Water plays a primary role in the functionality of biomedical polymers such as hydrogels. The state of water, defined as bound, intermediate, or free, and its molecular organization within hydrogels is an important factor governing biocompatibility and hemocompatibility. Here, we present a systematic study of water states in gelatin methacryloyl (GelMA) hydrogels designed for drug delivery and tissue engineering applications. We demonstrate that increasing ionic strength of the swelling media correlated with the proportion of non-freezable bound water. We attribute this to the capability of ions to create ion-dipole bonds with both the polymer and water, thereby reinforcing the first layer of polymer hydration. Both pH and ionic strength impacted the mesh size, having potential implications for drug delivery applications. The mechanical properties of GelMA hydrogels were largely unaffected by variations in ionic strength or pH. Loading of cefazolin, a small polar antibiotic molecule, led to a dose-dependent increase of non-freezable bound water, attributed to the drug's capacity to form hydrogen bonds with water, which helped recruit water molecules in the hydrogels' first hydration layer. This work enables a deeper understanding of water states and molecular arrangement at the hydrogel-polymer interface and how environmental cues influence them.
Keywords: drug; gelatin methacryloyl; hydrogel; ionic strength; mesh size; pH; swelling; water.