Specific Aim 1: is to measure the influence of progenitor cell mobilization on the PPCD proteomic profile investigators derived during our recently completed protocol (IRB # 12-1198). Ten CLI patients will be enrolled. All will wear the PPCD as before. All will receive 10 mcg/kg of Filgrastim (Nuopogen, Amgen Inc.) for a total of 5 doses. The FDA approved indication is stem cell mobilization. The dosimetry is less aggressive than the product label as the dosimetry is being spread out to once every 72 hours rather than once a day. The FDA has granted the PI an IND waiver for this dosing interval. As with our first protocol, cytometry will yield the distribution of progenitor CD34+ and VEGFR2+ cells and monocytes (CD14+). The distribution of mature circulating endothelial cells (CD31+) cells will be measured as a reference point. As with our first protocol investigators will do a proteomic survey measuring levels of cytokines associated with arteriogenesis, as well as serum nitrite (reflection of nitric oxide synthase activity).