Cochlear implants convert sound into electrical signals, which are used to stimulate the auditory nerve fibres in the cochlea of the inner ear. To fit the implant, an array of electrodes is fed down the cochlea around one and a half turns of its coiled, shell-like structure.
Surgeons do this work "by feel"- and if they hit an obstacle the operation has to be abandoned. "In roughly 10 per cent of cases you have an obstruction," says Halit Sanli, a biomedical engineer at the Sydney Cochlear Implant Centre in Australia.
What they really need is a miniature endoscope that would allow them to see around the obstruction. Standard endoscopes are too big: they comprise bundles of thousands of optical fibres and are about 5 millimetres wide.
So a team of photonics experts led by Martijn van Eijkelenborg of the Australian Photonics Cooperative Research Centre at the University of Sydney turned to "holey fibres", a technology that could provide critical connectors in the optical computers of the future.
These are single fibres with microscopic holes running right through them, like the lettering in a stick of seaside rock. Light is channelled down each hole, only emerging when it reaches the end of the fibre. Because the light entering each hole remains separate from light in all the other holes, the pattern of light leaving the fibre at the other end creates a discernible image.
The plastic between the holes also guides the light in the same way as the holes themselves, effectively doubling the number of light channels.
Van Eijkelenborg and his colleagues made the endoscope by drilling 112 holes into an 8-centimetre-thick plastic rod, stretching it until the fibre was just 0.5 millimetres wide. This creates an image consisting of 224 pixels.
A beam splitter ensures light sent down the fibre to illuminate the interior of the cochlea does not interfere with the image coming the other way. The image is "a pretty crude one", says van Eijkelenborg, who now hopes to boost the low resolution by creating fibres with up to 1000 holes before beginning full-scale development of the instrument for medical uses.
Rachel Nowak, Melbourne
New Scientist issue: 24th January 2004
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