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Making Smarter Materials: Shape-memory Polymers Designed for Biomedical
Applications
January 3, 2008
Researchers at the Georgia Institute of Technology are developing unique
polymers, which change shape upon heating, to open blocked arteries, probe
neurons in the brain and engineer a tougher spine.
These so-called shape-memory polymers can be temporarily stretched or compressed
into forms several times larger or smaller than their final shape. Then heat,
light or the local chemical environment triggers a transformation into their
permanent shape.
“My focus has been to optimize these polymers for many different biomedical
applications. My lab studies how altering the chemistry and structure of the
polymers affects their chemical, biological and mechanical properties,” said Ken
Gall, a professor in the George W. Woodruff School of Mechanical Engineering and
School of Materials Science and Engineering.
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.
Georgia Tech professor Ken Gall develops unique shape-memory polymers to open blocked arteries, probe neurons in the brain and engineer a tougher spine.
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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 earlier
this year.
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.
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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.”
Georgia Tech professor Ken Gall reaches inside a thermo-mechanical test frame, which is designed to measure properties of the polymers under environmental conditions simulating the human body.
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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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