## 2 Materials and Methods  
Static electricity due to friction is more readily observed in a dry environment, increasing the feasibility of micro plasma generation. This study used the following structure to intentionally generate micro plasma by external vibration. A long rod-shaped metal structure and a mass for resonant motion by external vibration were placed at one end (Figure 1a-c). The metal rod was fabricated from a shape memory alloy ensuring high elasticity and restoring force. The mass was fabricated using light polyethylene terephthalate (PET). The other end was a ring-shaped contact surface structure. The micro-crack contact surface was fabricated using a diamond tip moving horizontally to the surface. The resulting needle-shaped surface facilitated the concentration of electrons. Both the mass and the metal rod were designed as a resonant structure to generate as much motion as possible by a single application of external force. The resonant frequency can be calculated using the following equation:  
\[\text{f}(\text{Hz})=\frac{1}{2\pi}\sqrt{\frac{k}{m}}=\frac{1}{2\pi}\sqrt{\frac {3 EI}{mI^{3}}}. \tag{1}\]  
Here, f (Hz) is the resonant frequency, \(k\) is the spring constant, and \(m\) is the weight of a mass. \(E\) denotes the Young's modulus of bar, \(l\) is the length of bar, and \(l\) is the moment of inertia. In this design, the mass was 0.243 g in weight (\(m=0.243\) g), and the rod was 50 mm in length (\(l=50\) mm) and 0.3 mm in diameter (d = 0.3 mm). In the design of this study, the resonant frequency was set to 7.05 Hz, which belongs to the typical range of 1-10 Hz in nature. As a multiple number of contacts could occur in a single vibration, triboelectric charges could be effectively extracted from TENG to an electric circuit (Figure 1d). Charges accumulated in TENG were discharged as soon as the metal column and the internal surface of the ring were sufficiently close for the plasma discharge to occur. This occurred several times during a single external vibration event. The rough internal surface of the ring concentrated and easily emitted electrons, similar to a lightning rod or electron gun (Figure 1c). Figure 1e shows the micro plasma occurring in the structure designed in this study. If a TENG has sufficient basic power, such micro plasma can occur several times owing to a single external force. Micro plasma has different intensities depending on whether the internal surface of the ring is smooth or rough. The TENG output amplifier (TENG Amp.) was designed by applying this theory, as illustrated in Figure 2a,c. Two terminals for connecting electrodes and a casing for protecting the interior part were added to the above basic structure. Figure 2b,d-g show the shapes fabricated herein. The surface of TENG Amp. was observed to analyze the roughness by an optical microscope (VHX-7000, KEYENCE Corporation (Osaka, Japan)). The TENG output amplifier had dimensions of 30 cm \(\times\) 10 cm \(\times\) 5 cm. One of the two terminals was connected to the output terminal of the TENG, and the other terminal was connected to the electric circuit. The effects of the TENG output amplifier are explained along with the test results in the next section. Two types of TENG specimens were prepared for the test. The one specimen simply comprised a PDMS layer and an electrode. As shown in Figure 3a, the simple TENG specimen with the PDMS layer was fabricated by molding the PDMS layer with dimensions of 6 cm \(\times\) 6 cm \(\times\) 0.5 cm and placing it on a metal electrode. The other specimen was fabricated using a conductive sponge with a carbon-based porous PDMS structure. In this specimen, the CNT-PDMS conductive sponge was used as an electrode structure. Silicone rubber was adopted for negative charges. The contact/separation triboelectrification mode was applied. An Al plate connected to a pushing tester was used as a positively charged counterpart. The conductive sponge was fabricated using a process illustrated in Figure 3b. Coarse-grained sugar was used to fabricate a porous structure.