Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2020

Stockholders Equity [Abstract]

Weighted-Average Assumptions for Stock Options

For the six months ended June 30, 2020 and 2019, we used the following weighted-average assumptions in our Black-Scholes calculations:

Employee Stock Options:

	Six Months Ended June 30,
	2020			2019
Risk-free interest rate		1.3	%		2.5	%
Dividend yield		0.0	%		0.0	%
Volatility		74.5	%		76.3	%
Expected life	6.1 years			6.1 years

Board of Directors Stock Options:

	Six Months Ended June 30,
	2020			2019
Risk-free interest rate		0.8	%		1.9	%
Dividend yield		0.0	%		0.0	%
Volatility		75.3	%		74.3	%
Expected life	5.7 years			6.3 years

2017 ESPP:

	Six Months Ended June 30,
	2020			2019
Risk-free interest rate		1.0	%		2.5	%
Dividend yield		0.0	%		0.0	%
Volatility		71.9	%		64.1	%
Expected life	6 months			6 months

Stock-Based Compensation Expense

The following table summarizes stock-based compensation expense for the three and six months ended June 30, 2020 and 2019 (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Cost of sales - product	$	350	$	137	$	587	$	255
Research and development expenses		3,484		2,563		4,833		6,484
Selling, general and administrative expenses		12,287		11,663		17,982		26,184
Total	$	16,121	$	14,363	$	23,402	$	32,923