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| Fantastic Spastic Elastic Plastic |
| 2005-04-16 |
| Imagine a thin plastic thread inserted into a patient's arteries that, when exposed to light, transforms into a corkscrew-shaped stent to keep blood vessels open. Such shape-shifting materials are a staple of sci-fi fantasy, but two professors are working on bringing the concept to fruition -- perhaps at a hospital or toy store near you. Since the late 1990s, Robert Langer of MIT and Andreas Lendlein, of the University of Technology in Aachen, Germany, have been working to create plastics that can change shape when exposed to different wavelengths of light. Originally, the professors' material shifted shape with the introduction of heat; now it warps with certain wavelengths of light. Though it's uncertain when the process and its resulting products would be available, applications could range from improving minimally invasive surgical procedures to creating funky toys for kids. "I think there could be some very interesting applications in the medical field. We are thinking about stents, small tubes that are able to open up blood vessels ... and you could use a fiber optic to let the stent open so it stays at the place it should be," Lendlein said. The idea is simple: Shine a light on object A, it turns into predetermined shape B. Shine a different light on shape B, it molds back into its original form. Any light in a wavelength range above 260 nanometers will change the first shape to the second; any light in a range below 260 nanometers will change it back. Besides changing shape, the object could be made a little larger or smaller, as tests have shown the plastic in use is capable of stretching about 10 percent to 20 percent. Right now, the scientists are performing tests in Germany with skinny plastic polymer fibers. The scientists are using light to lengthen them or change their forms into shapes like spirals, and observing how long they'll last in a new position. In a phone interview from Germany, Lendlein said that in stress tests, the polymers held their shape for eight hours, while test objects sitting around his lab don't appear to have changed shape for weeks. Also, the shape-shifted objects have been tested to see if environmental stresses like temperature will affect their ability to hold a new shape. Lendlein said they've been fine at up to 50 degrees Celsius, and expects they would hold up at 80 or 100 degrees Celsius. "Our materials stay stable in the temporary shape," he said. Langer came up with the idea for shape-shifting plastics about seven or eight years ago, he said, as a way to enhance biocompatible plastics. He mentioned it to Lendlein, who was a visiting scientist at MIT in 1997, working under Langer. Over the next several years, the two set about creating actual items capable of changing form. Their first work utilizing heat was presented in 2001. On a molecular level, the plastic has been endowed with what Langer calls "photocrosslinkable" switches. If a light is shined on the polymer, these switches zip up like a zipper. Shining a different wavelength of light will cause it to unzip, Langer said. This metaphoric zipping and unzipping changes the object's shape. The "switches" are made from photosensitive chromophores, or groups of molecules that react to light. The researchers also had to measure the ultraviolet spectrum and test different wavelengths to see if they were absorbed by the chromophores they wanted to use. The physical transformation is determined by where the researchers hit the object with light, Lendlein said. For example, a corkscrew shape is made by lighting only the top of the polymer, which causes the top to elongate while the underside remains untouched, leading to curls in the material. In theory, the scientists could make any shape by just altering where the light hits the polymer, according to Lendlein. He said they're working on making knotting sutures right now, which they were able to accomplish in their past work using heat and polymers. Lendlein said they knew the process would work because they were already aware of which wavelengths caused reactions in different chromophores. The problem was embedding them in plastic. "But we had to link those photosensitive chromophores to the polymer spectrum," Lendlein said. The work is still just a prototype. Right now, it takes about 90 minutes for test objects to shift from one shape to another. For many uses, the reaction time will need to be sped up, Lendlein said, but he mentioned some applications -- a futuristic sunscreen that slowly releases UV blockers, for example -- that might require a lengthy time frame to achieve the desired effect. The pair's work is described in a paper, written by Langer and Lendlein with colleagues Hongyan Jiang and Oliver JÃŒnger, that appears in the April 14 issue of Nature. |
| Posted by:Sobiesky |