Study On Flexible Strain Sensor Based On Layered And Microstructured-graphene With A Through-layer Buckle Wavelength Gradient Design

Flexible strain sensors,a subfield of the emerging flexible electronics,show a great potential for applications in biomedical devices,human health and sports monitoring,the human-machine interface,soft robotics,and so forth.For the application as wearable sensors,flexible strain sensors have to simultaneously possess a high sensitivity and stretchability,because of the fact that human activities could range from vigorous articular movements to subtle physiological activities such as breathing,heartbeat,and the pulse.The former requires a high stretchability,whereas the latter requires a high sensitivity in order for the sensor to be responsive to subtle stimuli.However,for most reported flexible strain sensors,the coupling of high stretchability and sensitivity in a single strain sensor is still a big challenge.To tackle this issue,a through-layer buckle wavelength-gradient design is proposed and introduced into the multilayered graphene woven fabrics?GWFs?film,leading to a flexible strain sensor with Dragon Skin as the flexible substrate and multilayered and selectively buckled GWFs?MBGWF?as the sensing film.Specifically,a series of systematic research work has been conducted studying the preparation and tuning of the strain sensing performances of the free-standing GWFs and the fabrication,strain sensing performances,and working mechanism of MBGWF strain sensors and their applications as wearable sensors.Detailed research contents,approach,and conclusions are discussed below.?1?Preparation of free-standing GWFs sensing units and the controlling of their strain sensing performancesUsing Raman microscopy,we studied the influences of the composition of the gas mixture and growth duration in the CVD process on the properties of the as-grown graphene and their relationship with GWFs'free-standing capability on the water.The optimal CVD condition for the free-standing GWFs was determined to be a process at1000°C,with a gas mixture of CH₄:H₂:Ar=55:45:400 mL min^-1,and for a duration of25 min.Besides,the effects of stretching angle,pre-cracking,and buckles on the strain sensing performances of the GWFs sensing units were determined.Compared with that stretched on the stretching angle of 0°,the GWFs stretched on 45°showed an increase in the strain sensing range,but a lowered sensitivity.Also,the GWFs pre-cracked with18%strain showed an increased sensitivity but narrowed strain sensing range.Buckles played a great role in accommodating the strain loaded on the sensor and worked like a switch that controlled the on-off state of the sensing mode of GWFs sensing units.?2?Fabrication,strain sensing performances,and working mechanism of MBGWF strain sensors and their applications as wearable sensorsWe introduced a pre-stretching-transfer-partial release approach that could effectively build the as-devised through-layer buckle wavelength gradient into multilayered GWFs,based on which the MBGWF strain sensors were fabricated.The MBGWF strain sensors exhibit highly tunable electromechanical performances.Specifically,the sensor with 10-layer GWFs has a GF?gauge factor,defined as d??R/R₀?/d??of 2996 at the maximum strain of 242.74%and an average GF of 327.Besides,it also has an extremely low minimum detection limit of 0.02%strain,a fast signal response of less than 90 ms,and a high cyclic durability through more than 10000cycling test.Such excellent performances qualify it in accurately monitoring full-range human activities,ranging from subtle stimuli?e.g.pulse,respiration,and voice recognition?to vigorous motions?finger bending,walking,jogging,and jumping?.Through tracing the morphology evolution of the sensor under different stretching states,we determined that the working mechanism of MBGWF strain is a layer-by-layer cracking propagation process.Based on this mechanism,we established a model and studied the effects of key fabrication parameters on the sensing performances.