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A TINY pump implanted under the skin could one day help people with a type of hormonal deficiency boost their chances of having children, without the need for a bulky pump strapped to their body or courses of drugs lasting many years.

The disorder occurs when production of a substance called gonadotrophin releasing hormone (GnRH) goes awry. In men the hormone should normally be released around once every two hours from a part of the brain called the hypothalamus. In women, its release is more complex, changing frequency during the menstrual cycle from once an hour to once every four hours. If these patterns are disrupted, girls can grow up without a functioning uterus or ovaries and men can have extremely low sperm counts.

To treat the condition, patients are often given the hormone via repeated intravenous injections or by a pump that is kept strapped to their body. But now Gauri Misra and Ronald Siegel at the University of Minnesota are developing a tiny implantable pump that automatically switches the hormone flow on and off.

The pump is a small container topped with a thin polymer sheet, whose molecules are peppered with negatively charged chemical units called carboxylate groups. The container is filled with a solution containing GnRH hormone, plus hundreds of tiny polymer beads impregnated with the enzyme glucose oxidase.

When the container is immersed in a watery environment, as it would be under the skin, water molecules are attracted to the carboxylate groups and push the polymer molecules apart, creating pores that make the sheet permeable. This allows some of the hormone inside to escape. But as well as allowing hormone to flow into the bloodstream, the pores allow glucose from the surrounding body fluid to flow in and be broken down by the enzyme to produce positive hydrogen ions. These ions bind to the carboxylate groups in the polymer film, and neutralise them. This liberates the water that had been pushing the molecules apart, sealing the barrier and halting the flow of hormone.

But the barrier doesn鈥檛 stay closed for long. The hydrogen ions in the polymer film gradually diffuse into the surrounding fluid, exposing the carboxylate groups. The cycle then restarts as water is again attracted to the carboxylate groups.

So far the system has only been tested in the laboratory, using simulated body fluids. But Siegel has found that the pump can release hormones regularly every 4 hours for a week. By miniaturising the device and using a modified polymer, Siegel hopes to get the release frequency nearer to that in the body. An implantable pump could have a drug reservoir that could be topped up by an injection when needed, he says.

GnRH deficiency affects some people from birth, but it can also be caused by damage to the hypothalamus during procedures such as radiotherapy. Currently women with the hormonal condition require treatment over a period of months before they can consider conceiving a child. Men are often treated for longer: some strap portable pumps to their body to inject the hormone to induce puberty and then keep them fertile.