6. Intangible Assets

The gross carrying amount and accumulated amortization of the Company’s acquired identifiable intangible assets as of March 31, 2019 are as follows (in thousands):


	Weighted Average	Gross Carrying		Accumulated
	Amortization Period	Amount		Amortization		Total
Acquired technology	9 years	$	5,861	$	(2,314)	$	3,547
Customer relationships	10 years		260		(130)		130
Trademark	5 years		60		(60)		—
		$	6,181	$	(2,504)	$	3,677

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 and accumulated amortization of the acquired technology from December 31, 2018 to March 31, 2019 is due to changes attributed to foreign currency translation.

As part of the agreement to acquire the intellectual property from American Fuel Cell, the Company shall pay American Fuel Cell LLC 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 March 31, 2019 these milestones have not yet been reached.

Amortization expense for acquired identifiable intangible assets was $0.2 million for both the three months ended March 31, 2019 and 2018. Estimated amortization expense for subsequent years is as follows (in thousands):


Remainder of 2019	$	410
2020		546
2021		546
2022		546
2023		546
2024 and thereafter		1,083
Total	$	3,677