Convertible debt (Details) (USD $)	12 Months Ended			1 Months Ended			12 Months Ended		1 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013	Apr. 30, 2011 Common stock	Apr. 30, 2008 Common stock	Feb. 28, 2007 Common stock	Dec. 31, 2012 Common stock	Dec. 31, 2011 Common stock	Apr. 30, 2011 Convertible notes issued in 2007 and 2008	Apr. 30, 2008 Convertible notes issued in 2007 and 2008	Feb. 28, 2007 Convertible notes issued in 2007 and 2008	Apr. 30, 2008 Convertible notes issued in 2007 and 2008 Common stock	Feb. 28, 2007 Convertible notes issued in 2007 and 2008 Common stock
Convertible debt
Proceeds from private placement (before expenses)						$ 7,600,000
Debt instrument face amount		8,979,926							9,000,000	3,000,000	5,000,000
Convertible note term										4 years	5 years
Number of shares of common stock issued for convertible notes				4,049,776	1,333,333	2,272,727
Conversion price of convertible note (in dollars per share)												$ 2.25	$ 2.20
New shares of common stock issued, value	1,601,746	9,855,881				2,600,000	4,884	44,848
New shares of common stock issued						1,141,491	488,337	4,484,776
Share price of stock issued under private placement (in dollars per share)	$ 3.48	$ 2.27	$ 2.78			$ 2.28
Annual interest rate of convertible note (as a percent)										9.00%	9.00%
Interest payments of convertible note not paid through December 31, 2008										200,000	800,000
Shares issued in lieu of cash payments for accrued interest				127,318
Value of shares issued in lieu of cash payments for accrued interest				300,000
Shares issued in lieu of prepayment fee negotiated at the time conversion				297,682
Value of shares issued in lieu of prepayment fee negotiated at the time conversion				600,000
Prepayment fee		600,000
Convertible debt interest expense (non-cash)		$ 254,636