Hi everybody, I am Dr. Tina Turner.
I am a comprehensive ophthalmologist in Grosse Pointe with
the Henry Ford Health System.
I did my residency at U of M, go blue.
Today we're going to talk about viscoelastics.
Most of what I'm going to tell you is from this excellent textbook.
There's a newer edition of the text book, which is also wonderful.
My other reference is going to be this brain.
This textbook is an excellent reference.
This brain is a work in progress and prone to malfunction so bear with me.
Let's first start talking about the function
of viscoelastic devices in cataract surgery.
We basically use them to maintain the intraocular space so
that you've got room to move when inside the eye.
How did we do that in the past?
Does anybody know?
We basically used air which was less than ideal.
And we also use OVDs to protect the endothelium.
Why is that important?
Because we're born with about 3500 endothelial cells per square millimeter.
And we have around 2400 of those by the time we're an adult and we continue
to lose them throughout the rest of our life and they don't repopulate.
So the more protection, the better.
We also need to talk a little bit about rheologic properties in order to
understand the function of OVDs and how they do their job.
The first word we need to understand in rheology is viscoelasticity, and
that's a substance's ability to return to its original state after being stressed.
Just like these balls when they're compressed,
they return to their original round state.
We also need to understand viscosity.
Viscosity is a substance's resistance to flow.
Let's take a look at one of my all time favorite substances, ketchup.
Can you believe that ketchup is more viscous than honey?
Which partially explains this phenomenon, but only partially.
Ketchup is ultra resistant to flow.
However, it is also highly pseudoplastic, and
pseudoplasticity refers to a substance's ability to transform under pressure.
So, something that's thick at rest and thinner at higher shear rates.
So if you take that bottled ketchup and put it in a squirt bottle and
place it under a super high shear rate, it behaves like a low viscosity substance and
comes out a whole lot easier.
Don't you just love science, getting your ketchup out of the bottle quicker?
One of the other very lovable attributes of ketchup is its surface tension,
or coatability.
Surface tension is determined by a substance's contact angle and
that angle is determined by how a substance sits on a surface.
The substance with low surface tension has a low contact angle.
It has a better ability to spread and a better ability to coat.
Just like ketchup, unlike water, which has a high surface tension and
beads up and has a high contact angle on a surface.
But ketchup, super super coatable.
Excellent on your burger.
Now that we understand some of these fancy rheologic terms,
let's talk about the physical characteristics of some of the compounds
which determine their properties.
One of the characteristics that determine a viscoelastic property
is its chain length.
So, these are polymers that usually have pretty long chain lengths.
And if you increase the chain length,
will obviously increase the interactions that the molecule has within itself.
And so that can increase viscosity and
elasticity by increasing intrachain reactions.
And if you increase a substance's concentration of all these molecules,
you'll increase interchain molecular interactions and that also increases
viscosity and increases elasticity and decreases pseudoplasticity.