Krystyn Van Vliet - New materials to protect the brain
MIT graduate student Bo Qing studies synthetic gels that could be used in
better equipment to protect against traumatic injuries.
Denis Paiste | Materials Processing Center
March 24, 2015
better protective gear against severe impacts for civilians and
soldiers requires a detailed understanding of how soft tissues in the
body actually respond to such impacts, whether from concussions,
ballistic attacks, or blast wounds. MIT researchers are developing new
synthetic polymer-solvent gels, called tissue simulant gels, which mimic
the response of natural tissue.
Biological engineering graduate student Bo Qing is studying the
impact of traumatic force on brain tissue from rodents and modeling
synthetic substitutes to enable better insight into preventing such
injuries. "If we can design a material that mimics this impact response,
it would be very helpful to serve as an injury model and use to assess
new protective equipment that can minimize this harm," explains Qing,
who works under MIT Associate Professor Krystyn J. Van Vliet.
"We want to study how biological tissues like the brain, heart, and
liver respond to impact and then find synthetic mimics that can
recapitulate those responses because they will be very helpful for the
Army, for example, to devise new protective strategies and understand
how injury actually occurs," Qing says.
Qing is studying multilayered polydimethylsiloxane-based (PDMS) gels,
which are stretchy and transparent, as models for brain tissue. The Van
Vliet group previously identified specific cross-linked PDMS gel compositions that closely matched the impact response of heart tissue.
"These are, essentially, a PDMS chemically cross-linked network,
that's loaded with a PDMS solvent. There are a lot of different
variations of these gels where we can basically tune the
cross-linker-to-polymer-base stoichiometric ratio, the molecular weight
of the PDMS solvent, and the amount of solvent that's used for these
different type of gels," Qing says.
The work is supported by the U.S. Army Research Lab, which provides
some of the organogels. Qing synthesizes the top layer using a different
commercially available type of PDMS without the solvent. "In terms of
that top layer, some properties I'm varying are its stiffness as well as
its thickness to tune the overall material response to impact and match
that of the tissues we're interested in," he explains. Qing, 23, is a
second-year PhD student and expects to finish his doctorate in 2018. He
received his bachelor's degree at the University of California at
Working with substitutes for extremely soft, or compliant, tissue
like brain tissues poses some special challenges, Qing says. "When you
are dealing with synthetic gels, once you reach a certain stiffness
level, if you try to go below that, the gels just get very, very sticky,
so it makes it very difficult to work with, especially for me, because I
work with small volumes of material and analyze the response by
physically contacting the gel surface," he says.
Principal investigator Van Vliet says it is more difficult to
replicate soft tissue with engineered materials such as gels than to
mimic hard tissue like bone with metal or ceramic substitutes. "As you
get into these 'soft tissues,' like your heart, your brain, your liver,
those are more complex to fabricate," she explains. "You're using
polymers instead of metals and ceramics. There are a lot of parameters
that one can tune, but usually those parameters are coupled with each
other, so as the material gets less and less stiff, it also becomes
increasingly sticky, for example. That's not always true in the tissues,
but it's true for many of the engineered polymers."
"Bo is generating some of the first data on the brain tissue using
this method, and then using the same approach to understand how polymers
can be engineered to either match or protect the tissue," Van Vliet
says. "We could make a very, very complicated material that looked just
like brain tissue, had multiple layers to it, white and gray matter, but
that's not really the point. The point is to make as simple a material
as you can to mimic the mechanical behavior of interest so that you can
then ultimately manufacture a lot of it at scale."
High impact rates
Qing, who has a National Defense Science and Engineering Graduate
Fellowship, conducts impact tests on a nanoindenter on both rodent
tissue and gel substitutes to establish parameters for accurate
measurement. The experimental setup pushes a probe on a pendulum into
the material at high impact rates. "What you get is displacement of the
probe as a function of time, and from this output, we can analyze this
data to get things like the material's resistance to penetration based
on the maximum depth that the probe was able to penetrate into the
material," Qing explains. "From these different bounces, we can analyze
basically how much and how fast the impact energy is dissipated."
Each impact-indentation experiment takes approximately three seconds,
though setup time is much longer. Impact load is measured in
millinewtons. The tests are repeated at varying impact force and speed.
"I've been characterizing these impact responses of all kinds of tissues
and then designing synthetic gels that have the same impact response
properties," Qing says.
"There's a lot of design space that nature has covered that we don't
actually have access to right now," Van Vliet says. "But the community
is learning through experiments like Bo's how to tune, for example, the
stiffness of a material independently of how fast it can dissipate
impact energy. There's a lot be learned in terms of how to mimic tissues
and also how to keep it simple and inexpensive so that you can
manufacture such materials at large scale."