New Medical Devices Vanish Inside You
Although the physician who first
wanted to open blocked blood vessels
was described as “something of a
radical” by his colleagues, even he
might have been surprised by the idea
of a tiny plastic scaffold that holds
open an artery and then dissolves.
When Charles Dotter of Oregon Health
& Science University proposed in 1969
his “coil-spring endarterial tube” to
accompany what is now known as
angioplasty, he envisioned a tube or
coil made of metal, known as a stent,
that lasts forever.
For decades metal has been the norm
in stents. But in October dissolvable
versions reached an important
milestone, with the release of clinical
trial data in The New England Journal
of Medicine showing a degradable
device made by Abbott Vascular
performed as well as its traditional
counterpart. In theory, dissolvable
implants reduce the risk of
inflammation, blood clots and other
side effects. The report not only
represents the likely future of stents but
also a highly visible advance from the
emerging field of biodegradable
technologies. Researchers in the field
envision the day when most medical
hardware implanted into the body—
such as that used for joint repair or
surgical wound support—will last only
as long as it is needed.
The concept is not new—doctors have
imagined it since the introduction of
synthetic dissolvable stitches in the
1970s. In fact, the Abbott stent detailed
in the NEJM study is made of the same
polymer as the sutures. Until recently
progress has been plodding, partly
because the materials in degradable
devices have to satisfy a long list of
criteria, including strength, durability,
safety and even the ability to show up
on an x-ray.
Today, scientists have been
increasingly able to manipulate
degradable substances—many of them,
like the stuff of sutures, around for
decades—to perform specific functions,
such as delivering drugs, aiding organ
function and performing other tasks.
Like a sugar cube, these materials
gradually disintegrate in liquid, usually
into components that the body can
break down and excrete. The Abbott
stent, for example, is a polymer of
lactic acid, which metabolizes and
ultimately exits the body as carbon
dioxide and water.
The problem with most current medical
implants is that “there is no material
that can be placed in the body without
an immunological response over time,
with very few exceptions,” says
Joachim Kohn of Rutgers University,
with complications like inflammation
and pain. Given enough time all joint
replacements eventually fail, according
to doctors writing in the May 2014
issue of Mediators of Inflammation. Or
consider the track record of a surgical
mesh used after incontinence surgery
in women, in which the U.S. Food and
Drug Administration recently warned
that complications and failures are
“not rare.”
With approximately half a million
people receiving implants each year in
the U.S., stents have long been
attractive for developers of degradable
technology. The device’s purpose is to
support a vessel as it recovers from
angioplasty, which uses a small balloon
at the end of a catheter to widen a
narrowed artery. Modern stents are
coated with drugs that help prevent scar
tissue from forming and plaque from
reestablishing itself. Within a year the
vessel is fully restored but the implants
are not removed. That means the
artery can never return to its original
flexibility. Long-term risks, like the
return of blockage, are small but real.
More than a dozen companies have
degradable stents in development.
Manufacturers tend to stick to
materials that are already used in
medicine or known to be metabolized
by the body. Approval of an entirely
new polymer would add to the time and
expense of development. Whereas some
companies are developing stents made
of iron and manganese—common
nutrients that the body can easily break
down—Abbot’s stent is made of poly(L-
lactic acid), or PLLA, a lactic acid
chain that commonly used in medicine.
Once the entire drug is delivered, and
the vessel has healed, the degradable
stent gradually vanishes over the first
few years. “They are so different from
metal stents we don’t even call them
stents any more. We call them bio-
absorbable scaffolds,” says Gregg Stone
of Columbia University Medical Center,
one of the lead scientists in the study,
which was sponsored by Abbott.
