Fabrication And Mechanism Of Hydrogel Composites With Multiple Responsive Behaviors

Hydrogel is a kind of material with three-dimension network structure and possesses excellent hydrophilicity,low toxicity and benign elasticity modulus.Stimuli-responsive hydrogels can respond to the environmental changes,such as temperature,pH,light,electric or magnetic field,solvents,and change shape macroscopically.Hydrogels with responsive behaviors have been studied widely and developed extensive applications in soft actuation,sensors,bionic devices,and biological medicine.Anisotropic hydrogels can be preprogrammed into various structures with local sensitivity,thus realizing flexible control of responsive behaviors and giving rise to extensive attentions in many fields.Combining multiple responsive modes into one entirety hydrogel can provide materials more applications.However,the researches focus on single response modulation and pay fewer attention to anisotropic hydrogels with multiple environmental responses.Therefore,we fabricated multi-responsive hydrogel composites via chemical programming and studied its mechanism based on structural design.The main researches are as follows:1.Two kinds of single active layer hydrogel,N-type and D-type,were designed and fabricated based on one positive layer with poly(N-isopropylacrylamide)(PNIPAM)and poly(2-(dimethylamino)ethyl methacrylate)(PDMAEMA)and one negative layer with copolymerization of sensitive monomer and acrylamide.We systematically studied the responsive behavior of two type hydrogels.For N-type hydrogel,PNIPAM possesses certain LCST which endows temperature and ion responses.For D-type hydrogel,PDMAEMA can take place phase transformation and protonation in high temperature and acid.We added dyes into the hydrogels to indicate the response behavior that bromocresol green doped in D-type hydrogel can change color from blue to yellow accompanying with deformation in acidic solution.Additionally,we fabricated single-component and multi-component hydrogel composites.Through structure preprogramming different conjunction to combine hydrogel bulks,the composites can imitate flexible motions such as underwater writing,arms bending,and limbs actuation,coordinating with environments variation(temperature,pH,or ionic concentration),which provide guides to visual monitoring.2.We fabricated one kind of bilayer hydrogel PNIPAM/PDMAEMA with dual active layers and studied its temperature and pH dual response.The swelling difference between two layers in acid and ethanol provides hydrogel bidirectional bending behavior.Additionally,the hydrogel can revert back in NaOH solution quickly after deformation in high temperature based on response behavior of dual active layers.We studied the relationship between swelling property and curvature and found that deformation degree of the hydrogel can be estimated by difference between relative swelling ratios.Based on the bilayer hydrogel,we designed one four-arm gripper that can grasp and release the weight with the variation of temperature and pH.Additionally,we fabricated one circuit switch,which presented bidirectional bending behavior in acid and ethanol to lighten the LEDs selectively.This material can pave way for soft actuation and ionic circuits.3.Two kinds of anisotropic hydrogels with fluorescence,bilayer-type hydrogel and composite hydrogel,were fabricated via chemical copolymerization and physical doping separately.One synthesized coumarin-based fluorescence monomer was introduced into the bilayer hydrogel.We studied the photochemical property and fluorescence of the hydrogel,suggesting dimerization/cleavage under 365nm and 254nm ultraviolet lights.The composite hydrogel was fabricated through light curing with mask and congo red and bromocresol green dyes were introduced by physical doping.In acidic solution,the dot section(PDMAEMA)presents color change from red to blue and other sections(PNIPAM)present fluorescence intensity reduction.The composite hydrogel also possesses temperature/pH responsive behavior that can be preprogarmmed into various deformation shapes.4.We designed and fabricated nanocomposite hydrogels doped with graphene oxide(GO)and clay,PNIPAM-GO and poly(N,N-dimethylacrylamide)-GO(PDMAA-GO)and characterized their structural and mechanical properties.We found that the introduction of GO and clay guaranteed good mechanical strength and self-healing behavior under near-infrared(NIR)light.On the basis of two hydrogels,the bilayer hydrogel was fabricated that can realize local folding with the guide of NIR laser.Afterward,curtain-type and petal-type light-driven hydrogel valves were fabricated with the initial state closed,and then irradiated by NIR to control local deformation of valve,thus achieving solid/liquid transportation and controllable reaction trigger.Based on its self-healing property,we combined bilayer hydrogel and PDMAA hydrogel to fabricate "hinge-type"hydrogel,which can achieve fast opened and closed motions under irradiation and maintain good folding and recovery within multiple cycles.