Structure Design And Performance Of Thermal Responsive Soft Actuators

As a new method to obtain mechanical energy,the soft actuators are small-sized,lightweight,flexible and intelligent,which can respond to electricity,light,heat,humidity,chemical atmosphere and pressure.Soft actuators can be potentially applied in artificial muscles,cellular scaffolds,micromanipulators,soft robots,etc.Thermal responsive soft actuators that are driven by infrared light,joule heating,etc,can be combined with humidity-responsive and pressure-responsive actuating materials.Thermal responsive actuation materials include low melting point alloys,shape memory polymers,hydrogels,and carbon nanomaterials.By utilizing the phenomenon of thermal expansion and thermal contraction,deformations such as expansion,bend and rotation can be achieved through structural design for most materials.Actuator design generally includes membrane structure,fiber structure,and cavity structures.Based on different materials and structures,thermal responsive soft actuators still have the following issues:（1）The component selection of thermal expansion/thermal shrinkage materials is limited,and the lack of micro-nano-level fundamental design restricts its multi-functionality.（2）Most thermal responsive hydrogels have poor mechanical properties and can only work in water,which greatly limits their application.（3）Due to the limitation of thermal diffusion rate and actuating methods,most responsive polymer materials have slow response rate,small deformation,and limited driving methods.（4）The actuation usually needs continuous external driving source to maintain,which is called an unstable actuation.It consumes a lot of energy under long-term working conditions and can be easily affected by the environment.Aiming to address the above issues,this thesis generally includes the following topics:Coiled yarn actuator based on carbon nanotube/polyurethane:A high-speed drum is used to collect the electrospun fiber membrane,and the obtained sub-micron carbon nanotube/polyurethane fiber is highly oriented.Twisting the fiber membrane can further improve the orientation,and form the coiled yarns.By changing the loading during twisting,the tensile stress and stroke of the yarn can be controlled.Polyurethane has a negative coefficient of thermal expansion in the axial direction after orientation,and a positive coefficient of thermal expansion in the radial direction,so it will contract along the axial direction when the temperature rises.The carbon nanotubes are uniformly distributed in the polyurethane matrix,ensuring that the yarn has good mechanical strength.At the same time,due to the excellent infrared heat absorption and infrared heat radiation capabilities of carbon nanotubes,the coiled yarns exhibits rapid temperature changes under infrared irradiation,thereby achieving fast expansion and contraction.The coiled yarn actuator has a tensile stroke of 6.7% at 70℃,and can work stably for more than 1000 times.Bilayer actuator based on PNIPAM/MXene/alginate hydrogel:Poly（N-isopropylacrylamide）（PNIPAM） and polyacrylamide（PAM） hydrogels are used to make a bilayer actuator with an asymmetric structure.The water absorption and volume of the PNIPAM layer can change drastically near the phase transition temperature at28℃,while the PAM layer doesn’t have the responsive behavior,so the film can exhibits reversible bending deformation.As a hydrophilic 2D material,MXene sheets can be combined with the PNIPAM molecular chains through the hydrogen bonds,thus greatly improving the mechanical strength and elongation of the hydrogel.At the same time,MXene has excellent infrared heat absorption ability,so the hydrogel film can be driven by infrared radiation in the air.Alginate can greatly improve the mechanical properties of the hydrogel after being cross-linked by calcium ions.Compared with PNIPAM hydrogel,the breaking strength of PNIPAM/MXene/alginate hydrogel is increased by more than 3 times,the elongation is increased by more than 5 times,and the curvature of the actuator can reach 0.29cm^-1.Flexible bilayer actuator based on copper/reduced graphene oxide:By utilizing the reducibility of Cu and the oxidizing property of graphene oxide（GO）,a bilayer actuator with an asymmetric structure is fabricated.One layer is copper,and the other layer is the cuprous oxide/reduced graphene oxide（Cu₂O/rGO） composite.The thickness ratio of the copper layer and the Cu₂O/rGO layer can be adjusted by changing the redox time.The rGO obtained after the reaction uniformly covers the surface of the Cu₂O particles,and the separate Cu₂O particles are bonded together through van der Waals forces,Cu-C and Cu-O-C bonds,so that the film has excellent stability.After 50,000 cycles,there is no significant degradation in performance.Based on the difference in the coefficient of thermal expansion,the maximum curvature can reach 2.4cm^-1.The film inherits the high conductivity of copper,thus can be driven by a low voltage of 1V,and the temperature change within 2s can reach about 170K.rGO has excellent infrared heat absorption and infrared heat radiation capabilities,so that the film can be also driven by infrared irradiation,the temperature change reaches 60K within 2s,and the curvature change reaches 0.8cm^-1.The film can be cut into different shapes for potential application in flexible robotic arms and remote control.Soft bilayer actuator based on bistable electroactive polymer:A thin-film actuator with a double-layer structure is prepared based on bistable electroactive polymer（BSEP）.When the temperature changes near the phase transition point,the modulus of BSEP can change up to 3 orders of magnitude.By modifying the content of octadecyl acrylate in BSEP,the phase transition temperature and modulus variation ratio can be adjusted.As an electroactive layer,the BSEP layer can be driven by Maxwell pressure in the high-temperature phase to produce in-plane expansion.After forming a double-layer film with another soft passive layer,the film will bend out-of-plane.When the temperature drops back to the phase transition point,the film can maintain the actuation state after Maxwell’s pressure is removed.The bistable design means that the actuator can maintain the initial shape and actuating shape without continuous external driving sources.In addition,polyimide strips are used to adjust the bending direction.Finite element analysis （Abaqus Software） was used to simulate the deformation of the actuator,and the strain and stress distribution before and after actuation were analyzed.The above researches focus on promoting the mechanical strength,response rate,and functionality of the thermal responsive soft actuators,which also facilitate the application in artificial muscles,water/underwater soft robots,remote control,and so on.