6. Intangible Assets

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


	Weighted Average	Gross Carrying		Accumulated
	Amortization Period	Amount		Amortization		Total
Acquired technology	10 years	$	7,757	$	(2,483)	$	5,274
Customer relationships	10 years		260		(136)		124
Trademark	5 years		60		(60)		—
		$	8,077	$	(2,679)	$	5,398

The gross carrying amount and accumulated amortization of the Company’s acquired identifiable intangible assets as of December 31, 2018 are 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 June 30, 2019 is 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 June 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 June 30, 2018 and 2017, respectively. Amortization expense for the acquired identifiable assets was $0.3 million for both the six months ended June 30, 2019 and 2018. Estimated amortization expense for subsequent years is as follows (in thousands):


Remainder of 2019	$	370
2020		741
2021		741
2022		741
2023		741
2024 and thereafter		2,064
Total	$	5,398