Study On Key Devices For Flexible And Werable Sensing

Flexible and wearable sensors are widely used in a vast number of fields such as human health monitoring,prosthetics and robots.Flexible sensors can collect a massive amount of data,which is needed to be stored,computed,and analyzed by corresponding high-performance memories and processors.The conventional silicon-based nonvolatile memory can not meet the requirements of flexibility and ductility due to the characteristics of its rigid substrate.Flexible resistive random access memory(RRAM)featured with fast write/erase speeds,low power consumption,high device density,etc.,makes it have great application potential.Most of the flexible and wearable sensors are active sensors,either powered by micro batteries or external power supply.However,the use of micro-batteries as the power supply has a fatal shortcoming of not being able to last for a long time owing to its limited energy density.When external supply is used as the power source,it is very inconvenient for operation.Therefore,the new type of self-powered sensor is much more attractive in comparison with the active sensors.Flexible triboelectric nanogenerator(TENG)can not only converts the mechanical energy of human body motion into electricity but also has the sensing characteristics,is a new type of self-powered sensor.This work mainly aims to explore flexible RRAM and flexible and wearable self-powered sensors based on TENG that can be used for flexible and wearable sensing applications.The main work of this article is as follows:(1)This paper proposes a flexible and fully biodegradable RRAM.First,biomaterials gelatin was selected as the dielectric layer,and four different kinds of metal were selected as the top electrode(TE)to study its influence on resistive switching characteristics of the RRAM devices with metal TE/gelatin/tungsten(W)structure.The work function difference between the TE and bottom electrode(BE)metals is found to play a profound role in determining the characteristics of the devices and even reversing the set and reset processes.The devices with larger work function difference between the TE and BE metals usually have higher and more stable on/off resistance ratio.The resistive switching mechanism of the metal TE/gelatin/Tungsten(W)devices was measured and analyzed subsequently,the formation and fracture of metallic conductive filaments are confirmed to be responsible for the switching behaviors of the devices experimentally,and what metal element in the conductive filaments is determined by the electrochemical activity of metal electrodes as well as the work function difference of the metals used for the top and bottom electrodes.(2)Based on the above results,a flexible and fully biodegradable resistance random access memory(RRAM)was developed with Mg as the top electrode and the flexible and biodegradable polymer poly(lactide-coglycolide)acid(PLGA)as the substrate.The resistive switching mechanism and characteristics of the fabricated RRAM devices were studied,its mechanical flexibility,as well as the biodegradability,were also investigated.The research results show that the memory device is suitable for wearable and implantable biomedical applications.(3)A switchable textile-triboelectric nanogenerator(S-TENG)is proposed in this work,which can be used as a self-powered sensor.The RC circuit is composed of TENG,switch,and an external load,by closing the switch,TENG discharges and two valid signals including output voltage peak and time constant can be obtained.We first proposed the method of using the instantaneous capacitance of TENG(referring to the time constant with a given load)as the sensing parameter to eliminate the effect of environmental interferences on sensing results.When STENG is used for static sensing,it can be used as the self-powered weight scale,pressure sensor,and the grip strength sensor.When it is used for dynamic sensing,it can be attached on the inner side of the cloth or attached inside a shoe at the heel position to sense the variation bending angle of the elbow and the stepping force of foot with time,respectively.Moreover,the mechanical switch is replaced by an electronic switch to achieve a better sensing resolution.