Research On MXene-based Multi-functional Flexible Stress Sensors

With the continuous development of the social economy,traditional metal and semiconductor sensors are limited in the detection application field due to poor tensile properties and small sensing range.The flexible stress sensor has many advantages,such as lightweight,softness,flexibility,and adhesion,so it has attracted extensive attention in the fields of human motion detection,medical health,and human-computer interaction.However,the preparation process and sensing performance of the flexible stress sensor has some shortcomings,such as a complex preparation process,poor flexibility,low human compatibility,and low sensitivity.In this context,it is of great significance to fabricate flexible stress sensors with high sensitivity and high stability using an easy-to-operate process.In this paper,the two-dimensional transition metal carbide MXene/ Silver nanowires(Ag NWs)composite structure is selected as the conductive material,and the high elongation and UV-curable polyurethane(UV-PU)as the flexible substrate material.Different polyols and UV-PU with different R values were prepared by pre-polymerization and UV curing.By direct screen printing of MXene/Ag NWs based inks on a UV-PU flexible substrate,a flexible conductive film was obtained by dry molding and used for stress sensors.Based on the chemical bond theory,S-MXene was modified by a silane coupling agent γ-mercaptopropyl triethoxysilane(MPTES)to enhance the interaction force between conductive materials and improve the bonding with the substrate.Conductive materials with a uniform layer-by-layer assembly(LBL)structure was prepared by a indirect transfer method,and transferred after UV-PU casting and curing.Thus,a high-performance S-MXene /Ag NWs flexible stress sensor is prepared.The sensing performance of the sensor was tested by means of an electric translation table and a digital source table,and its sensing mechanism was studied and analyzed by means of microscopic characterization.The results show that the UV-PU with R value of 1.5 prepared from polyether polyols polypropylene glycol 2000(PPG)has good mechanical properties,the fracture strain is 681%,and the tensile strength is up to 1.9 MPa.The working range and sensitivity of MXene/Ag NWs flexible stress sensor prepared by direct screen-printing method need to be improved.When S-Mxene was used as the sensing material,an LBL S-MXene /Ag NWs flexible stress sensor was prepared by indirect transfer method,in which S-MXene and Ag NWs formed Ag-S coordination bond,enhancing the interaction force between conductive materials.In addition,the addition reaction between S-Mxene and PU substrate C=C improves the bonding between the conductive material and the substrate and reduces the slip of the conductive material on the substrate surface.Based on the disconnection mechanism and crack propagation mechanism,the LBL structure ensures that the conductive path is connected in the working state of the flexible stress sensor.Compared with the direct screen printing method,the S-MXene /Ag NWs flexible stress sensor has higher sensitivity(GF up to 7838),wider operating range(120%),and long-term functional stability(over 3000 cycles),good linearity,and excellent adhesion performance.By adjusting the number of layers of conductive material,the flexible stress sensor with the best stability is obtained.The sensor can be fixed to the human body,and can detect different range of human motion in real-time,including different amplitude of joint bending deformation,as well as weak facial expression changes and radial artery pulse signal of the wrist.