Body shop

‘Steve Austin: astronaut. A man barely alive. We can rebuild him. We have the technology. We can make him better than he was. Better… stronger… faster.’

So began one of the cult TV dramas of the 1970s, in which boffins transformed the damaged body of a test pilot into the ‘Six Million Dollar Man’ — a government agent with a right arm as strong as a bulldozer, a top running speed of 60mph, and, of course, a zoom lens for an eye.

In the mid-1970s the technology used to turn Austin from crash victim to bionic man wasn’t much more plausible than Doctor Who’s Tardis, or the transporter on the starship Enterprise. But today, while time travel and teleportation remain firmly in the realm of fiction, advances in engineering, surgery and artificial intelligence are combining to usher in a range of exciting prosthetic devices that closely mimic, and could one day even surpass, the behaviour of human limbs.

And while the latest crop of prostheses won’t turn their recipients into crime-fighting super-heroes, the bionic hands, arms, legs and feet currently under development have a far nobler purpose: to restore mobility and independence to people with lost limbs.

People like Donald Mackillop, a Scotsman who recently picked up a glass with his right hand for the first time in decades. Mackillop, who lost his hand in an industrial accident 30 years ago, has been fitted with i-Limb, a prosthesis that its Edinburgh developer, Touch Bionics, claims is the most advanced bionic hand in the world.

The company, which until a few years ago was part of the NHS, traces its roots to pioneering work on prosthetic limbs for Thalidomide victims in the 1960s. Technology director David Gow believes that the latest product, which will be fully launched later this year, represents a huge breakthrough.

Expected to cost around £8,000, i-Limb comes perhaps closer than any other prosthetic device to mimicking the gentle and economical precision of a human hand. This feat of technical elegance is largely down to the fact that each of the hand’s five digits are individually powered by miniature DC motors and tiny transmission systems.

Thus, while most prosthetic hands are capable only of a fairly rudimentary pincer grip, i-Limb is far more adaptable. ‘We have fingers that articulate and the ability to make more than one grip pattern,’ said Gow. ‘Therefore if we want the patient to have a precision grip they can have a precision grip, if they want a power grip where the fingers do all the gripping we can take the thumb out of the way.’

An important side issue is that the poor geometry of pincer grips means that you have to compensate with far higher gripping forces than we use. Thus, while most human tasks are conducted with forces of just 10-15N, existing prosthetic hands generate forces of around 100N. According to Gow, i-Limb currently falls about halfway between the two. ‘We believe we use the gripping forces appropriate to the task similar to the way the human hand does it,’ he said.

The hand is controlled using electrodes placed close to the skin. These collect electrical impulses from the muscles that are then used to trigger components in the prosthesis. This so-called myoelectric control technology has been used successfully for some time, and is relatively straightforward when applied to pincer-type prosthetic hands with just one motor. But i-Limb has five motors, raising considerable challenges, not only for the developers but also for the patient and the medical staff.

Gow explained that prosthetists will typically work with patients to find optimal muscle signals in two sites on the arm; the muscles that are used to extend or flex the wrist are commonly used. Once these sites have been found, it’s important that the two signals are kept independent of each other, that they are repeatable, and that the electrodes and sensors built into the socket worn by the patient coincide precisely with those sites to ensure good electrical contact.

Sounds tricky. But it gets a lot more complicated with i-Limb, where the performance of the hand and the requirement for multiple electrode sites raises considerable signal processing challenges.

But not all advanced prosthetics require such problematic connection with the body. Icelandic company Ossur is in the early stages of launching the Proprio foot, a £5,500 prosthesis with built-in intelligence.

According to Heidrun Ragnarsdottir, director of R&D at Ossur, the performance of the foot represents a massive improvement over previous systems. Until now the most advanced prosthetic feet available have been passive devices where any ankle movement results from the user putting weight on the prosthesis. The Proprio foot is, by contrast, an active system: accelerometers gather information about the outside world, which is then acted on by a microprocessor-controlled actuator consisting of a worm drive and a stepper motor.

Powered by an external rechargeable battery that helps to minimise the weight of the device and avoids creating an undesirable pendulum effect, the intelligent foot takes just 15 steps to ‘fingerprint’ an individual’s style of gait. Once it has done this, it’s able to use deviations in style to detect different types of terrain and adjust its position accordingly.

When walking on level ground, for instance, the foot will lift the toe during each swing, to prevent it from stubbing the floor. This is a common problem with existing prosthetic feet for which patients compensate by lifting their hip as they walk. Similarly, when walking up a slope the foot will gradually move into the slope angle, making it easier to walk up an incline. And when it detects stairs, the foot will lift the toe so that users can move their centre of mass more easily over the foot, once more avoiding the awkward and potentially damaging compensations that users of traditional prosthetic feet are forced to make.

Ragnarsdottir added that there are relatively few maintenance issues with the Proprio foot. Its ability to adjust to different walking conditions is expected to reduce the requirement for a prosthetist to be on hand to fine-tune the device when someone’s circumstances change. She said that the foot has an expected lifetime of three years, but that this could vary depending on how much use it gets.

Ossur’s efforts are not restricted to the foot. The company has developed a range of impressive prosthetic devices including sockets, shock absorbers, and, intriguingly, the Rheo Knee, a prosthetic knee that exploits the unusual properties of rheological fluid.

The viscosity of this fluid — a suspension of tiny magnetic particles — can be altered by the application of a magnetic field and used to adjust the stiffness of the knee instantaneously. The technology has been used before in both automotive suspension systems and damping systems for earthquake-proof buildings, but according to Ragnarsdottir, this is its first application in prosthetics.