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Inside a thermo-mechanical test frame, a
shape-memory polymer fractures as it stretches past its
deformation limit. This testing allows the researchers to
determine the maximum possible shape change from the permanent to
temporary shape and vice versa.
Photo by
Gary Meek
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The mechanical properties of these polymers
make them extremely attractive for many biomedical applications,
according to Gall, who described his research in this area during two
presentations at the Materials Research Society�s fall meeting in
November.
Engineers are always searching for materials that display
unconventional properties able to satisfy the severe requirements for
implantation in the body. Particular attention must be paid to the
biofunctionality, biostability and biocompatibility of these materials,
which come into contact with tissue and body fluids.
With funding from the National Institute of Biomedical Imaging and
Bioengineering of the National Institutes of Health (NIH), Gall
proposed replacing metallic cardiovascular stents with plastic ones
because polymers more closely resemble soft biological tissue. Plus,
polymers can be designed to gradually dissolve in the body.
�Metal stents are frequently covered in plastic anyway, so we set out
to remove the metal leaving just a polymer sheath,� explained Gall.
�Also, polymers are more flexible and do not stress the artery walls
like the metals.�
Gall�s research group has designed a shape-memory polymer stent that
can be compressed and fed through a tiny hole in the body into a
blocked artery, just like a conventional stent. Then, the warmth of
the body triggers the polymer�s expansion into its permanent shape,
resulting in natural deployment without auxiliary devices. This work
was published in the journal Biomaterials (see below).
For another project, Gall and graduate student David Safranski have
been investigating how altering a polymer�s chemistry changes its
properties, such as stretchiness. This project was funded by MedShape
Solutions, an Atlanta company that Gall co-founded to develop medical
devices primarily for use in minimally invasive surgery.
�You can tailor the polymer to moderate its strength, stiffness,
stretchiness and expansion rate,� noted Gall.
They found that by changing the chemistry of the polymer backbone to
include special side groups, they could increase of the amount of
strain the polymer could withstand before failing without sacrificing
stiffness. This discovery enabled the creation of polymers that could
stretch farther and also push harder during recovery.
Gall and graduate student Scott Kasprzak are exploring how these
polymers might be used as a deployable neuronal probe, with funding
from the National Institute of Neurological Disorders and Stroke of
the NIH.
�We�re looking for smart materials that can be synthesized in the size
range of 100 microns � similar to the size of a strand of hair � and
then be inserted into brain tissue,� explained Gall. �This type of
probe would need to slowly change shape inside the brain as to not
disturb any surrounding tissue.�
Another project in Gall�s laboratory is examining the use of these
polymers for the spine. Most spinal surgeries are currently not
performed arthroscopically, so Gall sees benefits in using these
shape-memory materials to enable minimally invasive spinal surgery.
With funding from the National Institute of Arthritis and
Musculoskeletal and Skin Diseases (NIAMS), Gall and graduate student
Kathryn Smith are developing shape-memory polymers for the spine that
are tough � meaning they stretch far and support a lot of weight like
native spinal disks.
�This would improve the deliverability and life of artificial disks
currently used in the spine. Essentially, we�re just trying to
engineer tougher synthetic polymers that can be easily delivered,�
explained Gall, who is collaborating on this project with Barbara
Boyan and Johnna Temenoff, both of the Coulter Department of
Biomedical Engineering at Georgia Tech and Emory University.
In addition to exploring different biomedical applications for
shape-memory polymers, Gall has also turned his attention to
manufacturing them. Walter Voit, a graduate student in the
Technological Innovation: Generating Economic Results (TI:GER) program,
is investigating how to produce shape-memory polymers at a low cost.
More specifically, Voit is examining different types of materials and
processing methods that can be used to commercially produce quality
polymers for lower cost medical applications.
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