Optical Microfiber Probe Based Bending Sensors And The Applications In Wearable Devices

With the rapid development of novel flexible materials,the flexible,biocompatible,noninvasive and skin-mountable wearable sensors have been attracting a lot of attention,and be promoted in many fields such as human-machine interaction,individual health monitoring and clinical diagnosis of dyskinesia diseases.In order to overcome the problems like electric leakage and electromagnetic interference from electrical wearable sensors,the researchers recently focused on the wearable sensors based on flexible optical fiber,which not only can endure large-scale bending deformation but can measure subtle strain/vibration with high sensitivity.Micro/nanofiber,a kind of miniature optic-fiber device with high sensitivity,low bending loss and high flexibility,can satisfy the above two requirements of pliability and high sensitivity,while overcoming the disadvantages of complex preparation process and high cost of flexible optical fiber.Therefore,micro/nanofiber is expected to become a potential candidate in wearable sensing devices.According to the requirements and development tendency of wearable sensors,two reflection-type sensing structures,multimode microfiber probe(MMMP)and single mode microfiber probe(SMMP)are proposed and experimentally demonstrated.In this paper,we mainly study the sensing principles and fabrication processes of two kinds of microfiber probe.In addition,we focus on the large measurement range bending sensor based on MMMP and the flexible wearable sensor based on SMMP,exploring their practical applications in the field of human motion detection and individual health monitoring.The main research content includes:(1)Based on the basic theory of optical waveguides for micro/nano fibers,we study the critical condition under two kinds conditions of non-adiabatic stretching and adiabatic stretching.We carry out a simulation analysis to study the HE₁₁ and HE₁₂ mode field distributions under non-adiabatic stretching.Finally,we study the refractive index profile and the mode transformation of bent micro/nanofiber.(2)A large measurement range bending sensor based on MMMP is proposed.We improve and simplify the fabrication process of MMMP,and make a simulation model to analysis its bending sensing principle.Furthermore,we experimentally study the bending sensing characteristics of MMMP,demonstrating it has a large measurement range of 0-11.82m^-1through monitoring the shift of resonance wavelength.(3)A wearable bending sensor based on SMMP is proposed and experimentally demonstrated.A two-step stretching method is proposed to simply fabricate SMMP,and the SMMP is encapsulated into a flexible wearable sensor using polydimethylsiloxane and Teflon.Furthermore,we research systematically the sensing characteristics of SMMP wearable sensor.The experimental results indicate it has a large measurement range of bending angle(0-150°),fast response time(8.4ms),high stability and good repeatability.In addition,the breaking strain of SMMP is increased to 6.92%after PDMS encapsulation,which is suitable for large-scale bending deformation like human joint motion.(4)We explore the practical applications of SMMP wearable sensor in human motion detection and individual health monitoring.The SMMP sensor with the size of5cm×1cm×1mm is mounted on the wrist joint and finger,and it successfully realizes the detection of joint bending motion and quantitative measurement of bending angle of finger,which is significant for accurate human motion detection.Additionally,we reduce the thickness of sensor's PDMS substrate to enhance the sensitivity to subtle strain,and noninvasively measure the pulse wave signal in the radial artery with high precision,which is expected to be applied to individual health monitoring.