For the first time, researchers from the University of Virginia and Toyota have described how human fat tissue behaves under forces typical of motor-vehicle crashes, closing a vulnerable gap in car safety research while offering a valuable resource for manufacturers seeking to make safer vehicles.

Prior to this study, published May 25 in Acta Biomaterialia, the mechanical qualities of human adipose tissue had been poorly described in scientific literature. But now the picture is clearer: Human fat reacts like water when subjected to shear strain, which is the force and alteration of objects sliding past one another. This quality helps govern the interaction between passenger and lap belt, and plays a significant role in accidents that inflict devastating internal injuries. 

"There weren't a lot of things known about fat in this realm before this study, so this area we're stepping into is completely blank, unknown," said lead author Zhaonan Sun, an R&D engineer at Align Technology, Inc., who completed this research as a graduate research assistant at the University of Virginia, independent of his current position. 

According to Sun, the study was inspired by a desire to improve the safety of cars, something that automotive engineers are constantly working toward to help stem the high number of road deaths. In 2019, for example, 36,096 people died in 33,244 fatal accidents in the U.S., according to the Insurance Institute for Highway Safety. 

"There's still plenty of work to be done to understand and make cars safer," said co-author Jason Kerrigan, the director of the University of Virginia Center for Applied Biomechanics. "It's certainly not a situation where we can't make any more gains because we can. We've seen continued gains over the last 30 or 40 years of this research in general, and I think there's a lot of room to go."

Car manufacturers have typically assessed vehicle safety using crash test dummies, though these tools represent only a small portion of the population, according to Kerrigan. Because human variation plays a significant role in how bodies respond during crashes, dummies cannot accurately simulate the way most people would get injured in a car accident, so now car manufacturers are turning to computational simulations to offer a more diverse perspective on bodily response to crashes. 

"Mechanical models are complicated to start with, and the human body is probably one of the most complicated mechanical structures there is," Kerrigan said in an interview with The Academic Times. "And so what's easy is to make something that looks kind of like the human body, but what's really difficult is to make it respond in a mechanical way like the human body."

So far, much work on these computational models has gone into predicting bone fractures and getting the geometry of the human body correct, but less has been done on understanding soft tissues, which exhibit some of the human body's most complex mechanical responses, according to Kerrigan. 

To understand how fat responds to the complex combination of forces sustained in a crash, the researchers subjected tissue samples from six different subjects to compression and shear tests. Compression describes the force pushing on an object, while shearing describes the strain of forces sliding around or over it, similar to how one can move the skin covering one's arm, according to Kerrigan.

The tests revealed that human fat reacts to compression and shear forces similarly to water. It stiffens when compressed, but flows, or slips, when shorn. 

Closing the information gap around fat matters when the interaction between fat and restraint systems can govern the impact of serious injury. For example, "submarining" is a particularly dangerous effect that can occur when a passenger may be reclining during a crash. The pelvis, located at an irregular angle, can slide out from under a lap belt, which then violently compresses the passenger's abdomen. This research is especially important as self-driving cars become more common, resulting in more drivers and passengers spending time reclined. 

By having an updated model that accounts for the shear and compression properties of fat, car manufacturers could potentially change a variety of parameters in their automobiles' restraint systems, including how much force to include in a seatbelt, the angle of the belt, how much air to put in an airbag and how fast the bag releases. 

According to Kerrigan, despite an unchanging outward appearance, lap belts have continued to advance over the last 25 years. Kerrigan and his colleagues are currently working with a manufacturer that is seeking to protect reclining occupants by developing a lap belt that grips the pelvis tightly at the moment of impact, though questions remain: How hard to pull? At what angle? Can the car sense the position of the occupant? 

"That's really the next advancement, where the car will say, 'Oh, I know who's in the car: He's this size, and he's a male and he's this age.' And so as a result, we know we can do these things," Kerrigan said. "So maybe now the restraint system will actually say, 'Oh, I'm in this kind of crash, too.' Maybe it senses the kind of crash and then adapts restraint for your body in your crash, maybe even based on how you're seated."

Sun has already developed a human body model using the information from this current study, and analyzed how the new data changes the results of simulations. He's so far found that fat, while important, isn't the only tissue that matters. Other tissues that cover the pelvis, such as skin, and the tissues that connect the pelvis to the skin also matter, because of the variation in body geometry influenced by these factors. 

For example, some passengers will have almost no tissue between their pelvis and their skin, where others will have more. This variation can greatly affect their body's response to a crash. 

Ultimately, additional study will be needed to better understand these relationships to continue improving car safety research. 

"It's a long journey," Sun said. "Our research is taking you to the airport, and then you've got to fly there, but we're basically giving you the ticket."

The study, "Multidirectional mechanical properties and constitutive modeling of human adipose tissue under dynamic loading," published May 25 in Acta Biomaterialia, was authored by Zhaonan Sun, Align Technology, Inc.; Bronislaw D. Gepner, Sang-Hyun Lee, Joshua Rigby, Patrick S. Cottler and Jason R. Kerrigan, University of Virginia; and Jason J. Hallman, Toyota Motor Engineering and Manufacturing North America, Inc.