German Scientists Develop Synthetics With Memory

Take an empty plastic yoghurt container and put it in the oven. Then gradually increase the heat so the container goes soft but doesn't melt. At a certain temperature the plastic will contract and the container will become a round disc. Actually, the container started off as such a disc and was eventually drawn into its later shape. The material "remembers" its original form, and Professor Andreas Lendlein, a chemist at the GKSS Research Center in Teltow, near Berlin, uses this sort of memory not for yoghurt containers, but for medical implants.

"Thus you can introduce a bulky implant in a compressed form into the body through a small cut, such as in keyhole surgery," Lendlein said. "By warming the room temperature to body temperature the implant unfurls and then the whole thing disintegrates after a certain (amount of) time. It dissolves so that you don't have to carry out a second operation to remove the implant."

The researcher calls the material a multi-functional polymer since it has two qualities: It remembers its original form and it can dissolve itself inside the body. Precision work was necessary to combine both characteristics in one material.

Intelligent plastics

Most synthetic materials are made of only one initial substance. Lendlein however mixes various materials to produce his polymer. Thus he created a special thread for surgeons that can make itself into a loop once it's inside the body.

"In most cases the clinics want it to just make a knot but not to pull it tight, since the tension that's located at the edges of the wound is decisive for healing successfully." Lendlein said. "If you knot it too loosely, more scar tissue builds up than you would want. If you knot it too tightly, you destroy more tissue than is actually necessary."

Synthetic materials with memories have a special internal construction: chains of molecules are connected on many different points to form a network. This material can, in a manner of speaking, be programmed in three steps.

First, as with all conventional plastics, the material is shaped into a permanent shape, such as a thread that's five centimeters (two inches) long. Then the thread is heated just enough to be deformed. When warmed, the molecule chains become mobile and they can be stretched to a certain degree. They are held together by the network of connected points, even when it's distorted.

Heat, cold and light waves

Stretching the material is the second step. The thread is pulled from five centimeters to 10 centimeters and then cooled to room temperature. An elastic material would spring back to its original length of five centimeters, Lendlein pointed out. His polymer, however, is "programmed" and will keep to the 10 centimeters almost exactly, he said.

The third step, cooling the material, stops molecule chains from moving anymore. Their structure is practically frozen. The molecules are drawn-out, under tension, and they can't release without warmth. As they warm up the thread gradually returns to its original length.

Currently Lendlein and his research group are developing synthetic materials that change their forms when bright light is shined on them.

Memory materials that react to warmth only change their form once and must then be reprogrammed. But Lendlein has been able to switch the first prototypes of non-sensitive fibers back and forth with different wave lengths of light. Increasing the number of these cycles is Lendlein's task for the coming years.