Smaller than a grain of rice, this new pacemaker could transform cardiac medicine. (John A. Rogers) Scientists said Wednesday they have developed the world's tiniest pacemaker, a temporary heartbeat regulator smaller than a grain of rice that can be injected and controlled by light before dissolving. While still years away from being tested in humans, the wireless pacemaker was hailed as a "transformative breakthrough" that could spur advances in other areas of medicine. Millions of people acros...