Ion-Mediated Gelation of Thermo-Responsive Cellulose Nanofibril/Poly(N-isopropylacrylamide) Hybrid Hydrogels with Tunable De-Swelling Kinetics

The tunability of the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAM) to lower or higher temperatures, as well as the ease of modulation of the LCST phase transition kinetics broadens the scope of application of PNIPAM-based materials in biomedical fields. This work reports a facile approach to formulate a smart, injectable cellulose nanofibril (CNF)/PNIPAM hybrid gel. Hofmeister salts are used to induce ion-mediated gelation of the nanofibrils and PNIPAM chains, resulting in an interpenetrating network (IPN) structure. From rheological measurements, the hybrid material displays excellent structural integrity at room temperature and tunable thermo-stiffening around body temperature. De-swelling kinetics can be modulated by varying the nature and concentration of the Hofmeister ion used. The successful realization of the IPN hybrid gel structure is dependent on the molecular weight of PNIPAM used. Moreover, the hybrid gels show good thermo-reversibility during thermal cycling, as well as excellent injectability and remarkable self-healing post-injection, owing to shear-thinning and thixotropic characters. Since rheology is a crucial technique in the analysis of soft matter and flow behavior is fundamental for the design and synthesis of application-specific viscoelastic materials, the work reported herein provides a rheological basis for careful design and synthesis of smart gels.