Monday, March 20, 2006

Beyond Self-Tying Sutures

Continued from page 1

By Kevin Bullis

In addition to Lendlein, others are using nanoparticles to trigger shape-shifting, including Richard Vaia of the Air Force Research Labs, the Cornerstone Research Group in Dayton, OH, and Composite Technology Development in Lafayette, CO. These groups use nanotubes to convert light and electricity into heat, and the nanotubes themselves change shape, adding to the effect. Vaia says Lendlein's paper "is a wonderful example of a growing area of work, of people creating nanocomposites not just to strengthen the polymer, but actually provide some unique functionality."

Some of Lendlein's earlier polymers, which were activated by directly applying heat, are being commercialized by a spinoff, Mnemoscience, in Aachen, Germany, and should be in clinics later this year, he says. In this application, strands of plastic form sutures that tighten uniformly when heated -- even tightening a loose knot to hold themselves in place. This could be useful in minimally invasive surgery, in which tying sutures by hand is difficult.

Lendlein's magnetically activated polymers, in addition to their potential usefulness in medical applications, could help on assembly lines: fasteners too small to work by hand could snap themselves into place, Lendlein says. Mather speculates that such a trigger also could be handy for deploying solar panels on satellites.

Currently, however, the magnetic field is too strong to use safely with living tissue; but Lendlein says optimizing the material for a lower field strength is an "engineering problem" and not a question of whether it is possible in principle. The magnetic field also limits its industrial applications -- it could not be used, for example, to assemble electronic components.

In addition to decreasing the necessary field strength, Lendlein is now working on applying the same magnetic trigger in other materials. Mather expects that the method will turn out to be a general principle that applies to many shape-memory polymers. And, for medical applications, toxicity tests will be needed to show that the nanoparticles are safe. Lendlein says early results look positive.

So far, shape-memory polymers only "remember" one position. "The thing that's coming over the horizon is two-way, reversible shape memory," says Mather, in which the polymer can shift between two positions. This could lead to artificial muscle or, according to Vaia, morphing skins on military airplanes.