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David Bradley ISSUE #25
July 2002

Stitched up

    Click image to see movie 1
Click image to see movie 1.
Plastic sutures that tighten themselves have been developed from a biodegradable polymer by scientists at the Massachusetts Institute of Technology and the University of Technology, Aachen, Germany.

MIT scientist Robert Langer and his colleague reckon the materials could have many medical applications. For example, the new plastics might be used as sheets to prevent adhesion between internal tissues after a surgical operation, or as a fastening to hold bone together. They could also be used as a spiral stent for holding open blocked arteries and other vessels (see movie 1). The self-tightening knotting suture can be seen in movie 2.

Langer and Andreas Lendlein, managing director of mnemoScience GmbH in Aachen have been working with polymer materials that can be "taught" to have a temporary shape at one temperature and switch to another permanent shape when heated higher than a switching transition temperature. A piece of material is manufactured by conventional methods like extrusion or injection molding from the melt. After cooling below the melting temperature of the hard segment but above the thermal transition temperature of the switching segment, the polymer is deformed in its temporary shape, which is being fixed by then cooling below the switching segment thermal transition temperature. When the material is warmed to body temperature again it adopts its permanent shape. Thus a sheet at body temperature might be formed into a thread that can be implanted through a surgical keyhole only to spring back to its sheet shape once inside the warmth of the body. A similar approach would be used for a screw-like structure that could hold bone together or an internal suture. A shape-shifting suture also circumvents the problem of tightening a knot to the right tension in a confined body space!

The problem, until now, has been that shape-shifting polymers have not been biocompatible or biodegradable and so have been wholly unsuitable for surgical applications. A plastic suture that helps the wound heal but does not dissolve away would not be so useful in medical applications.

The team has now used a biodegradable multiblock copolymer. One component of the polymer chain undergoes a phase transition at a different temperature than the other component. By manipulating the temperature and stress applied to the overall material, Langer and Lendlein end up with a material that forms a temporary shape at one temperature, and so can switch back to its permanent shape when heated.

Macrodiols with different thermal characteristics were prepared using ring-opening polymerization of cyclic diesters or lactones with a low molecular weight diol as an initiator. Oligo(sigma-caprolactone)diol (OCL) was then used as the precursor for the switching segment of the polymer that has a melting transition temperature around body temperature. While a crystallizable oligo(p-dioxanone)diol (ODX) has a higher melting transition temperature and so would become the hard component.

Science Express, 25th April, http://www.sciencemag.org/cgi/rapidpdf/1066102v1*

* Articles that provide a link to a particular paper will usually take you direct to the paper, although you may need a subscription or to make a pay-per-view to the journal to access the full text. For more information on any of the publishers and how to subscribe to any journals cited in RR please go direct to the publisher's home page (www.sciencemag.org).