Materials with Memory

Imagine a world of shape-shifters. A surgeon inserts a small lump of plastic into an anesthetized patient and, like magic, it expands into a life-saving mesh tube that keeps a formerly clogged artery open. Or maybe the shape-shifter starts as a microscopic metal claw with talons ready to pounce. Once the claw is in a patient’s body, a doctor zaps it with electricity and the device clamps down, as though it had muscles of its own, and performs a biopsy. Or think of something less health-oriented. Perhaps the shape-shifter is a tiny oscillating membrane that drives a motor in a missile’s guidance system. Maybe it’s the panel on a car door that got dented by a shopping cart–heating the door with a hair dryer makes it as good as new.

These are among a plethora of shape-changing products in development or under consideration in labs around the world. Known as shape-memory materials, they are metal alloys or polymers that accomplish similar feats in different ways. Both types of materials can be preprogrammed with a permanent shape that can be recovered after a deliberate or accidental change. In most cases, applied heat brings the material back to its pre-set form. The metal alloys accomplish this switch by undergoing an internal change in their crystal structure. In a shape-memory polymer, different components control its form at different temperatures.

Known as “smart” metal alloys and polymers, these materials may literally reshape technologies ranging from warfare to medicine.

Magic metals

Metal alloys are the older, more established class of shape-shifting materials. Thermally activated alloys have been around for decades, but they’re finding more and more uses. The most widely employed shape-memory alloy–a blend of nickel and titanium commonly known as nitinol–is used in robots, satellites, and even coffee pots. It also serves as an alternative to surgical steel in medical implants, says Greg Carman of the University of California, Los Angeles.

Doctors implant the material in patients as stents, which are mesh tubes that hold open damaged blood vessels, and as venacava filters, which are metal webs placed in clot-prone blood vessels to break clots up as they pass through. The size of the nickel-titanium stent that a physician feeds into an artery can be smaller than that of a stainless steel stent. This makes the shape-memory stent ideal for such a delicate procedure, says Carman.

Steel stents are usually mechanically sprung into an expanded configuration after insertion in the vessel. Once in place, a shape-memory stent warms to body temperature, changing its internal crystal structure and expanding.

One of the promising aspects of shape-memory alloys is that “you can make very, very microscopic tools” with them, says Carman. He works with thin films of nickel-titanium alloys that are one-fiftieth the width of a human hair. In the past few years, he and his coworkers have been testing what he calls a microgripper. This cage or claw just 100 micrometers wide opens and closes like a hand when Carman heats it by passing a small electric current through the device. He envisions tiny tools such as this going into the body, grabbing suspicious cells for testing, removing cancerous tissue, and even stitching up internal incisions.

This microscopic tool is possible because some nickel titanium alloys are what scientists call two-way shape-memory materials. They can transform from their temporary shape to a preset shape and then return precisely to the temporary shape. These shifts occur when a current is applied and removed. A thin nickel-titanium alloy film, for example, might cycle about 100 times a second.

“You can heat it up and cool it down real quick,” says Carman.

He and his colleagues are now using these films to design and build powerful motors about the size of four stacked quarters. A thin oscillating membrane of nickel-titanium moves fluid that in turn drives a piston. The devices are being designed for missile-guidance systems that need small motors to move small parts.

Despite the successes of nickel-titanium alloys, newer shape-memory alloys wait in the wings. A few years ago, scientists discovered that it’s possible to initiate a large, controlled shape change in certain metal alloys with a magnet rather than heat. But they don’t yet know the alloys’ capabilities and limitations. “People are trying to understand the phenomenon and the material,” says Carman.

One of the most studied of these materials, called ferromagnetic shape-memory alloys, contains nickel, manganese, and gallium. “But that might not be the best,” says Carman.

Robert O’Handley of the Massachusetts Institute of Technology is examining fundamental traits of the newly prized materials, which he thinks can change shape more quickly and extensively than heat-activated alloys. He and his coworkers are examining the basic properties of magnetically activated shape-memory alloys that the Navy would like to use in a variety of applications, such as suppressing loud or damaging vibrations in submarines.

When a large spinning turbine starts to vibrate, for example, small sensors on bearings within the machine would tell a computer that they’re off-center by, say, 20 micrometers. Parts made from magnetic shape-memory alloys could then push on the bearings to rebalance the machine. O’Handley is also exploring whether magnetic shape-memory alloys might be able to counter vibration or noise passively, by simply absorbing energy from vibrating parts.

