The inner layer of the membrane is small enough to prevent the cells from leaking out, but the outer layer is large enough to encourage blood vessels to grow along the membrane. The implanted cells need access to blood in order to sense and respond to changes in blood sugar, as well as to deliver the oxygen the cells need to survive.

While encapsulation protects the cells, it also introduces its own problems. "One of the fundamental challenges has been to identify materials that don't cause fibrosis, or scar tissue around material," says Dan Anderson, a chemical engineer at MIT. "That's particularly important here because [fibrosis] can starve cells of oxygen and inhibit their ability to respond to glucose." The company is currently using a prototype membrane from a company called Theracyte, but it is also working on its own customized version.

In the new research, scientists showed that animals whose own insulin producing cells were chemically destroyed could survive with the implant. "They have been completely controlled by the human graft for four months," says D'Amour. In fact, while mice typically have a higher resting blood glucose level than do humans, the animals with human insulin-producing cells had glucose levels that more closely resemble those of humans.

ViaCyte still has a number of issues to solve before its device can be tested in patients. It's not clear how the human immune system will react to the implants, an issue that ViaCyte is studying in collaboration with scientists at the University of California, San Francisco. For example, while the membrane is designed to protect the cells, patients may still require immunosuppressive drugs. Or the cells within the device may need to be tissue-matched to the recipient, much like whole organ transplants.

Living Cell Technologies, headquartered in Australia and New Zealand, has ongoing human tests of encapsulated pancreatic cells derived from pigs in Russia and New Zealand. While the results of the studies have not yet been published, reports from the company based on a small number of patients say the treatment so far appears safe and patients do not require immunosuppressant drugs.