Mechanical properties of model and natural gels have recently been demonstrated to play an important role in various cellular processes such as adhesion, proliferation, and differentiation, besides events triggered by chemical ligands. Understanding the biomaterial/cell interface is particularly important in many tissue engineering applications and in implant surgery. One of the final goals would be to control cellular processes precisely at the biomaterial surface and to guide tissue regeneration. In this work, we investigate the substrate mechanical effect on cell adhesion for thin polyelectrolyte multilayer (PEM) films, which can be easily deposited on any type of material. The films were cross linked by means of a water-soluble carbodiimide (EDC), and the film elastic modulus was determined using the AFM nanoindentation technique with a colloidal probe. The Young's modulus could be varied over 2 orders of magnitude (from 3 to 400 kPa) for wet poly(L-lysine)/hyaluronan (PLL/HA) films by changing the EDC concentration. The chemical changes upon cross linking were characterized by means of Fourier transform infrared spectroscopy (FTIR). We demonstrated that the adhesion and spreading of human chondrosarcoma cells directly depend on the Young's modulus. These data indicate that, besides the chemical properties of the polyelectrolytes, the substrate mechanics of PEM films is an important parameter influencing cell adhesion and that PEM offer a new way to prepare thin films of tunable mechanical properties with large potential biomedical applications including drug release.

Highlights d Influenza alters the production of SCFAs by the gut microbiota d The dysbiotic microbiota transfers susceptibility to respiratory bacterial infection d Supplementation with acetate restores the killing activity of alveolar macrophages d Activation of the SCFA receptor FFAR2 protects against bacterial superinfection

The buildup and secondary structure of poly(L-glutamic acid)/poly(allylamine) (PGA/PAH) multilayer films were investigated by means of optical waveguide lightmode spectroscopy, quartz crystal microbalance, and Fourier transform infrared spectroscopy in attenuated total reflection mode. The thickness and the mass of these films grow exponentially with the number of deposited bilayers. Moreover, PGA undergoes a random/R-helix transition when interacting with PAH during the film buildup process. This structural transition leads to (PGA/PAH)i films with an R-helix content (contribution of the R-helices to the amide I band) that switches regularly between 30% and 40% during the film buildup, when the multilayer is alternatively brought into contact with the PAH and PGA solutions. The secondary structure of the film is thus entirely driven by the last deposited layer. The independence of the R-helix content with the number of deposited bilayers also strongly suggests that the film is structurally homogeneous over its whole thickness.