Local dynamics of crosslinked hyaluronic acid gels at charged surfaces

Hydrogel adhesion is a complex process that involves chain dynamics, thermodynamics, chemistry, and topology. We have shown that local polymer gel characterization is possible with fluorescent differential dynamic microscopy (fDDM). Using fDDM in combination with fluorescence confocal microscopy, we have determined surface dynamics of crosslinked hyaluronic acid (HA) gels, equilibrated again 167 mM NaCl solutions, at positively and negatively charged surfaces. Due to the negative ionization of HA, the gels are repelled from negative glass surfaces and the gel network motion is slowed and hindered. To create positive surfaces, poly(l-lysine) is adsorbed to the glass surface. As the HA chains are adsorbed, network motion is no longer resolved until ~ 1 µm away from the surface into the gel, where network motion is faster than in the bulk. We show that for soft, HA gels, surface repulsion or adsorption can significantly change the local strand motion, and that these changes persist up to µm into the material. This large deformation and new gel equilibrium is surprising and establishes a need for a better theoretical description of how soft gels are perturbed by surface interactions, which will lead to a better understanding of gel adhesion and nanoparticle reinforcement of gels.