Research background. 
The wearable flexible sensor with long life, high operation safety and mechanical flexibility has a great prospect in the application of the next generation biomedical equipment. Recently, the integrated multi-function devices which can supply power to the equipment and detect the changes of electrical signals at the same time have aroused considerable interest. In order to ensure the continuous operation of the whole self-power supply system in the environment such as lack of sunlight, the fully integrated self-power supply equipment is composed of energy collection / conversion equipment, energy storage unit and functional equipment. However, most integrated self-powered devices are based on the use of different materials to achieve the function of integrated devices. In fact, when manufacturing integrated sensors and power supplies, it is necessary to optimize the synthesis process and cost of materials, so as to reduce the complexity and cost of device integration. Therefore, it is a great challenge to design functional materials with dual characteristics. 






Full text guide. 


In view of this, Jilin University has prepared a flexible and durable MXene/BP-based self-powered wearable system for accurate and continuous detection of human physiological signals. The layered MXene/BP, is obtained by using LbL self-assembly process and fabricated into a miniature supercapacitor by laser engraving technology to store the energy generated by solar cells to drive the operation of pressure sensors also based on MXene/BP. In this system, the flexible supercapacitor provides a volume capacitance of 896.87 F cm-3, and the flexible pressure sensor shows high detection sensitivity (77.61 kPa-1) and fast response / recovery time (10.9 ms). Therefore, the self-powered intelligent sensor system can be used for real-time monitoring of human pulse and can be used as a wearable wristband. This integrated method of advanced materials and equipment provides great potential for the next generation of self-powered electronic products. The results of related papers are published on Advanced Materials under the title "Flexible Self-Powered Integrated Sensing System with 3D Periodic Ordered Black Phosphorus@MXene Thin-Films". 

The introduction is as follows: 
Firstly, MXene/BP multilayer films were obtained by layered vacuum filtration of Ti3C2Tx MXene and BP nanosheet solutions. A flexible pressure sensor with wide pressure response is obtained by pressing the MXene/BP film with the flexible PET substrate coated with the ITO film layer as the top and bottom electrodes. 1 M PVA/H2SO4 gel was used as electrolyte to cast onto the surface of laser-crossed MXene/BP InterDigital electrode for the manufacture of solid-state supercapacitors. 

Figure 1: a schematic diagram of a self-powered intelligent sensor system based on MXene/BP. 

The characterization of MXene/BP multilayer films is as follows: 

Fig. 2: characterization of mixed MXene/BP films: a) Optical images and b) SEM cross-sectional images of mixed MXene/BP thin films; schematic diagram of internal structure of; c) mixed MXene/BP thin films;; d) EDS spectra,; e) XRD spectra,; f) Raman spectra; XPS spectra of P2p (g), Ti2p (h) and O1s (i). 

No matter how bent the flexible micro supercapacitor of MXene/BP hybrid film is, the micro supercapacitor shows excellent electrochemical stability, thus showing excellent mechanical stability. At the same time, the author also tests the flexibility and durability of the micro supercapacitor while maintaining the bending condition (figure 3h). After 10000 cycles, the capacitance retention is still as high as 91.74%. 

Figure 3: electrochemical performance of supercapacitors based on MXene/BP films and under different yielding conditions.

Based on the application of flexible pressure sensor, the working principle of MXene/BP pressure sensor is revealed at the level of molecular structure. At the same time, in order to further verify the response behavior of the flexible pressure sensor, the author measured the current change of the flexible sensor at different pressure levels. The sensing characteristics of the flexible pressure sensor based on MXene/BP are also studied, and the current change of the sensor corresponding to the pressure change is tested when the finger swings and bends at different angles. 

Figure 4: a flexible MXene/BP-based device for sensing and distinguishing the current signal response of stimuli. 
The conclusion of the article. 
MXene/BP thin films with periodic staggered layered structure were prepared by LbL self-assembly process and used as the core materials of micro supercapacitors and flexible pressure sensors.Micro supercapacitors based on MXene/BP exhibit high specific capacitance, excellent rate performance and long-term cycle stability. The flexible pressure sensor based on MXene/BP provides a sensitivity of up to ≈ 77.61 kPa-1 and a fast response time of 10.9 ms, which completely covers the working range of biological receptors. In addition, the author designs a self-powered integrated system composed of flexible pressure sensor, micro supercapacitor and solar cell array, which proves its ability to detect the state of human heart under physiological conditions.

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