# Breathable Materials for Triboelectric Effect-Based Wearable Electronics
## 3 Breathable Wearable Electronics Based on Triboelectric Effect
### Nanofiber Membrane-Based Wearable Electronics  
The direct use of textile is a convenient and easy way to design comfortable and wearable electronics. Nevertheless, the electric output performance and sensitivity of these electronics are barely satisfactory. According to existing literature, the nanostructure on the surface of the friction materials will increase the output of the triboelectric effect-based devices, as a larger effective contact area will be obtained [51; 52]. Therefore, a nanofiber membrane, which has both air permissivity and a nanostructure on its surface, was introduced to prepare the wearable electronics. As presented in Figure 6[53], polyvinylidene fluoride (PVDF) and thermoplastic polyurethanes (TPU) nanofibers, prepared through electrospinning, were chosen as friction layers of the device. Figure 6b,c present the SEM images of the surface of the PVDF and TPU nanofiber. The nanofiber membrane was formed by the pile of millions of nanofibers, which lead to nano-micro-holes between  
Figure 5: The textile-based wearable electronic device. (**a**) Illustration of the structure of the washable electronic textile (WET). (**b**) Photograph of the as-fabricated WET. (**c**) The SEM image of the nylon textile covered with CNT ink. (**d**) The magnified SEM image of the nylon textile covered with CNT ink. Figure adapted from ref. [38]. nanofibers. It is speculated that gas can diffuse through the holes. The surface is relatively rough, as well, because of the irregular pile of the nanofibers. At the same time, conductive gauze, and silver elastic textile, which act as breathable electrodes, were used as substrates to collect PVDF and TPU nanofibers. Consequently, the whole device is breathable and comfortable. Interestingly, the TPU membrane and its substrate, silver textile, are elastomer, which made the device stretchable. The stretchability and flexibility of the device is favorable for harvesting irregular human motion such as stretching or twisting the wrist, and is a more comfortable and durable method compared to traditional non-elastic materials. As mentioned above, nanofiber membrane-based electronics have high sensitivity compared with textile-based sensors. It has been demonstrated that sensors fabricated with nanofiber membranes were able to detect slight mechanical force. For example, our group put forward a self-powered and nanofiber-based triboelectric sensor (SNTS) for respiratory monitoring [54]. Figure 7a presents the structure of the SNTS. The friction material and electrode is PVDF nanofiber membrane (PVDF NM) and Ag nanoparticles (AgNPs), respectively. The SEM in Figure 7b is the surface of the PVDF NM coated with AgNPs, where little holes indicate the air permissivity of the device in terms of the micro-nano morphology of the materials. Based on a single-electrode working mode, the detailed process of the transfer of the electrons is described in Figure 7c. With the idea of simple structure and mass production, electrospinning and screen-printing are combined to fabricate the SNTS (Figure 7d). The SNTS used for respiratory monitoring, with light weight and small volume, is shown in Figure 7e. Figure 7f compares the air permissivity of the as-fabricated sensor with other commonly used materials. Although the gas permeability of the SNTS is worse than jeans, it is about 4.5 times higher than A4 printing paper, not to mention its superiority compared with cast film. Given features such small volume and light weight, the SNTS can be easily attached on the inner side of a mask to monitor respiratory conditions with high sensitivity. In previous work, nanofibers of PVDF NW was shown to be waterproof with high electronegativity. However, its application in wearable electronics is still unsatisfactory, as it will crack under high stretch stress or deformation [55]. Therefore, devices which can be stretched, bent, and compressed are necessary to meet the higher requirements of wearable electronics [56].