As of December 31, 2020 and 2019, our inventory consisted of the following components:



	December 31,		December 31,
(in thousands)	2020		2019
Purchased materials	$	3,531	$	3,966
Work in process		6,651		4,594
Finished goods		4,048		4,752
Inventory	$	14,230	$	13,312

As of December 31, 2020 and 2019, our property and equipment, net, consisted of the following components:



	December 31,
(in thousands)	2020		2019
Laboratory equipment and machinery	$	24,948	$	25,173
Leasehold improvements		29,931		29,902
Computer equipment		12,400		11,851
Software		4,940		4,747
Furniture and fixtures		2,434		2,422
Construction in progress		137		193
		74,790		74,288
Less: Accumulated depreciation		(49,891)		(44,218)
Property and equipment, net	$	24,899	$	30,070

Depreciation expense during the years ended December 31, 2020, 2019 and 2018 was $6.4 million, $7.3 million and $7.2 million, respectively.

As of December 31, 2020 and 2019, our accrued expenses consisted of the following components:



	December 31,
(in thousands)	2020		2019
Salaries and benefits	$	15,261	$	9,748
Accrued product development costs		415		67
Accrued Tenant Improvements for Menlo Park building		—		998
Inventory accrual		218		229
Accrued professional services and legal fees		726		943
Other		730		1,257
Accrued expenses	$	17,350	$	13,242

As of December 31, 2020, we had a total of $10.3 million of deferred revenue from our service contracts, $8.7 million of which was recorded as “Deferred revenue, current” to be recognized over the next year and the remaining $1.6 million was recorded as “Deferred revenue, non-current” to be recognized in the next 3 years. Revenue recorded in the year ended December 31, 2020 includes $7.6 million, respectively, of previously deferred revenue that was included in “Deferred revenue, current” as of December 31, 2019. Contract assets as of December 31, 2020 and December 31, 2019 were not material.

As of December 31, 2020, we had a total of $0.7 million of deferred commissions included in “Prepaid expenses and other current assets” which is recognized as the related revenue is recognized. Additionally, as a practical expedient, we expense costs to obtain a contract as incurred if the amortization period would have been a year or less.

As of December 31, 2019, a balance of $16.0 million aggregate principal amount of debt remained outstanding under the debt agreement with Deerfield entered into in February 2013 and was presented as “Notes payable, current” on the consolidated balance sheet as of December 31, 2019.

In February 2020, upon the maturity of the debt agreement, we repaid the remaining outstanding principal of $16.0 million and interest.

A number of features embedded in the promissory notes required accounting for them as a derivative, including the indemnification of certain withholding taxes and the acceleration of debt upon (i) a qualified financing, (ii) an event of default, (iii) a Major Transaction (as such term is defined in the Facility Agreement), and (iv) the exercise of the warrant via offset to the debt principal. These features represent a single derivative (the “Financing Derivative”) that was bifurcated from the debt instrument and accounted for as a liability at fair value, with changes in fair value between reporting periods recorded in other income (expense), net.

The estimated fair value of the Financing Derivative was determined by comparing the difference between the fair value of the promissory notes with and without the Financing Derivative by calculating the respective present values from future cash flows using a 6.5% discount rate at December 31, 2019. The estimated fair value of the Financing Derivative as of December 31, 2019 was $0.

In February 2020, after we repaid the remaining outstanding principal of $16.0 million and interest to Deerfield, the related Financing Derivative expired.

As of December 31, 2020 and 2019, our Other liabilities, current consisted of the following components:


	December 31,
(in thousands)	2020		2019
Accrued ESPP	$	2,037	$	—
Other		2,482		225
Other liabilities, current	$	4,519	$	225