Keeping track of vaccinations remains a major challenge in the developing world. Now a group of Massachusetts Institute of Technology researchers has developed a novel way to address this problem: embedding the record directly into the skin. Along with the vaccine, a child would be injected with a bit of dye that is invisible to the naked eye but easily seen with a special cell-phone filter, combined with an app that shines near-infrared light onto the skin. The dye would be expected to last up to five years, according to tests on pig and rat skin and human skin in a dish. The system - which has not yet been tested in children - would provide quick and easy access to vaccination history ... according to the study. Delivering the dye required the researchers to find something that was safe and would last long enough to be useful. The team ended up using a technology called quantum dots, tiny semiconducting crystals that reflect light and were originally developed to label cells during research. The work was funded by the Bill & Melinda Gates Foundation and came about because of a direct request from Microsoft founder and philanthropist Bill Gates himself. The researchers hope to add more detailed information to the dots, such as the date of vaccination. Along with them, the team eventually wants to inject sensors that could also potentially be used to track aspects of health such as insulin levels in diabetics.