Three and a half years ago, a freak fall from a third-floor balcony paralyzed Patrick Maher, then a 21-year-old University of Illinois at Champaign senior, from the waist down. Today, with a book-sized computer and some electrodes strapped to his body, he can stand on his own two legs and walk across the room.

Kristen Cloud, a 22-year-old Californian who had been gradually going deaf since birth and lost all hearing four years ago, stood before the assembled media at a Food and Drug Administration press conference in November and gave an emotional account of how a device surgically implanted in her ear had returned to her a sense of hearing.

A decade after a gunshot wound blinded a 33-year-old Ogden, Utah, man, a bit of fancy high technology made him see the light in a Canadian hospital operating room.

The lame walk, the deaf hear, and the blind see. High technology has begun to duplicate the miracles of the Bible.

But it is just a beginning. Of all the artificial organs being developed, only the artificial ear is now widely available. Even if the technological problems are solved so the other artificial organs and limbs can become commonplace, they will never be as good as the body parts they replace.

Still, the advances seem spectacular, and while most people consider "spare parts" medicine something for the future, it is here today.

Last year in the United States alone, more than 100,000 artificial hips were implanted in the legs of aging arthritics, about 40,000 artificial heart valves kept blood flowing through diseased hearts, nearly 650,000 intraocular lens implants cleared the vision of cataract-clouded eyes, and 89,000 Americans had their blood purified by artificial kidneys.

Physicians implant more than 2 million medical devices worldwide each year at a cost exceeding $2 billion, estimates Biomedical Business International of Tustin, Calif., a health-care consulting company.

"The use of artificial body parts has a very long and accepted history," says Dr. Warren Reich, professor of bioethics at Georgetown University's School of Medicine and Kennedy Institute of Ethics. "The wooden leg is a marvelous prototype. It is an artificially constructed part from a pretechnological era. It shows that we quite eagerly accept artificial parts that support our life and bodily functions."

But in the last few decades, spare-parts technology has become much more complex. The era of modern artificial organs began in wartime Holland in 1944, when an artificial kidney -- created by Dr. Willem J. Kolff, now director of the University of Utah artificial organ program, the largest in the United States -- cleansed the blood of a man with kidney failure.

Since then antique devices like wooden teeth and hearing horns have given way to sophisticated artificial alternatives for blood cells, livers, kidneys, pancreases, lungs, skin, spinal cords, arms, legs and even sensory organs like the eyes and the ears.

The Jarvik 7 artificial heart has been the most dramatic and the most famous of the artificial organs. William Schroeder, the 52-year-old Jasper, Ind., grandfather, became the second man to receive a permanent artificial heart implant on Nov. 25. He is doing so well two months after the implant that he and his family have begun to take responsibility for his personal care needs.

"The heart is running without a single flaw," says Humana Inc. spokesman Robert Irvine. As Schroeder gets stronger and his family learns to care for him, he will be moved out of the hospital to a specially equipped home a block from Humana Hospital Audubon, where the transplant was performed. No date has been set for Schroeder to leave the hospital, nor has a candidate been selected for the next artificial heart operation.

The development and use of artificial organs, like the Jarvik 7, are driven by four main motives, say several observers of this field: the suffering caused by various diseases, the serious shortage of natural organs needed for transplants, the fame created by building an artificial organ and the vast potential for profit.

Physical pain wasn't the only problem for Illinois student Patrick Maher. Paralyzed from the waist down in 1981, he suffered mentally as his legs wasted away.

"My legs had atrophied so much that I would not put shorts on to sit around on my own because I was so self-conscious," says Maher, now a 25-year-old international customer service representative for American Hospital Supply, Equipping and Consulting Organization in Evanston, Ill.

One and a half years ago, Maher joined a research project at Michael Reese Hospital & Medical Center and the Illinois Institute of Technology, both in Chicago, to develop a computerized bypass around his severed spinal cord so commands could get from his brain to the still-functioning nerves and muscles in his legs.

When anyone stands, muscles in the chest and the back contract. In the paralyzed person, the computer interprets those upper-body muscle contractions and then stimulates the leg nerves through electrodes attached to the skin. When the computer turns on the electricity, the leg muscles contract and bring the paralyzed person to a standing position.

Maher's muscles, however, had become too weak to support his weight. Each night, he would use the stimulators to exercise his legs -- two 45-minute sets with a 20-minute break in between -- to strengthen the muscles in his paralyzed limbs. Three months later, he could stand by himself.

"My legs had grown more than two inches on every point on the quadriceps thigh muscle ," Maher recalls. "I was pretty excited about that."

Maher and a half dozen other paraplegics have taken the technology one step further. By linking the computer to signals from muscles on the upper part of their body, they have learned to walk.

"These people are not going to take walks for pleasure," says Dr. Kate H. Kohn, chairman of rehabilitation medicine at Michael Reese. "But if you can get a paraplegic up out of his wheelchair to go into the bathroom, you have done him a great favor."

Though impressive, this artificial spinal cord will help few of the 10,000 to T 15,000 Americans who suffer spinal cord damage each year because it is limited to a specific kind of damage at a specific location in the back.

The artificial ear, on the other hand, could help as many as 200,000 of America's 2 million deaf.

This device, newly approved by the Food and Drug Administration for general use, produces a sense of sound by directly stimulating the inner ear's auditory nerve. The current model produces a scratchy clicking noise similar to static interference on a radio that is not quite tuned in. While that doesn't allow wearers to understand speech, it still allows them to distinguish a human voice from street sounds, hear a baby cry or understand simple words like yes and no.

It's called a cochlear implant because a surgeon places an electrode inside the fluid-filled inner ear, or cocholea, where sound vibrations are converted into nerve impulses. An external microphone and card deck-sized sound processor pick up the sounds and convert them into small electronic impulses that stimulate the auditory nerve.

