Birth control pills can work well if women remember to take them every day but missing doses can mean a surprise pregnancy. Now scientists have figured out how to pack a month's supply of contraception into one capsule.

The trick involves a tiny star-shaped gadget that unfolds in the stomach to gradually release the drugs.

The experimental capsule is still years away from drugstores, but researchers reported Wednesday that it had worked as intended in a test on animals. The Bill and Melinda Gates Foundation is investing 13 million U.S. dollars in the further development of the once-a-month pill in the hope of eventually improving family planning options in developing countries.

"It has a lot of potential," said Dr. Beatrice Chen, a family planning specialist at the University of Pittsburgh, who was not involved in the new research. "Birth control is not one-size-fits-all," and women need more options.

Today, women who want the convenience of long-lasting contraception can choose from various methods, from a weekly patch to a monthly vaginal ring or an IUD (intrauterine device) that lasts for years.

Initially, it was not clear that "the Pill" – one of the most popular forms of birth control because it is cheap and easy to use – ever could join this list. Pills of all sorts generally pass through the body in a day.

A team from the lab of Massachusetts Institute of Technology inventor Robert Langer engineered a fix to protect pills from the harsh environment of the digestive system.

"We developed this capsule system that looks like a starfish that can stay in the stomach several days, weeks, even a month at a time," said Dr. Giovanni Traverso of Boston's Brigham and Women's Hospital, a senior author of the study.

The star-shaped device has six arms, with each holding a certain dose of medication. The device is folded inside an ordinary-sized capsule. Swallow the capsule and stomach acid dissolves the coating, letting the star unfold. The structure is too big to fit through the stomach's exit but not big enough to cause an obstruction. As medication dissolves out of each of the arms, the device breaks down until it can safely pass through the digestive system.

Langer and Traverso's team first used the technology to try turning daily drugs for malaria and HIV into capsules that lasted a week or two. They also are experimental, but longer-lasting pills one day could help patients with serious diseases better stick with treatment.

A logical next attempt: A month-long oral contraceptive

First, scientists had to tweak the star-shaped device to make it stronger. They turned to long-lasting contraceptive implants for the materials to hold the hormone ingredient and let it gradually seep out.

Then they tested the contraceptive capsules in pigs, which have human-like digestive systems. The experimental capsules released the contraceptive fairly consistently for up to four weeks, and the amount in the pigs' bloodstream was similar to what daily tablets deliver, MIT lead authors Ameya Kirtane and Tiffany Hua reported in the journal Science Translational Medicine.

Lyndra Therapeutics Inc., a Massachusetts company co-founded by Langer and Traverso, is further developing the monthly pill and multiple other uses for the technology.

To be most useful, the capsule should be designed to emit three weeks of contraception and then allow for a woman's period, like a month's supply of birth control pills does, Traverso said. That would alert women when it was time to take another monthly dose.

Pittsburgh's Chen cautioned that more safety testing was required, including how well the experimental capsule breaks down and ascertaining the proper dosage for different hormones, before the technology is tried on women. Other issues include whether the device dissolves in the same way in different people.

But should the testing succeed, Chen said it would be exciting to try combining both contraception and HIV drugs into the same capsule, particularly for developing countries where women are at high risk of the virus that causes AIDS.