Hydration of hydrogels regulates vascularization in vivo

The key barrier to the clinical application of tissue engineering scaffolds is the limitation of rapid and sufficient vascularization. Adipose-derived stem cells (ASCs), especially multicellular aggregates, exhibited a promising angiogenic activity. Herein, we designed a series of poly(l-glutamic acid) (PLGA)-based hydrogels with tunable hydration to control the in situ formation of multicellular spheroids. Oligo(ethylene glycol)s (OEGs) were employed to regulate the hydration of hydrogels. The hydrogel cross-linked with ethylene glycol (OEG₁) supported the most excellent adhesion and proliferation of human ASCs in vitro. However, as the hydration of hydrogels strengthened, the adherent ASCs were gradually replaced with multicellular spheroids. Moreover, the in situ formation of spheroids was more effective in upregulating hypoxia-adaptive signals (e.g., hypoxia-inducible factor-1α, HIF-1α) and enhancing the secretion of angiogenic factors (e.g., vascular endothelial growth factor (VEGF) and fibroblast growth factor 2 (FGF-2)) compared to adherent cells in OEG₁ hydrogels. The hydrogel cross-linked with oligo(ethylene glycol)400 (OEG₉) carrying spheroids induced a high angiogenic response of host tissue in vivo, resulting in an improved system vascularization, compared to adherent cells in the OEG₁ hydrogel. These features indicated that the PLGA-based hydrogels were expected to be applied toward bone and fat tissue regeneration.