We've been interested in understand what is called,
"self-healing materials." Self-healing materials are materials
which actually mend themselves even when they get
broken. For example if we had these kinds of materials in
pipes, when there is a leak it will naturally mend that leak
and plug it. The reason why we went to blood in this case
is because that is a perfect example of a system in which
you get self-healing. Basically we're bleeding all the time
and our organism is self-healing its vessels continuously.
The focus of this work has been on the first stages of blood-
clotting, which by the way occurs even in the absence of a
lesion; cells can die and in that case they open a little
wound, or just normal stresses that you have in your body
will actually cause little places to not seal perfectly. And in
that case you need this cascade to start rolling in and plug
the leak. There's two principle components that are very
important in the first stages of clotting, that are actually
the focus of the work we are presenting here. And they
are a polymer which is a long chain, it's a really long chain
and you need this really long polymer because if you don't
you have a disease. [This polymer] is called von Willebrand
factor. The other key ingredient is something that looks
spherical, but you can think of an object that is about
one-hundredth the size of a hair and is called a platelet.
A platelet is a cell and you have quite a bit of them in your
blood. So, the idea is that these two objects start to interact
with each other and the way they start to interact is
regulated by the flow. So essentially von Willebrand factor
can be thought of as this scotch tape. Under normal
conditions it basically doesn't stick to anything, but
suddenly due to flow and some chemical activations, it
will actually stretch. And from the moment it's stretched
you have many parts of this molecule, which is basically
like spaghetti, that can actually stick to other things. It can
stick to the surface and it can also stick to, let's say platelets
that I'm going to think about as my hand. So it [the tape]
can stick here and because I'm in the flow this will roll
around and make a very strong binding. Now you can
repeat that if this is very long it can actually stick to one and
then stick to others and it will start to, what we call, linking
platelets. So that will be the starting point of this aggregate
that just keeps on growing and growing. And that big
aggregate is what we call the plug. We're very interested in
this plug formation, meaning the formation of this aggregate
of platelets and von Willebrand factor and the reason is
because it behaves in a very counter-intuitive way. Most
things tend to break when you have high flows. This
actually forms in high flows; basically the strong the flow
you put on it, the more propensity it has for aggregating.
This is very counter-intuitive, but it's very interesting from
the standpoint of self-healing materials. One of the cool
surprises that we found in this work is that the aggregation
phenomena was universal. What does that mean? It means
that if we think about a few parameters in the system we
could basically describe the behavior of it in terms of one
universal parameter that depended on them, and this allows
us to essentially move across multiple different physical
systems because we can now tell you what's going to
in all of them. We can tell you, for example, when
some system is going to dissolve or when some system is
going to aggregate, regardless if we're talking biological
systems or a synthetic system because it all comes down to
those parameters that come into the universal features of
the system. This could be very important when we're trying
to create materials at the molecular level and controlling
their molecular structure in a very precise way.