What happens to the brain when there is a sudden impact? Egg yolks may contain the answer

An increasing number of professional soccer players have been diagnosed with a neurodegenerative disease called chronic traumatic encephalopathy (CTE), most likely the result of repeated concussions or similar recurrent brain trauma over the course of their careers. It is also common in other sports such as boxing, Muay Thai, kickboxing and ice hockey. We can find clues about 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 immersed in liquid egg white encased in a hard shell is an example of what physicists call ‘soft matter in a liquid environment’. Other examples include the red blood cells flowing through our circulatory systems and our brain, surrounded by cerebrospinal fluid (CBR) in a hard skull. According to Villanova University physicist Qianhong Wu and his co-authors on this latest study, the most important feature is how much a kind of soft material deforms in response to external influences. They point to red blood cells as an example. It is the ability of red blood cells to change shape under stress (“erythrocyte deformability”)) with which they can push through small capillaries, for example, and also cause the spleen to remove red blood cells whose size, shape and overall deformability have changed too strongly.

In the case of traumatic brain injury, it is related to how much the brain deforms due to impact. The exact cause of CTE is still a matter of ongoing research, but the prevailing theory is that recurrent brain trauma can damage blood vessels in the brain, which can cause inflammation and the growth of clumps of a protein called Tau. Eventually, the clots in the brain spread, killing brain cells. Those suffering from CTE often experience memory loss, depression and in severe cases dementia, among others.

Previous studies have shown that deformation of soft material in a liquid environment occurs in response to sudden changes in the fluid field, such as shear flow or a sudden change in the circuit. Wu et al. was interested in the specific case of soft material in a liquid environment that was also closed in a sturdy container – like the yolk of an egg, surrounded by liquid egg white, wrapped all in a shell. They wondered if it was not possible to break the yolk without breaking the shell, as it is the case with most concussions that the brain can be damaged without cracking the skull.

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

They buy fresh eggs at a local grocery store, remove the egg yolks and egg whites and then place them in a clear, sturdy container, the better to monitor the distortion by recording the whole process with speed cameras. They built two separate appliances. One applied the so-called “translation impact” – that is, hitting the container directly – via a small hammer falling from a vertical guide rail (see Figure 1A in the gallery), with a spring at the bottom holding the move vertically. They used an accelerometer to measure the acceleration of the container.

For the second setup (see Figure 1B in the gallery), they connected the holder to an electric motor to study two types of rotational impact: the rotational impact accelerates and the rotational impact decelerates (ie if the outer casing accelerates or slow down, turn). They also peel off the membranes around the fresh yellow and hang them in petri dishes filled with water, the better to study how the membranes also respond to stress.

Wu et al. was somewhat surprised to find that, in the case of translational impact, there was almost no deformation of the yolk. Instead, the entire container (and its contents) moved as a single solid. In the event of a rapid rotational impact, the team found that the yolk would start in a spherical shape and then begin to stretch horizontally to form an ellipsoid. The egg yolk could maintain a stable ellipsoidal shape for a few minutes if the angular velocity was kept constant.

The most interesting results occurred in the case of the delayed rotation impact. Here, the yolk almost immediately began to deform significantly, expanding horizontally and increasing its radius in the middle – sufficient deformation to severely damage the yolk under prolonged stress.

“We suspect that rotation, especially retarded rotation, is more harmful to matter in the brain.”

To ensure that it is not primarily an effect of the yolk as biomaterial, Wu et al. performed the same experiment with synthesized soft capsules immersed in a calcium lactate solution, enclosed by a thin membrane of calcium alginate. They obtained similar results, confirming that ‘the dominant mechanism leading to the deformation of soft materials in a liquid environment is a result of mechanical forces rather than biological reactions’, they wrote.

Based on this, “We suspect that rotation, especially retarded rotation, impact is more damaging to brain dust,” Wu said, and that centrifugal force is likely to play a critical role. “The large distortion of brain material during this process causes the stretching of neurons and causes damage.” This may explain why a boxer can be knocked out by a sharp blow to the chin. “Since the chin is the furthest point from the neck, hitting the chin can cause the highest rotational acceleration / deceleration of the head,” the authors concluded.

“Critical thinking, coupled with simple experiments within the kitchen, led to a series of systematic studies to investigate the mechanisms that cause egg yolk distortion,” Wu said of the implications of their findings. “We hope to apply the lessons learned from it to the study of brain biomechanics, as well as other physical processes that include soft capsules in a fluid environment, such as red blood cells.”

DOI: Physics of Liquids, 2021. 10.1063 / 5.0035314 (on DOIs).

List by Ji Lang / Qianhong Wu