The study’s results are not what you might expect. Using an egg substitute as a model for the brain, the team discovered that rotational impact – especially decelerating rotation – rather than direct impact causes extensive trauma to the yolk inside.
“What we found in this paper is quite counter-intuitive and very surprising,” says Qianhong Wu from Villanova University, US, senior author of the paper published in the journal Physics of Fluids.
As researchers in brain biomechanics, Wu and his team turned to the humble egg to understand the fundamental physics behind traumatic injury.
“We came up with a very wild idea to compare an egg yolk bathed in the liquid egg white, enclosed in the hard shell, to the soft brain matter bathed in the liquid cerebrospinal fluid and enclosed in the hard skull,” says Wu.
“Soft matter bathed in a liquid environment widely exists in nature; besides the brain matter, the egg, [or] a red or white [blood] cell is another example.”
This led them to ask how they could deform or scramble an egg yolk without breaking the eggshell to help understand why brain matter is deformed during concussion even though the skull is unharmed.
The team designed two experiments using fresh egg yolk bathed in egg white and enclosed in a rigid, transparent, cylindrical shell, which they filmed with a high-speed camera.
The first experiment tested “translational impact” to the yolk, in which the container is exposed to a sudden and direct hit.
The second used a rotational impact test, which caused the container to rotate, either accelerating or decelerating.
After discovering that the egg yolk was very sensitive to rotation, especially deceleration, they explored further with fluid mechanics analysis to clarify.
“The large deformation of the brain matter during this process induces the stretch of the neurons and causes the damage,” write the authors. “This finding explains why a player in a boxing game will very likely faint if he is hit on the chin.”
As the chin is the farthest point from the neck, they explain that hitting it could cause the most forceful rotation of the head.
The study adds to their previous work, for which they designed a novel “smart brain” with similar properties to the human brain – apart from thinking – as a tool for gaining insights into traumatic brain injury.
The current discovery could help address concussion injuries in sports, Wu notes. Helmets could be designed to mitigate the rotational impact, for instance, and if rotational impacts are implicated, he suggests more cautious testing for concussive brain injury. “By saying so, I am not excluding concussion due to translational impacts, but it follows a different mechanism – we are working in this area,” he says.