amortization expense for acquired identifiable intangible assets was $0.2 million for both the three months ended September 30, 2019 and 2018. Amortization expense for the acquired identifiable assets was $0.5 million for both the nine months ended September 30, 2019 and 2018. Estimated amortization expense for subsequent years was as follows (in thousands):


Remainder of 2019	$	181
2020		724
2021		724
2022		724
2023		724
2024 and thereafter		2,036
Total	$	5,113