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muscle, setting a proper rate of beating for people whose hearts beat too slowly. Pacemakers are the most widely used functional bionic implants; some two million have been placed into patients around the world since 1960,including one implanted in U.S. vice president Dick Cheney in 2001. The earliest recorded portable models were devel- oped and demonstrated around 1930, but serious development began only in the 1950s.The history of these devices exemplifies many of the general issues associated with bionic implants, along with their benefits.

To provide the stimulating electrical current, some early pace- maker models used electrodes that lay on the skin but did not enter the body, but these were unsuited for long-term use because they burned the skin after a few days. Other versions developed in the 1950s used implanted wires; that is, the electronic part of the pace- maker was mounted outside the body, from where it sent pulses to a small wire that entered the body and made its way to the heart. But this arrangement, too, had serious drawbacks. It was easy for infection to develop at the entry points of the wires. The external electronic unit that generated the proper pulses was too bulky for easy portabil- ity and required so much power that it had to draw on conventional house current.This meant that the implanteesmobility was limited by the length of power cords, and the implantees were utterly at the mercy of power failures.

These deficiencies have been remedied through parallel advances in electronics and implantation procedures.The problem of infection could be avoided by implanting the entire unit in the body, but that wasnt possible until the introduction of transistors, which made the units much smaller.As a bonus, the transistorized units also drew less power than the earlier models, so that battery operation became prac- tical. The result was the first wearable battery-powered pacemaker, developed in 1957, and then the first fully implantable unit.The first successful implantation of a pacemaker, in which the unit operated in the implantee for nine months, was carried out in 1960.

Today, further advances in electronics, computation, and implant surgery support highly sophisticated pacemakers.The devices became

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