Controllable Shape Morphing Of Hydrogels Based On Natural Polymer

As a highly crosslinking polymer material,hydrogels can rapidly swell but not dissolve in water.Because of its high similarity to biological tissues for its physical and chemical properties,hydrogels have shown great potential in tissue engineering,drug delivery,and actuator.However,it is necessary to design the structure and shape of hydrogels in order to meet the requirements of applications,thus resulting to the complexity of preparation and design of hydrogels.Therefore,how to efficiently and simply design hydrogel’s structures to meet the application has become the current research hotspot.In this paper,a new method of tuning the deformation and actuation of hydrogel films is proposed.Based on the inherent structural characteristics of the materials,the deformations and/or behaviors of hydrogel films were well controllable.To confirm the effectiveness of the method,natural polymers based on hydrogels were prepared by natural polysaccharides（Chitosan,Starch and Sodium alginate）,which are very common in nature.The deformations and behaviors of hydrogel films were studied,respectively.First,the hydrogel films were prepared by the reaction of the chitosan（-NH₂）with glutaraldehyde（-CHO）solution（Schiff base reaction）.With the gravity effect and cross-linking agent concentration difference,the pre-polymer solution was crosslinked through the top-down method of crosslinking agent.Thus,the cross-linking density difference along the thickness of the hydrogel was constructed by the one-step method.The structural differences caused by crosslinking density could be adjusted by tuning the geometry of the hydrogel films.Therefore,pre-setting the geometry of the hydrogel films（thickness-h,length-L₀,width-W₀,and length-to-width ratio-L₀/W₀）would precisely adjust the three-dimensional deformation of the hydrogel films（such as diagonal rolling,helix,short-side rolling,and long-side rolling）.Combined with the"Kirigami"process and the p H-responses characteristics of chitosan,various complex three-dimensional shapes were simply prepared,and the bending behavior was also dynamically adjusted.In addition,the layer-structure of starch-based hydrogel films was formed in the evaporation process of starch solution.Then a simply starch-based hydrogel films actuator was constructed.The starch solution was blended with sodium alginate solution to improve the mechanical properties of films.Because of the existence of layer structure,the sensitivities of both sides of the composited films to temperature and humidity were different,which made starch-based hydrogels show thermo-and humidity-sensitivity.Therefore,an easy but interesting stimulus response membrane actuator was constructed.Finally,the materials and processes in this paper are very simple but effective,which provide inspirations for the applications of natural polymer materials in the field of hydrogel bionics.