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In the field of tissue engineering, in order to restore tissue functionality hydrogels that closely mimic biological and mechanical properties of the extracellular matrix are intensely developed. Mechanical properties including relaxation of the surrounding microenvironment regulate essential cellular processes. However, the mechanical properties of engineered hydrogels are particularly complex since they involve not only a nonlinear elastic behavior but also time-dependent responses. An accurate determination of these properties at microscale, i.e. as probed by cells, becomes an essential step to further design hydrogel-based biomaterials able to induce specific cellular responses. Atomic Force Microscopy (AFM) with contact sizes of the order of few micrometers constitutes an appropriate technique to determine the origin of relaxation mechanisms occurring in hydrogels. In the present study, AFM force relaxation experiments are conducted on chemically and physically crosslinked hydrogels respectively based on a synthetic polymer, polyacrylamide and a natural polymer, a bacterial exopolysaccharide infernan, produced by the deep-sea hydrothermal vent bacterium, Alteromonas infernus. Two distinct relaxation mechanisms are clearly evidenced depending on the nature of hydrogel network crosslinks. Chemically crosslinked hydrogel exhibits poroelastic relaxations, whereas physically crosslinked hydrogel shows time-dependent responses arising from viscoelastic effects. In addition, two relaxation processes are revealed in ionic physical hydrogel originating from chain rearrangement and breaking/reforming of the ionic crosslinks. The effect of the ionic strength on both the long-term elastic modulus and relaxation times of physical hydrogels was also shown. These findings highlight that physical hydrogels with well-defined time-dependent mechanical properties could be tuned for an optimized response of cells.

Acid milk gels with fat (3.5 or 10%) or without fat containing 0.2–1% of whey proteins (WP) or 0.2–1.5% of protein aggregates (fractal aggregates, microgels and mixed casein/WP aggregates with an 80/20 ratio) were investigated. The fat-containing systems were homogenized and the systems were preheated (90 °C, 11 min) and acidified to a pH of 4.6 by adding glucono-delta-lactone. The protein composition of the fat droplet interface was characterized by SDS-PAGE and the textural and rheological properties, microstructure, and whey separation of acid networks were determined. WP and fractal aggregates showed the best ability to improve the textural properties and microstructure of all acid milk gels and reduce whey separation. Increasing the concentrations in microgels and mixed aggregates did not lead to an increase in gel firmness or generate a strong impact on the protein network, but a high concentration in mixed aggregates could reduce whey separation. The fat droplet interface was made almost exclusively of caseins, even if the fractal aggregates could also be adsorbed when the interfacial surface was increased either through fat content or through an increase in homogenization pressure. Adding proteins changes the textural properties of the acid gels in all the systems, mainly due to their role in the continuous phase.

International audience; Bone Morphogenetic Protein (BMP-2) is an osteoinductive growth factor clinically used for bone regeneration. Tuneable sustained strategies for BMP-2 delivery are intensely developed to avoid severe complications related to supraphysiological doses applied. To address this issue, we investigated the ability of the bacterial exopolysaccharide (EPS) called Infernan produced by the deep-sea hydrothermal vent bacterium Alteromonas infernus, exhibiting both glycosaminoglycan-mimetic and physical gelling properties, to efficiently bind and release the bioactive BMP-2. Two delivery systems were designed based on BMP-2 retention in either single or complex EPSbased microgels, both manufactured using a microfluidic approach. BMP-2 release kinetics were highly influenced by the ionic strength, affecting both microgel stability and growth factor/EPS binding, appearing essential for BMP-2 bioactivity. The osteogenic activity of human bone-marrow derived mesenchymal stem cells studied in vitro emphasized that Infernan microgels constitute a promising platform for BMP-2 delivery for further in vivo bone repair.