R&D Digest: The monthly review of new technologies and medical device innovations.
Daniel Levi and Gregory Carman display different forms of thin-film nitinol used in combination with a collapsible heart valve designed for children.
Using a thinner form of nitinol and a smaller catheter, doctors and engineers have developed a heart valve for children that can be inserted into the body via a catheter. Surgeons wouldn't need to crack the chest cavity open to replace a valve, which could mean lower risk and a shorter recovery time for young patients. The technology was developed at the University of California, Los Angeles (UCLA).
“The reason we were interested in making heart valves with thin-film nitinol was solely because [the material] gives the physician the ability to collapse the valve and insert it with catheters rather than with surgery,” says Daniel Levi, MD, assistant professor of pediatric cardiology at Mattel Children's Hospital at UCLA. “The novelty is in the idea of using the material for transcatheter replacement and, to some degree, the design.” He adds that a group in San Antonio is working on a similar project, but with a different design and approach.
“I think you're going to find the thin-film form of this material in a number of different commercial applications in the next decade,” predicts Gregory Carman, professor of mechanical and aerospace engineering at UCLA. “As a society, we want to move away from surgical procedures. Typically that means going toward catheterization procedures.”
Thin-film nitinol is made through a sputter deposition process. Researchers hope nitinol valves will be more effective than
mechanical or bioprosthetic valves.
The heart valve is a bileaflet, or butterfly, design that opens from the middle rather than from the edges. The device's inner diameter is 12 mm and can be made even smaller. Most of those currently being tested are 14 mm ID, according to Levi. His work is targeted at children, because many conventional valves are “prohibitively bulky” for them. Pediatric patients have a smaller arterial system than adults, and thin-film nitinol can fit into a smaller catheter.
Although nitinol can be found in devices such as stents, a thinner film version of the material hasn't been used in a commercially available medical device, according to the researchers. Thin-film nitinol is less than 10 µm thick. To put its size in perspective, the thickness of a human hair is about 50 µm.
Nitinol is a material that has shape-memory and superelastic properties. “The metal behaves like rubber, so you can stretch it very long distances, and it springs back to its original shape,” says Carman. “It has long elongations like rubber, but it has the stiffness associated with aluminum or steel.”
The material exists at a low-temperature state in which it's very malleable, and at a higher temperature where it remembers shape and is somewhat stiff. At its higher-temperature, or austenitic, state, the material can be trained to form a particular shape. “Then you can cool it down and compress it into a catheter,” says Levi. “When it comes out of the catheter and hits body temperature or a high temperature, it converts into its austenitic shape and remembers the shape it's supposed to form. That's why it's so great, especially for transcatheter applications.”
The UCLA team makes thin-film nitinol through a process called sputter deposition. “The sputter process involves the deposition of atoms from a target material onto a substrate, such as silicon, in a vacuum environment,” explains Carman. “The atoms are dislodged from the target material by collision with ionized gas and [are] directed toward the substrate.” Then the atoms join on the surface as a noncrystalline material. After the atoms are deposited on the substrate (which takes about an hour), it is heated to crystallize the material into a uniform structure.
The researchers have also applied the thin-film nitinol to stents and conducted studies on the material's performance. Based on what the researchers have seen so far, Carman hopes that the material could help the valve perform better than mechanical or bioprosthetic valves. Mechanical valves have a tendency to cause blood clots, and bioprosthetic valves aren't as durable and can calcify, requiring replacement after 10–15 years.
“Our valve, because of its thin property, may not have that clotting issue. We haven't seen the agglomeration of platelets on the thin-film structure,” says Carman. “We believe that we won't see an issue with calcification, because tissue hasn't been harvested from some other source.”
The valves will be used in older children before neonates, and in the pulmonary position before in the aorta. Pulmonary valve replacement is needed when a person is born with only one valve. In that case, the body uses the single valve for the aortic valve.
Right now the biggest technical challenge will be to make a collapsible heart valve that performs as well as the noncollapsible versions. UCLA researchers are currently performing high-cycle fatigue testing and will need to show that the valve can open and close billions of times without deteriorating. Carmen and Levi are optimistic about their work but stress that it's still in the very early phases.
The UCLA research has been supported by a grant from the National Institute of Child Health and Human Development, which is part of the National Institutes of Health.