Chemistry Reference
In-Depth Information
There is a great need for strong materials such as alloys that can
snap back into shape. Medical applications include prostheses—
artificial limbs—and implanted devices such as heart valves. Most bio-
logical substances are “smart,” and the ability to replace lost or injured
tissues and organs with smart materials would be a tremendous medical
advance.
Some researchers are already making progress. Consider heart
valves, for example. A heart valve consists of flaps of hard tissue con-
trolled by muscles. The heart has four chambers, and four valves open
and close to keep the heart contraction from squirting blood through
the wrong vessel. (The opening and closing of these valves create the
sounds of a heartbeat.) In some people, the valves do not work correctly
due to a defect or injury. Some of these patients need surgery in which
one or more valves are replaced, usually with valves from a human do-
nor or perhaps taken from an animal (usually a pig), or a mechanical
valve.
Valve replacements do not always last a long time, and they are
complicated in children because the valves do not grow with the child.
Children may require several surgeries as they age, which are risky and
require lengthy recuperation. But in 2006, Gregory Carman and Lenka
Stepan at the University of California, Los Angeles (UCLA) and Daniel
Levi at UCLA's Mattel Children's Hospital, designed and built a heart
valve made with nitinol. The valve, consisting of a thin film of nitinol,
can be folded up and placed into a tube called a catheter. Physicians in-
sert the catheter into a blood vessel and carefully slide it into the heart.
When the nitinol valve is in place, the physician releases a catch and the
valve, made of shape-memory alloy, remembers its original shape and
unfolds, yet retains the strength necessary to function at the demanding
pace of about 90,000 beats a day.
Catheter procedures avoid the need for surgeons to open the pa-
tient's chest. As a result, the patient experiences a lot less pain and needs
far less time for recovery. These procedures have already become com-
mon in adults, but the bulky valves and catheters are not suitable for
children. With nitinol's memory and flexibility, the valve can be fitted
into a small space yet return to its needed size when placed in posi-
tion. Nitinol is biocompatible—it does not harm tissues—so it causes
no damage when implanted in the body.
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