Researchers have successfully tested a fully implantable glucose-monitoring device in pigs for nearly two years, according to new research published today in Science Translational Medicine. Scientists plan to file for approval from the U.S. Food and Drug Administration to begin human tests. Eventually, researchers aim to couple this kind of device with one that would automatically deliver insulin in response to changing blood-sugar levels.

The new device, about three centimeters in diameter and one centimeter thick, would be implanted into the chest in an outpatient procedure. It measures glucose levels in tissue and wirelessly transmits that information to an external receiver, such as a cell phone. Unlike existing continuous monitors, "there is nothing protruding from body," says David Gough, a bioengineer at the University of California, San Diego, who developed the technology. Gough cofounded a company called GlySens to commercialize this and other technology his lab has developed.

Insulin-dependent diabetics are supposed to measure their blood sugar several times a day with a finger stick. This involves the glucose in a small drop of blood reacting with chemicals on a disposable test strip. In the last few years, a growing number of people have started to use continuous glucose monitors, which measure glucose levels every few minutes via a sensor embedded into the skin. The sensor is attached via a wire to a small processing unit taped to the abdomen, which sends the information to a receiver in a pocket or worn on the belt.

The advantage of these devices is that they can show trends in blood-sugar levels, helping patients to better tailor their next dose of insulin. However, with existing models, the sensor needs to be replaced every three to seven days and frequently recalibrated with traditional finger sticks. Scientists have been working for more than a decade to develop a fully implanted device that can accurately monitor glucose over months or years without replacement.

Like finger sticks, the new implanted monitor measures glucose levels using an enzyme called glucose oxidase. When glucose levels are high, the enzyme carries out a reaction that consumes oxygen, which is detected via a neighboring oxygen sensor. Gough says one of the major challenges in developing a sensor to work over the long-term was stabilizing the enzyme, which tends to degrade over days. To fix this problem, researchers added a second enzyme designed to eliminate one of the toxic byproducts of the reaction.