(8) Intangibles

Intangible assets are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. No impairment charges were recorded for the years ended December 31, 2017, 2016 and 2015.

Amortization of capitalized patent costs associated with the application and award of patents in the U.S. and various other countries are capitalized and amortized on a straight-line basis over the term of the patents as determined at the award date, which varies depending on the pendency period of the application, generally approximating seventeen years.

Amortization of intangible assets acquired is calculated using the straight-line method over the estimated useful lives of the assets.

	Estimated Life	December 31,			December 31,
	(years)	2017			2016
Capitalized patent costs	17-20	$	7,978		$	7,281
Intangible assets acquired:
Purchased patents and intellectual property	3-10		250			250
Existing technology	5		1,560			1,560
Customer relationships	7		290			290
Backlog	2		760			760
Tradenames	3		290			290
Non-solicitation agreements	1		120			120
Gross intangible assets			11,248			10,551
Accumulated amortization			(4,867	)		(4,129	)
Intangibles, net		$	6,381		$	6,422

Amortization expense on intangible assets was as follows:

	Year Ended		Year Ended		Year Ended
	December 31,		December 31,		December 31,
	2017		2016		2015
Amortization expense	$	741	$	693	$	722

For intangible assets recorded at December 31, 2017, the estimated future aggregate amortization expense for the years ending December 31, 2018 through 2022 is approximately as follows:

	Amortization
Year ending December 31:	Expense
2018	$	450
2019		436
2020		378
2021		314
2022		300