Study Of The Tuning Of Mechanical Properties Of Tough Hydrogels Based On The Valence State Of Iron Ions And Its Applications

Tough hydrogels have great potential for applications in the areas of soft robotics and healthcare.For example,flexible devices made out of tough hydrogels need to exhibit relatively stable performance when subjected to large deformations,such as stretching,bending,twisting,etc.Therefore,the engineering of mechanical properties of tough hydrogels has attracted great interest in the scientific community.People are devoted to develop a simple and feasible method to tune the mechanical properties of tough hydrogels,such as toughness and stiffness,on demand.On the basis of recent findings,this thesis focuses on the study of the tuning of mechanical properties of tough hydrogels based on the valence state of iron ions and its applications,including mainly the following research contents:Firstly,the softening and shape-morphing of stiff and tough hydrogels by localized unlocking of the Fe³⁺trivalent ionically cross-linked centers was studied.The mechanical properties of hydrogels?such as stiffness,stretchability?are critical for a variety of applications in tissue engineering,soft robotics and medicine.Herein,we have developed a feasible method to fabricate ultra-soft and highly stretchable structures from stiff,less stretchable,and tough hydrogels,and applied these switchable hydrogels to programmable shape-morphing systems.The stiff and tough structures of hydrogels are fabricated via the mechanical strengthening of Ca²⁺-alginate/polyacrylamide?PAAm?tough hydrogels by adding Fe³⁺ions,which introduces the Fe³⁺ionically cross-linked centers into Ca²⁺divalent cross-linked hydrogels to form an additional and much stiffer trivalent ionically cross-linked network.The obtained stiff and tough hydrogels are immersed in an L-ascorbic acid?vitamin C?solution to reduce Fe³⁺to Fe²⁺ions.As a result,the trivalent ionically cross-linked networks transform into flexible divalent ones,leading to rapid softening of the stiff and tough hydrogels.In addition,the localized variation in stiffness of tough hydrogels can be achieved by sequential steps of precise patterning with a vitamin C solution,resulting in the stiffness-mismatched structures in hydrogels.By use of this strategy,the localized softening,unfolding and sequential folding of tough hydrogels into pre-programmed complex three-dimensional?3D?structures is demonstrated.Therefore,this strategy provides an effective way to tune the mechanical properties of tough hydrogels either in bulk or locally and to control their shape-morphing behaviors.Secondly,the site-specific Fe²⁺-oxidation-induced stiffening and shape-morphing of soft and tough hydrogels was studied.Living biological tissues are composed of structures with appropriately defined mechanical properties?e.g.,toughness and stiffness?toward specific biological functions.Here,we have developed a chemical manipulation strategy to locally alter the oxidation state of Fe ions from divalent to trivalent in tough hydrogels.The resulting trivalent ionically cross-linked networks become stiffer,which results in a significant increase in the stiffness of tough hydrogels.The mechanical strengthening of Fe²⁺/Ca²⁺-alginate/PAAm tough hydrogels is demonstrated via the oxidation with ammonium persulfate?APS?.Furthermore,by applying the surface patterning techniques,the mechanical properties of tough hydrogels can be locally enhanced and thus,can be used as anisotropic elements to guide the shape-morphing of tough hydrogels into complex three-dimensional structures.This approach opens up a simple strategy not only to alter the mechanical properties from pre-fabricated soft and tough hydrogels either in bulk or locally,but also to control their shape-morphing behaviors on demand.