Performance And Study On Preparation Of Flexible And Wearable Strain Sensor

Flexible and wearable strain sensors have received extensive attention owing to their potential applications in many fields,such as human motion monitoring,electronic skin and robotics.The conductive polymer composites(CPCs)with excellent stretchability and machinability,can be employed to transform strain signals into electrical signals through altering the conductive channels.CPCs are ideal materials for flexible wearable strain sensors.In this paper,to fabricate flexible and wearable strain sensors with a large strain range and high sensitivity,flexible polymer materials were employed as the substrate,and the conductive fillers consisted of reduced graphene oxide(RGO)and carbon nanotubes(CNTs)with the excellent electrical properties.RGO/thermoplastic polyurethane(TPU)conductive composites and polydopamine(PDA)/CNTs/elastic band(EB)conductive composites were prepared by prefabricating conductive networks.The main research contents and conclusions of the paper are shown as follows:(1)Preparation and investigation of properties of RGO/TPU strain sensorsTPU was selected as the substrate for the conductive composites owing to its excellent flexibility.RGO was used as conductive filler,which was attributed to its large specific surface area and excellent electrical properties.RGO/TPU conductive composites obtained special conductive network,which was prepared by electrospinning and ultrasonic technique.Clearly,microstructure morphology showed that RGO was coated on the surface of TPU fibers.RGO nanosheets were overlapped with each other,constructing excellent conductive paths on the surface of TPU fibers which could be employed as a skeleton owing to their three-dimensional network structure.When RGO/TPU strain sensor was stimulated by external strain,the change of conductive network consisted of two parts,including the variation of distance between conductive elements(RGO nanosheets)and the geometrical changes(the deformation of the TPU net and the single RGO-wrapped conductive fiber).RGO/TPU composites possessed excellent electrical properties(0.2 S/m)and mechanical properties(elongation at break of 670%).It was worth noting that RGO/TPU composites could be stretched,bended and twisted.In addition,RGO/TPU conductive composites obtained large strain range(200%),high sensitivity(155.7),fast response time(200 ms)and high durability(6000 cycles of stretching).RGO/TPU strain sensor could monitor full-range human motion,including large strain signal of human motion(walking,running,jumping,arm bending,etc.)and small strain signal(cough,phonation,pulse beat and subtle muscle movements of cheek,etc.).The monitoring signal exhibited fine stability and repeatability,showing a good application prospect of RGO/TPU in the field of flexible and wearable devices.(2)Preparation and investigation of properties of PDA/CNTs/EB strain sensorsA PDA/CNTs/EB conductive composite with a large strain range and high sensitivity was fabricated.EB was used as substrates of conductive composites duo to its excellent mechanical properties and CNTs were selected as conductive fillers owing to its large aspect ratio and excellent electrical properties.PDA with good adhesion performance was the surface protective layer.The micro-morphology analysis showed that CNTs formed conductive networks both on the surface and inside of EB.The PDA coated on the surface of CNTs/EB could prevent the CNTs from falling and guarantee the electrical properties of PDA/CNTs/EB more stable.Meanwhile,the response mechanism of PDA/CNTs/EB conductive composites was investigated.During stretching,some minor crack and gap appeared on the surface of PDA/CNTs/EB,leading to the change of the conductive network structure.With the increase of strain,the number and area of gap also increased.At the same time,the CNTs conductive network of PDA/CNTs/EB was damaged,resulting in a climbing resistance value.The shoulder peak phenomenon occurred during stretching,which was due to the competition between the destruction and reconstruction of the conductive network.In addition,the mechanical hysteresis(H_M)of PDA/CNTs/EB was related to the intensity of the shoulder peak.When the H_M value was more than 10%,the shoulder peak phenomenon of PDA/CNTs/EB appeared.In addition,the intensity of shoulder peak would increase with the increase of H_M.The results of tensile sensitivity tests indicated that PDA/CNTs/EB had a large strain range(920%),high sensitivity(gage factor(GF)of 129.2 at the strain of 780%-920%),fast response speed(220 ms)and good durability(10000 cycles).PDA/CNTs/EB conductive composites could monitor the movement of human muscles and joints and the results were stable and repeatable,which provided ideas for the preparation of flexible and wearable sensors with large strain range and high sensitivity.