What happens to the brain during a sudden impact? Egg yolks might contain the answer

An increasing number of professional soccer players have been diagnosed with a neurodegenerative disease called chronic traumatic encephalopathy (CTE), possibly the result of repeated concussions or similar repetitive brain trauma during their careers. It is also common in other high contact sports like boxing, Muay Thai, kickboxing, and ice hockey. We could find clues to the underlying physics by studying the deformation of egg yolks, according to a new article published in The Physics of Fluids. This in turn could one day lead to better prevention of such trauma.

Egg yolk submerged in liquid egg white enclosed in a hard shell is an example of what physicists call “soft matter in a liquid environment”. Other examples include the red blood cells that circulate in our circulatory systems and our brains, surrounded by cerebrospinal fluid (CBR) inside a hard skull. The extent to which a type of soft matter deforms in response to external impacts is a key characteristic, according to University physicist Villanova Qianhong Wu and his co-authors of the latest study. They cite red blood cells as an example. It is the ability of red blood cells to change shape under stress (“erythrocyte deformability”) this allows them to squeeze through tiny capillaries, for example, and also triggers the spleen to remove red blood cells whose size, shape, and overall deformability have been too heavily altered.

In the case of traumatic brain injury, it is linked to the deformation of the brain in response to the impact. The precise cause of CTE is still a matter of ongoing research, but mainstream theory holds that repetitive brain trauma can damage blood vessels in the brain, causing inflammation and the growth of clumps of a protein called Tau. Eventually, these clumps spread throughout the brain, killing brain cells. People with CTE often experience memory loss, depression, and in severe cases, dementia, among other symptoms.

Previous studies have shown that the deformation of soft matter in a liquid environment occurs in response to sudden changes in the fluid field, such as shear flow or a sudden change in the flow path. Wu et al. were interested in the specific case of soft matter in a liquid environment that is also enclosed in a rigid container – like the yolk of an egg, surrounded by liquid egg white, all enclosed in a shell. They wondered if it was possible to break the yolk without breaking the shell, since it is the case with most concussions that the brain can be damaged without cracking the skull.

To answer this question, Wu et al. set up a simple preliminary experiment with a Golden Goose Egg Scrambler, a new kitchen appliance that allows users to scramble an egg directly into the shell. Wu’s team applied rotational forces to scramble the egg and were intrigued by how the egg yolk warped and broke as the shell remained intact. This prompted them to conduct additional experiments to gain insight into the fundamental physics of the flows behind the effect.

They bought fresh eggs from a local grocery store, removed the egg yolks and whites, and then placed them in a transparent rigid container, to better monitor the deformation by recording the entire process with high-speed cameras. . They built two separate devices. We administered what is called a “translational impact” – that is to say, hitting the container directly – via a small hammer falling from a vertical guide rail (see figure 1A in gallery), with a spring at the bottom allowing the container to move vertically. They used an accelerometer to measure the acceleration of the container.

For the second configuration (see figure 1B in the gallery), they connected the container to an electric motor to study two types of rotational impact: acceleration of rotational impact and deceleration of rotational impact (i.e. (i.e. when the external content speeds up or slows down as it spins). They also peeled off the membranes surrounding the fresh yolks and hung them in Petri dishes filled with water, to better study how these membranes respond to stress as well.

Wu et al. were somewhat surprised to find that in the event of a translational impact, there was almost no deformation of the yolk. Instead, the entire container (and its contents) moved as one rigid body. In the case of an accelerated rotational impact, the team found that the yolk started out in a spherical shape and then began to stretch horizontally to form an ellipsoid. The yellow could maintain a stable ellipsoid shape for several minutes if the angular velocity was kept constant.

The most intriguing results have occurred in the case of the decelerating rotational impact. Here, the yolk began to deform significantly almost immediately, extending horizontally and increasing its radius in the center – a deformation enough to seriously damage the yolk under sustained stress.

“We suspect that the impact of rotation, especially deceleration rotation, is more harmful to brain matter.

To ensure that this was not primarily an effect of yellow as a biomaterial, Wu et al. conducted the same experiment with synthesized soft capsules immersed in a solution of calcium lactate, enclosed by a thin membrane of calcium alginate. They obtained similar results, confirming that “the dominant mechanism leading to the deformation of soft matter in a liquid environment is the result of mechanical forces instead of biological responses,” they wrote.

Based on this, “we suspect that the impact of rotation, especially deceleration rotation, is more harmful to brain matter,” Wu said, and that centrifugal force probably plays a critical role. “The large deformation of brain matter during this process causes neurons to stretch and causes damage.” This could explain why a boxer can be knocked unconscious by a heavy blow to the chin. “Since the chin is the furthest point from the neck, hitting the chin could cause the head’s highest rotational acceleration / deceleration,” the authors concluded.

“Critical thinking, along with simple experiments in the kitchen, has led to a series of systematic studies to examine the mechanisms that cause egg yolk to deform,” Wu said of the implications of their findings. “We hope to apply the lessons learned to the study of brain biomechanics as well as other physical processes involving soft capsules in a liquid environment, such as red blood cells.”

DOI: Fluid Physics, 2021. 10.1063 / 5.0035314 (About DOIs).

Ad image by Ji Lang / Qianhong Wu