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Materials


plastics Shape-


shifting


Advances in plastics that change their confi guration in response to external stimuli could revolutionise the design of many existing medical devices, as well as fostering entirely new products in healthcare. As academic and industrial research into the shape-memory effects of certain polymers is growing apace, Jim Banks speaks to John Hardy, senior lecturer in materials chemistry at Lancaster University, to learn the science behind the materials and how they could underpin a new generation of medical devices.


modify their physical characteristics – shape, stiffness and size to name a few – with the application of a huge range of external stimuli, they open up a host of possible applications across many industries, and the medical devices sector is no exception. At the molecular level, SMPs have a matrix of polymer that resembles a fishing net. This matrix can be deformed – stretched or bent into a different shape – but unlike a normal polymer, which will return to its original configuration when the tension is removed, an SMP can be fixed at its stretch point by applying a stimulus, often heat, only returning to its original shape when another stimulus releases the tension to unlock it. Imagine an elastic band that is stretched to its full extent and stays there until its temperature changes or an electrical charge is applied.


S Medical Device Developments / www.nsmedicaldevices.com


hape-memory polymers (SMPs) are the very definition of smart materials. Able to


“In the


fishing net structure, the points at which the strands interlock are


crystalline or hard in some way compared to what connects them, not dissimilar to spider silk,” says John Hardy, senior lecturer in materials chemistry at Lancaster University.


“The toughness comes from the ability to stretch and unfold the individual sheets in the matrix, and peeling back the individual sheets means they absorb a high level of energy.” With SMPs, there are soft and hard segments and reversible transformations are possible because the soft elements that link the hard primary structure elements


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