Warrants (Tables)

9 Months Ended

Sep. 30, 2013

Text Block [Abstract]

Outstanding Warrants to Purchase Common Stock

The following table summarizes outstanding warrants to purchase common stock as of September 30, 2013 and December 31, 2012:

	Number of Warrants
Liability-classified warrants	As of September 30, 2013		As of December 31, 2012		Exercise Price		Expiration Dates
Issued in Series B Preferred Stock offering		1,320		1,320	$	2.50		Jul. – Nov. 2015
Issued in Series D Preferred Stock offering		14,800		39,800	$	2.50		Dec. 2015 – Mar. 2016
Issued in Series E Preferred Stock offering		60,000		120,000	$	2.50		May – June 2017
Subtotal		76,120		161,120
Equity-classified warrants
Issued in June 2011 equity financing		6,113		6,113	$	22.50		June 2016
Issued in March 2010 and Preferred Stock offerings		4,209,357		4,209,357	$	6.25 – 7.50		Oct. 2015 – June 2018
Issued with Convertible Notes		1,125,578		1,125,578	$	2.50		June 2018
Issued to placement agents in August 2011 equity financing		50,123		50,123	$	13.75		August 2016
Issued in August 2011 equity financing		565,759		579,759	$	18.75		August 2016
Subtotal		5,956,930		5,970,930
Total		6,033,050		6,132,050

Calculated Aggregate Fair Values and Net Cash Settlement Value

The following table summarizes the calculated aggregate fair values as of the dates indicated along with the assumptions utilized in the Black-Scholes option pricing model for each calculation.

	September 30, 2013			December 31, 2012
Calculated aggregate value (in thousands)	$	205		$	374
Exercise price per share of warrant	$	2.50		$	2.50
Closing price per share of common stock	$	4.38		$	3.75
Volatility		68	%		70	%
Expected term (years)		3.4			4.0
Risk-free interest rate		0.8	%		0.6	%
Dividend yield		—	%		—	%