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A prick from a used hypodermic needle is not just painful—it can also
transmit deadly diseases. So Donald Highgate and Roy Rengstroff have come up
with the ingenious idea of a needle that becomes blocked and blunted immediately
after it has been used for the first time. They suggest coating the tip of a
hypodermic needle with a hydrophilic material that absorbs water and swells up
(GB 2 284 554). When the coating gets wet, it will change shape, permanently
blocking the tip of the needle and making it impossible to use again.