Design And Functionalization Of Flexible Strain Sensing Composites Based Two Dimensional Materials

Flexible strain sensors have great potential for applications in artificial electronic skin and biomedical systems,etc.Strain-sensitive materials are critical component for flexible strain sensors.The preparation of flexible strain-sensitive composites with high sensitivity and wide detection range has always been a great challenge in the field of strain sensing.Two-dimensional conductive nanomaterials characterized by good flexibility,outstanding electrical conductivity and mechanical property are regarded as ideal flexible strain sensing materials.In this paper,two-dimensional conductive nanomaterials(graphene or MXene)were adopted as strain sensitive materials.After assembly with flexible cellulose acetate fibers(CAF)or polyurethane(PU)fibers,highly flexible and conductive two-dimensional nanomaterial-based strain sensitive composites were acquired.Through controlling the alignment of the fiber bundles and constructing the microstructured conductive fiber network,highly sensitive and flexible strain sensing composites were presented.The influence of the microstructure of the sensing layer on sensing properties was systematically investigated.The main findings are as follows:(1)Reduced graphene oxide(RGO)nanosheets with good electrical conductivity were assembled onto the CAF surface to fabricate flexible and conductive CAF/RGO composite fiber array.Results show that the mechanical sensing properties of the CAF/RGO composite fiber array are directly associated with the fiber alignment and the intrinsic crimpness of CAF.Under the tensile deformation,the fiber arrays perpendicular to the strain axis are separated from each other,thus leading to the decrease of the contact area between the fibers,which makes the relative conductance perpendicular to the array direction changes significantly.At the same time,the intrinsic crimpness of the fibers ensures that the CAF/RGO fiber array can maintain the structure integrity under large tensile deformation(e.g.75%).The CAF/RGO composite fiber array exhibits good electrical response to a variety of deformations with high sensitivity(GF=-25)low detection limit(tensile deformation: 0.05%;pressure: 2 Pa;vibration: amplitude 8 μm),wide sensing range(tensile deformation: 0-75%;pressure: 2 Pa-36 k Pa)and good cyclic stability.At the same time,the fiber array is also sensitive to complex physical deformations and weak physiological signals,such as joint activities,respiration and pulse.(2)To further improve the strain sensing properties,MXene/PU fiber film with good elasticity and conductivity was fabricated through electrospinning combined with assembly techniques.Crack or wrinkle microstructures were introduced into the MXene layer via a pre-stretching and fracture-stretching process to further improve the sensing performance.The results show that the porous network structure of the fiber membrane and the wrinkle patterns on the MXene layer can effectively buffer the strain;while the crack patterns of the MXene layer can act as mechanical weak points,which is extremely sensitive to mechanical stimuli.Therefore,the as-prepared MXene/PU conductive fiber film shows excellent comprehensive strain sensing performance.Both the strain sensing range and sensitivity can be effectively regulated by adjusting the pre-stretching and fracture-stretching deformation.The as-prepared MXene/PU conductive fiber film exhibits good sensing response to various strains,such as stretching,compression,bending and vibration.The sensitivity of tensile deformation reaches 1000 and the sensing range can reach 120%.