Design,Fabrication And Modeling Of Graphene Epidermal Pressure Sensor

With the rise of wearable devices,epidermal sensors that can attach to the skin to measure physiological signals of the human body have attracted wide attentions.The epidermal pressure sensor,which can monitor heart rate in real time,has become a hotspot of research.The contact resistance pressure sensor stands out among all types of devices due to its advantages of high sensitivity and fast response time.However,the inherent disadvantage of small linear work range of this kind of pressure sensor limits its application for real-time heart rate monitoring.Consequently,how to extend its linear operating range while ensuring high sensitivity and fast response time is a challenge for its application in real-time heart rate monitoring.It is generally believed that the pressure response of such devices is largely controlled by the microstructure of the contact surface.Therefore,the key is how to design the appropriate microstructure to meet the requirement.Unfortunately,it is still an open question due to the lack of theoretical models.In this regards,we carried out systematical investigations on the contact resistance type epidermal pressure sensor fabricated by the laser direct writing technology.This dissertation consists of five chapters,and the content of each chapter is presented briefly as follows.In chapter one,we briefly introduce the characteristics and types of epidermal electronic devices,then summarize the research progress of the epidermal pressure sensor,and finally present the purpose and content of this dissertation.In chapter two,we design a contact resistance epidermal pressure sensor based on an asymmetric double-layer conductive structure.The device is composed of two layers:one is the interdigital electrode layer and the other is the conductive layer.Both layers are made from laser-reduced graphene.The initial resistance of the device depends on the area where the upper and lower layers overlap,which can be readily adjusted by the length,width and number of the interdigital electrodes.The sensitivity and working range of the device can be regulated by the initial resistance.The produced sensor has a sensitivity of～2 kPa-1,a linear work range of～200 Pa,and a response time of～0.15 ms.More importantly,the device can be pasted on the skin to detect the finger touching and the wrist bending,as well as to monitor the pulse in real time.In chapter three,a new method to convert PDMS into graphene by laser irradiation is proposed,and a highly sensitive contact conductance type epidermal pressure sensor is fabricated by this method.The Raman results show that the surface of PDMS can be converted into graphene by laser irradiation.The produced graphene has high electrical conductivity and excellent mechanical properties,and thus can be directly used to fabricate pressure sensors as a flexible conductive layer.The as-fabricated pressure sensor has high sensitivity(～480 kPa-1),fast response/relaxation time(2 μs/3 μs)and excellent cyclic stability(>4000 cycles).Moreover,the device can be attached to the skin of the wrist to monitor the pulse.In addition,we also fabricated a 7×7 pressure sensor array to detect the spatial distribution of pressure,which demonstrated its potential applications in the field of electronic skin.In chapter four,based on the height cumulative distribution function,a theoretical model for the contact conductance pressure sensor is established concisely.The model includes the effect of microstructure of contact surface and substrate on the pressure response.As a consequence,it can not only calculate the response of the conductance to the applied pressure directly from the AFM topological data of the contact surface,but also deduce the relationship between the conductance and the pressure from the geometric parameters of various microstructures.Therefore,this simple model is a powerful tool for designing the performance parameters of the contact conductance type pressure sensor through varying the microstructures on the contact surface and substrate.In chapter five,we summarize the main innovation points and shortcomings of this dissertation,and prospect the important works that is worthy of being carried out in the future.