Architecture Design and Performance Adjustment for Nano-silver Based Wearable Force Sensitive Sensing Materials

Author:Chen Song

Supervisor:liu zuo


Degree Year:2019





Wearable Force Sensitive Sensing Materials(WFSM)is a kind of flexible conductive composites that can be confomability attached onto different parts of human body and deform freely to transform external stress(including pressure,tension,shearing force and twisting force)into electrical signals.However,the research of WFSM recently is still in its infancy.Specifically,the relationship between structures and properties,force sensing mechanism and theoretical system are still unclear.Hence,it still remains great challenges to increase both the wearability and the force sensing performance.As a result,this research focuses on the structural design of WFSM,the construction of conductive networks and the regulation of its force sensing performance and wear performance,providing a theoretical basis for the practical application of high performance WFSM.The high-stretchability and high sensitivity under high strain of WFSM are achieved by designing the“sandwich”structure.The AgNPs/GE/TPU conductive composites with"sandwich"structure were prepared by swelling and in-situ reduction method.The conductive outer layer was composed of a synergistic network of GE and AgNPs,and the core layer is highly elastic and insulating.The special structure endows the composite with an initial volume conductivity of 1.4×105 S m-1 and a high stretchability of 1000%.Under high stretching,the conductive network still works because the GE can bridge between the AgNPs cracks.WFSM made by AgNPs/GE/TPU conductive composites exhibits excellent strain response,bending response,high sensitivity under high stretching(a sensitivity of 476 at the strain of 500%)and high stability(more than 1000 cycles at the strain of 50%)and successfully applied to the detection of different fingers’bending and vocal cord vibration.The ultra-high sensitivity of WFSM under micro-strain is obtained by the design of"micro-crack"structure.The urchin-like AgNWs@reduced graphene oxide(AgNWs@rGO)hybrid particles were successfully prepared via hydrogen bonding interaction and reduction of graphene oxide(GO).AgNWs@rGO/TPU composite was prepared by casting liquid TPU on the AgNWs@rGO hybrids.Own to the formation of microcracks introduced by pre-stretching process,the as-prepared WFSM shows high-sensitivity under micro strain(1%).The study about the sensing mechanism shows that the resistance changes under 1%tensile exhibit three distinct stages:the first stage is the mutual slip of the stacked conductive network;the second stage is the slight slip among the AgNWs of on the edge of the hybrid particles;the third stage is the tunneling resistance change induced by the occurrence of cracks.For the 10%pre-stretched WFSM(AgNWs:GO=4:1),the sensitivity is as high as 20(Δε<0.3%),1000(0.3%<Δε<0.5%),4000(0.8%<Δε<1%).The high sensitivity makes WFSM successfully applied in detecting pulse beat and physical vibration.High sensitivity pressure sensing is achieved through the design of a"multi-dimensional hierarchy"structure.Because of the great difference in Young’s modulus between polyvinyl alcohol(PVA)and VHBTM elastic tape,AgNWs/PVA/VHBTM conductive composite with multi-dimensional hierarchical structure(micro-wrinkle structure in pre-stretch direction,arch structure perpendicular to the stretched direction)was obtained by pre-stretch and release.Due to the existence of micro-wrinke structure,the resistance change of the composite during tensing shows typical three-stage,corresponding to the contact resistance change,the wrinkle extension,and the tunneling resistance change;the composite and the interdigitated electrode are further assembled and prepared for pressure detection.Due to the presence of the arched structure,the WFSM exhibits an ultra-high sensitivity of 4.54 kPa-1(0-220 Pa)under pressure,and the sensitivity in this process is independent of the degree of pre-stretching,and is also independent of the tensile state during use,showing the potential for stable use in a dynamic stretching environment.In addition,when the pressure exceeds 220 Pa,the WFSM still exhibits excellent sensing performance due to an increase in the contact area of the wrinkle structure with the electrode under pressure.A highly wearable WFSM is achieved by the preparation of a modulus-adjustable,self-adhesive flexible substrate.The PDA/PVA-Fe3+flexible matrix was prepared by hydrogen bonding interaction and the cross-linking with Fe3+.It exhibits good adhesion,where its adhesion to metal is up to 30.2 kPa;the shear and peel strength to PET substrates is up to 44.1 kPa and 61.3 N/m,respectively,the shear and peel strength on PLA substrates are up to 55.8 kPa and 68.9 N/m,respectively.At the same time,the system has a Young’s modulus adjustable property,so the WFSM prepared by compositing it with AgNPs has a modulus similar to that of the skin and has good skin adhesion and exhibits excellent wearability.In addition,with the crack morphology of the AgNWs network under tension,the as-prepared WFSM has a high sensitivity of 2012(20%stretch)and has been successfully applied to various parts of the human body for detection.In addition,the adhesion and modulus-adjustable substrate also shows a good application potential in self-locking actuators.