Fabrication And Application Of Flexible Sensors Based On Laser-induced Graphene

For decades,researchers have dedicated themselves to fabricate electronic skin by developing tactile sensing technology,that exhibits the functions like human skin.Electronic skin requires highly diverse and interdisciplinary expertise,in which flexible sensors are the key component to achieve tactile sensing.However,the development of flexible sensors is facing up with lots of challenages and difficulties.First,flexible sensors need to be wearable and conformal to different shapes,and biocompatibility,long-term durability,and structural robustness should be achieved as well.Second,flexible sensors should meet the requirements of applications in complex environments and be able to distinguish a variety of different external physical stimuli,such as stretching,pressing,bending,and twisting,etc.With breakthroughs in material engineering,the development of sensitive materials like graphene,CNTs,and Ag NWs had made it possible to fabricate flexible sensors with excellent performance,simple construction,low weight,low cost,and mass production.Among them,laser-induced graphene(LIG)has the advantages of simple synthesis,superior mechanical properties,and excellent conductivity,and it is one of the optimal active materials for the preparation of flexible sensors.In this thesis,we reported the techniques of preparing wearable flexible sensors based on LIG.LIG could be transferred to a flexible substrate(such as silicone rubber and polydimethylsiloxane(PDMS))because of the porous structure of LIG,and then LIG-based conductive elastic composite was obtained,which achieved scalability and conductivity simultaneously.After several operations,flexible sensors with different functions can be prepared.The main contents of this work are as follows:(1)The mechanism of preparing LIG on polyimide(PI)film was studied,the impacts of parameters(e.g.,the power of laser and scanning speed of laser head)on the morphology of LIG were discussed,and the properties of the prepared LIG were characterized.The efficiency of LIG transfer to flexible substrates was studied,and the properties of the conductive elastic composite were characterized.(2)A technique of preparing flexible piezoresistive strain sensor based on a three-dimensional(3D)wavy LIG was proposed,the PI film was fastened onto a 3D-printed mold with wavy structure,3D wavy LIG could be obtained by using CO₂ infrared laser to scan on PI film,and then transferred to the silicone rubber substrate,flexible strain sensor with 3D wavy LIG was finished after packaging.(3)The stress concentration of the strain sensor was studied when strain load was applied.Due to the wavy structure of LIG,the stress was concentrated at the trough of the wavy LIG,which improved the sensitivity of the strain sensor and reduced the damage of stress on the LIG layer.Therefore,the strain sensor had achieved high sensitivity(GF=37.8 in the range of 0?31.8%,which is better than planar-LIG one),low hysteresis(?1.39%),and a large operating range(0?47.7%).In addition,the strain sensor had been proved to have a stable dynamic response and long-term durability,with a signal deviation of only 2.32%after 5000cycles.(4)A technique of preparing flexible capacitive pressure sensor based on LIG/PDMS foam was proposed,PDMS foam can be prepared due to the solubleness of citric acid monohydrate(CAM)and superwettability of PDMS in ethanol,and LIG/PDMS foam was obtained after LIG was transferred to PDMS foam,so it was a flexible capacitive pressure sensor as well.(5)The influence of different porosity and thickness on the performance of the pressure sensor was studied.The pressure sensor with large porosity and small thickness is more sensitive to the pressure load and had a faster dynamic response.The pressure sensor had the advantages of integrated structure and structural robustness and achieved high sensitivity(GF=?0.026 k Pa^-1 in the range of 15-40 k Pa),low hysteresis(?9.762%),and short response time(?120 ms).Over 5000 cycles of durability testing,the signal varied by only 4.548%.Moreover,the pressure sensor was able to recognize multiple physical stimuli.According to the electrode resistance and capacitance between electrodes,four external physical stimuli like stretching,pressing,bending,and twisting could be distinguished.(6)The practicability of the flexible sensor was studied.And it could be used in the applications such as muscle contraction/relaxation detection,joint movement detection,touch switch,and heart pulse detection.By patterning design,the capacitive pressure sensor array was prepared for sensing 2D pressure distribution.The above methods are featured by simple process,low cost,and excellent efficiency.The prepared flexible sensors can be utilized into wearable medical devices or electronic skin,which have dramatic development potential and promising prospects.