Local deformation and dynamics of cross-linked hyaluronic acid gels at charged interfaces

Hydrogel adhesion is a complex process that involves chain dynamics, thermodynamics, chemistry, and topology. Using fluorescent confocal microscopy in combination with fluorescent differential dynamic microscopy (fDDM), we have determined surface deformation and dynamics of cross-linked hyaluronic acid (HA) gels, equilibrated against 1-1000 mM NaCl solutions, at positively and negatively ionized surfaces. Due to the negative ionization of HA, the gels are repelled from negatively ionized glass surfaces creating a fluid separation layer and repulsion remains unaffected by salt concentration. At these interfaces, the gel network motion is slowed, as determined with fDDM in 167 mM ionic strength. To create positively ionized surfaces, poly-L-lysine is deposited on the glass surface. At higher salt concentrations, surface ionization has little effect, while in lower salt concentrations, the softer gels are compressed 4-6 times by the surface forces. In lower salt concentrations, the surface interactions are less screened and the gels are softer, leading to greater deformation. These results reveal that gel deformation and interfacial dynamics are governed by a delicate interplay between gel modulus, surface ionization, and ionic strength, underscoring the need for new theoretical models to predict soft gel behavior at interfaces and enabling the rational design of gel-based adhesives, coatings, and biointerfaces.