Angiogenesis, often thought of as the first step of wound healing, is an organized series of events, beginning with vessel destabilization, followed by endothelial cell proliferation and migration, ending with vessel maturation. Vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) have been shown to be important in vascular permeability and endothelial cell proliferation, and migration (early stage angiogenesis), while platelet derived growth factor (PDGF) and sphingosine 1-phosphate (S1P) have been shown to stimulate vascular stability (late stage angiogenesis). For this reason, it was hypothesized that inducing angiogenesis by sequentially delivering angiogenic growth factors, controlling their presence and absence, would better mimic the temporal role of each factor during the progression of native angiogenesis in situ. To this end, we utilized a delivery system based on porous cellulose hollow fibers that, for the first time, permits sequential delivery of an early stage factor followed by a late stage growth factor in vivo, where previous attempts have only resulted in different rates of delivery. Our delivery system addresses the idea that factors involved in one stage of angiogenesis may inhibit other stages of angiogenesis, causing absence of one factor to be just as important as the presence of another factor. Using a modified murine Matrigel plug model, it is apparent that delivery strategies where VEGF alone is delivered before S1P alone as well as delivery strategies where bFGF alone is delivered before PDGF alone, not only lead to greater recruitment of endothelial cells, but also higher maturation index of associated vessels. Sequential delivery was also optimized by examining varying delivery schedules. Additionally, the hollow fiber delivery system, was analyzed for its transport properties, where it was discovered that transport from the lumen of the hollow fiber to the surrounding environment was not only based on diffusion of the factor, but osmosis-driven convection as well. Sequential delivery strategies such as this one have potential to improve wound healing strategies involving angiogenesis as well as other types of tissue formation that occur in a series of organized stages
