Multi-scale characterization of hydrogels of biological interest*

Hydrogels are three-dimensional elastic materials consisting of a porous matrix swollen with a large amout of water. They are characterized by a strong porosity and thus constitute water reservoirs which can absorb or release water when they are subjected to external stimuli. They must be able to withstand the stresses due to drying (water removal) and swelling (water imbibition). For these reasons, gels are used in many biomedical applications (controlled drug release systems).

We study here the way to form a homogeneous bead of colloidal hydrogel. We control the gelation kinetics of silica nanoparticles as well as the drying. This process leads to the formation of a gel with well-defined mechanical properties and permeability. A phase diagram evidences two regions in the parameter plane, i.e. the physicochemical properties versus the evaporation rate: a region where the drop undergoes an isotropic shrinkage and forms an homogeneous gel and a region where mechanical instabilities (cracking, buckling) appear. We proceed to a multiscale analysis from the microscopic scale by studying the structure of silica gels (X-ray scattering) to the macroscopic scale through mechanical properties measurements (indentation testing).

Finally, we have shown that the addition of a model protein (Bovine Serum Albumine) in the hydrogel bead, results in the reinforcement of the structure, involving a controlled deformability. Such a process has potential applications in drug delivery