Fabrication And Performance Study Of Flexible And Highly Sensitive Piezocapacitive Sensors With A Different Structured TPU Dielectric Layers

With the aging of population and the repeated occurrence of COVID-19,the prevention and monitoring of chronic diseases and their complications has become a major challenge for society today.And now,the shortage of medical resources has provided hitherto unknown historical opportunities for the development of community and household wear monitoring equipment.Mobile wearable monitoring equipment can monitor human pulse,blood pressure and sleep in real time,which provides important equipment guarantee for the prevention and early warning of patients with chronic diseases and complications.As the core of wearable monitoring equipment,sensors are an important component for the collection of physical signs,especially pressure sensors,which can recognize all kinds of human movements and collect all kinds of symptom information,and play an important role in accurately judging the accumulation degree of many chronic diseases.Pressure sensors are mainly divided into four types:capacitive,resistive,piezoelectric and triboelectric.Capacitive pressure sensors can not only measure dynamic pressure and static pressure,but also possess the benefits of simple structure,low power consumption,excellent stability and small temperature effect.They are ideal sensors for wearing monitoring equipment.However,due to the limitation of Young’s modulus of common elastomer dielectric layer and preparation process,capacitive pressure sensor is difficult to achieve high sensitivity.Therefore,this topic has carried out a series of studies on enhancing the sensitivity of low-cost capacitive flexible pressure sensors,aiming to explore the simple,low-cost,stable and efficient method to fabricate high-sensitivity capacitive flexible pressure sensors.The main research contents of this paper are as follows:（1）Fabric template method:A capacitive flexible pressure sensor based on a microstructured thermoplastic polyurethane（TPU）dielectric layer was designed by two-step replication method with the inexpensive and uniformly structured stainless-steel mesh cloth as the template.With this method,the obtained microstructure of dielectric layer is regular and easy to control compared with natural template.And this method does not need complicated preparation process and expensive professional equipment in contrast to photolithography and chemical etching,which is in line with the development concept of green environmental protection.The results of the morphological analysis of the dielectric layer showed that the wavy microstructure was successfully transferred from the stainless-steel mesh cloth template to the TPU dielectric layer by the two-step replication method.The experimental results showed that the sensor displayed excellent sensing performance with the 300-mesh stainless-steel mesh cloth as the template,and mainly reflected by the sensitivity of0.182k Pa^-1,fast response time（78ms）,small detection limit（15Pa）and good durability under1000 loading/unloading cycles.Finally,the practical application of this kind of flexible sensor in monitoring human movement such as finger bending,fist making,mouse clicking,and swallowing was successfully demonstrated.（2）Sacrificial template method:The porous structure was introduced into the TPU dielectric layer using table salt as the sacrificial material,while the continuous carbon fiber was adopted as the electrode material,so that a one-piece capacitive flexible pressure sensor based on a porous structured TPU dielectric layer was prepared by continuous fiber 3D printing technology.This method has the advantages of low manufacturing cost,one-piece molding,and can be used to prepare flexible piezocapacitive sensors on a large scale,as well as the unique advantages of 3D printing such as controllable shape and one-piece molding.This is the first time that continuous fiber 3D printing technology has been applied in the preparation of flexible pressure sensors.In the one-piece flexible pressure sensor,the capacitive pressure sensor was constructed at the intersection by the cross-printing path of the upper and lower layers.Under pressure,the porous structure formed by salt exudation would lead to the sensor containing more compression amplitude and increase the effective dielectric constant.As a result,the sensor with the porous structure exhibited higher sensitivity compared to the unstructured sensor(0.32N^-1).The experimental results showed that when the salt content is 40wt%,the sensor showed high sensitivity,fast response/recovery time（60/80ms）,good durability（1000cycles）,and magnificent pressure resolution.The sensor was used to achieve the detection of limb operation,such as finger bending,knee bending,elbow bending and so on.Lastly,the 4×4 sensor array prepared by this sensor enables the perception of the spatial distribution of external pressure.（3）Electrospinning method:Similar to the fabric template method,the stainless-steel mesh cloth with regular structure was adopted as the collector of electrospinning fiber membrane,and the template sacrifice method is used to construct the internal pore structure of electrospinning membrane,which results in the dielectric layer with both external microstructure and internal pores.This preparation method has the advantages of simple process,low cost and large-scale preparation.When preparing the dielectric layer,the dual structure TPU dielectric layer is formed by electrospinning technology.The prepared pressure sensor is composed of a dual-structured TPU dielectric layer and two conductive fabrics on the upper and lower sides as electrodes.Benefiting from the prolific air in the dielectric layer,the designed sensor has been representing outstanding sensing performance,including high sensitivity of 0.28k Pa^-1,fast response time of 65ms,and high-grade durability after 1000cycles.Further,the sensor can detect human limb movements and object grasping signals.In summary,this topic prepared three kinds of high-sensitivity structured capacitive flexible pressure sensors based on microstructured TPU dielectric layer,porous structured TPU dielectric layer and dual-structured TPU dielectric layer,respectively.Eventually,the sensors showed excellent sensing performance and practicality in detecting human motion signals,indicating their application potential in the field of flexible wearable.This work can provide new inspiration for the subsequent studies on the rapid preparation of low-cost,high-sensitivity capacitive flexible pressure sensors,and provide new possibilities for expanding the applications of mobile wearable monitoring equipment in the fields of real-time monitoring of human pulse,blood pressure and sleep.