Fabrication of ultrastiff and strong hydrogels by in situ polymerization in layered cellulose nanofibers

Integrating high stiffness, strength, and toughness on par with those of soft tissues into synthetic hydrogels is extremely challenging. We have proposed a method to overcome this problem: in situ polymerization of a polymer matrix in layered cellulose nanofibers. In an attempt, ionically cross-linked poly(acrylamide-co-acrylic acid) is fabricated in a wet cellulose nanofiber cake. The resulting hydrogels, called ionically cross-linked nanocomposite (ICN) hydrogels, exhibit a readily adjustable elastic modulus (11.9–190.0 MPa) and high fracture strength (generally > 10 MPa), which are comparable with those of skin and ligament. The high frictional force between the cellulose nanofibers and matrix is responsible for the stiffness of ICN hydrogels; while the tough matrix and weak direct interfibrillar interactions enable good stretchability. We expect that various kinds of cellulose nanofiber/polymer nanocomposite hydrogels with excellent mechanical properties and/or other features can be fabricated by simply changing the monomers.