Stock-based compensation (Tables)

3 Months Ended

Mar. 31, 2016

Stock-based compensation

Summary of Stock Based Compensation Expense

The following table summarizes the stock-based compensation expense recognized in the three months ended March 31, 2016 and 2015 (in thousands):

	Three Months Ended
	March 31,
	2016		2015
Cost of goods sold	$	589	$	370
Research and development		971		493
Sales and marketing		887		453
General and administrative		992		740
	$	3,439	$	2,056

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 March 31,
Stock options	2016	2015
Weighted-average expected term (years)	4.96	5.25
Weighted-average volatility	64%	63%
Risk-free interest rate	1.76%	1.65%
Expected dividends	—  %	—  %
Stock appreciation units
Weighted-average expected term (years)	3.03	1.91
Weighted-average volatility	61%	57%
Risk-free interest rate	0.65%-1.47%	0.25% – 1.10%
Expected dividends	—  %	—  %
ESPP
Weighted-average expected term (years)	0.70	0.70
Weighted-average volatility	70%	58%
Risk-free interest rate	0.08%-0.33%	0.03% – 0.13%
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 three months ended March 31, 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	12,300		10.40	16,900		9.19
Exercised/Converted	(349,954)		3.52	(8,001)		7.65
Cancelled/Forfeited	(30,108)		6.05	(7,618)		7.36
Balance at March 31, 2016	4,640,035	$	4.41	1,214,967	$	7.49