Body Watch : The Point of This Research Is to Rely Less on Hypodermics : Medicine: Drug delivery via ultrasound is now being tested on hairless rats. Human trials could begin soon.

Apainless ultrasound drug-delivery system may soon replace the dreaded hypodermic needle in many instances.

That could be a real shot in the arm for millions of people--such as diabetics who require frequent injections of insulin--for whom syringes are an unending torment that also carry the risk of infection.

For years, scientists have sought non-invasive ways of getting crucial drugs to diffuse across the skin and into the bloodstream. Among other advantages, such a trans-dermal delivery system would allow the chemicals to seep gradually into the body, rather than arrive in a single big dose. But many medically important molecules--including insulin and interferon--are simply too big to breach the impermeable barrier of human skin, even when helped along by conventional ultrasound, researchers reported in Science.

The outermost section of skin--called the stratum corneum--though only a few thousandths of a millimeter thick, is made up of tough layers of flat, dead cells surrounded by membranes made of fats. The arrangement is virtually impenetrable. Yet researchers have long known that some smaller molecules such as cortisone can wiggle through if the skin is bombarded with ultrasound.

Robert Langer, a Massachusetts Institute of Technology chemical engineer, and his colleagues set out to see why that happens. They found that the acoustic waves produce an effect in the outer skin called cavitation--the formation of tiny vapor-filled bubbles that occurs, for example, when a propeller moves through water. Those bubbles break down the orderly structure of that skin just enough to let certain drugs through. But ultrasound at conventional therapeutic frequencies of 1 million cycles per second only opens up the tissue enough for relatively small molecules.

Since cavitation is greater as the frequency gets lower, the MIT crew tested a 20,000-cycle-per-second ultrasound generator on human cadavers and hairless rats. In both cases, the skin was disordered enough to allow insulin and interferon to pass through. Trials on human volunteers could begin soon.