Study On Strain Sensing Properties Of 3D Printed Conductive Hydrogels

Conductive hydrogel flexible strain sensors are widely used in real-time monitoring of human movement and physiological signals due to their excellent flexibility and stretchability,which exhibit broad application prospects in the fields of health medical equipment and wearable electronic devices.However,conductive hydrogel flexible strain sensors prepared by traditional techniques,such as freeze-thaw and ultraviolet polymerization methods with the help of molds,possess the disadvantages of high preparation cost,time consuming,and low design freedom,which greatly limit the personalized application and development of flexible strain sensors.Compared with traditional techniques,the photopolymerization 3D printing technology avoids the process of mold opening and de-molding and exhibits the advantages of low cost,fast forming speed,and high structural design degree,which provides a new strategy for the preparation of hydrogel flexible strain sensors.Regarding the problems of current photopolymerized 3D printed hydrogel flexible strain sensors,such as the limited selection of photosensitive resins,poor biocompatibility of hydrogels,low sensitivity,lack of water retention,poor resilience and wearability,this study developed a photopolymerized 3D printed hydrogel with promising biocompatibility.Then,different composite strategies of conductive fillers and hydrogels were constructed to improve the sensitivity of strain sensors.Subsequently,physical and chemical crosslinking networks were designed and regulated to optimize the service performance of hydrogels such as water retention and resilience.Finally,various structures were designed to enhance the wearable,mechanical,and sensing properties of strain sensors.The main research contents and results of this paper are as follows:Based on the selection principles of photopolymerized 3D printed hydrogel slurries,the P(ACMO),P(HEMA),and P(PEGDA)hydrogels were prepared,and the mechanical,self-adhesive,and biocompatible properties of the hydrogels were compared.The internal crosslinking mechanism and tensile fracture mechanism of different hydrogels were studied.The force-bearing points of the P(ACMO)hydrogel during the tensile process were weak hydrogen bonds between chain segments and water,and the reversible break/recombination of hydrogen bonds endowed the P(ACMO)hydrogel with promising mechanical tensibility.Additionally,the P(ACMO)hydrogel showed promising self-adhesive and biocompatible properties.The exploitation of photopolymerized 3D printed P(ACMO)hydrogel laid a foundation for the subsequent introduction of conductive fillers and the preparation of flexible strain sensors.Different types of filler-embedded and coating-loaded conductive hydrogels were designed,and the electrical,mechanical,and sensing properties of electrically,ionically,and electrically/ionically hybrid conductive P(ACMO)hydrogels were compared.The sensing mechanism of different conductive hydrogels was studied.The conductive path of a single filler-embedded or coating-loaded conductive hydrogel was limited during the process of strain,and the conductive mechanism was always electrical or ionic conduction,while the hybrid conductive hydrogel exhibited the conversion of the mechanism of electrical and ionic conduction during the process of strain,which significantly improved the sensitivity of the strain sensor.The designed metal coating-loaded/Na Cl-embedded P(ACMO)conductive hydrogel showed the sensitivity coefficients of 2.4×10~5and 5.0×10~4in the 0-20%and 20-100%strain ranges,which laid a foundation for the design and development of high-performance hydrogel flexible strain sensors.Based on the physical and chemical crosslinking mechanism of hydrogels,the effects of contents of glycerin and PEGDA on water retention,self-adhesion,and resilience of hydrogels were explored,and the mechanism of water retention,self-adhesion,and resilience of hydrogels was revealed.Due to the water absorption and bridging of glycerin,the water retention and self-adhesive properties of hydrogels have been significantly improved.The synergistic mechanism of covalent crosslinking networks formed between P(ACMO)and PEGDA and hydrogen bonds formed between P(ACMO)/PEGDA and glycerin/water endowed the hydrogel with excellent resilience.The initial residual strain of the hybrid conductive hydrogel at100%tensile strain was only 9.3%.The optimization of service performance such as water retention,self-adhesive,and elastic properties of the P(ACMO)conductive hydrogel laid a foundation for the design and development of high-performance wearable hydrogel strain sensors.Hydrogels with different structures were designed,and the stress and strain distribution of hydrogels with different structures were analyzed by the finite element method.The influence of different structures on the weight,permeability,mechanical,and sensing properties of hydrogels was investigated.The finite element analysis showed that the strain distribution of the hydrogel with the grid structure was uniform during the tensile process.The mechanical properties test also showed that the hydrogel with the grid structure exhibited excellent resilience,and the initial residual strain was only 7.8%under the 100%tensile strain.Additionally,the grid structure design also improved the wear permeability,sensitivity,and sensing stability of the hydrogel strain sensor.The photopolymerized 3D printed conductive hydrogel flexible strain sensor with grid structure has been preliminarily applied in finger rehabilitation monitoring,sign language recognition,interactive human-machine interfaces,and other fields.