Langer worked with Judah Folkman at Boston Children's Hospital to isolate the first angiogenesis inhibitor, a macromolecule to block the spread of blood vessels in tumors. Macromolecules tend to be broken down by digestion and blocked by body tissues if they are injected or inhaled, so finding a delivery system for them is difficult. Langer's idea was to encapsulate the angiogenesis inhibitor in a noninflammatory synthetic polymer system that could be implanted in the tumor and control the release of the inhibitor. He eventually invented polymer systems that would work. This discovery is considered to lay the foundation for much of today's drug delivery technology.