Strain-stiffening composite hydrogels through UV grafting of cellulose nanofibers

Strength, toughness and strain-induced stiffening behaviors of hydrogels are essential for the case of load-bearing applications. However, these mechanical properties are rarely integrated into a synthetic hydrogel. To this end, we adopted the method of UV grafting to fabricate cellulose nanofiber (CNF)-based composite hydrogels. Strain-stiffening behavior was particularly focused on owing to the great significance of this feature for soft tissues. Poly(2-hydroxyethyl methacrylate) (pHEMA) matrix was grafted from the surfaces of CNFs under UV irradiation in the absence of any initiator, resulting in a polymer-grafted-nanofiber architecture. The synergistic alignment of CNFs and pHEMA networks during stretching led to strain-stiffening behavior. To enhance this behavior, pHEMA chains were further grafted from pHEMA-grafted CNFs instead of pure CNFs. As a result, the ratio of elastic modulus before fracture to initial elastic modulus was as high as 15.4. This was the first time to report CNF-based hydrogels with excellent mechanical properties by the method of UV grafting. The strategy of double grafting was also an effective approach to obtain nanocomposites with interesting features.