Adhesives are made of polymers because, unlike other materials, polymers ensure good contact between surfaces by covering asperities, and retard the fracture of adhesive joints by dissipating energy under stress. But using polymers to 'glue' together polymer gels is difficult, requiring chemical reactions, heating, pH changes, ultraviolet irradiation or an electric field. Here we show that strong, rapid adhesion between two hydrogels can be achieved at room temperature by spreading a droplet of a nanoparticle solution on one gel's surface and then bringing the other gel into contact with it. The method relies on the nanoparticles' ability to adsorb onto polymer gels and to act as connectors between polymer chains, and on the ability of polymer chains to reorganize and dissipate energy under stress when adsorbed onto nanoparticles. We demonstrate this approach by pressing together pieces of hydrogels, for approximately 30 seconds, that have the same or different chemical properties or rigidities, using various solutions of silica nanoparticles, to achieve a strong bond. Furthermore, we show that carbon nanotubes and cellulose nanocrystals that do not bond hydrogels together become adhesive when their surface chemistry is modified. To illustrate the promise of the method for biological tissues, we also glued together two cut pieces of calf's liver using a solution of silica nanoparticles. As a rapid, simple and efficient way to assemble gels or tissues, this method is desirable for many emerging technological and medical applications such as microfluidics, actuation, tissue engineering and surgery.

We discovered that the free radical polymerization of N,N-dimethylacrylamide in water can lead, above a certain concentration, to gels without any added difunctional crosslinker. These so called ''selfcrosslinked'' hydrogels were prepared and their weak mechanical properties were improved by introducing silica nanoparticles. From swelling experiments performed at equilibrium in aqueous media, it was shown that silica particles behave as adhesive fillers and strongly interact with PDMA chains. These interactions are responsible for the reinforcement of mechanical properties. From initial elastic moduli, determined in the preparation state, we show that the elastic behaviour of these hydrogels mainly originates from entanglements and from physical crosslinks that can be controlled by the polymer concentration and the ratio between silica particles and polymer chains, respectively. The mechanical behaviour was characterized using: monotonic tensile tests, loading-unloading cycles at large strains and stress relaxation experiments in order to investigate long time behaviour. The introduction of silica highly increases the stiffness of the network without greatly reducing its extensibility, implying that strong interactions take place between PDMA chains and silica surfaces. Non-linear behavior was pointed out: softening at small deformations and hardening at high deformations which is related to finite chain extensibility. All these effects have been shown to strongly depend on the silica content. The analysis of hysteresis and residual strains induced by cycles, clearly indicate that contrary to chemical crosslinkers, hybrid interactions increase the dissipative process.Physico-chimie des Polym eres et des Milieux Dispers es, UMR 7615,