Stock Purchase Warrants (Tables)

3 Months Ended

Mar. 31, 2018

Warrants and Rights Note Disclosure [Abstract]

The following table describes the outstanding warrants as of March 31, 2018:



	August 2013 Warrants	September 2016 Warrants	December 2017 Warrants
Exercise price	$4.80	$2.25	$4.27
Expiration date	August 16, 2018	September 9, 2022	December 6, 2023
Total shares issuable on exercise	365,150	117,074	53,902

Schedule of assumptions used in calculating estimated fair value of warrants

The assumptions used by the Company are summarized in the following tables:



August 2013 Warrants	March 31, 2018			December 31, 2017
Closing stock price	$	9.95		$	5.45
Expected dividend rate	—		%	—		%
Expected stock price volatility	75.4		%	63.7		%
Risk-free interest rate	1.95		%	1.65		%
Expected life (years)	0.38			0.62



September 2016 Warrants	September 9, 2016
Closing stock price	$	2.20
Expected dividend rate	—		%
Expected stock price volatility	89.8		%
Risk-free interest rate	1.4		%
Expected life (years)	6.00



December 2017 Warrants	December 6, 2017
Closing stock price	5.10
Expected dividend rate	—	%
Expected stock price volatility	86.4	%
Risk-free interest rate	2.2	%
Expected life (years)	6.00

- Definition

Tabular disclosure of warrants or rights issued. Warrants and rights outstanding are derivative securities that give the holder the right to purchase securities (usually equity) from the issuer at a specific price within a certain time frame. Warrants are often included in a new debt issue to entice investors by a higher return potential. The main difference between warrants and call options is that warrants are issued and guaranteed by the company, whereas options are exchange instruments and are not issued by the company. Also, the lifetime of a warrant is often measured in years, while the lifetime of a typical option is measured in months. Disclose the title of issue of securities called for by warrants and rights outstanding, the aggregate amount of securities called for by warrants and rights outstanding, the date from which the warrants or rights are exercisable, and the price at which the warrant or right is exercisable.

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