Research On The Fabrication And Actuation Of Graphene Oxide-based Stimuli-responsive Actuators

Stimuli-responsive actuators are smart devices that can convert various environmental stimulation(such as light,heat,humidity,electric/magnetic fields,pH and chemicals)to mechanical motion by fast and reversible morphology changes.They have revealed great potential in cutting-edge applications including soft robots,micro-electromechanical systems,automatic Lab-on-a-Chip(Lo C)systems and human-machine interface systems.To date,various functional materials have been successfully developed and employed for stimuli-responsive actuators,such as metal,semiconductor,polymer,magnetic material,piezoelectric material,shape memory alloy and carbon material.Among these materials,graphene and its derivatives have been considered as appealing candidates for developing stimuli-responsive actuators due to their unique physical/chemical properties.However,based on the recent works on graphene oxide actuators,we find that the research on graphene oxide actuators is still in elementary stage.In order to realize a real soft robot,there are still many problems to be solved:(1)At present,most of the graphene oxide actuators are driven by single stimulus,which are limited in complex environment.Therefore,it is necessary to develop dual-responsive or multi-responsive actuators.(2)Despite the fact that rapid progress has been made in graphene oxide actuators,current research works are focused on the optimization of response time and the degree of deformation.At present,graphene oxide actuators are only capable of simple deformation,such as bending.Less attention has been paid to the refined control of their deformation.In order to be a real soft robot,it is not enough that the actuator can only achieve simple deformation.It is necessary to develop actuators that can realize complex deformation.(3)Generally,bilayer or multi-layer structure has been widely used for design and fabrication of graphene oxide actuators.However,there may be interlayer separation after frequent actuation,resulting in a poor stability.Therefore,it is necessary to develop stimuli-responsive actuators based on solo material system to improve their stability.(4)Soft robot is actually an intelligent system with the functions of sensing,actuation and energy storage.At present,most of the actuators can only execute actions.Therefore,in order to realize the transformation from actuator to robot,function integration is an urgent problem to be solved.Based on the above-mentioned challenges and problems,we take graphene oxide(GO)as the main material,and propose strategies such as complementary integration of materials,graphene oxide actuator swarm,wrinkles induced by mechanical stress under moisture,heterogeneous healing to realize the dual-response,programmable complex deformation,fabrication of single component actuator with customizable deformation,and integration of actuators/sensors.A series of soft robots have been fabricated.The main results are listed as follows:1.A complementarity strategy for design and fabrication of dual-responsive graphene oxide actuators.We combined the composite layer of nano-size graphite(Nano-G)and polyvinylidene fluoride(PVDF)with GO,forming a bilayer graphene actuator.In response to moisture,GO that can absorb a large amount of water molecules and swell is a positive layer,whereas Nano-G@PVDF is a negative layer.Due to the difference of hygroscopic expansion,the actuator bends towards the Nano-G@PVDF side.On the contrary,under light irradiation,GO that has very small coefficient of thermal expansion(CTE)is negative;whereas Nano-G@PVDF layer is positive,since it is photothermally active and has a relatively large CTE.The large difference in thermal expansion makes the bilayer structure bend towards the GO side.The two layers are complemental for moisture and photothermal actuation.In this way,we demonstrated a bi-directional walking robot and a smart claw array for controllable moisture and light manipulation.The complementarity strategy is a universal concept for designing and fabricating dual-/multi-responsive graphene actuators.2.Programmable deformation of patterned graphene oxide actuator swarm.Inspired from the collective coupling and coordination of living cells,we developed a graphene oxide actuator swarm that enables programmable deformation by integrating SU-8 pattern arrays with GO.The SU-8 micropattern arrays with specific geometries and orientations can couple with the GO film,forming a swarm of bimorph actuators,in which an individual SU-8/GO bilayer can serve as an actuator “cell”.Under external stimulation,each SU-8/GO actuator deforms individually,and the deformation of the entire structure is the collective coupling and coordination of the actuator swarm.In this way,by controlling the size,shape and orientation of the SU-8patterns,more complex deformations can be programmed due to the collective coupling and coordination of the actuator swarm.As a proof of-concept,we further developed several moisture responsive paper robots that can realize multiform deformations under moisture actuation,including smart “dancer robots” and a smart“caterpillar robot”.The present method may open up a new way for precisely controlling the deformation of graphene-based actuators.3.Moisture actuation with customizable deformations based on sole graphene oxide.We prepared a graphene oxide-based actuator that enables customizable deformations.The nanowrinkles on the one side of GO paper induced by the humidity assisted mechanical stress endows GO with the capability of customizable deformations and reversible actuation.Essentially,the stacking GO can be considered as a swarm of single-atom layer 2D nanosheets that are interdependent and inter-constraint with each other through relative weaker interaction,as well as being interactive with external stimuli.Therefore,the capability of customizable deformations is dominated by the stacking arrangement of the GO nanosheets and their unique interaction with guest molecules such as water molecules.With the capability of customizable deformations and the self-healing property,GO has emerged as a novel 2D smart material for robotics.As a proof-of-concept,several moisture responsive soft robots have been demonstrated,including “Venus Flytrap”robot,“Drosera” robot,crawling robot,turning robot and somersault robot.This work not only achieves complex deformation but also avoids the problem of interlayer separation,which greatly promotes the development of graphene-based actuators.4.Heterogeneous self-healing assembly of MXene and graphene oxide enables producing free-standing and self-reparable smart devices integrated with actuators and sensors.The strong interaction of GO and MXene with water molecules endows self-healing ability of MXene and heterogeneous self-healing properties between GO and MXene.The slipping of GO or MXene nanosheets caused by the adsorption and desorption of water molecules and the fracture/recombination of hydrogen bonds account for the self-healing/heterogeneous self-healing process.Notably,MXene is highly conductive,whereas GO is a dielectric material with tunable electronic properties.The heterogeneous self-healing assembly of GO and MXene significantly facilitates the fabrication of free-standing soft electronics and robots.As a proof-of-concept,typical electronic devices including generators,humidity sensors,pressure sensors,moisture-responsive actuators and robots that enable self-healing have been demonstrated based on these two 2D materials.This work provides a new idea for the fabrication and integration of graphene-based smart devices.