| The booming progress of soft robotics,wearable electronics and artificial intelligence benefits from the selection and design of actuator materials which can reversibly change their size,external dimension,volume,or physical attributes in response to multifarious external stimuli.Intelligent actuators based on a variety of active materials,such as one-dimension(1D)and two-dimension materials(2D,carbon nanotubes,MXene and graphene),polymers(such as shape memory polymers,gels and liquid crystal elastomers),ceramic and alloy,etc.,have received widespread attention.Although several soft actuators have demonstrated excellent performance in terms of tensile strength,bending angle,response time and actuation speed,and some have even been used to create swimming and crawling robots,there are still some great challenges need to be solved in practical applications.Although several basic investigations into the preparation of multi-responsive actuators have been undertaken,few of them exhibit substantial deformations and controllability,which are urgently required in actual applications.Multiple stimulus responses and stable execution performance are essential for flexible actuators to adapt to complicated environments.Therefore,it is urgent to fabricate a smart and excellent performance flexible actuator which can respond to various stimuli(humidity,heat,or light,etc.).In this work,a bidirectional multi-stimuli responsive soft actuator based on MXenecellulose nanowhiskers(CNWs)-polyvinyl alcohol(PVA)/polydimethylsiloxane(PDMS)multilayer membranes is proposed(MXCP-P).Based on high photothermal conversion capacity of MXene and large difference of coefficient of thermal expansion(CTE)between PDMS and MXene,the MXCP-P achieves an ultra-large amplitude and fast response driven by natural light.The microstructure and mechanical properties of the actuator were studied,and the humidity and photoactuation properties of the actuator were investigated.According to the changes of humidity and optical power density,the application of MXCP-P actuator is discussed.The specific research content is as follows:1.The study of preparation and structure study of multi-stimuli-responsive flexible actuators(1)In this study,MXene was used as the base material,and cellulose nanowhisker(CNWs)with low cost and high aspect ratio was used,and polyethylene pure(PVA)with strong adhesion was used to build strong hydrogen bond interaction between MXene and CNWs.The mixed solution of MXene/CNWs/ PVA was used to fabricate the upper and lower layers,and pure MXene solution was used to develop the middle layer to prepare MXCP composite film.In addition,polydimethylsiloxane(PDMS)was used as the thermo-actuated layer,which was coated on the MXCP film and cured at high temperature.Finally,the MXCP-P flexible actuator with multi-stimulus response and bidirectional bending was prepared.(2)The microstructure of MCCP thin films was investigated in detail.The results show that the MCCP films have gradient interlayer spacing and excellent mechanical properties.The possibility of MXCP-P as an actuator was initially explored.The results show that the multilayered MXene are more suitable as actuator fillers for heat transfer.2.Study on humidity actuation performance of flexible actuator with multi-stimulus response.(1)The hygroscopic mechanism and humidity response of the actuator were studied.MXCP and MXCP-P films realized faster response and larger wet deformation in the human palm due to their high sensitivity to moisture.(2)The content of MXene in MXCP film and the thickness of PDMS in MXCP-P actuator are discussed.A low content of MXene will lead to a small weight of the film itself,which is not conducive to practical application.A high content of MXene will lead to a large thickness of MXCP film,which is not conducive to deformation.Similarly,the optimal thickness of PDMS layer is also selected.(3)A reasonable closed space with adjustable humidity was designed to investigate the wet actuation performance of the actuator.The results show that MXCP-P has excellent humidity response,large shape variable,different bending angles under different moderate conditions,and good controllability and cycling stability.3.Study on photothermal actuation performance of flexible actuator with multistimulus response.(1)The effect of PDMS thickness on photothermal performance was discussed.The results show that the PDMS thickness has little effect,which further confirms that the PDMS thickness discussed in the wet-induced part is the best.(2)The photothermal properties of MXCP-P film are systematically discussed,including whether the temperature field of photothermal conversion of the actuator is uniform,the maximum temperature under different optical power densities,the time required to reach the maximum temperature,and the cycle stability.The results show that the actuator has good photothermal conversion capability.(3)The thermal actuation performance of the MXCP-P actuator was characterized,including the thermal braking mechanism,bending deformation under natural light,and the subsequent simulated sunlight test for quantitative research.The results show that the deformation and temperature change of the actuator are basically synchronous under fixed optical power density,and the maximum surface temperature increases linearly with different optical power density,and the cycle stability is good.In addition,the weight variation of the actuator was also measured under the light cycle,indicating that the photothermal behavior of the actuator is the synergistic effect of the dehydrating contraction of the MXCP layer and the endothermic expansion of the PDMS layer.4.Research on the application of flexible actuator with multi-stimulus response.(1)When the rolling robot is exposed to light,the center of gravity shifts due to the curvature change of the part under the light,and the initial curvature is restored after it is away from the light based on the excellent mechanical properties of the actuator.Based on the above mechanism,the rolling robot achieves continuous motion.(2)A Venus flytrap robot is designed.Different from the above application,this paper uses humidity and light and heat stimulation to display the predatory effect.When stimulated by humidity,the Venus flytrap opens its leaves and prepares to hunt.When light strikes,the Venus flytrap closes its leaves to complete its hunt.(3)MXCP-P actuator can not only be applied to traditional rolling robots and flytrap robots with release structure,but also can be skillfully designed and cooperated with different stimulus sources for some complex deduction and application.This kind of design and innovation not only opens up a new way for the intelligent development of Chinese traditional culture,but also opens up a new field for the application of actuators. |
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