Once when your body wore out, that was it. Now, as Charles Arthur and Steve Connor report, scientists are finding ways to replace bits and pieces as readily as a mechanic swaps parts of a car
In California, where the technology of the future tends to be built and where everything is a "ware", they like to call humans "wetware". The good news is that increasingly, we can upgrade our bodies, just like computers. At the moment attention is on external "hardware" to make things better - devices and attachments, many of which use x-rays, electrodes, and ultrasound scans to improve the body, often after it has been damaged by injury or disease. But in the years to come it will be the software, comprising our own cells, that will be used to make life more comfortable for the ageing population.

The first piece of add-on hardware for the body was probably the tooth filling, which dates back thousands of years: ancient skeletons have been found with objects crushed into holes in teeth. The next were spectacles, in the Middle Ages. But the first significant internal implant was the hip joint, first developed in 1905, which replaces the worn-out head of the femur where it rotates in the hip. Thousands of replacement hips have been issued; some people are even replacing their replacements.

Much more is available: heart valves from pigs, breast implants of silicone or soya, tendons made of carbon fibre, artificial eye lenses, tiny amplifiers for the inner ear, mechanical larynxes, plastic blood vessels, even artificial skin.

Among the more eagerly examined projects is one that would literally let the blind see. Last October a team from the Daresbury Laboratory, near Warrington, announced that with scientists at De Montfort University in Leicester they had developed a form of light-sensitive silicon that is being connected in the laboratory to nerve tissue.

That is the first step towards "silicon implant" glasses that would connect directly to the wearer's nerves. Dr Lorraine Buckberry, senior lecturer in biochemistry at the university, said: "It is our aim that this finding could help the blind see again."

There's nothing like having a modest target, after all. But American researchers are keen to follow similar lines: another team at Johns Hopkins University in Baltimore is trying to connect electrical circuits to silicon, and thence to nerves. There have already been successful first tests in which a matrix of light-sensitive electronic receptors is attached to the cells in the retina.

Thanks to the enormous "plasticity" of the brain - its ability to "rewire" itself to find meaning in a jumble of new data - even these primitive signals appear as comprehensible images to the 15 volunteer patients. Mark Humayun said: "Some of patients haven't seen for four decades. It takes a while for them to see the letters, but once they pick up even a single dot they make quick progress."

Professor Roy Bakay, from Emory University in Atlanta, Georgia, implanted tiny glass transmitters into the "motor cortex" brains of two severely- disabled people, who were then able to operate a computer simply by thinking. The motor cortex is where thoughts about movements are focused; by thinking of different parts of their paralysed bodies, the volunteers were able to direct a cursor on a computer screen, up and down, or left and right, and use it to produce simple messages on the screen.

Yet there are some parts of the body which may lie beyond the reach of hardware replacement for decades - if not for ever. The complex chemical factory of the liver, for example, has never been mimicked outside the body, unlike the heart of kidneys. Thus the list for liver transplants keeps growing. The greater likelihood is that long before engineers can devise a tiny, artificial liver that can be routinely implanted into failing bodies, the biologists will have beaten them - with the software of a liver grown from the patient's own cells. CA

Transplant surgeons have long dreamt of it, but growing human tissues and body organs from scratch is fast becoming a reality. Breakthroughs in growing living cells outside the body have enabled scientists to contemplate the possibility that they may one day be able to exchange body parts such as livers and kidneys in the same way that car mechanics replace radiators and carburettors.

Last November scientists announced that they had identified the elusive "stem cells" of the embryo which are capable of developing into any one of dozens of tissue types. The discovery opened the door to dramatically different forms of transplant surgery.

"These cells could be grown in the laboratory and used to regenerate failing tissue. Because such cells do not age, they could be used to generate virtually a limitless supply of cells and tissues for transplantation," said Thomas Okarma, vice president of research at Geron, the American biotechnology company with an interest in anti-ageing therapies.

Stem cells could be grown in the laboratory and stimulated to differentiate into one of the specialised tissues of the body. It is envisaged that with the right "scaffolding" tissues could be put together to grow new organs such as livers and hearts. Doctors could call upon a virtually unlimited supply of spare parts to keep an ageing body going.

Evan Snyder, assistant professor of biology at Harvard Medical School, is using stem cells isolated from embryonic brain tissue to grow new nerve cells. He intends to implant them into the brains of people who have suffered tissue damage from stroke or illnesses such as Alzheimer's and Parkinson's. "We hope to show that we can integrate them in a seamless fashion into the fabric of the brain," Professor Snyder said.

Although the problems of growing new organs outside the body are more formidable, they may not be insurmountable. Charles Vacanti, director of tissue engineering at the University of Massachusetts in Worcester, is attempting to use coral as a scaffold on which to build replacement bones. Many people do not realise that bone is a living tissue and coral has lots of interconnected channels for bone cells to grow in. As the bone develops, the coral degrades away, eventually leaving pure tissue behind. Researchers at the Massachusetts General Hospital in Boston hope to build new blood vessels by growing cells around an artificial polymer. Making the synthetic vessels water-tight and strong enough to sustain the pressure of the human blood supply poses special difficulties but the group claims to have produced artificial arteries and veins able to withstand a blood pressure 20 times greater than normal.

Michael Sefton, director of tissue engineering at the University of Toronto, wants to build a "heart in a box", complete with chambers, valves and interacting muscles. He says the $5bn (pounds 3bn) project is "the Holy Grail" of transplant biology. He accepts it will be at least 10 years before anything meaningful comes out of existing research.

Even more ambitious is the prospect of combining stem-cell research with cloning technology. Austin Smith of Edinburgh University has proposed that it may be possible for every child to have their own supply of stem cells for use in later life. This would be done by taking the nucleus of a baby's skin cell and fusing it with an unfertilised egg to produce an early embryo clone from which it would be possible to extract embryonic stem cells. Such cells would not suffer the tissue rejection problems of normal transplant operations. If this ever became possible, the day of an unlimited supply of soft body parts would truly have arrived. SC

HIP Computer technology will translate x-rays into 3-D mock-ups of joints, to ensure replacement hips fit perfectly.

HAND Electrodes placed along the arm, controlled by an external processor can make paralysed muscles move again

EYES "Silicon implant" glasses will connect directly to the wearer's nerves.

BRAIN A cyberknife will use x-ray images to track the precise location of a tumour during surgery and focus a radiation beam.

INTESTINE New software will enable a series of 2-D images to built up an endoscopy tour through the gut.

THROAT Prefabricated implants of different sizes can restore the voice of a throat cancer victim.

WOMB Portable ultrasound machines will enable scans to be taken anywhere of a pregnant woman. The device will send images back to hospital via satellite.

BONES Coral to be used as a scaffold to enable bone cells to grow. As it forms, the coral itself degrades, leaving only living tissue behind.

HEART AND LIVER Lab-grown stem cells will be stimulated to develop into specific organs.