Common sense says that when you stretch a substance, say a piece of rubber, it becomes longer in the direction of the pull but thinner in the transverse directions. But there are a very few substances such that when you pull on them they actually increase in cross-section. These oddities are said to be auxetic. The word in this sense is believed to have been coined by Professor Ken Evans of Exeter University in an article in Nature in 1991. Such auxetic substances, which now include certain ceramics and polymers, have potential as molecular sieves and filters, as cushioning (for example, inside crash helmets), and possibly other uses still being investigated. The word itself is far from new. It has been used since medieval times as a term in rhetoric in which repetitive language is used to amplify or magnify a statement. In plant physiology, auxetic substances tend to increase cell growth without cell division; the related term auxin refers to a substance that does this, also called a plant hormone. All these terms stem from the Greek word meaning “increase; grow”. The noun is auxesis.
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