Prepaid Expenses and Other Current Assets (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Prepaid Expenses And Other Current Assets [Line Items]
Equipment maintenance and services	$ 747,273	$ 424,328
Prepaid insurance	365,167	251,418
Interest receivable	140,363	106,695
Clinical and research expenses	15,354	13,714
Other	36,730	82,304
Total	$ 1,304,887	$ 878,459