Every day you walk around with your brain gently crawling around in your skull. Much like a soft egg yolk floating in a cloud of bright egg whites.
All it takes is a sudden shock or a blow, and your brain is pushed aside at an alarming rate. Whether it hits the skull or makes a turn, the damage can be serious, as we know from people who have experienced a traumatic brain injury.
But exactly what happens to the brain at that moment of impact? How does it move?
Research examining the biomechanics of brain injuries usually involves crash test dummies on the way to an accident, athletes wearing mouthguards or helmets with motion sensors, or models simulating the human brain.
Now scientists have thrown eggs into the mixture.
How an egg yolk reacts when different forces are applied. (Lang et al., Fluid Physics, 2021)
What started as a curiosity in a kitchen for a team of engineers, with an egg cookie tool for home cooks, led them to study the fundamental physics that controls the movement of soft materials in a liquid environment and ‘ use an egg to mimic the brain.
“Critical thinking, coupled with simple experiments in the kitchen, has led to a series of systematic studies to investigate the mechanisms that cause egg yolk deformity,” said biomedical engineer Qianhong Wu of Villanova University in Pennsylvania.
Although their approach was somewhat unusual, the results of this study help us to understand how soft matter, such as brain tissue, moves and deforms when exposed to external forces.
The more we know about and can account for concussion forces affecting the brain, the better researchers can improve vehicle safety systems, design headgear for protection and help sports players 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 head shake to the head is enough to cause changes in the functioning of brain cells, studies have shown.
Causing the brain injuries, co-rotation was proposed as a mechanism for brain injury in the 1940s. Easy to imagine if you think of a fist blow to the chin that throws the head back, or someone who gets a whiplash from a tackle.
But there is often confusion about concussion mechanics, as there are different ways to measure head impacts and use the information to predict brain injury.
Early research efforts looked at straight or ‘linear’ impacts, where the brain is pushed in one direction and bounces off the skull. Then the focus focuses on rotational forces that rotate the brain within the skull.
Needless to say, it is difficult to measure in what way the brain can turn such an impact, because we can not look into people’s moving heads.
But scientists can still learn something by recreating the brain, stuck in its cerebrospinal fluid and using similar materials.
In this study, the researchers began to measure the material properties of an egg yolk and its outer membrane so that they could later quantify the stress in which the eggs are located during laboratory experiments, which contained two setups.
“To damage or deform an egg yolk, one would try to shake and rotate the egg as quickly as possible,” the study authors write in their paper, so that eggs are burst in a clear container and three types of impact are subject.
The team observed how egg yolks compressed and stretched in different directions with an accelerated rotational impact, and also how it almost did not change at all with a direct hold in the container.
When a rotating egg-filled container was sharply stopped, the egg yolk deformed “tremendously” with the delayed rotational impact, and it took about a minute before the twisted yellow resumed its original round shape.
“We suspect that rotation in particular [decelerating] rotation, the impact is more harmful to the brain, ”said Wu.
The results of this study were in parallel with previous investigations regarding vehicle collision tests and impact of the pendulum head, which found that rotational head impacts are a better indicator of traumatic brain injury risk than linear acceleration.
These findings reflect the general consensus that the brain is more sensitive to rotational motion than linear motion.
But that does not mean we should completely discount the direct impact, as other researchers are proposing new measures of injuries that combine the measures of line and rotational head acceleration to determine the concussion risk.
Brain injuries are certainly complicated and many are unfortunately not detected. At least with this clever experiment we can see the brute impact for ourselves.
The study was published in Physics of liquids.