Stock Purchase Warrants

The Company has historically issued warrants to purchase shares of the Company’s common stock in connection with certain of its common stock offerings and in September 2016 the Company issued warrants in connection with the updated debt agreement (September 2016 Warrants) discussed in note7. The warrants issued in August 2013 (August 2013 Warrants) include anti-dilution price protection provisions that could require cash settlement of the warrants and accordingly requiring the warrants to be recorded as liabilities of the Company at the estimated fair value at the date of issuance, with changes in estimated fair value recorded as income or expense (non-cash) in the Company’s statement of operations in each subsequent period. The September 2016 Warrants meet the requirements for equity classification. The following table describes the outstanding warrants:



	August 2013 Warrants	September 2016 Warrants
Exercise price	$4.80	$2.48
Expiration date	August 16, 2018	September 9, 2022
Total shares issuable on exercise	724,950	117,074

On September 9, 2016, the Company issued 117,074 warrants to two holders in conjunction with the loan agreement described in note 7. The initial valuation of the September 2016 Warrants was recorded as debt issuance costs and is being amortized over the remaining life of the loan agreement to interest expense. The September 2016 Warrants are treated as equity instruments recorded at fair value with no subsequent remeasurement. Pursuant to the warrants, the holders may exercise their warrants for an aggregate of 117,074 shares of the Company’s common stock.

The fair value of the warrants described in the table above is measured using the Black-Scholes valuation model. Inherent in the Black-Scholes valuation model are assumptions related to expected stock-price volatility, expected life, risk-free interest rate and dividend yield. The Company estimates the volatility of its common stock based on historical volatility that matches the expected remaining life of the warrants. The risk-free interest rate is based on the U.S. Treasury zero-coupon yield curve on the grant date for a maturity similar to the expected remaining life of the warrants. The expected life of the warrants is assumed to be equivalent to their remaining contractual term. The dividend rate is based on the historical rate, which the Company anticipates to remain at zero.

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



August 2013 Warrants	December 31, 2016			December 31, 2015
Closing stock price	$	3.00		$	2.58
Expected dividend yield	—		%	—		%
Expected stock price volatility	97.9		%	91.4		%
Risk-free interest rate	1.03		%	1.31		%
Expected life (years)	1.62			2.63



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