The cost that has been charged against income for stock-based compensation is set forth below (in thousands):

	Three Months Ended				Six Months Ended
	June 28, 2019		June 29, 2018		June 28, 2019		June 29, 2018
Employee stock options	$	2,162	$	897	$	3,592	$	1,516
Restricted stock		77		59		159		110
Restricted stock units		311		494		1,415		1,092
Nonemployee stock options		29		148		54		181
Total stock-based compensation expense	$	2,579	$	1,598	$	5,220	$	2,899

The Company recorded stock-based compensation costs in the following categories (in thousands):

	Three Months Ended				Six Months Ended
	June 28, 2019		June 29, 2018		June 28, 2019		June 29, 2018
Cost of sales	$	22	$	4	$	36	$	7
General and administrative		1,018		655		1,796		1,174
Marketing and selling		690		366		1,861		826
Research and development		849		573		1,527		892
Total stock-based compensation expense, net		2,579		1,598		5,220		2,899
Amounts capitalized as part of inventory		276		145		465		267
Total stock-based compensation expense, gross	$	2,855	$	1,743	$	5,685	$	3,166

The Amended and Restated Omnibus Equity Incentive Plan (“the Plan”) provides for various forms of stock-based incentives. To date, of the available forms of awards under the Plan, the Company has granted only stock options, restricted stock, unrestricted share grants, and restricted stock units (“RSUs”). Options under the Plan are granted at fair market value on the date of grant, become exercisable generally over a three-year period, or as determined by the Board of Directors, and expire over periods not exceeding 10 years from the date of grant. Certain option and share awards provide for accelerated vesting if there is a change in control and pre-established financial metrics are met (as defined in the Plan). Grants of restricted stock outstanding under the Plan generally vest over periods of one to three years. Grants of RSUs outstanding under the Plan generally vest based on service, performance, or a combination of both. As of June 28, 2019, there were 1,639,474 shares available for grant under the Plan

The fair value of each option award is estimated on the date of grant using a Black-Scholes option valuation model applying the weighted-average assumptions noted in the following table. Expected volatilities are based on historical volatility of the Company’s stock. The expected term of options granted is derived from the historical exercises and post-vesting cancellations and represents the period of time that options granted are expected to be outstanding. The Company has calculated an 8% estimated forfeiture rate based on historical forfeiture experience. The risk-free rate is based on the U.S. Treasury yield curve corresponding to the expected term at the time of the grant.

	Three Months Ended						Six Months Ended
	June 28, 2019			June 29, 2018			June 28, 2019			June 29, 2018
Expected dividend yield		0	%		0	%		0	%		0	%
Expected volatility		53	%		53	%		53	%		53	%
Risk-free interest rate		2.05	%		2.81	%		2.41	%		2.70	%
Expected term (in years)		5.67			5.72			5.67			5.72

A summary of stock option activity under the Plan for the six months ended June 28, 2019 is presented below:

	Stock Options (in 000’s)			Minimum Exercise Price		Maximum Exercise Price
Outstanding at December 28, 2018		3,920
Granted		801
Exercised		(126	)
Forfeited or expired		(16	)
Outstanding at June 28, 2019		4,579		$	3.50	$	43.84
Exercisable at June 28, 2019		3,026

A summary of restricted stock and RSU activity under the Plan for the six months ended June 28, 2019 is presented below:

	Restricted Stock (in 000’s)			Restricted Stock Units (in 000’s)
Unvested at December 28, 2018		11			322
Granted		11			19
Vested		(11	)		(202	)
Forfeited or expired		—			(5	)
Unvested at June 28, 2019		11			134