Stock-based compensation (Tables)

6 Months Ended

Jun. 30, 2017

Stock-based compensation

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, 2017 and 2016 (in thousands):

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2017		2016		2017		2016
Cost of goods sold	$	324	$	719	$	471	$	1,308
Research and development		511		556		1,173		1,527
Sales and marketing		313		365		777		1,252
General and administrative		738		590		1,337		1,582
	$	1,886	$	2,230	$	3,758	$	5,669

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 with the following assumptions:  

	Three Months Ended		Six Months Ended
	June 30,		June 30,
Stock options	2017	2016	2017	2016
Weighted-average expected term (years)	5.99	5.25	5.99	5.20
Weighted-average volatility	65%	63%	65%	63%
Risk-free interest rate	2.08%	1.21%-1.38%	2.08%	1.21%-1.76%
Expected dividends	—  %	—  %	—  %	—  %
Stock appreciation units
Weighted-average expected term (years)	2.33	2.84	2.39	2.94
Weighted-average volatility	71%	62%	71%	62%
Risk-free interest rate	0.76%-1.50%	0.59%-0.95%	0.51%-1.50%	0.59%-1.47%
Expected dividends	—  %	—  %	—  %	—  %
ESPP
Weighted-average expected term (years)	0.68	0.70	0.68	0.70
Weighted-average volatility	55%	70%	55%	70%
Risk-free interest rate	0.45%-0.91%	0.08%-0.39%	0.45%-0.91%	0.08%-0.39%
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 six months ended June 30, 2017:

	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, 2016	4,301,340	$	5.18	2,089,473	$	10.15
Granted	45,126		9.20	183,177		9.48
Exercised/Converted	(402,977)		3.76	(516,478)		7.89
Cancelled/Forfeited	(44,977)		9.15	(89,926)		10.65
Balance at June 30, 2017	3,898,512	$	5.33	1,666,246	$	10.75