# Plasma-Sprayed Flexible Strain Sensor and Its Applications in Boxing Glove
## 1 Introduction  
Flexible and stretchable electronic devices, which contain flexible sensors, can be used in wearable, lightweight, portable, biocompatible, economical, and environmentally friendly products. Flexible sensors can be attached to the surface of objects and measure the strain distribution on the detection surface. Thus, they are widely used in several applications, such as bionic skin [1, 2, 3, 4], health monitoring in sports [5, 6, 7, 8, 9], and communications [10, 11, 12]. The design and manufacture of the sensing layer is the most important aspect of flexible resistance-strain sensors. Several methods are used for the preparation of these sensing layers, such as deposition, soft lithography, screen printing, and inkjet spray printing. [PERSON] et al. [13] obtained a stable and stretchable EGaIn strain sensor (Figure 0(a)) by printing a liquid metal on a platinum-catalyzed silicone elastomer (Ecoflex(tm) 00-30) to fabricate the sensor. The sensor exhibited a nonlinear sensitivity and a maximum gauge factor (GF) of \(<\)8. [PERSON] et al. [14] used an elastomer composite filled with carbon black as the sensing layer, while the composite sensing layer was mass-produced using a thin-film casting technology and CO\({}_{2}\) laser ablation technology, which facilitated the rapid multilayer fabrication of resistive flexible sensors. The fabricated sensor exhibited a high strain amplitude, low hysteresis, and high repeatability (Figure 0(b)). However, its GF in the resistive mode was 1.62-3.37. As shown in Figure 0(c), [PERSON] et al. [15] used a very simple flexible sensor preparation scheme, which involved using a tape as a mask and a hand-held spray can for spraying. However, the GF (1-1.2) of the prepared sensorwas not high. [PERSON] [16] also used a spray method to manufacture CNT coating films as a sensing layer and study the hysteretic dependence on strain under cyclic loading. [PERSON] et al. used 3D-printing technology to prepare a flexible sensor with a filament made from a TPU/MWCNT mixture and explored how directional sensitivity was affected by the type of pattern design [17]. [PERSON] et al. [18] fabricated an integrated glove sensor using a direct ink-writing method, and this integrated manufacturing method could improve manufacturing efficiency. Previous studies indicated that flexible strain sensors prepared using deposition methods exhibited several issues, such as an insufficient bonding force between the sensing layer and the substrate, as well as easy peeling of the sensing layer, which reduced the sensitivity of the fabricated flexible sensors. Therefore, in this study, a method using low-temperature plasma to treat the sprayed material during the spraying process was proposed to improve the sensitivity of the flexible sensors. Here, multiwalled carbon nanotubes (MWCNTs) and silica gel (Ecoflex 0050) were used as the sensing layer and substrate, respectively, of the flexible, stretchable resistive-strain sensors. The effects of low-temperature-plasma treatment on the performance of the flexible sensor, the effects of pattern parameters on the sensitivity of the flexible sensor, and other properties of the flexible sensor were studied. Boxing gloves are an essential part of the protective gear used in martial arts and fighting competitions. They protect the hands of the boxer and allow them to hit the opponent more efficiently while giving the opponent a certain degree of protection [19]. The development of smart gloves that can monitor the hitting force in real time during hitting will help the judges in determining the hitting force and technique. This will create a fairer environment for boxing competitions and relieve the pressure of enforcing the rules by the judges. Numerous scholars throughout the world have carried out research work on boxing intelligence. [PERSON] et al. [20] virtualized the real training process using somatosensory interaction and recorded training information in the information system, which provided a new method for virtual boxing training, games, and teaching.