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Getting under the skin - easily

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The outer layer of our skin is normally impermeable, keeping body fluids in and everything else out. Now researchers have found that acoustic waves can make the skin temporarily permeable, so doctors can sample body fluids or inject medication without piercing the skin. They can generate these acoustic waves using ultrasound or a shockwave from a pulsed laser.

Clinical trials are already under way on an ultrasonic system for measuring glucose levels in diabetics, developed at the Massachusetts Institute of Technology in Cambridge. And just a short distance away, Massachusetts General Hospital in Boston is testing a drug delivery system that uses laser technology.Sound waves temporarily change the structure of the impermeable outer layer of skin, which consists of dead skin cells encased in layers of fat molecules and is between 10 micrometers and 15 micrometres thick. A few minutes of low-frequency ultrasound - sound greater in frequency than 20 kilohertz at the threshold of human hearing - creates tiny cavities.

These holes allow large molecules to pass through the fat layer for several hours, says Joseph Kost, a professor of chemical engineering from Ben Gurion University of the Negev in Beersheeva, Israel, who is on sabbatical at MIT. At a meeting of the Acoustical Society of America in Seattle next week, he will reveal how the technique can deliver drugs and perform tests.

With MIT's Bob Langer and Samir Mitragotri, Kost is focusing initially on glucose testing.

Physicians recommend that diabetics test blood glucose at least three times a day, but few jab their fingers that often to draw blood. The noninvasive acoustic technique makes it much easier - all patients have to do is place a glucose-monitoring patch over the permeable spot.

Initial tests used a desktop device, but Kost and Langer have formed a company to develop a hand-held, battery-powered ultrasound generator for use at home. They plan clinical tests of a similar system to deliver insulin or other drugs.

Meanwhile, Apostolos Doukas at Massachusetts General is using laser pulses to generate the acoustic waves that make the skin permeable. He fires a short laser pulse at a plastic disc laid on the skin, producing a stress wave that lasts 0.2 microseconds to create the temporary pores. Drugs applied between the disc and the skin can then diffuse through the pores while the barrier remains open.

"For large molecules the barrier recovers in a couple of minutes, for small molecules it can be 15 minutes to half an hour," Doukas says. As with the ultrasonic technique, patients in the trials said they felt no pain.

Small pulsed lasers can also penetrate the skin, but they drill holes which, like needle pricks, can cause pain and take time to heal.