Biomimetic smart polymer goes from hard to soft

Materials World magazine
1 Apr 2008

A new polymer nanocomposite that turns from hard to soft and vice versa on exposure to chemical stimuli, such as water, could be used in bulletproof vests and implants for artificial nervous systems to treat strokes or spinal cord injuries, say US researchers. The material mimics the skin of sea cucumbers.

Developed by an interdisciplinary team at Case Western Reserve University, Cleveland, USA, the material mimics the skin of sea cucumbers. ‘These sea creatures can reversibly and quickly change the stiffness of their skin,’ explains Dr Jeffery Capadona, Associate Investigator at the Department of Veteran Affairs Medical Center. ‘Normally it is soft, but, for example in response to a threat, the animal can activate its body armour by hardening its skin.’

Holding firm

The nanocomposites developed comprise rubbery polymers, such as ethylene oxide/epichlorohydrin copolymer or polyvinyl acetate, into which strong and rigid nanofibres are embedded.

The fibres are cellulose whiskers extracted from another sea creature called tunicates (22nm in diameter and 2.2µm in length), which are dispersed into dimethyl formamide solvent and mixed with the polymers for solution casting. The petri dishes are then placed in a vacuum oven to evaporate the solvent and dry the resulting films, which are compression moulded to create 300-400µm thin materials.

Capadona says, ‘The ratio of whiskers to polymer mediates the mechanical properties of the materials. Because of the high density of strongly interacting surface groups, the whiskers [aggregate]. In this state, the material is strong and rigid. If it is exposed to water, the material swells slightly, and the sulphate-modified whiskers adsorb enough water molecules to "unglue" the fibres, [making] the material 1,000 times softer.’

The polymers used adsorb enough water to expose the fibres, without soaking them.

From the bottom up

‘Scientifically it holds together’, comments Alan Windle, Professor of Materials Science at the University of Cambridge, UK. But he argues, ‘It is not a wonder material. Paper behaves like this. It would be fascinating to compare it to the properties of paper, which are not quite as reversible. Paper does get soggy’.

He adds, ‘What is important is that the researchers have engineered the material to achieve these properties – it is another example of bottom-up technology. They are taking the building blocks and [putting] them together at the nanoscale. The properties themselves therefore become much more controllable, which gives confidence in the material for critical applications’.

One market for the new composite is smart intra-cortical microelectrode implants for artificial nervous systems.

‘Current electrodes are made of stiff materials such as metals, ceramic and silicon,’ explains Professor Dustin Taylor of the University’s Department of Biomedical Engineering. ‘While a high stiffness electrode is advantageous during insertion, the micro-motion of rigid electrodes within the soft cortical tissue chronically inflicts trauma on the surrounding neurons.’

A mechanically adaptive polymer that becomes soft once placed into cerebral spinal fluid could be a solution.

Professor Pankaj Vadgama, a specialist in biomaterials at Queen Mary, University of London, UK, adds, ‘There would be potential for other clinical applications where a soft resident material is required following initial insertion, particularly in light of accelerating developments in robotic and minimally invasive surgery’.

The US team will explore the long-term biocompatibility of the material, and look at fabricating and in vivo testing of microelectrodes made from the polymer. Researchers have also begun work on electrically switchable materials.