# Self-Powered, Hybrid, Multifunctional Sensor for a Human Biomechanical Monitoring Device
## 3 Results and Discussion  
Figure 2a illustrates the working mechanism of the SPMS, showing the principle of electric moving action. When applying an external force, triboelectric positive and negative charges are induced in the Al and PTFE, respectively, after the contact. Almost simultaneously, a piezoelectric potential is generated owing to the deformation of the PENG. To balance the piezoelectric potential, the piezoelectric current flows along with the external load from the bottom to the top Al electrode. After removing the external force, the piezoelectric potential disappears due to the recovery of the PENG, which causes the piezoelectric current to flow reversely. Almost simultaneously, the triboelectric pairs of Al and PTFE separate from each other. To balance the triboelectric potential, the triboelectric current flows from the top to the bottom electrode. When an external force is applied again, the triboelectric current flows reversely. The SPMS is composed of piezoelectric structures and a warped and zigzag-structured triboelectric nanogenerator. The stainless  
Figure 1: (**a**) The schematic diagram of the self-powered multifunctional sensor (SPMS) under the leg rehabilitation device. (**b**) The material of the sensor is based on a processed stainless steel piezoelectric material and three-layers of polytetrafluoroethylene (PTFE) and Al films. (**c**) Schematic diagram of the spherical triboelectric nanogenerator (TENG) with a multilayered structure. (**d**) A schematic illustration of the functional components of the sensor and the basic diagram of the piezoelectric nanogenerator (PENG) and TENG working methods. (**e**) Scanning electron microscope images of the surface on the PTFE. (**f**) Micomorphology of #1000 grit sandpaper surface. steel based PENG provides piezoelectricity. Furthermore, the TENG includes Al foil, PTFE tape, and copper tape electrodes that provide triboelectricity. The testing method was as follows: The total height of the device was fixed at 10 mm. The device was placed on the base and cyclical press/release operations were performed with a displacement of 7.5 mm to obtain an open-circuit voltage and short-circuit current. Both piezoelectric and triboelectric charge generation and electricity generation processes can be described by the compression/deformation/touch/release processes. When the piezoelectric stainless steel actuator begins to deform, the charge accumulation process in the opposite direction among the positive and negative charges can balance the induced dipole moment. Similarly, the mechanically actuated triboelectric signals can be generated from the SPMS while the three layers of PTFE films and the Al films as rubbing in the origami zigzag structure, generating triboelectric output. Physical contact between the PTFE and Al films can generate free electrons by injecting them into the Al film and produce triboelectric charges of opposite signs. When the piezoelectric actuators are forced to contact each other by the underlying electrostatic layers, the force is rebounded because of the natural spacer generated by the special polyimide actuator, and then it returns to the original position. The stability test of the SPMS is illustrated in Supplementary Information Figure S3, indicating the SPMS is highly robust and stable. The SPMS is capable of individually/simultaneously converting biomechanical energy to electricity. As compared with the TENG, the PENG has a much larger output voltage and current. As seen in Figure 2b, the piezoelectric voltage and current output were measured as \(\sim\)84 V and \(\sim\)293 \(\upmu\)A, respectively. Figure 2c shows the triboelectric output. The average voltage and current output of the triboelectric were recorded as 22 V, 285 nA, respectively. Based on the measurement results of the piezoelectric output and the quantitative triboelectric output, it can be found that the voltage and current have a superimposed effect in Figure 2d.