Gel–Gel\nPhase Separation in Clustered Poly(ethylene\nglycol) Hydrogel with Enhanced Hydrophobicity

The development of hydrophobic poly(ethylene glycol)\n(PEG) hydrogels,\nwhich are typically hydrophilic, could significantly enhance their\napplication as artificial extracellular matrices. In this study, we\ndesigned PEG hydrogels with enhanced hydrophobicity through gel–gel\nphase separation (GGPS), a phenomenon that uniquely enhances hydrophobicity\nunder ambient conditions, and we elucidated the pivotal role of elasticity\nin this process. We hypothesized that increased elasticity would amplify\nGGPS, thereby improving the hydrophobicity and cell adhesion on PEG\nhydrogel surfaces, despite their inherent hydrophilicity. To test\nthis hypothesis, we engineered dilute oligo-PEG gels via a two-step\nprocess, creating polymer networks from tetra-PEG clusters with multiple\nreaction points. These oligo-PEG gels exhibited significantly higher\nelasticity, turbidity, and shrinkage upon water immersion compared\nto dilute PEG gels. Detailed characterization through confocal laser\nscanning microscopy, rheological measurements, and cell adhesion assays\nrevealed distinct biphasic structures, increased hydrophobicity, and\nenhanced cell attachability in the dilute oligo-PEG gels. Our findings\nconfirm that elasticity is crucial for effective GGPS, providing a\nnovel method for tailoring hydrogel properties without chemical modification.\nThis research introduces a new paradigm for designing biomaterials\nwith improved cell-scaffolding capabilities, offering significant\npotential for tissue engineering and regenerative medicine.