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Stock Purchase Warrants (Tables)
9 Months Ended
Sep. 30, 2012
Stock Purchase Warrants  
Schedule of assumptions used in calculating estimated fair value of warrants

 

October 2007 Warrants

 

December 31, 2011

 

September 30, 2012

 

Closing stock price

 

$

1.82

 

$

1.63

 

Expected dividend rate

 

0

%

0

%

Expected stock price volatility

 

93.6

%

56.2

%

Risk-free interest rate

 

0.1

%

0.2

%

Expected life (years)

 

1.25

 

0.50

 

 

January 2010 Class A Warrants

 

December 31, 2011

 

September 30, 2012

 

Closing stock price

 

$

1.82

 

$

1.63

 

Expected dividend rate

 

0

%

0

%

Expected stock price volatility

 

86.1

%

73.6

%

Risk-free interest rate

 

0.5

%

0.3

%

Expected life (years)

 

3.50

 

2.75

 

 

December 2010 Warrants

 

December 31, 2011

 

September 30, 2012

 

Closing stock price

 

$

1.82

 

$

1.63

 

Expected dividend rate

 

0

%

0

%

Expected stock price volatility

 

83.6

%

70.7

%

Risk-free interest rate

 

0.6

%

0.4

%

Expected life (years)

 

3.96

 

3.21

 

 

Summary of change in estimated fair value of warrant liabilities

 

Warrant Liabilities

 

 

 

Balance at December 31, 2011

 

$

16,625

 

Warrants exchanged for common stock

 

(10,382

)

Decrease in fair value

 

(3,289

)

Balance at September 30, 2012

 

$

2,954