By Roland Piquepaille
You certainly know that it's possible to alter the shape of plastics and polymers by heating them. But now, a team of American and German researchers have found a way to change plastics shape with light, according to this MIT news release. These special polymers can move to new shapes by being exposed to light of specific wavelengths. And they'll retain this stable new shape until they're illuminated with another source of light of a different wavelength. This discovery has many potential applications, particularly for medical applications, such as expandable strings keeping blood vessels opened during surgery. Read more...Here is the introduction of this news release.
Picture a flower that opens when facing the sunlight. In work that mimics that sensitivity to light, an MIT engineer and his German colleagues have created the first plastics that can be deformed and temporarily fixed into shape by light.
Here is how this works.
Key to the work: "molecular switches," or photosensitive groups that are grafted onto a permanent polymer network. The resulting photosensitive polymer film is then stretched with an external stress and illuminated with ultraviolet light of a certain wavelength. This prompts the molecular switches to crosslink, or bind one to another.
The result? When the light is switched off and the external stress released, the crosslinks remain, maintaining an elongated structure. Exposure to light of another wavelength cleaves the new bonds, allowing the material to spring back to its original shape.
"This is really a new family of materials that can change from one shape to another by having light shined on them," said Institute Professor Robert Langer of MIT.
And what are some possible applications of these new materials?
Imagine, for example, a "string" of plastic that a doctor could thread into the body through a tiny incision. When activated by light via a fiber-optic probe, that slender string might change into a corkscrew-shaped stent for keeping blood vessels open.
The team is also looking at other medical and industrial applications, such as "paper clips that relax when you don't need them anymore."
This research work has been published by Nature in its April 14, 2005 isuue under the title "Light-induced shape-memory polymers." Here is a link to the abstract and below is the text of this abstract.
Materials are said to show a shape-memory effect if they can be deformed and fixed into a temporary shape, and recover their original, permanent shape only on exposure to an external stimulus. Shape-memory polymers have received increasing attention because of their scientific and technological significance.
In principle, a thermally induced shape-memory effect can be activated by an increase in temperature (also obtained by heating on exposure to an electrical current or light illumination). Several papers have described light-induced changes in the shape of polymers and gels, such as contraction, bending or volume changes. Here we report that polymers containing cinnamic groups can be deformed and fixed into pre-determined shapes -- such as (but not exclusively) elongated films and tubes, arches or spirals -- by ultraviolet light illumination.
These new shapes are stable for long time periods, even when heated to 50 °C, and they can recover their original shape at ambient temperatures when exposed to ultraviolet light of a different wavelength. The ability of polymers to form different pre-determined temporary shapes and subsequently recover their original shape at ambient temperatures by remote light activation could lead to a variety of potential medical and other applications.
Finally, if the subject interests you, here are two references to previous papers about shape-memory polymers, "Biodegradable, Elastic Shape-Memory Polymers for Potential Biomedical ApplicationsScience" published by Science in May 2002 (free registration for access to the full paper) and "Shape Memory Polymers: Biodegradable Sutures," published by Materials World in July 2002.
Sources: Elizabeth Thomson, MIT News Office, April 14, 2005; and various websites
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