Plastics with a Memory

Self-repairing fenders and intelligent implants – shape-memory polymers as materials of the future

With a bang, the fender is dented and has to be replaced. Wouldn`t it be nice if the dent could simply – presto! – disappear? Such “intelligent” materials are already being developed, relate Andreas Lendlein and Steffen Kelch in an overview of the field in Angewandte Chemie.

Shape-memory polymers, that`s the magic words: after an undesired deformation, such as a dent in the fender, these plastics “remember” their original shape. Heating them up gives their “memory” a boost – the dent could thus be removed with a hair-dryer.

Polymers with shape memory have both a visible, current form and a stored, permanent one. Once the latter has been produced by conventional methods, the material is molded into a second, temporary form by skillful heating, deformation, and finally cooling. The plastic maintains this shape until the permanent form is recalled by a predetermined external stimulus. The secret behind these clever materials lies in their molecular network structure, which contains meltable “switching segments”. Raising the temperature activates the switching: the crystallized switching segments melt and the material resumes its original form.

In the case of the fender, we are just interested in one shape: the undamaged original shape. The impact results in a temporary form, which changes back to the original form upon heating – the plastic repairs itself.

The two researchers have high hopes that especially interesting applications will be found for shape memory polymers that are tolerated physiologically. These are just the thing for buttonhole surgery, the gentle operative technique of the future. It is conceivable that large implants in a compressed state could be introduced into the body in a minimally invasive fashion, and could subsequently “remember” their original shape. Such materials can also be made to be completely biodegradable, eventually disappearing out of the patient`s body.