Multi-Stimulus-Responsive Hydrogels And Assembly Deformation Through Host-Guest Interaction Of Cyclodextrin

Hydrogels are three-dimensional network structures formed by physical or chemical crosslinking,exhibiting high water content and moderate water retention.Some organisms in nature(such as mimosas)will curl when exposed to external force,and the stimuli-responsive hydrogels in hydrogels also have similar functions,which can produce specific responses to different external stimuli,resulting in polymer chain conformation transitions or polymer network changes.As a result,significant changes such as bending deformation,color change and phase transition are produced,so they can be used as drug-controlled release materials,tissue engineering materials,environmental sensors,intelligent actuators,etc.However,the problems of complexmonomer synthesis,slowresponse speed,and difficulty inremote control limitthe application of such hydrogels.The disadvantages of poor mechanical properties,sirgle function,and inability to achieve gradual deformation also restrict the development.Therefore,it is very important to prepare a multifunctional smart hydrogel by introducing a variety of functional units or components,which is of great research significance to improve its precise controllability,response rate,and mechanical properties.In this paper,different stimuliresponsive hydrogel modules based on thermosensitive hydrogels were explored to obtain fastresponsive physical/chemical cross-linking hydrogels,and then the macro-scale bilayer hydrogels were realized through host-guest interaction.Anisotropic deformation is generated by utilizing the difference in responsiveness of different hydrogels to the same stimulus.In this way,single-function hydrogels are assembled into multifunctional hydrogels with various properties such as deformation,color change,or fluorescence,accompanied by a great increase in response rate.It is verified that there is a host-guest interaction between β-cyclodextrin(β-CD)and the isopropyl group of N-isopropylacrylamide(NIPAM)through theoretical simulation calculations and experimental methods,and a variety of stimuli-responsive hydrogels were synthesized using the host-guest interaction as a physical cross-linking method.Taking advantage of the excellent photothermal conversion ability of graphene oxide(GO)nanoparticles for near-infrared(NIR),GO/P(NIPAM-β-CD)hydrogel with fast light response and temperature response was prepared,and the swelling kinetic curve and NIR response volume change curve was studied.And the swelling behavior was investigated by Fickdiffusion model.The hydrogel has the ability of remote photocontrollable shrinkage and deformation ability(The deformation time is less than 30 s),and the existence of hydrogen bonding and host-guest interaction also endows the hydrogel with rapid self-healing ability in air at room temperature(The self-healing time is less than 10 s).Then,Mo7/P(NIPAM-β-CD-DAC)with photoreversible color changing ability was prepared by electrostatic interaction between acryloyloxyethyltrimethylammoniumchloride(DAC)and ammonium molybdate(Mo7).At the same time,P(NIPAM-β-CD-TPE)hydrogels with reversible fluorescence properties were prepared by introducing the organic substance tetraphenylene(TPE).The former can achieve a reversible change from colorless to blue within 5 s under ultraviolet(UV)irradiation,and the latter can realize the appearance and disappearance of fluorescence by switching the UV light source on and off and the solvent type.Meanwhile,the reaction rate,controllability,repeatability,and image display capability of the UV responses of these two hydrogels are investigated in detail in this paper.The bilayer structure hydrogel is assembled through the host-guest interaction betweenβCD and isopropyl of NIPAM.The connection between the bilayer hydrogels is firm,and the bilayer hydrogel actuator is obtained.The GO/P(NIPAM-β-CD)hydrogel was designed as its active control layer,and the other two hydrogels were passive layers.Due to the differences in responsiveness to NIR light sources,bilayer hydrogels with light-controlled controllable deformationand photochromic orfluorescence capabilities were synthesized,respectively.And their deformation and discoloration process can be precisely controlled by controlling the illumination parameters,including the realization of gradual deformation and other forms.The above in the bilayer actuator systems are all PNIPAM-based hydrogels with lower critical solution temperature(LCST)properties with active contraction layers and passive bending layers.Furthermore,fatty alcohol polyoxyethylene ether-9(AEO-9)was introduced into the poly(acrylamide-acrylic acid)(P(AAm-AAc))system to prepared AEO-9/P(AAm-AAc-DACβ-CD)hydrogels with upper critical solution temperature(UCST)properties,which greatly improved the response rate since the temperature responsiveness with the opposite trend greatly enhances the driving force of water molecules in and out of the hydrogel.Therefore,the time of bending process is reduced to less than 30 s.