"Hot Stuff," Feature Article, December 2002

FEATURE FOCUS: Smart Materials

hot stuff

Advanced materials are moving out of the lab and into the commercial world.

by John DeGaspari, Associate Editor

Once regarded as laboratory curiosities, smart materials are beginning to make their mark on some high-profile commercial applications. This class of materials encompasses a broad range of ceramics, metal alloys, gels, and polymers. What sets them apart is their ability to adapt when they're
exposed to external stimuli, such as temperature, magnetism, or electricity.

Piezoelectric crystals and magneto-rheological fluids have been around for years. Now, newer exotic alloys and polymers with intriguing properties are joining them. Some of the more recent arrivals in the lab, such as carbon nanotubes, are still the subject of basic research. Yet others have made the transition into commercial applications.

Joe Constance, an analyst with Frost & Sullivan in San Antonio, believes that interest in smart materials is linked to the trend toward miniaturization. "By combining the ability to miniaturize actuators and create these novel materials, people are able to incorporate features on the nanoscale that had not been possible before," he said. Constance is the author of a report just issued by Frost & Sullivan, covering research and early commercialization efforts for a range of smart materials.

Lord Corp. provides Rheonetic MR fluid for Delphi's MagneRide primary suspension shocks, which eliminate electromechanical valves.

In October, the U.S. Department of Energy's Brook- haven National Laboratory in Upton, N.Y., announced its intention to build a Center for Functional Nanomaterials for research into the fabrication and uses of nanoscale materials. Brook- haven's center will be one of five run by the DOE and will complement a network of university centers that are backed by the National Science Foundation.

While there remains plenty of research to be done for years to come, engineers have been finding applications for smart materials. Some say it is a hard sell.

According to Marthinus van Schoor, president of Midé Technology Corp., a Medford, Mass., engineering company that develops applications for a variety of shape memory materials, customers are often reluctant to try new materials in already successful products.

What's more, many shape memory materials are more expensive than traditional materials, because economies of scale have not yet been established. Quality of some materials, too, may be uneven.

Despite these hurdles, some classes of smart materials are currently making inroads in a variety of commercial applications.

Magnetic Attractions

Take MR fluids, for instance. Under normal conditions, magneto-rheological fluids are free-flowing with a viscosity akin to motor oil. In the presence of a magnetic field, the fluid can become a near-solid in milliseconds. It can return instantly to its fluid state when the magnetic field is withdrawn.

Jacob Rabinow invented MR fluid while he was at the National Bureau of Standards in the 1940s. An early picture of a demonstration of the material's capabilities showed an MR fluid device supporting a 117-pound woman suspended on a swing. For decades, until the technical infrastructure grew up around it, MR fluid did not get past the point of novelty.

Sophisticated algorithms, fast control circuits, and sensors eventually brought its possibilities closer to usefulness, said Lynn Yanyo, manager of marketing and sales at Lord Corp.'s materials division in Cary, N.C.

Lord Corp. has developed a number of commercial applications for its Rheonetic MR fluid, which has been under development by the company since 1991. Yanyo said that it was a challenge to use the fluid in systems that were stable and would not abrade. She said it took six years to resolve the issues. Lord's magneto-rheological fluids are suspensions of micron-size iron particles in a carrier fluid, which is often some kind of oil.

Jacob Rabinow (right) and colleagues demonstrated MR fluid in a 1940s-era National Bureau of Standards photo.

In 1997, the company put MR fluid into a seating shock absorber system for trucks. The suspension uses the controllable magnetic fluid to provide varying levels of firmness and adapts to changing levels of shock and motion 500 times per second, according to the company. The system reacts in real time, eliminating compromises of passive systems, which essentially try to satisfy many conditions with one setting, Yanyo said.

Delphi Corp. in Troy, Mich., a supplier of automotive components to General Motors, also had a long-standing interest in MR fluid and was investigating it around the same time, according to Yanyo. In 1999, Delphi contacted Lord to supply MR fluid for its new MagneRide semi-active automotive suspension system.

