Engineer-clinician collaborations yield innovative,
Funded via the W.H. Coulter
Translational Research Partnership in Biomedical Engineering,
these research projects recently concluded in year one of the
partnership. This partnership fosters early-stage collaborations
between University of Wisconsin biomedical engineering researchers
and practicing physicians that will enable researchers to deliver
their advances more quickly to patients.
'Balloon' effect: Blocking aneurysms
earching for a safer, more efficient alternative
to today’s imprecise, time-intensive methods for treating cerebral
aneurysms, a group of UW-Madison biomedical engineers and physicians
has developed a system that not only shaves hours from the current procedure
time, but also plays an active role in healing the area.
Essentially, a cerebral aneurysm is a “balloon”
or bulge that forms along a weak wall of an artery or vein in the brain;
about 40 percent of patients whose aneurysm ruptures die within 24 hours.
According to the National Institute of Neurological
Disorders and Stroke, all cerebral aneurysms have the potential to rupture
and cause bleeding within the brain—though most
go unnoticed until they rupture or are detected on brain images for
Coil embolization is one common method for treating
a cerebral aneurysm. Somewhat like winding a rubber-band ball, the process
requires surgeons to feed wire coils into the bulged-out area via a
catheter until the aneurysm is full and the coil “ball”
blocks blood flow to the aneurysm. “The hope is that this bundle
of wire is going to instigate a wound-healing response or scar formation
around the aneurysm that will wall this part off from the blood vessel
and no longer have a risk of rupture,” says Assistant Professor
However, she says, the process can take up to eight
hours and, because blood flowing past the coils could put even more
pressure on the aneurysm, it actually may increase the risk of rupture.
Neurosurgery resident Roham Moftakhar
had an idea for an alternative device. He recruited Biomedical Engineering
and Engineering Physics Associate Professor Wendy
Crone and Masters to the project because of their expertise, respectively,
in shape-memory metals and biomaterials. Together, they devised a method
that strips the process down to a catheter, a single wire coil, and
an expandable “balloon”—a bioactive polymer shell.
“Probably one of the most important things to our surgeon is that
this only involves putting in a single coil,” says Masters. “It
could dramatically—by orders of magnitude—decrease the time
to occlude these aneurysms.”
For the wire coil, the group chose nickel-titanium,
a shape-memory alloy already in use for aneurysm treatment. When the
wire is cool—somewhat below body temperature—it straightens
easily and users can thread it into a tiny catheter. Inserted into the
body via the catheter, the wire warms. At body temperature, it wants
to coil, but the catheter keeps the wire straight until it reaches its
target. “This coil, as it’s exiting from the catheter, expands
the ‘balloon’ to then create instant contact with the surrounding
aneurysm shell,” says Masters.
Rather than choose just a biocompatible material for
the balloon, the group decided to use a bioactive material to stimulate
and accelerate healing. But the researchers’ list of required
material properties was long: It had to be biocompatible, hemocompatible,
highly elastic, able to be made into thin films or capsules, and, ideally,
contain elements that promote wound healing. “When we started
thinking about this, we realized there wasn’t a good existing
material to meet these criteria,” says Masters.
So Masters engineered one. She began with polyurethane,
an elastic biocompatible polymer widely used in medical devices. To
make the material hemocompatible, she incorporated varying amounts of
hyaluronic acid, an antithrombotic molecule—or, an anti-coagulant—that
stimulates wound healing. “We found that even at our lowest hyaluronic
acid incorporation, we get complete elimination of platelet adhesion
to the materials, so these are highly hemocompatible,” she says.
“The resulting polyurethane hyaluronic acid copolymer also has
1,500 percent elongation, perfect for making it into thin films for
the ‘balloon.’” The researchers currently are testing
their system in in vitro aneurysm models
they constructed from silicone.
Although she tailored the new hemocompatible material
for the group’s aneurysm occlusion device, Masters is excited
about the potential polyurethane hyaluronic acid copolymers have in
applications like using native biomolecules to deliver bioactivity.
As a result, she has spun off several additional research projects.
“You can really tailor the physical and biological properties
on your materials based on how much of the hyaluronic acid you put in
them,” she says.