Five years ago, Jeffrey Karp sat down to a dinner party with Massachusetts General Hospital dermatologist R. Rox Anderson. The two started talking, and by the end of the evening, Karp—himself a bioengineer at the nearby Brigham and Women’s Hospital—knew he wanted to make a collaboration happen between them. But on what? As he thought, he twisted his ring—his nickel allergy had flared up, and the skin on his finger was chafed and raw. Twist, and think; twist, think. And then it hit him: up to 45 million people in the United States shared his allergy, and the best measures they had to control it were small-molecule chelating creams with a dangerous tendency to leach into the skin.