General Motors has specified the MagneRide on the 2002 Cadillac Seville STS and the 50th Anniversary edition of the Chevrolet Corvette for 2003. It also specified the system on two 2004 Cadillac models: the SRX sport utility and XLR roadster.

Darin Dellinger, senior vehicle development engineer at Delphi, said that the MagneRide system resulted in at least a 40 percent part reduction, mostly of valve parts, and also eliminates traditional shock absorber fluid. In its place is essentially a cylinder filled with MR fluid and surrounded by a coil.

The MR fluid system requires the setting of a few physical parameters, and most settings are made electronically, with a laptop computer. Dellinger added that there is a flexibility in setting performance characteristics that could not have been accomplished with traditional shocks.

Dellinger said that advances in sensor technology and rugged controllers helped to open the way for MR fluid technology. The MagneRide system is coupled to an array of sensors, including four that monitor wheel-to-body position, and to a control module. Damping is adjusted on each corner of the car about every millisecond. Dellinger's biggest initial concern with the technology was durability, he said. Computer modeling concepts developed in the early 1990s addressed many of those issues, he said.

Dellinger described MagneRide as an active suspension system in a passive package. That is, it does not supply active power to lift or push down a wheel, but when it goes over bumps, it dissipates energy intelligently. "We are able to use selective damping to get a lot of the benefits of active damping without all of the terrible packaging and mass," he said.
Lord Corp. has expanded MR fluid applications into other areas as well, including tactile feedback in control-by-wire systems, and seismic and wind mitigation systems for buildings and bridges.

An actuator produced by AdaptaMat incorporates magnetic shape memory material to proportionally control pressure in a pneumatic valve.

Meanwhile, a company in Helsinki, Finland, AdaptaMat Ltd., is developing a magnetic shape memory material that is claimed to have a much larger stroke than other "smart" actuator materials when exposed to a low magnetic field. The material is a crystalline alloy of nickel, manganese, and gallium.

AdaptaMat's marketing manager, Emmanouel Pagounis, said the company has commercial materials with a strain of 6 percent, and has produced test pieces that have a strain of 10 percent. That is about 100 times larger than some other actuator materials, including piezoelectrics, he said. The company claims that the material can be made to elongate, bend, or produce torsion. Other properties include microsecond response time, long fatigue life, and a wide operating range, between -70°C and 100°C. Pagounis said the company is working to increase the material's maximum operating range.

He said that the magnetic shape memory, or MSM, alloy was developed in the mid-1990s. The company was established in 1996 and is working with researchers at Helsinki University of Technology as well as at Massachusetts Institute of Technology and the University of Maryland. He said the company is developing the material for valves for fluid and pneumatic control, and is working with an automotive manufacturer on a fuel injector. It also developed a high-precision linear motor. Potential applications include couplers, positioning devices, vibrators, and sensors.

Artificial Muscle

SRI International, a research organization in Menlo Park, Calif., is developing an electroactive flexible polymer that it says has potential in actuation, sensing, and electric power generation. The material came out of research, begun in 1992, to develop an artificial muscle-like material for use in microelectromechanical devices. Today, most of its promising applications are on a larger scale, said Ron Pelrine, the director for SRI's transducer program.

The material acts like a capacitor. It is an elastomer sheet with electrodes on both sides. When voltage is applied to the material, the positive charge on one electrode is attracted to the negative charge on the other, squeezing the material. This provides the actuation mechanism, said Pelrine.

SRI International claims that its electroactive polymer shows potential as valve actuators in camless engines, offering high power-to-weight, variable lift, and low cost.

Philip von Guggenberg, director of business development, said that SRI has produced strains in the material of up to 380 percent—almost five times the original size. The company is working with acrylic and silicone sheet, which can be manipulated in a way—essentially stretched—to produce a high degree of actuation. Carbon-based electrodes are sprayed on the polymer. The electrode material has a chemistry that can sustain high strains without cracking, von Guggenberg said.

