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, Massachusetts. 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 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
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,” says Doukas. 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.