For many decades, neurologists have hit a wall that seemed impenetrable. It is called the blood-brain barrier and it is a complex network of blood vessels and immune cells that protects the most important organ in the body: the brain. This wall only opens to let in oxygen and a few other nutrients, but it is impregnable to most pathogens, something essential for survival. But this also has a bad side: its effect means that practically all medications designed to cure brain diseases, including cancer, fail to reach their destination.