Often designed as hydrogels of modified polysaccharides (PS), biomedical adhesives are pertinent alternatives to suture. Biocompatible and offering adapted mechanical properties, their underwater adhesion needs to be strong. PolysacAdh proposes to design biocompatible hybrid hydrogels of un-modified PS by complex coacervation. Known for limiting bacteria growth, chitosan (CS), cationic PS, will be used as major constituting polymer. To form complexes with CS, hyaluronic acid (HA) or alginate (ALG) will be the guest anionic polymers. When its degree of acetylation (DA) is high enough (>45-50%), CS has a specific solution behavior: contrary to low DA CS, it soluble in aqueous solution at physiological pH. Upon addition of HA or ALG, the interpolymer interactions are favored detrimentally to the interactions with water. In order to prevent the formation of kinetic complexes, the hydrogels will be prepared from homogeneous solutions of high ionic strength (screened interactions); the latter will be dialyzed to control the formation of the interactions. The mechanical properties of the formed materials, the chemical structure of the polymers (molar mass, DA, choice HA or ALG, mannuraonate/guluronate ratio of ALG), the relative concentrations CS/HA or CS/ALG, the content of the dialysis bath (pH, presence of multivalent cations) and the presence of nanoparticles (chitin nanofibrils or CS-coated magnetite) will be studied. In contact with controlled polysaccharide-grafted surfaces (biomimetic of the extracellular matrix), the structural, mechanical and adhesive properties of these systems will be optimized. PolysacAdh is divided in three main work packages. 1) Preparation and characterization of hydrogel adhesives based on PS complexes: the conditions for obtaining hydrogels by complex coacervation will be studied. The parameters cited above will be optimized to get the most interesting mechanical and tack (on various substrates, in-air or immersed) properties. The syneresis and the mechanical properties of the hydrogels will be evaluated; the microstructure will be analyzed by scattering techniques, and Raman spectroscopy will be used to find a signature of the interactions. 2) Preparation and characterization of hybrid hydrogel adhesives based on PS complexes: focusing on the most promising hydrogels obtained in (1), the interactions will be diversified by the addition of nanoparticles, incorporated prior to the dialysis step. The aim of their presence is to reinforce the interactions and the dissipative properties of the hydrogels, as well as the interactions with the substrates. 3) Modification and characterization of PS thin films presenting a tunable adhesion: starting from CS-grafted films with various DA (already made, PhD ending in 2021), PS-coated substrates whose surface is controlled (top surface being either CS of controlled DA, HA or ALG, or mixt layers) will be developed. By using the layer-by-layer technique on the grafted CS, the substrates will present a top layer adapted for testing the hydrogel adhesives formed in (1) and (2). These surfaces will be characterized by contact angle measurements, ellipsometry and infrared spectroscopy. Their behavior in solution will also be characterized by quartz microbalance and their swelling by neutron reflectivity. The used substrates (glass, quartz, silicon) show a similar surface chemistry and will be adapted to the characterization technique. To carry out the project, a PhD student will be involved in (1) and (2) and a one-year post-doc will take care of (3). The budget also includes 22 k€ of equipment, 70 k€ of materials and functioning, 6 k€ of travels and 30.6 k€ of overheads.