What do mantis shrimp, boat propellers and traumatic brain
injury all have in common? The answer is cavitation bubbles.
Mantis shrimp kill their prey by springing out their claws at
extremely high speeds. The claw accelerates as fast as a 22
caliber bullet and reaches speeds of 23 meters per second.
These high speeds cause an area of high pressure in front of
the claw and an area of low pressure behind the claw as the
water tries to move out of the way. The low pressure behind
the claw causes the water to instantly vaporise into a gas.
To understand why this happens, we have to first look at the
phase diagram of water. We all know that increasing the
temperature of water causes it boil and turn into a gas.
However, dropping the pressure also does the same thing.
As you lower the pressure, it reaches a point where the
water will start to vaporise into a gas regardless of the
temperature. This can be seen by putting water into a
vacuum chamber. As the pressure drops, the water will
start to boil even at room temperature. So when the mantis
shrimp's claw moves through water, the negative pressure
behind the claw causes the water to momentarily vaporise
and form little gas bubbles called cavitation bubbles.
When the water pressure normalises again, these little
bubbles instantly collapse releasing a powerful shockwave
and heat which helps stun and kill nearby prey. A
similar thing happens near boat propellers. The spinning
blades of propellers move quickly through the water
causing an area of low pressure on the trailing edge of
blade. This causes cavitation bubbles to form and collapse
which, again, creates these shockwaves and heat. Even though
these bubbles are tiny and the amount of energy it releases
is small, over time there are enough bubbles collapsing that
it can erode away even the strongest of materials like metal.
So how does this relate to traumatic brain injuries? First we
have to understand that your brain is surrounded on all sides
by a fluid called cerebrospinal fluid or CSF for short. You
can think of it like the brain is floating in the skull within
the CSF. So say if you received a hammer to the front of the
head, well the inertia of the brain will cause it to collide
with the front of the skull. The collision causes bruising of
the brain tissue which is called a contusion. On top of this,
the sudden acceleration and deceleration causes different
parts of the brain to move at different speeds, leading to
shearing forces. The shearing forces damage a part of the
brain cell called the axon leading to something called diffuse
axonal injury or DAI for short. So, when the brain collides
with the skull causing contusions and DAI on the side of the
trauma, this is called a coup injury. On the other hand, a
contrecoup injury occurs when the brain rebounds and hits the
opposite side of the skull. Like if you're in a car which
suddenly brakes, you get this whiplash effect against the seat
behind you - that's basically how a contrecoup injury works.
However, it's never that simple because I wouldn't be making
this video if it was. There's a lot of debate on how these
contrecoup injuries occur and a popular theory for this is
cavitation bubbles. Scientists think that after the brain
moves forward after trauma, it leaves behind an area of
negative pressure in the CSF at the back of the head. And, as
we know from earlier, if the pressure drops in a fluid below
a certain threshold, then that fluid will vaporise into a gas
and form cavitation bubbles. Just like how mantis shrimp kill
its prey, the bubbles collapse, sending out shockwaves and
heat which can damage nearby brain tissue. You can even
simulate this at home where you get a glass bottle with some
water in it and then hit the top with a rubber mallet. The
water rushes to one side due to inertia, leaving behind a low
pressure zone at the bottom. And, yep you guessed it, forming
cavitation bubbles. The bubbles collapse sending out
shockwaves which break the bottle. What's interesting is that
this even happens even without direct trauma to the head. A
common form of brain injury especially for soldiers is blast
injuries such as those from explosive devices. The initial
shockwaves produced by the explosive device travels through
the brain and gets converted to kinetic energy and heat which
damages the brain tissue in a similar way to direct trauma.
During World War 2, scientists modeled shockwaves with this
simple but not so simple mathematical equation called the
Friedlander equation. The model shows that this initial
shockwave creates this spike in pressure, however once that
shockwave has passed, there's this period of time where the
pressure becomes negative. You can think of it like a spring,
if you pull on a spring hard enough and create enough
pressure, when you let go it will rebound and go beyond the
starting position. Same thing is happening here. The shockwave
comes, creates a huge positive pressure and then it rebounds
into the negative. And as you now know, when there's negative
pressure in fluid - it causes the water to vaporise which
produces cavitation bubbles! So these blast injuries cause an
initial injury from the first shockwave but then the
cavitation bubbles it produces creates secondary shockwaves
which can cause further damage to the brain. In fact one study
has shown that the secondary shockwaves produce a pressure
that is five times greater compared to the initial shockwave.
It's important to note that although there has been a lot of
theoretical and experimental support for cavitation
occurring, it's never actually been directly observed in
humans or animals. It's still a theory, but a really
interesting and plausible theory with tons of support behind
it. The reason it's so hard to study for scientists is because
it's a multi-disciplinary topic that spans the complexities
of biology, engineering, physics and mathematics. Often
studies done by physicists and engineers don't have realistic
biological parameters like having accurate CSF flow or brain
tissue properties. And the opposite is also true, in that
biological studies often fail to account for the complex
physics and engineering problems involved. However, in those
rare instance we do get experts from multiple disciplines
collaborating, we get cool new medical treatments and
technologies that can help millions of people. An example, is
shockwave lithotripsy which is a medical device that produces
shockwaves that are designed to create cavitation bubbles in
the body to break up kidney stones. For traumatic brain
injuries, further collaboration between multi-disciplinary
teams can help solidify our understanding of the role of
cavitation in brain injuries
so that one day we can better prevent or treat these conditions.
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