Every day you walk around with your brain gently bobbing in your skull. Like a soft egg yolk floating in a cloud of clear egg whites.
All it takes is a sudden jolt or strike, and your brain is pushed aside at an amazing speed. Whether it collides with or twists the skull, the damage can be dire, as we know from people who have suffered traumatic brain injuries.
But what exactly happens to the brain at that moment of impact? How does it move?
Research into the biomechanics of brain injury usually includes crash test dummies headed for an accident, athletes wearing mouthguards or helmets equipped with motion sensors, or models simulating the human brain.
Now scientists have tossed eggs into the mix.
How an egg yolk reacts when different forces are applied. (Lang et al., Physics of Fluids, 2021)
What started as a kitchen curiosity for a team of engineers, with an egg scrambling tool for home cooks, led them to study the fundamental physics that controls the movement of soft matter in a liquid environment, using an egg to mimic brain.
“Critical thinking, along with simple experiments in the kitchen, has led to a series of systematic studies to investigate the mechanisms that cause yolk deformation,” said Qianhong Wu biomedical engineer of Villanova University in Pennsylvania.
Although their approach was somewhat unusual, the results of this study help us understand how soft matter, such as brain tissue, moves and deforms when exposed to external forces.
The more we know about and explain how the concussion affects the brain, the better researchers can improve safety systems in vehicles, design headgear for protection, and help athletes improve their technique to prevent injuries.
Inside the skull, the brain rests in a shock-absorbing fluid called cerebrospinal fluid.
The most common and mild form of traumatic brain injury (TBI) is concussion, and the term actually comes from a Latin word meaning ‘violent shaking’. But even one sub-concussion to the head is enough to induce changes in brain cell function, studies have shown.
In terms of the cause of brain injury, head rotation was proposed in the 1940s as a brain injury mechanism. Easy to imagine when you think of a blow to the chin that throws the head back, or someone getting whiplash from a tackle.
But there is often confusion about concussion mechanics, because there are several ways to measure head impacts and use that information to predict brain injury.
Early research efforts looked at rectilinear or ‘linear’ impacts, where the brain is bumped in one direction and bounced off the skull. Then attention turned to rotational forces that twist the brain in the skull.
Needless to say, it’s hard to measure how the brain can actually twist on such an impact, because we can’t see into people’s moving heads.
But scientists can still learn something by recreating the brain, snuggled in the cerebrospinal fluid, with similar materials.
In this study, the researchers began by measuring the material properties of an egg yolk and its outer membrane, so that they could later quantify the stress the eggs experienced during two-setup laboratory experiments.
“To damage or deform an egg yolk, you would try to shake and rotate the egg as quickly as possible,” the study authors write in their paper, so the eggs were cracked in a clear container and subjected to three types of impact.
The team observed how egg yolks were compressed and stretched in different directions with an accelerating rotational impact, as well as how they barely changed with a direct hit to the container.
When a rotating egg-filled container was stopped abruptly, the yolk became “hugely” deformed by the decelerating rotational impact, and it took about a minute for the warped yolk to return to its original round shape.
“We mainly suspect that rotation [decelerating] rotation, impact is more damaging to brain matter, ”Wu said.
The results of this study parallely with previous research involving vehicle crash tests and pendulum head collisions, which found that spinning head collisions are a better indicator of risk of traumatic brain injury than linear acceleration.
These findings reflect the general consensus that the brain is more sensitive to rotational movements than to linear movements.
But that doesn’t mean we should dismiss linear effects altogether, as other researchers have proposed new injury statistics that combine measurements of linear and rotational head acceleration for assessing concussion risk.
Brain injuries are certainly complicated, and many unfortunately go unnoticed. With this clever experiment we can at least see the brutal impact ourselves.
The study is published in Physics of Fluids.