Superelastic, Hygroscopic, and Ionic Conducting Cellulose Nanofibril Monoliths by 3D Printing.

Compressible and superelastic 3D printed monoliths have shown great promise in various applications including energy storage, soft electronics, and sensors. Although such elastic monoliths have been constructed using some limited materials, most notably graphene, it has not yet been achieved in nature's most abundant material, cellulose, partly due to the strong hydrogen-bonding network within cellulose. Here, we report a 3D-printed cellulose nanofibril monolith that demonstrates superb elasticity (over 91% strain recovery after 500 cycles of compressive test), compressibility (up to 90% compressive strain), and pressure sensitivity (0.337 kPa) at 43% relative humidity. Such a high-performance CNF monolith is achieved through both hierarchical architecture design by 3D printing and freeze-drying and incorporation of hygroscopic salt for water absorption. The facile and efficient design strategy for a highly flexible CNF monolith is expected to expand to materials beyond cellulose and can realize much broader applications in flexible sensors, thermal insulation, and many other fields.