"Life-like in motion, durable in construction, the nearest approach to nature in action. Satisfaction guaranteed." With these words one James I. Lyons of Chicago marketed his aluminum pneumatic feet - the latest thing in prosthetic design in 1894.

Today, prostheses are still being marketed in these terms, but the reality of carbon fiber materials, microchip computer technology and modern surgical techniques have made 1990s prosthetic innovations like the "dynamic ankle" and "energy storing feet" a rather closer approximation of the natural world - enough, for instance, to allow users to run the 100 meters in under 12 seconds or climb the world's highest mountains.This is just one relatively minor area where medical science has revolutionized the possibilities for those requiring new limbs, organs or body tissue. The combination of advances in computer technology, material sciences, laser surgery, and the understanding of chemicals, genes, viruses and bacteria have brought us within reach of what were once seen as the wilder realms of science fiction.

Culture has traditionally been suspicious of the artificially created being, of man usurping the powers of God to create life. From Mary Shelley's 1818 novel "Frankenstein" (about a man constructed from body parts) to Paul Verhoeven's "Robocop" (a mechanical being with a human brain), the artificial man has been either a figure generating fear and loathing or a soulless destroyer that can apparently be harnessed for good or evil.

But now, for the first time, we are coming close to turning the fictions into reality and manufacturing a human; and the challenge for the medical and scientific world is to wean a skeptical public away from its fears and fantasies.

Underlying the public's paranoia about science's ability to create a mechanical man - or, worse, an organic man - is the question of where that leaves humanity, the belief in the uniqueness of every being. As with cloning, the creation of tissue and the sophistication of mechanical limbs pose dilemmas about life and mortality, about what makes us what we are.

As the bionic man of TV drama comes closer to reality, so the ethical arguments increase. Every new step into the unknown will be argued over: does it represent a dream of progress, or a nightmare of dehumanization?

Take, for instance, the claims of the eminent brain surgeon, Dr. Robert J. White. Earlier this year he spoke of the possibility of conducting what he called "a total body transplant" - removing the head of one patient and attaching it to the body of another.

A decade ago this kind of talk from a respected physician would have been dismissed as outlandish or downright silly, but no longer: he has already achieved it with monkeys - within limits. White says he has perfected techniques of minimizing blood loss and reconnecting the major arteries, allowing one of the animals to survive for two weeks with the ability to see, hear, smell, taste, breathe and eat.

What he could not manage - and probably will not for another 20 years - is to reconnect the spinal cord, allowing the monkey to feel sensation in its new body.

Less dramatic but still astonishing was the announcement earlier this month that a team of surgeons at the University of Louisville were preparing for a 16-hour hand transplant operation through which they would connect all the tendons, muscles, veins and arteries from one wrist to another.

Over the past decade, the practice of transplanting organs from one human to another, and even multiple organ swaps (heart, lung, liver; bowel, kidney, liver, pancreas for example) has become routine. It is also possible to isolate the required sections of certain organs (the hormone-producing islets of the pancreas, for example), rather than removing and replacing the entire organ.

Across the board, survival rates have risen dramatically. In 1980, for example, 40 percent of kidney transplant patients died within a year; in 1998 this is down to 10 percent, while three-quarters survive more than five years, mainly as a result of advances in the development of immuno drugs.

Some of the most significant developments in the realm of immunology have been taking place through advances in xenograft technology (swapping a body part from one species to another). In one development, British researchers have modified the gene structure of pigs to prevent the rejection of their organs by humans.

Rejection rates are particularly high with tissue transplants, and there are major difficulties in acquiring suitable donors. However, another development along science fiction lines may provide a solution. Self-transplantation (cultivating cells from the patient's own body and then transplanting them back into the affected area) has become a realistic possibility with bone and tendon (clinical trials are under way) and is not far off with cartilage.

Another alternative for organ transplants is a mechanical solution: replacing the heart with a machine, for example.

"It may be possible one day, but the problem will be the power source," says Dr. Robert Johnson, a consultant surgeon in London. "But what we have seen since the early 1990s have been significant developments with heart pacemakers as a result of sophisticated microchip technology."

This kind of technology has also been at the nub of developments in medical science along the lines of "RoboCop." Recently, for example, Dr Robert Gow, director of the rehabilitation engineering service at Edinburgh University, produced the results of 35 years of research - a bionic arm, attached directly to the shoulder, which can be controlled by thought.

Some $16,000 worth of microchips, position control circuits, motors, gears, pulleys and a powerful battery allow for separate movement of individual, artificial finger, wrist, elbow and shoulder joints merely through thinking about which part to move. Microsensors pick up electrical pulses still sent by the brain to absent arm muscles.

Similarly, in 1995 a British company, Blatchfords, produced the first "Intelligent Knee."

"Previous prosthetic knees were free-swinging or used a hydraulic system which meant amputees had to put a lot of effort into making the leg move faster because it couldn't detect speed changes, but this is controlled by a microprocessor and is therefore able to detect when you want to speed up and to respond accordingly," says Ben Blease, a research prosthetist with the company.

Richard Hirons, of the British Association of Prostheses and Ortheses, says there have been four major advances within prosthetics during the 1990s: "The most important has been microprocessor control, through which electrodes can carry instructions from the brain to the artificial limb. This is aided by the use of lighter and stronger materials like carbon fiber and significant developments in the technology involved in fitting a prosthesis to the residual limb and also in material technology used to design life-like skin to cover the prostheses."

Computers have also opened the possibility for the deaf to hear and the blind to see. Cochlear implants (electronic devices for people with profound hearing loss) over the past five years have been married with advanced microchips, enabling the efficient encoding of speech, which is then presented to the inner ear to simulate hearing.

The far reaches of the virtual world of science fiction would seem to be represented by ELVIS - shorthand for the Low Vision Enhancement System. This is a video display worn on the head like a crash helmet which covers the eyes. It was developed in a partnership between NASA and the Low Vision Clinic at Johns Hopkins University in Baltimore and involves three cameras, providing a view of the surrounding environment and a zoom system for those with minimal vision.

But scientists are currently preparing to go way beyond ELVIS and are experimenting with attaching electrodes to the nerve centers within the visual cortex. Using microchip technology to simulate sight, this will potentially allow someone who is blind to record images comparable with normal vision.

This is certainly bringing us close to the computer vision of the "Terminator" robots, or as Victorian prosthesis designer James I Lyons would have it, "the nearest approach to nature in action."