Stock-based compensation (Tables)

3 Months Ended

Mar. 31, 2017

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, 2017 and 2016 (in thousands):

	Three Months Ended
	March 31,
	2017		2016
Cost of goods sold	$	147	$	589
Research and development		662		971
Sales and marketing		464		887
General and administrative		599		992
	$	1,872	$	3,439

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
	March 31,
Stock options	2017	2016
Weighted-average expected term (years)	—	4.96
Weighted-average volatility	—  %	64%
Risk-free interest rate	—  %	1.76%
Expected dividends	—  %	—  %
Stock appreciation units
Weighted-average expected term (years)	2.45	3.03
Weighted-average volatility	71%	61%
Risk-free interest rate	0.51%-1.47%	0.65%-1.47%
Expected dividends	—  %	—  %
ESPP
Weighted-average expected term (years)	0.70	0.70
Weighted-average volatility	54%	70%
Risk-free interest rate	0.45%-0.59%	0.08%-0.33%
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, 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	—		—	101,082		9.96
Exercised/Converted	(133,287)		3.68	(8,332)		8.50
Cancelled/Forfeited	(6,666)		11.61	(13,675)		10.93
Balance at March 31, 2017	4,161,387	$	5.22	2,168,548	$	10.14