Xyloglucan-cellulose nanocrystal-chitosan double network hydrogels for soft actuators

Hydrogels are materials consisting in a three-dimensional hydrophilic polymer network swollen by a large amount of water. An efficient strategy to elaborate hydrogels consists in establishing double polymer networks in order to achieve high strengthening effect associated with other properties such as transparency or tailored swelling capacities. In this work, we prepared cellulose nanocrystals (CNC)-based hydrogels with double network architecture. The first network, formed by CNC and xyloglucan (XG), takes advantage of entropic adsorption of XG on the CNC surface while the second network relies on electrostatic interactions between cationic Chitosan (Chi) and anionic CNC. Hydrogels with different compositions were successfully prepared. Their rheological properties, stability and swelling capacities in acidic and alkaline solutions were evaluated. Internal organizations of hydrogels were investigated by fluorescence microscopy after polymer labelling and polarized optical microscopy (POM). Finally, hydrogels demonstrated excellent mechanical properties and tuneable swelling capacities that can be leveraged for the implementation of bilayer actuators. Therefore, we further prepared films composed of two hydrogels layers, each one containing a different XG/CNC ratio. Bilayered films bended in water due to the asymmetric swelling of layers and the extent of bending can be modulated by the XG/CNC ratio.