Stock-based compensation (Tables)

6 Months Ended

Jun. 30, 2019

Share-based Payment Arrangement [Abstract]

Summary of Stock Based Compensation Expense

The following table summarizes the stock-based compensation expense recognized in the three and six months ended June 30, 2019 and 2018 (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2019		2018		2019		2018
Cost of goods sold	$	609	$	629	$	1,210	$	1,279
Research and development	787		829		1,668		1,602
Sales and marketing	599		642		1,277		1,580
General and administrative	1,010		1,039		2,188		2,025
	$	3,005	$	3,139	$	6,343	$	6,486

Schedule of Share-based Payment Award, Employee Stock Purchase Plan, Valuation Assumptions

The Company estimated the fair value of certain stock-based awards using a Black-Scholes-Merton valuation model with the following assumptions:  

	Three Months Ended June 30,		Six Months Ended June 30,
Stock options	2019	2018	2019	2018
Weighted-average expected term (years)	6.00	6.02	6.00	6.02
Weighted-average volatility	67%	66%	67%	65%
Risk-free interest rate	2.27%	2.62%	2.27%	2.27% - 2.62%
Expected dividends	—%	—%	—%	—%
Stock appreciation units
Weighted-average expected term (years)	1.55	2.00	1.61	2.04
Weighted-average volatility	62%	67%	63%	67%
Risk-free interest rate	2.27%-2.44%	1.73% - 2.39%	2.27%-2.63%	1.03%-2.39%
Expected dividends	—%	—%	—%	—%
ESPP
Weighted-average expected term (years)	0.69	0.71	0.69	0.71
Weighted-average volatility	60%	61%	60%	61%
Risk-free interest rate	2.36%-2.59%	1.20% - 1.93%	2.36%-2.59%	1.20%-1.93%
Expected dividends	—%	—%	—%	—%

Estimated Fair Value of Certain Stock-Based Awards Using Black-Scholes-Merton Valuation Model

The fair value of market-based RSUs was measured on the grant date using Monte Carlo simulation model with the following assumptions:

Market-based restricted stock units	Assumptions Used
Weighted-average volatility	66%
Risk-free interest rate	2.79%
Expected dividends	—%

Summary of Stock Option and Restricted Stock Unit Activity

The following table summarizes the Company’s stock option and RSU activity, excluding market-based RSUs, during the six months ended June 30, 2019: 

	Stock Options				Restricted Stock Units
	Number of Shares		Weighted Average Exercise Price		Number of Units		Weighted Average Grant Date Fair Value
Balance as of December 31, 2018	3,202,745		$	5.73	2,486,028		$	7.87
Granted	84,336		4.33		332,034		5.28
Exercised/Converted	(166,071	)	3.45		(423,229	)	7.12
Cancelled/Forfeited	(158,011	)	6.16		(186,353	)	7.73
Balance as of June 30, 2019	2,962,999		5.80		2,208,480		7.63