The Parkinson's procedure at Radcliffe Infirmary is routine now, and successful in 90% of patients: damage or death occurs, says Aziz, in only one in a thousand cases. But implant technology is in its infancy. Aziz believes we're on the brink of a revolution, driven by innovations in materials, by nanotechnology (the engineering science of the very small) and by the discovery of an extraordinary symbiosis between silicon and nerve cells.
I am sitting in a Cambridge laboratory with a sandy-haired 38-year-old researcher, Ed Tarte, while he demonstrates a design for a tiny electrochemical device he calls an "interface implant", which is set to make Aziz's electrical probe look decidedly low-tech. "If a working mechanism like this had been implanted in time in the spinal cord of Christopher Reeve," he tells me, "he might be walking and moving normally today."
Made of polymer, and containing hundreds of tiny "tunnels" through its length, the size of the implant will be a centimetre long with a diameter no bigger than that of an average pencil lead. Its parts and operations are so small and complex that he can only display them magnified on a screen. "We're attempting," he says, "to bridge the scar tissue of a spinal-cord injury. It will connect healthy nerve fibres above the scar to those below which have lost contact with the brain, a bit like mending a broken connection in an electric circuit."
Tarte heads a team in Cambridge University's materials science department, which works with researchers in the chemistry department and the Cambridge Centre for Brain Repair at Addenbrooke's hospital. He is a nerdy, ageless fellow with the air of an internet-cafe denizen. But this exterior masks his membership of a remarkable elite of scientists who work on the interface between microchip wizardry and living bodies — the coming reality of bionic man.
The expertise of the likes of Ed Tarte ranges across a daunting combination of specialities: superconductivity, quantum physics, computer science, neuroscience, magnetic resonance technology, biochemistry, physiology, materials science and neurology. There is no overall name for their ilk, but "bionicists" might fit the bill.
There are two kinds of pioneering techniques powering the expansion of the "interface" field: miniaturisation, and the "love affair", as some scientists call it, between synthetic materials and living tissue. Since 2000 the semiconductor industry has been producing nanochips with features measuring less than 100 nanometres: about one-thousandth of the thickness of a human hair. This descent into the microscopic realm spells massively enhanced computer power as well as the ability to make implants that can reside non-invasively within our exquisite nervous systems. At the same time, scientists have been coaxing brain and nerve cells to grow in intimate combination with silicon and polymer circuits, etched by laser technology to create an ideal habitat for the proliferation of living nerve cells that develop branch-like connections with each other.
At the Max Planck Institute for Biochemistry, just outside Munich, Peter Fromherz has exploited the neurones of snails and leeches to make a prototype "neurochip". These neurones, which are large compared with mammalian ones, are puffed onto silicon chips and placed over transistors. As the neurones grow and make connections, the transistors amplify their tiny electrical blips. Fromherz started last year with just 20 neurones; now he is planning a prodigious 15,000-cell neurone-transistor chip. He is creating, in short, a living computer, further reducing the barrier between live and inanimate matter. Living circuitry of this kind could one day be the basis of computer-controlled artificial limbs, for it heralds the prospect of mechanical computers speaking to our biology. But there are other extraordinary strategies much closer to realisation.
Tarte explains a spectacular recent advance in his own research. When he first started working on the "interface" to bridge the scar-tissue barrier of an injured spinal cord, he created a device he nicknamed "shotgun". The idea was to allow the living nerve fibres to find their own way through a series of micro-electrode tunnels to connect with severed fibres beyond the scar-tissue barrier and stimulate signalling and regrowth. The process, however, was not sophisticated enough. "Now we have engineered electrodes with chemical properties that attract specific types of nerve fibres to precise locations within the device, in order to match them with their corresponding nerve-fibre types below the scar tissue," he says. The comment evinces the importance of creating implants that mimic the chemical as well as electrical properties of the nervous system. Pharmaceutical therapies target brain chemicals, and electrodes target the delicate electrical blips of nerve cells; the new generation of implants aims to influence both in concert. In Tarte's new design, nerve fibres regulating finger and hand movement, for example, will now make exact connections with their corresponding fibres below the scar tissue.
But the key to huge advances in implants is an array of new synthetic materials. The initiated talk of "DNA chips", "biocompatible monomers", "plasma transistors", materials that possess "shape memory". A crude illustration would be the kind of plastic water tank that you can squeeze through a narrow trap door and then expand to its proper size up in the attic. In new implant technology, materials change shape and size at different temperatures. Keyhole surgeons aim one day to insert implants with a diameter of no more than a piece of string, which will then expand at body temperature. At the outer reaches of "shape memory" are liquid materials that harden inside the body. Researchers are working on a "smart" gel that will form an implant to obtain feedback control from the pancreas of a diabetes sufferer.
Behind the pioneering work on new materials and miniaturisation is the ambition of medical scientists to take on ever greater clinical challenges. Crude "interfaces" for loss of function have been in use since the invention of false teeth and peg legs. From contact lenses to silicone breast implants, from artificial hearts to pacemakers, the post-war era has seen incredible advances in prosthetic aids.
The most impressive example involves the implant of electrodes to compensate for deafness. Some 50,000 cochlear aids have been inserted involving the implant of a chip that stimulates amplified aural signals. Composed of an induction coil and an electrode array, the implant converts sound waves into weak electric currents, which are delivered to the vicinity of the auditory nerve in the inner ear. The auditory nerve is stimulated and transmits impulses to the brain, which recognises them as auditory signals. Users often describe the sound characteristics as "synthetic", but this perception changes over time, and even the profoundly deaf have benefited.
The biggest challenge is the bionic eye. At the Massachusetts Eye and Ear Infirmary at Harvard Medical School, a group is working on vision computer chips. The strategy: insert two chips in the vicinity of the optic nerve, which is assumed to be intact. One will contain a solar panel that will start up a laser beam in response to photons of light. The beam will then strike a second panel that sends signals along the optic nerve to the appropriate visual cortex in the brain. At the far reaches of research are plans to create an electronic eye simulating the cones and rods in the living eye, opening up potential for an eye beyond the capabilities of the human eye; operating, for example, at both telescopic and microscopic levels. But will the marriage of silicon, microprocessing and the nervous system promise the reality of a bionic human, as in the TV series The Six Million Dollar Man? Michio Kaku, the American guru of the future, has doubts. "Although it may be possible to connect the human body to a mechanical arm," he says, "the stunts of the bionic man would put intolerable stresses on our skeletal system, rendering most superhuman feats impossible."
