Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2020

Stockholders Equity [Abstract]

Weighted-Average Assumptions for Stock Options

For the three months ended March 31, 2020 and 2019, we used the following weighted-average assumptions in our Black-Scholes calculations:

Employee Stock Options:

	Three Months Ended March 31,
	2020			2019
Risk-free interest rate		1.5	%		2.5	%
Dividend yield		0.0	%		0.0	%
Volatility		73.6	%		76.4	%
Expected life	6.1 years			6.1 years

Board of Directors Stock Options (1):

	Three Months Ended March 31,
	2020
Risk-free interest rate		1.5	%
Dividend yield		0.0	%
Volatility		73.3	%
Expected life	6.3 years

(1) We did not grant stock options to our Board of Directors in the first quarter of 2019.

2017ESPP:

	Three Months Ended March 31,
	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 months ended March 31, 2020 and 2019 (in thousands):

	Three Months Ended March 31,
	2020		2019
Cost of sales - product	$	237	$	118
Research and development expenses		1,349		3,921
Selling, general and administrative expenses		5,696		14,521
Total	$	7,282	$	18,560