Californian Kristen Cloud was born with deformed cocholea that gradually lost all sensitivity to sound until she became completely deaf at age 18. In November, she gave a tearful testimonial during an FDA press conference as she explained how the implant has helped her.

"I didn't expect too much," she said, choking back tears. "To this day, I gained more out of the implant than I ever expected." It helps her read lips better and may even have saved her life, when she heard an ambulance coming up behind her as she crossed a street.

Tests with improved implant models that stimulate the auditory nerve with a dozen or more frequencies, instead of the single frequency now used, have shown that some completely deaf people can hear and recognize up to 75 percent of words spoken to them without any visual cues.

"Our goal," says Dr. William House, founder of the House Ear Institute in Los Angles and developer of the first implant approved for general use, "is to make them hear."

As badly as Cloud wants to hear again, people like the 33-year-old Utah student who lost his sight in a shooting accident want to see again.

To restore a semblance of vision, a surgeon at the University of Western Ontario Hospital implanted a one-inch-square sheet of Teflon containing a grid of 64 platinum electrodes on the surface of the student's brain where the vision centers are located.

Wires connected the electrodes to a computer, which sent tiny electric shocks to the part of the brain that controls sight. With each electric pulse, the man could "see" white points of light. He described the light as "distant stars." The white points of light could form a pattern, a letter or a scene.

Using these electrodes, the computer generated Braille characters directly in his mind's eye, and, with the computer connected to a video camera, he was able to distinguish between a vertical and horizontal bar.

It's not real vision -- more like the pictures created on an electronic scoreboard at a football stadium. But someday those scoreboard pictures may become clear enough to allow the blind to recognize a face or navigate in a world whose horizons are now limited to the reach of a cane.

Dr. William Dobelle, director of the Institute for Artificial Organs on Long Island, has been working on the artificial eye -- which he prefers to call electronic vision -- for more than a decade.

Although little has changed since the artificial eye prototypes were first tested in the mid-'70s, Dobelle envisions the day when surgeons will place tiny cameras in the sockets of sightless eyes, connect them to an improved computer that interprets the video signal (the computer would be located in the frames of eyeglasses) and install a large enough set of electrodes on the brain for the blind person to make out objects in the world around them. Nothing like that now exists.

So far, all of the artificial organs have been either purely mechanical, such as the heart or the artificial arm, or electronic, such as the artificial eyes and ears. But artificial organ researchers like Dr. Pierre Galletti, vice president of biology and medicine at Brown University, predict that future replacements will combine living tissue with some artificial component. His work on the artificial pancreas is the best example.

A major form of diabetes develops when the insulin-secreting cells -- called islet cells -- in a person's pancreas fail to produce enough of the hormone to control the amount of sugar in the blood. Diabetics who fail to produce insulin must try to control their blood sugar levels by injecting the insulin hormone every day, but this often fails to provide complete control and problems result.

In the late 1970s, researchers began experimenting with insulin pumps, devices worn on a belt that continually infuse small amounts of insulin through a needle in the skin. But the pumps also fail to control blood sugar as well as the pancreas. Transplanting a whole pancreas, or even the insulin-secreting islet cells, would solve the problem, but rejection-fighting drugs must then be used -- and they cause side effects.

Dr. Franklin Lim, formerly at the Medical College of Virginia in Richmond and now head of Lim Technology Laboratories Inc., developed a technique for wrapping islet cells in a semipermiable plastic that allows body fluids to flow in and insulin to flow out, but protects the cells from rejection. Lim says a mouse can be cured of diabetes by seeding its abdomen with a few hundred of these plastic mini-organs.

All of these technologies produce excitement among the people they may one day help, but in reality all have many technical problems and are not ready for widespread use. And even if the problems are solved, artificial devices will never be as good as the real thing.

"It's hard to beat biology," says Dr. Robert Mann, a long-time artificial organ researcher at the Massachusetts Institute of Technology. "Biology has had at least 10 million years or so to evolve the human form, and multimillions of years from scratch."

"Most people don't recognized that there are problems that go along with transplants," says Roger W. Evans, a research scientist at the Battelle-Seattle Research Center in Seattle, who has been studying organ transplant programs for the federal government. People who have had either a biological or an artificial organ transplant, he says, "don't live the life of a person who is perfectly normal. They live the life of a chronically ill person."

The devices themselves can cause problems, too; they may not last long enough. Even well-accepted spare parts like the artificial heart valve or the artificial hip, which have been in common use for more than a decade, wear out and fail because the body is a hostile, salt-water environment that actively tries to destroy or build scar-tissue walls around foreign objects.

"Total hips are a dramatic success story," says Dr. Steve Gordon, who oversees hip research for the National Institute of Arthritis, Diabetes and Digestive and Kidney Diseases. But they don't last forever, he admits. In the short run, the hip implants can take aging arthritics, hobbled by the extreme pain of degenerating joints, and have them back on their feet, virtually pain free, in three weeks.

But 10 percent of the hips fail within a decade and half of them fail in two decades, says Larry L. Hench, professor of materials science at the University of Florida in Gainesville.

Hip implants fail at the point where living bone comes into contact with steel and synthetic cement, Hench says. A kind of grout-like cement, which holds the hip implant in place, produces bone-killing heat as it sets. If the hip's alignment is a little off, the body's weight can cause the dead bone to break off. This loosens the implant, causing pain and, ultimately, failure of the joint. Once the hip fails, it's hard to implant another one.

Gordon admits that surgeons "are not putting them in people 25 to 40 years old. Because you are not sure that the total hip would last 25-30 years, you are less likely to do that surgery."

Hench puts it more bluntly: "There is a competition between the rate of implant failure and the length of the patient's life. There is a race to see which will be lost first."