Stock-based compensation (Tables)

9 Months Ended

Sep. 30, 2016

Stock-based compensation

Summary of Stock Based Compensation Expense

The following table summarizes the stock-based compensation expense recognized in the three and nine months ended September 30, 2016 and 2015 (in thousands):

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2016		2015		2016		2015
Cost of goods sold	$	297	$	339	$	1,605	$	1,119
Research and development		2,981		363		4,508		1,357
Sales and marketing		2,352		275		3,604		1,175
General and administrative		3,146		459		4,728		1,767
	$	8,776	$	1,436	$	14,445	$	5,418

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

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

	Three Months Ended September 30,		Nine Months Ended September 30,
Stock options	2016	2015	2016	2015
Weighted-average expected term (years)	5.9	5.6	5.8	5.4
Weighted-average volatility	65%	63%	65%	64%
Risk-free interest rate	1.01%-1.15%	1.63% – 1.85%	1.01%-1.76%	1.37% – 1.65%
Expected dividends	—  %	—  %	—  %	—  %
Stock appreciation units
Weighted-average expected term (years)	2.6	3.5	2.8	3.6
Weighted-average volatility	62%	60%	62%	62%
Risk-free interest rate	0.45%-0.71%	0.28% – 1.38%	0.45%-1.47%	0.25% – 1.57%
Expected dividends	—  %	—  %	—  %	—  %
ESPP
Weighted-average expected term (years)	—	—	0.7	0.7
Weighted-average volatility	—  %	—  %	70%	58%
Risk-free interest rate	—  %	—  %	0.08%-0.39%	0.03% – 0.14%
Expected dividends	—  %	—  %	—  %	—  %

Summary of Stock Option and Restricted Stock Unit Activity

The following table summarizes the Company’s stock option and RSU activity during the nine months ended September 30, 2016:

	Stock Options			Restricted Stock Units
		Weighted			Weighted
		Average			Average
	Number of	Exercise		Number of	Grant Date
	Shares	Price		Units	Fair Value
Balance at December 31, 2015	5,007,797	$	4.34	1,213,686	$	7.46
Granted	358,186		12.22	1,071,230		12.18
Exercised/Converted	(962,366)		3.62	(211,388)		7.02
Cancelled/Forfeited	(59,961)		4.99	(29,694)		7.36
Balance at September 30, 2016	4,343,656	$	5.14	2,043,834	$	9.99