4.	Balance Sheet Components

Property and Equipment

Property and equipment consisted of the following (in thousands):

	December 31,
	2017			2016
Prototype	$	11,929		$	6,405
Machine and equipment		7,831			6,057
Leasehold improvements		4,438			4,371
Furniture and fixtures		558			368
Software		1,142			1,028
Construction in progress		-			5,498
Property and equipment, gross		25,898			23,727
Less: accumulated depreciation and amortization		(14,334	)		(12,167	)
Property and equipment, net	$	11,564		$	11,560

Depreciation and amortization expense related to property and equipment was $2.2 million, $1.6 million and $1.1 million during the years ended December 31, 2017, 2016 and 2015, respectively.

Intangible Assets

Intangible assets consisted of the following (in thousands):

	December 31,
	2017			2016
License cost	$	512		$	512
Patents		104			104
Intangible assets, gross		616			616
Accumulated amortization		(538	)		(519	)
Intangible assets, net	$	78		$	97

Intangible amortization expense was $19 thousand, $115 thousand and $168 thousand during the years ended December 31, 2017, 2016 and 2015, respectively, which were recorded in general and administrative expenses in the consolidated statements of operations and comprehensive loss.

At December 31, 2017, the estimated future amortization expense of purchased intangible assets was as follows (in thousands):

Year Ended December 31,	Estimated Future Amortization Expense
2018	$	19
2019		19
2020		19
2021		10
2022		3
Thereafter		8
Total amortization expense	$	78

Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	December 31,
	2017		2016
Accrued payroll and related benefits	$	3,944	$	4,274
Accrued accounts payable		2,671		1,202
Tax payable		149		13
Accrued legal and accounting		322		509
Other		121		336
Total accrued liabilities	$	7,207	$	6,334

Deferred Revenue

Deferred revenue consisted of the following (in thousands):

	December 31,
	2017			2016
Deferred revenue:
Product	$	18,861		$	5,050
Services		1,182			1,561
Distribution rights		3,346			3,822
Total deferred revenue		23,389			10,433
Less: current portion of deferred revenue		(20,151	)		(6,515	)
Noncurrent portion of deferred revenue	$	3,238		$	3,918