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NEWS OF THE WEEK
MATERIALS
February 5, 2001
Volume 79, Number 6
CENEAR 79 6 pp.5
ISSN 0009-2347
EDITED BY JANICE LONG AND STEVE RITTER
[Previous Story] [Next Story]

SUITE OF SHAPE-MEMORY POLYMERS
Forgoing metal component means new materials are programmed in seconds

MAIRIN BRENNAN

 
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PHOTOS COURTESY OF ANDREAS LENDLEIN

TRANSFORMED Shape-memory polymer converts from a temporary shape (top) to its parent shape (bottom) in 45 seconds at 65 ºC.

Picture this: you've just been involved in a fender bender. You grab your heat gun, apply it to the dent, and the dent disappears. The fender has "remembered" its original shape.

This futuristic fender isn't metal, of course. It is made from a shape-memory polymer like those recently constructed by researchers at Massachusetts Institute of Technology and the German Wool Research Institute (GWRI), in Aachen [Proc. Natl. Acad. Sci. USA, 98, 842 (2001)].

The research team's goal was to develop a polymer system that was easier to shape and offered more applications than the shape-memory materials already in use. The latter include the nickel-titanium alloy Nitinol, used, for example, in making orthodontic wires that self-adjust, flexible eyeglass frames, and pliant guidewires and tools for "bloodless" surgery.

Nitinol and other shape-memory alloys are able to undergo a so-called martensitic phase transformation that enables them to switch from a "temporary" shape to a "parent" shape at temperatures above a transition temperature. Below that temperature, the alloy can be bent into various shapes. Holding a sample in position in a particular parent shape while heating it to a high temperature programs the alloy to remember the parent shape. Upon cooling, the alloy adopts its temporary shape, but when heated again above the transition temperature the alloy automatically reverts to its parent shape.

There's plenty of room for improvements in such materials, notes lead author Andreas Lendlein, head of the department of biomaterials at GWRI. "Programming a metal alloy is not only a time-consuming procedure, it also involves heat treatment at temperatures of several hundred degrees Celsius," he says. "Another drawback for shape-memory alloys is that the maximum deformation they can undergo is only about 8%. Furthermore, they are much more expensive than polymers."

Lendlein, GWRI graduate student Annette M. Schmidt, and MIT professor of chemical engineering Robert Langer began prospecting polymers for more versatile materials (see also page 30). They identified two monomeric components that, when combined, generate a family of polymers that display "excellent" shape-memory properties. Moreover, the polymers are programmed into shape in seconds at about 70 ºC. And they can withstand deformations of "several hundred percent," Lendlein says.

One component, oligo(e-caprolactone) dimethacrylate, furnishes the crystallizable "switching" segment that determines both the temporary and permanent shape of the polymer. By varying the amount of the comonomer, n-butyl acrylate, in the polymer network, the cross-link density can be adjusted. In this way, the mechanical strength and transition temperature of the polymers can be tailored over a wide range.

"Sometimes it doesn't make sense to have shape transition at 32 ºC. You might want to have it at 42 ºC instead," Lendlein says. "With a polymer, you can easily adjust the transition temperature. With a metal, that's difficult. Now, with one set of monomers, you can have a whole set of shape-memory materials."

"These researchers have cleverly exploited the nature of polymer networks," comments Ulrich W. Suter, a professor of macromolecular chemistry at the Swiss Federal Institute of Technology, Zurich. "Clearly, there are a number of medical applications for this clever invention."

Indeed, homopolymers of both monomers are known to be biocompatible, opening the door for such applications. In addition, poly(-caprolactone) is biodegradable. For biomedical applications, biodegradability represents an additional advantage over Nitinol, Lendlein says. The new polymers have various potential medical uses, including stents--such as those used to keep blood vessels open--and catheters and sutures with more give than those currently available.

Lendlein is getting set to market the polymers via Aachen-based mnemoScience, a company he formed with Langer to develop medical products from the new materials. But the polymers have potential applications in other fields, he notes. Materials that repair themselves, for example, would be a very interesting development, he suggests. "You could make auto bodies out of such a material." Then dents would be erasable.



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