Meanwhile other researchers had devised a pacemaker in 1952 that was about the size of a large radio; the patient had to be hooked up to an external power source. A few years later electrical engineer Earl Bakken devised a battery-powered handheld pacemaker that allowed patients in hospitals to move around. In 1958 Rune Elmqvist and Åke Senning devised the first pacemaker to be implanted in a human patient. Greatbatch's major contribution in the late 1950s was to incorporate recently available silicon transistors into an implantable pacemaker, the first of which was successfully tested in animals in 1958. By 1960 Greatbatch's pacemaker was working successfully in human hearts. He went on to improve the battery power source, ultimately devising a lithium battery that could last 10 years or more. Such pacemakers are now regulating the heartbeats of more than three million people worldwide.

Both the pump and the pacemaker are examples of a key application of engineering to medicine: bionic engineering, or the replacement of a natural function or body organ with an electronic or mechanical substitute. One of the foremost champions in this field was Dutch physician Willem Kolff, inventor of the kidney dialysis machine. Though severely hampered by the Nazi occupation of his country during World War II, Kolff was able to build a machine that substituted for the kidneys' role in cleansing the blood of waste products. Like Gibbon's heart-lung device, it consisted of a pump, tubing, and a rotating drum, which in this case pushed blood through a filtering layer of cellophane. Ironically, the first patient to benefit from his dialysis machine was a Nazi collaborator.

After the war Kolff moved to the United States, where he continued to work on bionic engineering problems. At the Cleveland Clinic he encouraged Tetsuzo Akutsu to design a prototype artificial heart. Together they created the first concept for a practical artificial heart. To others it seemed like an impossible challenge, but to Kolff the issue was simple: "If man can grow a heart, he can build one," he once declared. These first efforts, beginning in the late 1950s, did little more than eliminate fruitless lines of research. Later, as a professor of surgery and bioengineering at the University of Utah, Kolff formed a team that included physician-inventor Robert Jarvik and surgeon William DeVries. After 15 difficult years of invention and experimentation, DeVries implanted one of Jarvik's hearts—a silicone and rubber unit powered by compressed air from an external pump—in Barney Clark, who survived for 112 days. Negative press about Clark's condition during his final days slowed further progress for a while, but today more sophisticated versions of artificial hearts and ventricular-assist devices, including self-contained units that allow greater patient mobility, routinely serve as temporary substitutes while patients await heart transplants.

Kolff was not done. With his colleagues he helped improve the prosthetic arm—another major life-improving triumph of "spare parts" medicine—as well as contributing to the development of both an artificial eye and an artificial ear. Progress in all these efforts has depended on advancements in a number of engineering fields, including computers, electronics, and high performance materials. Computers and microelectronic components, for example, have made it possible for bioengineers to design and build prosthetic limbs that better replicate the mechanical actions of natural arms and legs. And first-generation biomaterials—polymers, metals, and acrylic fibers among others—have been used for almost everything from artificial heart valves and eye lenses to replacement hip, knee, elbow, and shoulder joints.