Anisotropic Hydrogels Prepared By Limited Swelling And Its Application

Most hydrogels were formed by polymerization of monomers in water or by self-assembly of macromolecules,and their structure was usually isotropic due to their uniform dispersion in solution.In contrast,human skin,muscles,joints and cartilage tissues or plant stalks had well-defined hierarchical structures,and their anisotropic orientation reached macroscopic scale,which played an important role in living organisms,making it possible to perform specific functions such as mass transfer,lubrication,muscle contraction,etc.;and compared to isotropic hydrogels,anisotropic hydrogels were widely used in smart actuators,smart valves and gates,soft robots and Bionic skin sensors and other fields.Therefore,it was crucial to prepare hydrogels with anisotropic structures.Up to now,there were four main types of anisotropic structures:gradient structure,oriented structure,patterned structure and bilayer structure.And so far,the following methods were usually used to prepare hydrogels with anisotropic structures:（i）Control the distribution of one-dimensional or two-dimensional molecular materials under applied conditions such as gravity,magnetic field,electric field and polymerize to form.（ii）Orient the chains of a polymer network under the action of external forces,such as stretching,drying and so on.（iii）By controlling the cross-linking region through the mask,local hydrogels with different properties could be prepared,such as photomask method,ionic crosslink lithography（ICL）and so on.（iv）The photo-polymerization process was characterized by a gradual decrease in light intensity with increasing solution depth,which would lead to differences in the polymer network formed after the polymerization of monomers.Besides,it also applied to directed ion diffusion.（v）By combining directional freeze-casting and a subsequent salting-out treatment,the hydrogels could have unidirectional channels which assigned anisotropy to them.（vi）Though hierarchical aggregation,two different components of the hydrogel were attached together to form the hydrogel with bilayer composite structure.However,there were some problems in the above molding methods,such as poor interfacial stability,single response and function,and complex molding process.Based on various challenges,it was necessary to develop a method for the preparation of anisotropic hydrogels that were interface-free,multifunctional,designable,and had a simple molding process.The interface was generated by layered curing,so the interface could be eliminated by diffusion distribution of the second component in the first network followed by curing.In this study,based on the super-hydrophilic nature of the precursor hydrogel,the limited growth after absorption of the external solution made the second component present a gradient distribution in the first layer network.In addition,the matrix hydrogel and external solution could be designed on demand,and the growth direction was controllable and diverse.Firstly,the feasibility of gradient anisotropic hydrogels prepared by limited swelling growth method was verified.Though matrix screening,it was found that hydrogels（HD hydrogels）with only ion-based monomers could grow,and a series of studies were conducted on the influence of external environment on swelling growth process,and based on this method,two kinds of hydrogels HD₂₀/HPA₂₅ and HD₂₀/HPA₅₀were successfully prepared.It was proved that the hydrogel prepared by this method had anisotropic structure through the piecewise mechanical property test,distribution of element,electrical conductivity test,swelling ratio,all-band UV test and temperature change 550nm transmittance test.Secondly,the gradual smart response hydrogels were prepared by combining the method with temperature stimulated response（NIPAM）hydrogels.Its phase transition changed gradually with increasing temperature,and the transition temperature transition window of each section of the gel was tested by variable temperature 550 nm transmittance.By limiting the thickness and the concentration of external soaking solution,the gel transition temperature range and transition window could be regulated,and the optimal gel coverage temperature was 14-26,and it also had excellent environmental,time and cycling stability.In addition,by changing the base hydrogel(HD₂₀hydrogel to HDy₂₀ hydrogel)and introducing Dy elements with photothermal response,the transition from single temperature response to dual temperature-light response could be achieved.Finally,anisotropic hydrogels with gradient stress structure were prepared by changing the limited direction（growth direction）.The hydrogels were applied to pressure sensors,which could realize the detection range of high sensitivity and width detection range（486 Pa-1.17 MPa）.Besides,it could be suitable for low temperature environment（-10）and various material surfaces.Its electrical sensing had excellent cycling performance.