Space-Age Alloy Mends Blood Vessels And Bones
June 18, 2001 07:12 AM ET
By Amanda Cooper
VALENCIA, Spain (Reuters) - Repairing broken bones or patching up faulty
blood vessels could soon be easier than ever before, thanks to a metal as
supple as a bendy toy.
Nitinol -- an alloy of nickel and titanium -- can adopt different shapes
and bounce back to original form depending on temperature much like the evil
metal humanoid in action movie Terminator II regroups despite having been
smashed to bits.
"Nitinol has two useable features -- one is the change in shape that
comes with the change in temperature, and the other...is that it can exert a
constant pressure on the point to which it is attached over a wide range of
shapes," said Dr. Jaime Prat of the Institute of Biomechanics of Valencia in
Spain.
Nitinol, a shape memory alloy, might not have the advanced self-healing
powers of the Terminator II's fictional alloy, but its extraordinary elasticity
makes it the most mechanically similar to organic materials such as human hair
or tendons.
Nitinol can be deformed by cooling or heating and then recover its
original shape when re-heated or re-cooled.
It takes its name from the chemical symbols of its two components
--nickel (Ni) and titanium (Ti)-- and the U.S. Naval Ordnance Laboratories
(NOL) where it was discovered in the 1960's when scientists noticed that a
nickel-titanium alloy ingot produced in a vacuum furnace changed shape after it
was left in sunlight.
FILTERS BLOOD
Nitinol's main use is in the production of blood filters, which trap and
eliminate clots that can cause a stroke or heart attack. The filters are used
in patients who are unable to take conventional blood-thinning drugs to control
their blood flow.
From its raw state, the nitinol is molded into a filter shape and then
cooled and given another shape to make it easy to introduce into the body.
A surgeon then guides it to a blood vessel, where the alloy adopts its
original filter shape, as its temperature rises to meet the body's temperature
of 37 degrees Celsius, leaving the device lodged safely in place.
After many years of peering at the outside world from the shelter of a
laboratory, nitinol made its way into the human body in the 1990s in
cardiovascular applications including filters and stents -- small tubes made of
metal mesh.
These are used to treat conditions such as arterosclerosis and aneurisms,
which damage the walls of blood vessels.
Since a nitinol stent can easily be shrunk and inserted into the body,
only minimum invasive surgery is required.
"We are at the stage where a stent can be shrunk to just three
millimeters in diameter," said Professor L'Hocine Yahia, director of the
biomechanics and biomaterials research group of the Ecole Polytechnique of
Montreal in Canada.
"Another metal such as stainless steel once it is deformed, cannot come
back, there is no spring-back, and this is another reason why we use nitinol,"
Yahia said.
Until now, the most widespread method of treatment has been using
stainless steel or other titanium alloys, in part because of worries about
toxicity which some scientists dismiss.
"We don't know why this (nitinol) is not more widely used," Dr. Jose Luis
Peris, head of biological research at the Institute of Biomechanics of Valencia
said.
"Nitinol is far more biocompatible than other alloys."
BONE-HARD SUBSTITUTE
Nitinol's "spring-back" and ability to apply a constant pressure also
make it an attractive option in plates and clamps used in the treatment of bone
fractures.
"Everywhere where you need to replace bone...there is a potential to
apply this metal," Yahia said.
"We can approach original bone resistance. We can match the mechanical
property of the bone which is required in order to have optimal replacement."
Nitinol bone implants have not yet reached clinical trials in humans, as
many health authorities in North America and Europe have questioned its safety.
But countries such as Russia and China have used nitinol in humans for many
years.
The benefits of using titanium in bone tissue are well-known -- it is
corrosion-resistant, light and very strong.
But nickel is highly poisonous and its toxicity initially caused
scientists to snub nitinol as a viable material for human implants, and this
remains one of the reasons why it is not widely used in North America or
Europe.
NO ORDINARY ALLOY
But in its defense, nitinol does not behave like other alloys and this
only adds to its appeal as an implant material.
"Nitinol was not created by metallurgists, but by subatomic physicists,"
Yahia of Montreal said.
Once in the body, the alloy becomes coated with titanium oxide, which
poses no threat to humans and no trace of nickel remains on the surface.
The nickel ions in the compound cannot escape into the body due to their
strong inter-atomic bonds with the titanium.
"Stainless steel is about 40 percent nickel. It has a smaller content but
because the nickel is free in the stainless steel there is some risk of release
of these nickel ions which is not the case with nitinol," Yahia said. Unlike
stiffer alloy plates used in fractures, nitinol lets the broken bone bear more
of the body's load while mending, making it less liable to break once the plate
is removed.
Nitinol's elasticity makes it unable to act as a load-bearer and it is
unsuitable for generalized use in fractures.
But it could one day be key in treating back problems such as scoliosis
-- curvature of the spine -- or replacing discs that have deteriorated.
Researchers are currently developing nitinol foam pads which may be used
to replace damaged discs using the alloy's ability to bend, twist and absorb
shock without deforming in any way.
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