All tied up. Shape memory sutures knot themselves when exposed to heat.

Shape-Shifting Sutures

Surgeons may soon seal hard-to-reach wounds with the aid of novel shape-shifting threads that know how to tie themselves and never need to be removed. The new "smart" biodegradable plastic fiber can knot itself when heated to a few degrees above body temperature. The researchers say the same material could be made to last much longer and one day be used for self-repairing medical devices and shrink otherwise bulky implants--such as screws that hold bones together.

A growing class of materials possess "shape memory"--they hold one form at a certain temperature and transform into another shape when heated. Until now, no such plastics had been used in medical devices, nor had any proved to be biodegradable. The new material, described online 25 April in Science by synthetic chemist Andreas Lendlein of mnemoScience GmbH in Aachen, Germany, and biomedical engineer Robert Langer of the Massachusetts Institute of Technology, is composed of two polymers, each already used separately in clinical applications such as drug delivery.

When combined, the two polymers react to temperature. The higher temperature shape is the plastic's "permanent" form, which it assumes after it's heated. After it cools, the plastic can be stretched or scrunched into temporary forms up to four times larger or smaller than the permanent form. To make the sutures, the researchers took a fiber of their shape-shifting material and, after warming it for a few minutes, cooled it down and stretched it. They used the thread to loosely stitch a wound on a rat. When heated to 40°C, the suture tightened with just the right amount of tension, avoiding damage to the surrounding tissue.

Medical devices that can fit through tiny holes in the body and expand into designed shapes on demand would be invaluable, says laparoscopic surgeon Frederick Finelli at George Washington University Medical Center in Washington, D.C. But he cautions that surgeons might feel uncomfortable not having direct control over shape-changing implants. "If the size is predetermined and you have it in and you heat it up and then it turns out it's too small, what do you do then? These are the things surgeons worry about."

Related sites
Movies of shape-shifting threads
mnemoScience Gmbh
Robert Langer
intelliMat, a guide to intelligent materials