Biomimetic Anti-fatigue-fracture Conductive Hydrogel For Flexible Sensors

Conductive hydrogels are widely used in wearable sensing devices,electronic skin,human-machine system,and soft robotics due to their excellent stretchability and resistance-strain responsiveness.Hydrogels as flexible sensors are often subjected to thousands of cyclic loads during long-term applications.However,the generation of fatigue cracks is difficult to be avoided.Therefore,it is important to study how to resist the expansion of fatigue cracks in order to maintain hydrogel’s stability and improve the service life of sensor parts.In addition,in order to overcome the limitation of single detection mode of hydrogel,hydrogels can also be prepared as pressure sensors,while the conventional hydrogel pressure sensors exhibit low sensitivity and are limited in many applications.To address the above two problems,how to avoid fatigue fracture during long-term cyclic loading and how to enhance the pressure sensing sensitivity are hot topics of current research.Inspired by human muscle tissue,two biomimetic anti-fatigure-fracture conductive hydrogels were designed and applied as strain sensors to investigate their stability under long-term cyclic loading.Based on the investigation of the anti-fatigure-fracture conductive hydrogels,and inspired by the microstructure of the sensory receiver in the dermis of the skin,this topic further constructs random height distribution microstructures on the surface of the hydrogels by the template method to improve the sensitivity of the hydrogel pressure sensors,and also examines the stability of the pressure sensors under long-term cyclic loading.First,the crystalline cellulose nanofiber/carbon nanotube nanohybrids were introduced into the polyampholytic electrolyte hydrogel system based on the features of human muscle tissue,and prepared anti-fatigue-fracture conductive hydrogels by a one-step free radical copolymerization method.The nanohybrids have a structure similar to muscle tissue.When the hydrogel undergoes tensile deformation,the nanobybrids are oriented along the tensile direction to hinder the fatigue crack expansion.The addition of the nanohybrids improved the anti-fatigue-fracture performance of the hydrogel by 1.7 times as demonstrated by 2000 cycles of cyclic tensile/release load fatigue tests.The fatigue threshold of PNDU-CNF@CNT_1%hydrogel is 187 J/m²,which is much higher than the fatigue threshold range of conventional hydrogels and the currently reported anti-fatigue-fracture conductive hydrogels.The anti-fatigue-fracture conductive hydrogel was applied to strain sensors and showed excellent stability during long-term use.Second,inspired by the sensory receiver microstructure in the human skin dermis,the biomimetic microstructures were constructed on the surface of the above conductive hydrogel using sandpaper as a template.Applying this surface microstructured hydrogel as the pressure sensor,the sensitivity was improved nearly 18 times compared to the flat surface hydrogel pressure sensor.The stability of the pressure sensor during long-term use was also investigated,and the biomimetic microstructure hydrogel pressure sensor was explored for detecting human activities,heavy object presses,and its application to handwriting tablets.Finally,a strategy to induce polymer crystallization for the preparation of anti-fatigue-fracture conductive hydrogels was designed in this project.The crystalline polyvinyl alcohol hydrogel was regulated by a freeze-thawing method,and then immersed in lithium chloride solution to obtain anti-fatigue-fracture conductive hydrogel.The possible crack extension under cyclic loading was hindered by crystalline regions.The anti-fatigue-fracture performance of the hydrogel was verified by 2000 cycles of cyclic tensile/release load fatigue tests,and the application of the hydrogels in strain sensors was also explored.