Artificial eggs made from seaweed could provide women with an alternative to the Pill.
US government scientists have created tiny, sticky beads that trap sperm to prevent pregnancy.
The work is at an early stage but it offers hope of a contraceptive that is free of the mood swings, weight gain and other side-effects of hormone-based drugs used by millions of women.
The ZP2 peptide beads, as they are officially called, could also be used to choose the best sperm for use in IVF.
The beads are made from a seaweed extract, and are so small that eight million would fit on the nail of a little finger.
They are coated in a substance found on the outside of woman's eggs that sperm latch on to.
Inserted into the womb in their millions, they acts as decoys, trapping the sperm before they reach the eggs.
In experiments in mice, animals with bead implants failed to get pregnant, despite being continuously mated. Further work showed the beads created a "dramatic" barrier to human sperm, the journal Science Translational Medicine reports.
Importantly, the contraceptive effect was long-lasting, so is likely to be attractive to women who don't want to have to remember to take a pill every day.
Professor Allan Pacey, a Sheffield University fertility expert, said the technology could help IVF doctors find the best sperm by looking at which ones bind particularly well with the beads - and therefore a woman's egg.