In Plastics' Day in Surgery, Red Herring reports that an international team of U.S. and German researchers has developed a new kind of plastic that can shift between three different shapes when the temperature increases. Even if these polymeric triple-shape materials have not emerged from the lab, they could eventually be employed as removable 'stents' and self-closing fasteners used by surgeons and more generally by the healthcare industry. But read more...
This research work has been done partially at the MIT in Professor Robert Langer's research lab. Please note that I've already covered a previous Langer's project in "Light Used to Design Shape-Shifting Plastics" (April 14, 2005).
This research work has been published online before print by the Proceedings of the National Academy of Sciences (PNAS) under the name "Polymeric triple-shape materials" (November 20, 2006). Here is a link to the abstract.
Shape-memory polymers represent a promising class of materials that can move from one shape to another in response to a stimulus such as heat. Thus far, these systems are dual-shape materials. Here, we report a triple-shape polymer able to change from a first shape (A) to a second shape (B) and from there to a third shape (C). Shapes B and C are recalled by subsequent temperature increases. Whereas shapes A and B are fixed by physical cross-links, shape C is defined by covalent cross-links established during network formation.
The triple-shape effect is a general concept that requires the application of a two-step programming process to suitable polymers and can be realized for various polymer networks whose molecular structure allows formation of at least two separated domains providing pronounced physical cross-links. These domains can act as the switches, which are used in the two-step programming process for temporarily fixing shapes A and B. It is demonstrated that different combinations of shapes A and B for a polymer network in a given shape C can be obtained by adjusting specific parameters of the programming process.
Below is a series of photographs illustrating this triple-shape effect. On the left is a tube which could be used as a stent and on the right is fastener consisting of a plate with anchors. From top to bottom, you can see the shape evolution when the temperature increases to 40°C (in B) and 60°C (in C). (Credit: MIT/GKSS Research Center). This image has been extracted from the PNAS paper mentioned above.
In "Morphing Materials Take On New Shapes," Technology Review describes this process in plain English.
Lendlein says the key to the new structures was developing two types of polymers that have distinct melting points. At room temperature, the material holds its first shape. But when heated above a certain temperature, areas throughout the material soften, allowing it to change to an intermediate shape. At a yet higher transition temperature, the rest of the material softens, allowing the structure to take its final shape.
But what would be these shape shifting plastics useful for? They could eventually be employed as removable stents and self-closing fasteners used by surgeons and more generally by the healthcare industry, as Red Herring reports.
Although the technology hasn’t moved off the lab bench, Mr. Langer says its existence is the first step toward new applications, such as "intelligent stents," for example.
Stents are small metal tubes used to prop open clogged heart arteries after they have been cleared. The material could in theory enable a stent that takes an oval shape for insertion, then transforms to a fully inflated round shape upon implantation.
It could also be an option for placing stents in hard to reach places where a single heart vessel branches into two. Cardiologists usually have to alter a stent’s shape to fit the angle of the vessel by using multiple stents that often overlap one another to cover the lesion.
Finally, for more information about this morphing process, here is a link to the full PNAS paper (PDF format, 6 pages, 1.29 MB) and to some supporting information, which include several figures and two short movies worth viewing.
Sources: Red Herring, November 22, 2006; Kevin Bullis, Technology Review, November 21, 2006; and various websites
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