Originally Published MPMN June 2002

EDITOR'S PAGE

Research in Shape-Memory Polymers Reaches a New Plateau

There are several alloys with shape-memory capabilities, but only nickel-titanium has proven to be biocompatible. Consequently, nitinol, as it is commonly called, has enjoyed a near-monopoly in medical device applications where shape-memory properties are desirable. But competition may be on the way. Recently developed shape-memory polymers, which exhibit greater deformation capabilities and easier shaping procedures at a reduced cost, already show considerable promise as alternative materials. Now comes word that researchers have added biodegradability to the recipe, potentially opening up new areas of application. A paper on this topic authored by Andreas Lendlein, director of the German company mnemoScience, and MIT professor of chemical engineering Robert Langer, who is also a partner in the firm, was published on-line April 25 by Science magazine.

Programming shape-memory properties into nitinol is a delicate process. A minute change in the percentage of the nickel composition can engender a shift of several degrees in the material's transformation temperature. Moreover, its transition temperature and mechanical properties can be affected by oxygen, carbon, nitrogen, and other common contaminants. Processing nitinol is a time-consuming and exacting task that, in the end, produces a material with a maximum deformation of about 8%. By contrast, biodegradable shape-memory polymers can be programmed in seconds, according to Lendlein, and they can achieve deformations of several hundred percent. The transition temperatures and mechanical properties can be altered with minor changes to the polymer's chemical structure and composition, adds Lendlein.

"Sometimes it doesn't make sense to have shape transition at 32°C," Lendlein recently told reporters at Chemical and Engineering News. "You might want to have it at 42°C instead. With a polymer, you can easily adjust the transition temperature. With one set of monomers, you can have a whole set of shape-memory materials."

In their paper, Lendlein and Langer cite the development of a degradable suture to illustrate the biomedical applications of the shape-memory thermoplastic.

"A challenge in endoscopic surgery is tying a knot with the instruments and sutures that are currently available," they write. "It is especially difficult to manipulate the suture in such a way that the wound lips are pressed together under the right stress. A possible solution is the design of a smart surgical suture, whose temporary shape is obtained by elongating the fiber with controlled stress." The suture can be loosely applied in its temporary shape, shrinking on command when it is exposed to its transition temperature. The suture was tested four times on two different animals.

"For these tests, the fibers were elongated by 200% during programming and were able to generate a force of 1.6 N upon actuating the shape-memory effect in vitro," they write. The feasibility study, conclude Lendlein and Langer, suggests that this material has the potential to reshape the way in which implants are designed and could lead to new surgical procedures.

The paper is available through www.sciencexpress.org. Lendlein and Langer can be contacted via e-mail at [email protected].

Norbert Sparrow

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