Applications And Finite Element Modeling Of Hydrogels

Hydrogel has been a focal area of research due to its unique stimuli responsiveness and great biocompatibility since first synthesized 60 years ago.Especially in the past 20 years,hydrogel has attracted more and more attention because of its potential applications.People have developed various types of hydrogels with different characteristics and functions and applied them to tissue engineering,drug delivery,cell culture,artificial organs,water treatment,sensors,actuators,coatings,wearable electronic devices,soft robots and so on.This thesis mainly introduces two experiments of gel made by 3D printing and multistep lithographic polymerization and their potential applications.Also,the mechanisms of gel deformation are analyzed by simulating the swelling processes using a finite element method.The first experiment is about 3D printing strip gel swelling.It is found that gel has interesting deformation behaviors in water: bending,recovery,reverse-bending,recovery and remain unchanged when swelling in water.The two successive bending behavior in opposite directions are unexpected and interest us most.We reveal the series of gel deformation and analyze the stress distribution and solvent concentration distribution inside the gel using a finite element method.It is found that the peculiar gel deformation behaviors are caused by the following factors: the flow of precursor solution before complete curing results in an asymmetrical cross-section of the printed gel filaments.The nonuniform axial stress on the cross section due to the interdiffusion of the two solvents.The asymmetric cross section results in the separated centroids between the part under tension and that under compression,and the resultants of tensile and compressive stresses create a couple that bend the gel.Interdiffusion of the two solvents with different kinetics.In the gel,there two solvents diffusive in opposite directions(rapid inward diffusion of water molecules and slow outward diffusion of organic solvent molecules).The diffusion process is first dominated by water and then by organic solvent(at this time,the water is close to swelling equilibrium).The alternations of two solvents lead to the switching of bending directions.Our finite element simulation confirms this mechanism and agree well with experimental results.In the second application,the gel structures are made by multistep lithographic polymerization based on the idea of kirigami design.A soft and highly swollen gel is made into a two-dimensional kirigami structure and confined within a non-swelling gel frame.After swelling,the swelling gel buckles or twists due to the constrained expansion.Structures with different configurations can be designed by selectively controlling the buckling direction of different parts of the high-swelling gel.For example,hierarchically integrated kirigami designs can create three-dimensional multi-level gel structures which can be applied to the design of circuit multi-contact switches.Finite element simulations aid the design and verified the mechanisms.Smart hydrogels have good application prospects in biomedical equipment,flexible electronics and soft robots.The two mechanisms demonstrated in this thesis may be of significance in further widening the applications of hydrogels.