Micromechanics of surface-grafted hyaluronic acid gels

To design models of tissue surfaces, films of soft gels of hyaluronic acid (HA) (a ubiquitous constituent of the extracellular matrix) are covalently grafted to glass substrates functionalized with aminosilane monolayers. Gelation is achieved by coupling of carboxyl groups of HA through carbodiimide. The elasticity of the gel is controlled through the HA concentration. The magnetic bead microrheometry and colloidal bead deformation field mapping techniques are applied to measure the surface viscoelastic moduli and the effective Young moduli of the HA gel as a function of the gel density. For this purpose, magnetic beads and nonmagnetic beads are coupled to the surface of the HA films. Soft homogeneous films exhibiting Young elastic moduli between 3 and 250 Pa were generated. The shear deformation field induced by tangential force pulses applied to the magnetic beads is measured and analyzed in terms of the theory of elastic deformation of half-spaces by local forces. An unconventional vortex-deformation field is observed for gel film thicknesses of several hundred micrometers, which is attributed to the nonlinear elasticity of the gels. We finally show that amoeba-like cells of the slime mold Dictyostelium discoideum spontaneously adhere to the HA film, while fibroblast adhesion can be mediated through coupling fibronectin to the surface.