Bio-inspired water resistant and fast multi-responsive Janus actuator assembled by cellulose nanopaper and graphene with lignin adhesion

Flexible actuators with responses to various environmental stimuli have attracted great attention, but the issues of weak layer-to-layer bonding and low water resistance of cellulose-based actuators severely restrict their practical application. Herein, inspired by the internal structure of lignocellulose and reversible opening movement of pine cones by external humidity stimuli, a novel Janus film consisting of the insulated lignin-containing cellulose nanopaper (CNP) layer and the conductive reduced graphene oxide (rGO) layer was delicately designed and well fabricated through lignin adhesion between the two layers. Characterization showed that the obtained Janus film with a tensile strength of 141 MPa could be used as a flexible actuator, which could be operated based on water absorption and dehydration of the CNP layer as well as the light-to-heat conversion and high conductivity of rGO layer. Thus, the as-fabricated Janus film with excellent durability (folding resistance (>1000 times), good wet stability, and durable electrical actuation (>500 cycles)) had quick multi-responses to humidity, near-infrared radiation and electrical stimulation. Particularly, the resultant Janus film had the fastest response speed (1.875°/s/V) under the electrical stimuli compared to the reported carbon/polymer-based flexible actuators with comparable weights. Therefore, this CNP-based Janus film can be potentially used in many fields such as humidity sensor, bionic artificial arm/driving motor, and it could be helpful to develop facile and environmentally friendly strategies for the construction of multi-responsive Janus actuator/intelligent devices.