A way to stuff a microchip full of detectors for a whole range of medical conditions has been developed by US chemists. They say their device is a step towards a universal handheld diagnosis system that can look for signs of hundreds of conditions in a single liquid sample and give an instant read out.

Christine Keating and colleagues at Pennsylvania State University created their chip by coating a series of nanowires with DNA sequences that match those from disease-causing bacteria or viruses - whether MRSA, hepatitis, or HIV.

If DNA from one of those pathogens is present in a sample, it will bind to the nanowire with the matching sequence, a process that changes the wire’s conductivity.

Detecting an individual pathogen becomes a matter of spotting when the conductivity of a particular nanowire changes - something that can be achieved by fixing each nanowire to a transistor on a conventional microchip.

Electrical attraction

Manoeuvring a wire just 700 nanometres wide and 8 micrometres long into position is no easy task, of course, but Keating and colleagues Theresa Mayer and Thomas Morrow have come up with a solution.

They start with a conventional microchip, designed with a line of pill-shaped depressions called “microwells” in the centre, each flanked by an electrode on either side. Those electrodes create an electric field gradient that can be used to coordinate the tiny wires’ movements with surprising ease, says Keating.

“Our DNA-coated rhodium nanowires ‘feel’ the electric field at a distance and are then drawn towards the region spanning our guide electrodes until they reach a microwell, which they then snap into due to the higher field strength there,” she says.

The process is very accurate, with nanowires slotting into the right place 99% of the time.

A video (.mov format) shows the manipulation method in action.

Safety in numbers

To make a chip able to detect, say, hepatitis C, the chemists introduce multiple nanowires coated with the necessary DNA and switch on the electrodes intended to become hepatitis C detectors. To add the ability to detect another disease, the process is repeated using further electrodes.

Having many nanowires for each disease is important for avoiding false negatives and false positives, says Keating.

In tests, the team created a chip that was able to detect the presence of DNA from hepatitis B, hepatitis C, and HIV, although it signalled a positive match by glowing, rather than using electrodes. A full prototype with the transistors necessary to read out the conductivity of individual nanowires is currently in progress.

The team’s way to manoeuvre tiny objects with electrodes could also provide a way to put together future computer chips with ever smaller components, Mayer says.

“This work sounds exciting,” says Geoff Thornton of the London Centre for Nanotechnology at University College London. “We don’t have a means of positioning a nanowire precisely where we want it right now.”

Journal reference: Science (DOI: 10.1126/science.1165921)

  *  14:53 16 January 2009 by Paul Marks
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