As a gung-ho polymath in an era of debilitating specialization, de Gennes saw no bounds to the integrative role of materials science. As he remarked in 1995, 'I've battled for a long time to have three cultures in my little school: physics, chemistry and biology. Even at a time when there are not many openings for bioengineers in industry, this triple culture is already very important for physical and chemical engineers.'
When a group on these lines started work at the Institut Curie in Paris, one of its first efforts was to try out an idea for artificial muscles proposed by de Gennes in 1997. These would not directly imitate the well-known but complex protein systems that produce muscle action in animals. Instead, they would aim for a similar effect of strong, quick contractions, in quite different materials-the liquid crystals.
Discovered in 1888 by Friedrich Reinitzer, an Austrian botanist, liquid crystals are archetypal untidy materials, being neither solid nor liquid yet in some ways resembling both. They were only a curiosity until 1971 when Wolfgang Helfrich of the Hoffmann-La Roche company in Switzerland found that a weak electric field could line up mobile rod-like molecules in a liquid crystal, and change it from clear to opaque. This opened the way to their widespread use in display devices. De Gennes suggested that similar behaviour in a suitably engineered material could make a liquid crystal contract like a muscle.
In this concept, a rubbery molecule is attached to each end of a rod-like liquid-crystal molecule. Such composite molecules tangle together to make a rubber sheet. The sheet will be longest when the liquid-crystal components all point in the same direction. Destroy that alignment, for example with a flash of light, and the liquid-crystal central regions will turn to point in all directions. That will force the sheet to contract suddenly, in a muscular fashion. By 2000 Philippe Auroy and Patrick Keller at the Institute Curie had made suitable mixed polymers, and they contracted just as predicted, as artificial muscles.
'We are now in the era of smart materials,' Keller commented. 'These can alter their shape or size in response to temperature, mechanical stress, acidity and so on, but they are often slow to react, or to return to their resting state. Our work on artificial muscles based on liquid crystals might open the way to designing fast-reacting smart polymers for many other purposes such as micro-pumps and micro-gates for micro-fluidics applications, and as ''motors'' for micro-robots or micro-drones.'
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