Stock-based compensation (Tables)

3 Months Ended

Mar. 31, 2019

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of Stock Based Compensation Expense

The following table summarizes the stock-based compensation expense recognized in the three months ended March 31, 2019 and 2018 (in thousands):

	Three Months Ended March 31,
	2019		2018
Cost of goods sold	$	601	$	650
Research and development	881		773
Sales and marketing	678		938
General and administrative	1,178		986
	$	3,338	$	3,347

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 March 31,
Stock options	2019	2018
Weighted-average expected term (years)	—	6.00
Weighted-average volatility	—	65%
Risk-free interest rate	—	2.27%
Expected dividends	—	—%
Stock appreciation units
Weighted-average expected term (years)	1.67	2.10
Weighted-average volatility	64%	67%
Risk-free interest rate	2.45%-2.63%	1.03% - 1.98%
Expected dividends	—%	—%
ESPP
Weighted-average expected term (years)	0.73	0.73
Weighted-average volatility	61%	61%
Risk-free interest rate	2.36% - 2.59%	1.20% - 1.31%
Expected dividends	—%	—%

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:

	Year Ended December 31,
Market-based restricted stock units	2018
Weighted-average volatility	66%
Risk-free interest rate	2.79%
Expected dividends	—%

Summary 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 three months ended March 31, 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	—		—		33,598		7.10
Exercised/Converted	(47,730	)	3.50		(41,627	)	7.88
Cancelled/Forfeited	(9,688	)	11.53		(59,100	)	8.13
Balance as of March 31, 2019	3,145,327		5.75		2,418,899		7.85