Extremely stiff and strong nanocomposite hydrogels with stretchable cellulose nanofiber/poly(vinyl alcohol) networks

Many load-bearing applications of hydrogels, such as human tissue replacements, require extremely high stiffness, strength and fracture energy, which are rarely achieved simultaneously. Here, we report cellulose nanofiber-based nanocomposite hydrogels, exhibiting highly increased stretchability but with no sacrifice in stiffness when compared with pure rehydrated cellulose nanofiber networks. The simple processes include suffusing a cellulose nanofiber cake with poly(vinyl alcohol) solution, drying at room temperature, annealing at 80 °C and 120 °C, and rehydration in water. During the drying and annealing processes, strong interactions (hydrogen bonds) between cellulose nanofiber and poly(vinyl alcohol) are formed instead of interfibrillar hydrogen bonds, resulting in more stretchable cellulose nanofiber networks. Thus, the optimized sample reaches an elastic modulus of 47.92 ± 0.99 MPa, a fracture strength of 15.91 ± 0.48 MPa and a fracture energy of 2302.9 ± 42.0 J/m², which are almost equal to some human tissues. In addition, it contains high water content (66.8%), which is also close to the values of skin and cartilage. This article provides a simple method for preparation of load-bearing hydrogels from nanocelluloses.