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WASHINGTON - Tiny capsules made from seaweed and iron may help diabetics whose bodies reject insulin-cell transplants.

Researchers trying to understand why those transplants work for some people with Type I diabetes, but not for others, found success in experiments with mice and pigs.

Type I diabetes is the type usually beginning in childhood. It occurs when a person's immune system destroys the cells in the pancreas that make insulin.

The American Diabetic Association estimated that 20.8 million Americans have diabetes, though not all cases have been diagnosed. Between 5 percent and 10 percent of those are Type I, according to the National Diabetes Education Program.

Insulin helps the body process sugar. Without it, sugar levels in the blood rise and can result in complications such as blindness or kidney failure.

Injections of insulin can help if the patient carefully monitors blood sugar levels. Transplanting new insulin cells might be more effective, unless they, too, are destroyed.

Insulin-cell transplants remain an experimental procedure. The Collaborative Islet Transplant Registry reports just 319 cases in North America between 1999 and 2005.

In an effort to learn what happens to transplanted cells, researchers from Johns Hopkins University encapsulated them in a matrix made from alginate - derived from seaweed - and an iron-containing material so they could track the cells magnetically.

"It's very exciting, because now you will be able to see what's going on with all these cells. We hope it will help us understand the disease process and what's been going on," Dr. Aravind Arepally, an assistant professor of radiology and surgery at Hopkins, said in a telephone interview.

Their findings were published online Sunday in the journal Nature Medicine.

The porous capsules had openings large enough to let insulin out for the body to use, but not big enough for immune cells to get in and attack the transplants.

In the first experiment, the capsules - less than one hundred twenty-eighths of an inch across - were implanted in diabetic mice. The researchers said the blood sugar levels of the mice returned to normal in about a week. More than half of the mice that did not receive transplants died.

Researchers then moved on to swine. Capsules were implanted in the liver rather than the pancreas because the liver has more blood vessels that can carry the insulin to the rest of the body.

The team threaded a long needle-like tube into a large vein near the upper thigh and guided the tube upward, across and into a neighboring blood vessel and then into the liver.

Three weeks later, the capsules were still in place and were releasing insulin at usable levels, the researchers reported.

Co-author Jeff Bulte, professor of radiology and chemical and biomolecular engineering, said the hope is that the capsules will reduce the need for anti-rejection drugs in people receiving transplants.

Arepally said the researchers are beginning a longer-term trial in pigs and are working with a private company to begin the process of seeking Food and Drug Administration approval.

Dr. Larry C. Deeb, president of the American Diabetes Association, said it is fascinating that researchers could track the implants.

"That doesn't mean you can make it work to cure diabetes," he said. "These are the kinds of things where you do research and find something interesting and see where it leads you."

"I tell my patients that we're beating down the doors, slowly but surely," in the search for a cure, said Deeb, a pediatric endocrinologist in Tallahassee, Fla.

The research was funded by the National Institutes of Health and the Howard Hughes Medical Institute.