# Plasma-Sprayed Flexible Strain Sensor and Its Applications in Boxing Glove
## 2 The Controllable Plasma Spraying Method for Conducting Nanoparticles  
### Low-Temperature-Plasma Spraying Method and Device  
To simplify the fabrication method for flexible strain sensors and improve their sensitivity and stability, a programmable controllable low-temperature-plasma spraying method was proposed to fabricate high-sensitivity flexible resistive-strain sensors. Plasma [23] is a fully or partially ionized gaseous state of matter at high temperature or under specific excitation conditions. It is composed of ions, electrons, and a collection of unionized neutral particles. Low-temperature plasma is widely used in the field of material modification due to its low discharge temperature and simple operation. It has a significant effect on local material modification and hardly causes any damage to other structures. Figure 2 shows the schematic diagram of the low-temperature-plasma spraying device used in this study. It mainly consisted of an aerosol generator, a dual-medium jet device, a three-dimensional (3D) motion platform and its control, a low-temperature-plasma experimental power supply, an argon gas source, a rotameter, connecting wires and pipes. A liquid aerosol generator is a two-fluid atomizer used for generating uniform and stable aerosols with a particle size distribution (i.e., median total aerodynamic diameter, MMAD) of 1-4 \(\upmu\)m. Here, a dispersion containing the sensing layer material (CNTs) was placed in the liquid aerosol generator (Figure 2a). There are three main types of plasma jet generators: single-electrode, double-electrode, and three-electrode structures [24; 25; 26]. Because the spray material contains non-working gas components, a dual-medium jet device with coaxial dual jet channels is required. The inner channel sprays the material, and the outer channel generates plasma. Therefore, in this study, a dual-medium jet plasma tube was designed (Figure 2b). The plasma jet took the form of a ring-to-ring discharge. The device consisted of an inner-layer quartz glass  
Figure 2: The plasma spraying device. (**a**) introduction of the plasma spraying device (**b**) the process of the low-temperature-plasma spraying; (**c**) the temperature of the low-temperature-plasma. tube, an outer-layer quartz glass tube, a collector, a main air inlet, and an auxiliary air inlet arranged on the jet tube. To enable full contact between the spraying material and the plasma after being ejected from the nozzle, the outer nozzle was extended 2 mm compared to the inner nozzle. Figure 2c indicates that the temperature of the low-temperature-plasma spraying was 31.5 \({}^{\circ}\)C.  
The control board of the 3D platform was connected to a computer to achieve motion control. The setup was mainly composed of control software, a control board, a driver, a linear slide table, and a DC switching power supply (48 V). The control program was generated by the 3D design and processing software (UG). The Grbl Controller open-source control software was adopted here. The MEGA2560 control board, which is commonly used in 3D printers, was selected for the 3D computer numerical control milling machine control software. Figure 3 shows the 3D platform control interface and the 3D platform control panel. The step speed of the stepping motor we used was 1000 r/min, the speed of the synchronous belt was 0.1-1 mm/s, and the repeatability was 0.05 mm. The movement of the moving part was achieved by using a CCM synchronous belt linear slide table. Figure 4 shows the preparation process for the flexible sensor. The process was as follows. (1) A layer of silica gel was spin-coated on a circular acrylic plate so that the thickness of the silica gel film reached approximately 0.5 mm. (2) The CNT-ethanol dispersion was prepared at a mass fraction of 5%. (3) The path code was generated according to the set path, and the spraying device was then connected to the host computer to start the spraying path program.