The retina is hard to penetrate with drugs because it has a protective barrier.Credit: Paul Whitten/Science Photo Library Scientists have used pig semen to develop eye drops that can stop tumour growth in the retina and preserve vision, a study1 in mice shows. The work is published today in Science Advances. It is hoped that the drops could be developed to treat children with retinoblastoma, a cancer of the retina. The condition is typically treated with injections of drugs into the eye, chemotherapy or laser therapy, all of which can damage non-cancerous parts of the eye. Yu Zhang and colleagues at Shenyang Pharmaceutical University in China wanted to develop a method that could penetrate the barrier around the retina and deliver the drugs without causing any unnecessary damage. The eye drops contain exosomes — tiny particles released by almost every type of cell — engineered to carry molecules that kill cancer cells. Mice treated with the eye drops showed signs that the tumours had stopped growing.…