4.	Balance Sheet Components

Property and Equipment

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

	December 31,
	2016			2015
Prototype	$	6,405		$	6,492
Machine and equipment		5,807			7,128
Leasehold improvements		4,621			1,532
Furniture and fixtures		368			350
Software		1,028			832
Construction in progress		5,498			1,851
Property and equipment, gross		23,727			18,185
Less: accumulated depreciation and amortization		(12,167	)		(10,879	)
Property and equipment, net	$	11,560		$	7,306

Depreciation and amortization expense related to property and equipment was $1.6 million, $1.1 million and $853 thousand during the years ended December 31, 2016, 2015 and 2014, respectively.

Intangible Assets

Intangible assets consisted of the following (in thousands):

	December 31,
	2016			2015
License cost	$	512		$	500
Patents		104			104
Intangible assets, gross		616			604
Accumulated amortization		(519	)		(404	)
Intangible assets, net	$	97		$	200

Intangible amortization expense was $115 thousand, $168 thousand and $167 thousand during the years ended December 31, 2016, 2015 and 2014, respectively, which were recorded in general and administrative expenses in the consolidated statements of operations.

At December 31, 2016, the estimated future amortization expense of purchased intangible assets was as follows (in thousands):

Year Ended December 31,	Estimated Future Amortization Expense
2017	$	19
2018		19
2019		19
2020		19
2021		10
Thereafter		11
Total amortization expense	$	97

Accrued Liabilities

Accrued liabilities consisted of the following (in thousands):

	December 31,
	2016		2015
Accrued payroll and related benefits	$	4,274	$	1,938
Accrued accounts payable		1,202		1,880
Sales tax and medical device excise tax payable		13		219
Accrued legal and accounting		509		857
Accrued interest		—		—
Other		336		519
Total accrued liabilities	$	6,334	$	5,413

Deferred Revenue

Deferred revenue consisted of the following (in thousands):

	December 31,
	2016			2015
Deferred revenue:
Product	$	5,050		$	5,050
Services		1,561			911
Distribution rights		3,822			—
Total deferred revenue		10,433			5,961
Less: current portion of deferred revenue		(6,515	)		(5,616	)
Noncurrent portion of deferred revenue	$	3,918		$	345