BMe Research Grant
Although nanofibers have already been used to make TENGs, and some of TENGs made from polymers can also be used as a breathing sensor, there has been little research where a TENG can be used as a filter and a respiration monitoring system simultaneously. Therefore, the aim of my research is to develop a smart mask filter, which not only has high filtration efficiency but is also capable of monitoring respiration (Figure 2). My first task was to create a TENG configuration that can generate voltage signals based on different breathing parameters. To obtain stable and obvious voltage signals, the structural parameters of TENG need to be optimized. However, during the recording of the voltage signals, the shape of the signal spectrums was not as periodic as that of the theoretical ones due to the noise from the device. For this reason, I had to optimize the scanning interval time of the device to eliminate the disturbances caused by noise. The long-term stability of voltage output from the TENG is very important for TENG filter applications. Furthermore, in my research, the filtration efficiency of the nanofiber filter will also be examined and compared with the commercially available KN95 masks. Finally, the output voltage signals obtained from the TENG were analyzed based on the different respiratory parameters to detect the respiratory state.
Figure 2 The demonstration of the structure of the smart mask filter
Figure 5 Voltage generation mechanism of the TENG
The deformation of the PAN nanofiber membrane at different thicknesses was measured by 3D DIC method. The thickness was controlled by the electrospinning time, which was set to 40 min, 80 min, and 120 min. The center point has the maximum deformation with the highest displacement. It shows that the displacement of the center point increased with decreasing nanofiber membrane thickness (Figure 6). The deformation results served as a reference for setting the distance between the two layers.
Particle filtration properties of nanofiber membrane
In my previous research, I combined nanofibers with 3D printed mesh as a supportive structure to impart the nanofiber filter with good mechanical properties. During the 3D printing process, the higher nozzle temperature can increase the tensile strength and decrease the breaking strain because of the better fusion between the adjacent layers. A thin, single layer of nanofibers can outperform a classical surgical mask (Figure 7). Furthermore, the multiple layers of nanofibers can have a similar filtration performance as KN95/N95 and FFP2 filters [S3]. The results showed that nanofiber membrane is a good material for mask filters.
Figure 7 (A) Filtration efficiency of nanoporous filters with various particle size, (B) pore size distribution of nanoporous filter printed with different temperatures, (C) the filtration efficiency (wt%) of nanoporous filters with various stacking layers, (D) scanning electron microscope image of the nanoporous filter after filtration. Inset is the bent single-layered filter.
Respiratory sensing with TENG
The different respiratory parameters (respiratory frequency and respiratory quotient) were recorded successfully with the TENG sensor. Respiratory frequency can be represented by the time interval between adjacent positive (or negative) peaks. Respiratory quotients can be calculated from exhalation time and inhalation time. By adjusting respiratory parameters on a ventilator, the obtained signals showed consistent respiratory results with the input respiratory parameters of the ventilator. The durability and stability of the TENGs are also crucial and important factors for their practical application. There was no noticeable change in output voltage signals after 40 hours of use, demonstrating that the TENG has excellent durability and reliability (Figure 8).
Figure 8 The output voltage of the TENGs according to different breathing parameters: (a) characterization of one respiratory cycle, (b) different respiratory frequency, (c) different quotients (the ratio of inhalation and exhalation), (d) the durability of the TENG
Nanofibers can be used in the mask industry as high-performance filtration media to filtrate nanosized particles because of their large surface area, high porosity with small pore size. With a very thin nanofiber membrane, they can have the same filtration efficiency as FFP2/ FFP3 masks. In addition, the mask itself can also be used as a sensor to monitor the user’s breathing. In a pandemic time, the development of a smart mask with respiratory sensing is urgent and critical. Especially for the patients infected with Covid-19, who must wear the mask at all time to protect themselves and prevent the transmission of the virus. A smart mask could provide a critical early-warning sign when a patient’s condition worsens, receives anesthetic or opioid pain relief. In addition, being a self-powered sensor that does not require a power supply for its operating state, it is very convenient and comfortable for users.