stress of life
Analyzing a medical device at an early design stage makes it easier to save lives later.
Implanting a stent has become a common treatment
for cardiovascular disease, especially in cases of heart attack. The little
metal implant is a reticulated cylinder that holds a blood vessel open,
so blood can flow in a healthy manner.
Angioplasty, a treatment that reopens a blood vessel but does not use
a mechanical device to keep the passage free, has a high incidence of
restenosis, the reblocking of a vessel as it recloses. There is a risk
of restenosis with stents, too, because the tissue of the vessel can grow
through the spaces in the stent wall to close the pipeline for the blood.
According to Pennhealth.com, a Web site operated by the University of
Pennsylvania Health System, stents have reduced the frequency of restenosis
to fewer than 20 percent of patients, less than half the rate for angioplasty.
Engineers keep designing devices to cut the restenosis rate even more.
CardioVasc Inc. of Menlo Park, Calif., for instance, decided to wrap a
stent in a polymer sheath to inhibit the tissue invasion that can cause
blockage. It looked like a good idea. But would it hold up?
According to Leon Rudakov, director of engineering at CardioVasc, stents
are usually made by laser-machining stainless steel or another alloy to
create intricate networks of interconnecting struts. The device will be
crimped to compress its diameter, and later, after a cardiologist has
placed it, the stent will be expanded by a balloon inside it. The catheter
and balloon are withdrawn, and the stent remains.
The tubular structures with walls only a few thousandths of an inch thick
must be flexible and durable, to keep arteries open for millions of heartbeats.
A
polymer membrane surrounding a stent prevents tissue growth that can restrict
healthy flow of blood.
Predicting the fatigue life of the device is complicated by the nature
of the geometry and by the plastic deformations that occur at the strut
junctions during crimping and subsequent expansion. These actions cause
high stress concentrations in small junction areas. The resulting deformations
and associated material properties are highly nonlinear, thus further
complicating determinations of stress and fatigue life predictions.
Rudakov worked with Zachary Pursell, engineering manager at MSC Software
Inc. in Santa Ana, Calif., to predict behavior of the product at an early
stage of design.
Pursell used MSC's Marc finite element analysis to do the math for areas
of high stress and deformation, and to predict the stent's fatigue life.
To prevent tissue growth from forming inside the stent, CardioVasc's device,
called NuVasc, combines a stent with an ultrathin, porous ePTFE membrane
surrounding it. This sleeve, coated with synthetic peptide for better
vessel healing, provides a barrier between the open surface of the stent
and the vessel wall. Within days, the vessel tissue bonds itself to the
device, giving rise to the name "stent graft" for the device.
This design is also useful in performing other cardiological procedures,
such as repairing bypasses or restoring perforated vessels, Rudakov said.
According to Pursell, material properties of the stent and polymeric sleeve
were highly nonlinear, as were the geometries of the bodies as they were
subjected to crimping and elastic recoil during use. For purposes of analysis,
Ru- dakov and Pursell decided that 500 million cycles—a dozen or
more years worth of heartbeats—was an infinite life in determining
the factor of safety against fatigue failure.
For fatigue life prediction, alternating stresses resulting from cyclic
pressure loads were determined from the FEA solution. A custom code, based
on the Goodman fatigue method, was used with Marc to evaluate the factor
of safety for each element of the model.
Early analysis indicated areas of higher-than-expected stress concentrations.
Designers changed the radius in certain areas of the strut and modified
the stent thickness, for a considerably higher factor of safety.
Rudakov said that the CardioVasc device is in use in Europe and may start
clinical tests in the United States later this year.
This article was prepared by staff writers in collaboration
with outside contributors.