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Beyond Self-Tying Sutures

As shape-memory polymers find commercial application, one researcher has activated them remotely using magnetism.
March 20, 2006

Shape-memory polymers – materials that transform themselves into a pre-determined shape when activated – will reach clinics beginning this year in the form of self-tying sutures, fast-adjusting orthodontic braces, and other devices. But these polymers require direct triggering by light or direct heat. Now Andreas Lendlein and colleagues have created shape-memory polymers that are triggered remotely by a magnetic field – making it possible to activate them anywhere in the body.

“The Lendlein work is pioneering – the first magnetic shape-memory plastics,” says Robert Langer, chemical engineering professor at MIT, who has worked with Lendlein in the past. The new method, described this month in the Proceedings of the National Academy of Sciences (PNAS), could lead to medical implants, such as polymer stents, that doctors can insert in a compact form, and then remotely trigger to take their final shape once in the body. “You can get to any position in the body with magnetics,” says Lendlein, a researcher at the Institute for Polymer Research in Teltow, Germany.

Shape-memory polymers have two components: one is like a spring, which can be temporarily compressed. The other component, surrounding the spring, is something like a waxy glue. The researchers heat this glue-like substance until it “melts,” then compress the spring, holding it in place until the wax hardens again. Then they can let go, and the wax holds the spring in its compressed position. Finally, the researchers heat the wax-like component until it softens again, and the spring springs back into position.

[For images of the shape-memory polymer, click here.]

Lendlein adds a third component to his polymers: nanoscale particles of magnetite surrounded by a layer of silica. An alternating magnetic field interacts with these particles, causing them to heat up and trigger the shape-change. Incorporating the magnetite particles was an important accomplishment. “He figured out how to get these small particles very well dispersed – this is not easy to do,” says Patrick Mather, professor of macromolecular science and engineering at Case Western Reserve University. “This is a big challenge to nanomaterials in general.”

Mather says shape-memory polymers have started to take off in the last five years, as academics and companies make them with a greater range of mechanical properties, such as elasticity, and as industry begins to imagine ways to use them. Mather’s work has led to orthodontic braces with brackets that open when triggered, speeding up adjustments and decreasing “chair time” (the time a patient has to sit in a doctor’s office). It has also led to strands of plastic that when activated apply a very specific force, allowing orthodontists to tune their treatments with more precision than the old rubber-band method, Mather says.

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.

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