Every year, more than 250,000 people get a new lease on life when pacemakers begin tapping out a steady rhythm for their irregularly beating hearts. Doctors had long been searching for a way to help such patients when the implantable cardiac pacemaker was marketed in 1961; a simple mistake pointed the way.

In 1956, a University of Buffalo electrical engineer named Wilson Greatbatch was using some early silicon transistors to build a circuit to help the nearby Chronic Disease Research Institute record fast heart sounds. He accidentally installed the wrong resistor into the circuit, and it started to pulse in a recognizable "lub-dub" rhythm. Greatbatch was already aware of a problem called "heart block," in which the organ's natural electrical impulses don't travel properly through the tissue; he quickly realized that this circuit was exactly what was needed to steady these sick hearts.

At the time, clunky external cardiac pacemakers existed, but they plugged into wall outlets and had external electrodes that burned the skin. Greatbatch's circuit formed the basis for a painless, implantable device. But he found little enthusiasm for his invention until April 1958, when he met William Chardack, chief of surgery at the Buffalo Veterans Administration Hospital, who immediately saw the pacemaker's potential. Three weeks later, on May 7, Chardack and Greatbatch successfully implanted their first model in a dog. However, bodily fluids seeped past the electrical tape used to seal the gadget, shorting it out after only four hours.

Greatbatch recast the pacemakers in epoxy blocks, and within a year prototypes lasted four months. The team began looking for its first human patient-but Greatbatch's employer, Taber Instrument, did not want to take on the potential legal liability of the unproven device. So armed with $2,000, he set out on his own. He hand made 50 pacemakers in a barn workshop, and in April 1960, Chardack implanted the first of 10, seen above, into patients. That year, Minneapolis-based medical electronics firm Medtronic licensed Greatbatch's invention; it remains the top manufacturer of cardiac pacemakers.