New Smart Plastic Has Good Memory

What if it were possible to create plastic objects in one shape, deform them to fit other requirements, and then have them return to their original `shape when needed? For years, nitinol and other alloys with these memory properties have been used in everything from catheter wires to eyeglass frames and orthodontia. But these metals have limitations, including restricted pliability and resource-intensive fabrication. That is why Robert Langer and Andreas Lendlein, two scientists working at Massachusetts Institute of Technology (Cambridge, MA; www.mit.edu), set out to create a shape-memory plastic that would be easier to mold and control. And as announced in the January 30, 2001, issue of Proceedings of the National Academy of Sciences, the pair has succeeded in creating a mixture of polymers that reliably remembers its shape when properly combined and treated.

This novel material, composed of oligo-dimethacrylate and n-butyl acrylate, works similarly to existing shape-memory alloys. The plastic is formed in its parent shape and heated to a high temperature, locking in that particular arrangement as the unit's natural structure. Upon cooling, the piece can be molded into any desired temporary shape that is within the material's mechanical limitations. This structure is retained until the material is subjected to its transition temperature, when it rebounds to its original shape. This process can be repeated an unlimited number of times.

Though these new plastics may work in a similar fashion to their metallic predecessors, they hold several distinct advantages. Programming metal to remember its shape is a time-consuming process that requires temperatures of several hundred degrees Celsius. Shape-memory plastics, on the other hand, can be conditioned in seconds at temperatures around 70°C. Plastics can also be bent to a greater degree and still return to their original shape. The maximum deformation with metal is about 8%; with plastics, it is 300–400%. By varying the ratio of the two polymers, plastic also offers the benefit of an easily adjustable transition temperature. "Sometimes it doesn't make sense to have the material transition at 32°C," Dr. Lendlein explains. "You might want 42°C instead. With this polymer, you can easily adjust the transition temperature. With a shape-memory metal, that's difficult." It is also thought that it may be possible to make shape-memory plastics respond to transition stimuli other than heat. Light, ultrasound vibration, and moisture have been cited as possible alternatives.

Because the material is also biocompatible, shape-memory plastics have numerous potential medical uses. They could increase the flexibility in stents, stitches, and catheters. They could also be used to produce the opposite effect in devices that contract as a result of an environmental stimulus. Lendlein and Langer have set up a new company called mnemoScience (Aachen, Germany; www.mnemoscience.de) to explore these and other medical and commercial applications. Among some of the nonmedical uses being considered: car bumpers that snap back to their original shape after an accident.

Zachary Turke