# Design and Performance Research of a Wearable Airbag for the Human Body
## 5 Parameter Analysis of Wearable Airbag  
The gas mass of the inflaton and length of the pulling strap are two important design parameters for the wearable airbag system. The change in the gas mass directly affects the pressure inside the airbag, which determines the stiffness of the airbag. If the pressure is too large, the airbag will be too rigid, and this will not provide good protection of the human body. If the pressure is too small, the airbag will be too soft, and the airbag has a risk of being smashed through. In addition to the importance of choosing the right gas mass, the thickness of the airbag inflation also significantly influences the protective performance of the airbag. The thickness of the deployed airbag at the back of the head is decided by the length of four pulling straps, as shown in Figure 14. When the airbag is inflated, the pulling straps are gradually stretched out and the strap length controls the final thickness of the inflated airbag. HIC\({}_{15}\) of the head injury index is chosen to evaluate the protection effectiveness of different gas masses and strap lengths, as well as their combinations.  
\begin{table}
\begin{tabular}{c c c c c} \hline  & \multicolumn{2}{c}{**Head**} & \multicolumn{2}{c}{**Chest**} \\ \cline{2-5}
**Injury Indicators** & **HIC\({}_{15}\)** & \begin{tabular}{c} **3 ms Resultant** \\ **Acceleration of the** \\ **Head (g)** \\ \end{tabular} & \
\begin{tabular}{c} **Intracranial Pressure** \\ **(kPa)** \\ \end{tabular} &
\begin{tabular}{c} **3 ms Resultant** \\ **Acceleration of the** \\ **Chest (g)** \\ \end{tabular} \\ \hline Without airbag & 1124.4 & 111.2 & 261.5 & 43.6 \\ With airbag & 619.6 & 66.8 & 168.6 & 34.8 \\ Improvement rate & 45.9\% & 40.0\% & 35.5\% & 20.2\% \\ \hline \end{tabular}
\end{table}
Table 1: Comparison of injury indicators.  
Figure 14: Shape of the head airbag under pulling strap control.  
Figure 13: Resultant acceleration curves of the chest.  
In the original design of the wearable airbag system, the gas mass is 24 g and the length of pulling straps are 5 mm. Here, 20 g, 22 g, 24 g, 26 g, and 28 g are selected as the five levels of gas mass, and 5 mm, 10 mm, and 20 mm are specified as the length of pulling straps. Figure 15a shows the gas pressure with different gas masses, with a fixed pulling strengp length of 5 mm. As expected, when the gas mass increases, the airbag pressure also increases. Figure 15b shows the inflation thickness of the head airbag at different strap lengths, with a fixed gas mass of 24 g. The central thickness of the head airbag increases with the increase in the pulling streng length.  
Figure 16 shows the HIC\({}_{15}\) head injury index distribution under different combinations of gas mass and pulling strengp. For a given gas mass, HIC\({}_{15}\) decreases with an increase in the pulling strengp length between 5 mm and 20 mm. With an increase in gas mass, the head injury index HIC\({}_{15}\) decreases first and reaches a minimal value between 24 g and 26 g. HIC\({}_{15}\) begins to increase after the gas mass is more than 28 g. It is found that, when the gas pressure of the airbag is not high enough, the upper and lower surfaces of the airbag are smashed through and the HBM's head hits directly on the hood, as illustrated in Figure 17a. If the gas pressure is sufficient, the inflated airbag maintains a good shape and prevents direct contact between the HBM's head and the vehicle hood, as illustrated in Figure 17b. However, when the gas mass rises to 28 g, the pressure inside the airbag is close to 200 kPa, which significantly increases the rigidity and weakens the airbag's protection effect. Furthermore, the gas mass 26 g and the pulling streng length, 20 mm provide the best protection performance for the HBM's head, and can protect the pedestrian from head injury.  
Figure 15: (**a**) Airbag internal pressure under different gas masses; (**b**) Central thickness of the inflated head airbag under different pulling strengp lengths.