Hacking the body's delivery service

Optimized to travel in the body without attracting undue attention from the immune system, each tiny package is "an ideal drug carrier", says Juliane Nguyen, a bioengineer at the University of North Carolina at Chapel Hill. Among the most popular carriers are liposomes - spheres of lipid molecules, usually 100-200 nanometres in diameter, that can fuse with the cell membrane to deliver their cargo. According to Steven Stice, a stem-cell biologist at the University of Georgia, Athens, and co-founder of nearby biotechnology company Aruna Bio, exosomes from a human neuronal stem-cell line called AB126 cross the blood-brain barrier and home in on sites of injury. Cheng's team has found a human exosomal molecule, called miRNA-21-5p, that reduces the rate of heart-muscle cell death and improves blood-vessel growth and tissue repair after heart attacks in mice.