Chemotherapy is brutal. The drugs are delivered in near-toxic doses to penetrate tumor ­tissues, indiscriminately vanquishing the patient’s appetite, hair, and immune system alongside cancer cells. Scientists have tried everything from nanoparticles to homing-beacon bacteria to better target the disease. Now researchers at Germany’s Institute for Integrative Nanosciences are deploying a natural-­born infiltrator: sperm.

The team had been working for years on a robotic power-boost for slow swimmers—originally intended to help couples with low sperm motility conceive. But when they discovered that the sperm’s double-­lipid-lined head could also be coaxed into carrying a payload of chemo drugs, they shifted their focus from inducing life to slaying reproductive cancer.

The little guys won’t be going into battle naked. Using a two-photon laser, the scientists created a set of sperm harnesses—tiny tubes with flexible arms that cling to the head. By coating the harnesses in iron, they were able to steer the machines using an external magnetic field. As the ironclad Trojan horses collide with tumors, the force of impact causes them to shed their harnesses, burrow into the tissue, and drop their killer payload. When the researchers sicced the spermbots on a cancerous mass, they killed more than 90 percent of the rogue cells.

So far, the technique has been tested using bull sperm in microfluidic plastic chambers, not yet in animals or humans. Still, the technology reveals a promising new army of drug carriers—like guided missiles for cancer.