# Design and Performance Research of a Wearable Airbag for the Human Body
## 4 Analysis of Wearable Airbag Protection Performance
### Protective Performance of Wearable Airbag  
When pedestrians are involved in a collision with a car, severe head injury is a major cause of death [28]. A widely utilized head injury criterion is HIC, which is calculated by integrating the acceleration of the head during a given time. In this paper, HIC\({}_{15}\) is chosen as the head injury metric. According to the protection of vulnerable road users (VRU) in the European New Car Assessment Program (E-NCAP), HIC\({}_{15}\) is used as a pedestrian head injury indicator which should be less than 1000.  
\[HIC_{15}=\text{max}\left\{\left[\frac{1}{t_{2}-t_{1}}\int\limits_{t_{1}}^{t_{2 }}a(t)dt\right]^{2.5}(t_{2}-t_{1})\right\} \tag{5}\]  
where \(a(t)\) is the resultant acceleration of the head, and the time interval between \(t_{2}\) and \(t_{1}\) is 15 ms. The head accelerations of the HBM in the two scenarios with and without airbag are extracted and filtered, respectively.  
When the lower leg of the pedestrian contacts with the bumper, the impact of the vehicle causes a high acceleration of the lower limb of the HBM. After the whole human body rotates, the head hits the engine hood, which causes serious injury. Figure 10 shows the acceleration curve of the lower leg of the pedestrian when impacted. As shown in Figure 11, the HIC\({}_{15}\) value of the HBM without the airbag is 1124.4, while the HIC\({}_{15}\) of the HBM with the airbag is 619.6. That is a 45.9% decrease in the HIC\({}_{15}\) of the HBM's head. In addition, another indicator of head injury is 3 ms acceleration, which refers to the maximum acceleration of the head that can last for 3 ms. According to the European New Car Assessment Program (ENCAP), when a head acceleration of 88 g or more can last for 3 ms, it can lead to serious head injury. The 3 ms resultant head acceleration of the HBM wearing the airbag also decreased from 111.2 g to 66.8 g.  
Figure 10: Lower limb acceleration of the HBM.  
In 1980, [PERSON] [29] proposed the injury limit of intracranial pressure based on studies of animal and human cadaveric experiments, and finite element brain models. Severe injury occurs when the peak intracranial pressure exceeds 235 kPa, moderate injury occurs when the peak is between 173 and 235 kPa, and very mild or no injury is caused when the peak pressure is below 173 kPa. Figure 12a,b illustrates the peak intracranial pressure of the HBM without the deployment of the wearable airbag system at 174 ms and the peak intracranial pressure of the HBM with the deployment of the wearable airbag system at 179 ms, respectively. The peak intracranial pressure of the HBM without the deployment of the wearable airbag system is 261.5 kPa and the pedestrian will be severely injured according to [PERSON]'s injury criteria. In contrast, the peak intracranial pressure of the HBM wearing airbags is 168.6 kPa, a 35.5% decrease in peak pressure, and the pedestrian would not be injured or just injured mildly. Therefore, the wearable airbag system can provide significant protection for the pedestrian's head.  
Regarding the chest protection performance of the wearable airbag system, the 3 ms resultant acceleration of the chest is specified as the injury indicator of the chest. Figure 13 shows the resultant acceleration curves of the chest of the HBM in both scenarios, with and without the deployment of the wearable airbag system. The value of 3 ms resultant acceleration of the pedestrian's chest is 43.6 g for the HBM without the deployment of the wearable airbag system and 34.8 g for the HBM with the deployment of the wearable airbag system, a 20.2% drop. The injury indexes for the head and chest are summarized in Table 1, and we can immediately see the effectiveness of the wearable airbag system in protecting the pedestrian's head and chest.  
Figure 11: Head acceleration histories and HIC\({}_{15}\) of the HBM with and without the protection of the wearable airbag.  
Figure 12: Intracranial pressure map: (**a**) without airbag; (**b**) with airbag.