Multifunctional 2D hydrogels based on graphene and cellulose derivatives

Through the self-assembly of the biomaterial hydroxypropyl cellulose (HPC) and the two-dimensional material graphene oxide (GO), a new type of two-dimensional material that can be electrically and thermally controlled was constructed. The reversible hydrophilic/hydrophobic switching of HPC enables the 2D hydrogel excellent water transport capacity under temperature stimulation, which also leads to significant changes in the optical properties. Briefly, the ability of HPC to switch between hydrophobic and hydrophilic states enables it to exhibit birefringent responses in the two-dimensional (2D) environment of GO and layers. When the HPC undergoes a phase transition, it induces a change in the ordering and arrangement of the GO surface bound to it. Next, a composite material consisting of conductive reduced graphene oxide (rGO) and 2D hydrogel was developed. Using the conformational and alignment changes of HPCs as switches to mediate electrical energy transfer via Joule heating and thermal stimulation, electrothermal control valves were constructed to modulate optical properties and water transport. The as-prepared conductive 2D hydrogel exhibits excellent mechanical properties (Young's modulus of 2.5 GPa) and excellent electrical conductivity (176S cm-1) without sacrificing the superior swelling capacity. Furthermore, based on the motion of the GO layer regulated by the phase transition of the polymer chains, we fabricated a 2D rGO/HPC hydrogel-based artificial muscle. Attribute to the conductive rGO region, the movement of artificial muscle can be controlled by applying bias. Here, a general and sustainable method to synthesize new low-dimensional robust multifunctional hydrogels is demonstrated. The sustainable method for self-assembly of HPCs in the two-dimensional confinement states of GO and rGO presented here is applicable to the entire family of polymers with low critical solution temperatures (LCST). These stimuli-responsive composite 2D materials are considered excellent candidates for fluid transport systems, smart actuators, and bio-engineering applications.