The Navy might also use magnetic shape-memory alloys to send out the “pings”–or underwater vibrations–for submarines’ sonar systems, O’Handley says. A piece of the alloy would be mounted on the outside of the ship, connected to a stiff rubber part. A pulse of electricity would create a magnetic field that changes the alloy’s shape and moves the rubber to create the signal.

O’Handley hopes that shape-memory alloys might have advantages over the materials now used to create such underwater sonar. Those generally require phone-booth–size equipment and high voltages, which produce electrochemical reactions in salt water that promote local corrosion, he says.

Civilians also stand to benefit from the new ferromagnetic alloys. Researchers in Finland, for example, have expressed interest in using such materials for reducing vibrations in paper-mill machinery.

One word: plastics

Although shape-memory polymers have been under investigation for many years, they’ve made it into only a few trivial products, such as fork and spoon handles that can be softened under hot water and then molded to fit a person’s hand. Most of these products don’t benefit from their shape-memory properties, but only from the way that the materials are pliable without melting.

Yet consider how a shape-memory polymer might change the rules for auto-body repair. During manufacture, a polymer-based car door–or bumper or side-mirror casing–would have its permanent shape molded at a high temperature. A component of the polymer dictates the shape at that temperature but loses its dominance when the structure cools, says Steffen Kelch of the Institute of Chemistry at the GKSS Research Center in Teltow, Germany. Should the part later get dented, instead of painstaking bodywork, the repair would amount to heating the damaged part to the temperature at which the key component reasserts its control of the part’s shape. That temperature, called the switching temperature, would not be as high as the original manufacturing temperature.

In the June Angewandte Chemie International Edition, Kelch and Andreas Lendlein, head of the GKSS Institute of Chemistry, argue that products taking true advantage of shape-memory materials are likely to become far more prevalent in the next few years. Medical implants are particularly promising because polymers can be designed to be compatible with tissue and to degrade.

A significant step toward marketable medical applications occurred when Lendlein and Robert Langer of the Massachusetts Institute of Technology created biodegradable polymer strands that can knot themselves (SN: 4/27/02, p. 262: Self-Sutures: New material knots up on its own). The researchers first formed the strands at 90C into the shape of tight sutures. At cooler temperatures, the researchers used physical force to reshape the strands into unlooped, pliable threads. After the researchers loosely stitched the threads into rats’ skin, the polymer strands warmed to the animals’ body temperature, about 40C. There, the polymer component responsible for the pre-set shape took charge, and the sutures resumed their knotted conformation, Lendlein and Langer reported in the May 31 Science.

Shape-memory polymers like this one might serve in a variety of medical implants for the body, suggests Langer. One of the most important of these could be stents. A highly compacted shape-memory polymer might spring open once it’s inside the blood vessel and warmed to its switching temperature. In some medical applications, polymers would be designed to be degradable.

“I’m a big believer in the polymers,” says Langer. In 1998, he and Lendlein cofounded a company called mnemoScience in Aachen, Germany, to explore the development of these materials. “I think if you look at the evolution of medicine, [implants] have gone from very crude kinds of things, like metals or ceramics, more toward polymers and ultimately towards cells,” he says.

“They’re developed to be more and more natural.”

Materials scientists are now looking at ways to make shape-memory polymers with new properties, says Kelch. One area of focus is on finding polymers that change shape in response to triggers other than heat, such as magnetic fields or light.

Another active area of research is on designing two-way shape-memory polymers. Although some shape-memory metal alloys possess this trait, no polymer that heat transforms from its temporary shape to a preset shape will return precisely to that shape when it cools down. In this regard, “at the moment, shape-memory alloys are superior to the polymers,” says Kelch.

Two different materials

With both metal-alloy and plastic shape-memory materials available, engineers have the luxury of testing which one is better for any given application. “It’s like having a buffet of different materials to choose from,” says O’Handley.

“Medically, polymers are just more versatile,” says Langer. “You can make them more biocompatible.”

Moreover, “where you need elastic materials, the polymers are better,” Kelch adds. Some polymers stretch to 10 times their length, whereas metals are relatively inelastic, he says. Also, the polymers can undergo much more dramatic shape changes than the metal alloys can as they convert between their permanent and temporary forms.

On the other hand, metals are tougher than polymers and remain hard and tough at temperatures at which polymers become soft and weak. Scientists can also use a wider range of temperatures to switch metals from one shape to another.

Carman concludes, “There’s a place for different materials.”

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