<p>This perspective shows how control of water at polymer surfaces can be used to design better biomaterials. We group interfacial water into three states—free, intermediate, and nonfreezing—and tie each to clear measurements. A thin, mobile layer rich in intermediate water lowers protein damage and early immune signals, while the underlying network keeps strength and permeability. We translate this idea to natural polymers and to practical knobs: ion choice, gentle crosslinking, drying and annealing, and ultrathin neutral or zwitterionic coatings. The same rules guide coatings, cell capsules, dressings, printed gels, scaffolds, and sensors. Simple simulations link processing to expected signals, allowing screening before experiments.</p> Graphical Abstract <p></p>