The material has a good deal of design freedom, said Pelrine. It can be rolled, stacked, or used as a single sheet.

"Artificial muscle" polymer developed by SRI may have use as flat, lightweight speakers that could be incorporated into the headliners of cars.

SRI has identified a number of potential applications. The material would make a very lightweight speaker, for instance. It can be stretched over a frame and be made to expand and contract rapidly when an ac signal is applied to it. The film has a frequency response of 20 kilohertz.

The system also can be used to generate electricity, by applying mechanical energy to the polymer. The film can be made to push the positive charge away from the negative, raising the voltage between the two electrodes, Pelrine said. SRI had one project to put such a device in the heel of a shoe, to generate power when a person is walking. Von Guggenberg estimated that enough electricity could be generated to power a cell phone—about one to two watts of power per step.

Smart Suture

A biodegradable shape memory polymer has come out of research at MIT and the University of Technology in Aachen, Germany. A company, mnemoScience GmbH, based in Aachen, has been formed to market the material. Andreas Lendlein, managing director of the company, described it as a multiblock copolymer.

The material has a hard segment and a switching segment, each with different thermal properties. Lendlein explained that the material has two melting points. The higher one determines if the material is a solid or a melt. The second, lower melt point is where the material changes shape. The company is working with two versions: a thermoplastic elastomer as well as a thermosetting crosslinking polymer that would use a photocuring process.

SRI said that a device using electroactive polymer could, when placed in the heel of a shoe, generate enough electricity to power a cell phone.

The material could be produced on standard equipment, such as extruders or injection presses, with some additional steps to program the material's shape behavior.

One potential near-term application is a "smart" suture that can be trained to knot itself when it is heated, and, what's more, tie itself in a knot with the proper amount of tension. This can be useful in endoscopic surgery, where creating a knot in confined spaces can be difficult and can result in scarring or necrosis of the surrounding tissue.

Shape memory alloy fast response actuators from Midé Corp. are smart components that can be used as elements in larger systems.

Another application could be stents, in which the polymer is produced in a long fiber that changes into the corkscrew shape typical of a stent. The company is starting off with low-risk applications and working to get approvals from regulatory agencies. It is still in pre-clinical trials.

Lendlein added that the company is also investigating non-medical industrial applications.

Lendlein emphasized that the material is not toxic, and degrades into compounds that are already present in the body, although the degraded material would have to be present in low volume to be tolerated by the body.

Piezoelectric materials are among the most widely used in smart applications, according to Marthinus van Schoor of Midé Corp. Piezo materials, which change shape when an electric charge is applied to them, have been accepted into a number of industrial and consumer applications, including accelerometers, fast positioning of mirrors, and damping of sports equipment.

Midé Corp.'s ferromagnetic shape memory alloy servovalve actuator has 100 Hz bandwidth, spool displacement of 40 mils, and a force of 5 lbs.

Van Schoor said the company is developing piezo materials in several potential applications. One area is to develop fast servovalves for hydraulics or diesel injection. He claims to have developed a servovalve running at 380 Hz.

Piezo materials also can be used to harvest energy. "A lot of things vibrate," he said. "When they vibrate, you can take a reverse effect, in which the mechanical signals are converted into electrical signals. You can take that electrical signal to charge a battery and use that power."

Midé currently has a program with the Army in which sensors are distributed on a helicopter. They charge themselves and communicate to a computer through an RF link.

Midé is also working with a smart gel called NIPA. The gel absorbs or rejects water according to temperature changes. The gel is being used in a wetsuit, where it is embedded in an open foam. As the temperature drops, the gel absorbs water inside the suit. As the temperature rises, the gel rejects the water, converting the suit back to a conventional wetsuit.

It's difficult to predict just how successful many of these materials will be in cracking the commercial arena. Early successful applications are likely to smooth the transition for the many materials that will become available in the coming years.