Stock-based compensation (Tables)

9 Months Ended

Sep. 30, 2019

Share-based Payment Arrangement [Abstract]

Schedule of stock based compensation expense

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

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018
Cost of goods sold	$	441	$	553	$	1,651	$	1,832
Research and development	715		1,016		2,383		2,618
Sales and marketing	575		931		1,852		2,511
General and administrative	1,220		1,541		3,408		3,566
	$	2,951	$	4,041	$	9,294	$	10,527

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 September 30,		Nine Months Ended September 30,
Stock options	2019	2018	2019	2018
Weighted-average expected term (years)	6.00	0.00	6.00	6.02
Weighted-average volatility	68%	—%	67%	65%
Risk-free interest rate	1.82%	—%	1.82%-2.27%	2.27% - 2.62%
Expected dividends	—%	—%	—%	—%
Stock appreciation units
Weighted-average expected term (years)	1.45	1.87	1.56	1.99
Weighted-average volatility	65%	65%	63%	66%
Risk-free interest rate	1.75%-2.12%	1.73% - 2.52%	1.75%-2.63%	1.03%-2.52%
Expected dividends	—%	—%	—%	—%
ESPP
Weighted-average expected term (years)	0.00	0.00	0.69	0.71
Weighted-average volatility	—%	—%	60%	61%
Risk-free interest rate	—%	—%	2.36%-2.59%	1.20%-1.93%
Expected dividends	—%	—%	—%	—%

Schedule 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 nine months ended September 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	103,001		4.34		1,938,591		4.87
Exercised/Converted	(264,857	)	3.66		(996,340	)	8.70
Cancelled/Forfeited	(208,277	)	6.52		(220,524	)	7.51
Balance as of September 30, 2019	2,832,612		5.82		3,207,755		5.82

Schedule of 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	—%