Condoms are known to be highly effective at preventing unwanted pregnancies and sexually transmitted diseases. And although they've been around for more than 400 years, over the last half-century their design has remained pretty much the same — until now.

The Bill and Melinda Gates Foundation, which works to improve global health, has offered million-dollar challenge grants to scientists to design a better condom for the next generation.

Among the winners are two Boston doctors from the Boston University School of Medicine: radiology professor Ducksoo Kim and third-year radiology student Karen Buch. They're working on a nano-particle coating for condoms, and their hope is to solve the biggest challenge of condoms: despite their effectiveness, many people don't use them.

Karen Buch, radiology resident at BU School of Medicine working on a nano-particle coating for condoms