| Number of Shares | Price per Share | Weighted Average Exercise Price | ||||||||
| Balance at December 31, 2011 | 426 | $1.80 – $745.50 | $ | 117.60 | ||||||
| Warrants issued | 4,633 | $6.45 – $8.85 | $ | 7.80 | ||||||
| Warrants exercised | (20 | ) | $1.80 – $7.95 | $ | 2.85 | |||||
| Balance at December 31, 2012 | 5,039 | $1.80 – $745.50 | $ | 17.10 | ||||||
| Warrants issued | 3,548 | $6.32 – $7.59 | $ | 6.98 | ||||||
| Warrants exercised | (285 | ) | $1.80 – $8.85 | $ | 7.27 | |||||
| Warrants expired | (27 | ) | $745.50 | $ | 745.50 | |||||
| Balance at December 31, 2013 | 8,275 | $5.47– $735.00 | $ | 10.04 | ||||||