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Embryos have been formed from sperm that have been made magnetic to allow remote guidance towards an egg. The technique may allow IVF to be performed inside the body so it involves fewer invasive procedures and potentially has better success rates.

“Our ultimate idea is to do assisted reproduction in vivo, taking advantage of the body as a natural incubator,” says at nanoscience institute CIC nanoGUNE in Spain.

Conceiving naturally is harder when sperm numbers or motility are low, because too few sperm make it into the fallopian tubes to fertilise an egg. Assistance can be provided by IVF, in which sperm are placed with an egg in a dish, or directly injected into an egg, to form embryos. However, IVF involves hormone injections, egg retrievals and embryo transfers, which are invasive and can have unpleasant side effects. Many IVF cycles are also unsuccessful, partly because the artificial environment in which the sperm and egg meet and the multiple manipulation steps can affect embryo viability.

To address these issues, Medina-Sánchez and her team wanted to develop a way to transport sperm through the female reproductive tract into the fallopian tubes, where they could fertilise an egg under natural conditions. They decided to try making sperm magnetic so they could be directed through the body using weak external magnetic fields. “We consulted with IVF specialists because we wanted to develop a sperm-preparation protocol that could be integrated into the usual IVF workflow,” says Medina-Sánchez.

To trial this approach, they incubated cattle sperm with tiny magnetic beads made from iron oxide and polystyrene. About 30 beads stuck to the head of each sperm, making them magnetic, while still allowing their tails to move freely. Tests showed that the magnetic beads did not affect the sperm’s swimming speed or overall health.

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Next, the researchers incubated the magnetic cattle sperm with eggs in a dish, and found that they formed healthy embryos at the same rate as non-magnetic sperm. The magnetic beads simply fell off the sperm as they penetrated the egg, and did seem not to affect early embryo development.

Finally, the team showed that the magnetic sperm could be moved in different directions and guided towards an egg in a dish using an external magnetic field.

“It’s a really exciting advance and a great proof of concept,” says at Adelaide University in Australia. “However, there are still a few hurdles to jump before it could be considered clinically.” These include showing that the magnetic sperm can indeed be directed into the fallopian tubes and fertilise eggs outside of a lab dish, and that the resulting embryos successfully implant in the endometrium and produce live, healthy offspring, she says.

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The magnetic beads show up on ultrasound, which should make clusters of the sperm easy to visualise inside the body, says Medina-Sánchez. Encouragingly, her team has already shown that whole embryos embedded in tiny magnetic structures can be guided into the fallopian tubes of mice using external magnets.

Once the magnetic beads fall off the sperm, they should be naturally cleared by the body’s waste-removal systems, but otherwise, they could be sucked out using magnets coupled to catheters, says Medina-Sánchez. “We think it’s very promising,” she says.