Other researchers are developing prosthetic skin which might wrap around a bionic limb and feed back sensory information to the nervous system, in theory enabling users to detect and feel objects.The current generation of bionic hands can pinch or grasp using two or more electrodes fitted inside the portion of the prosthetic which fits over the stump.
These electrodes are positioned to pick up signals from the user’s peripheral nerve system that are naturally amplified by muscles in the stump.
Progress is almost continuous. German company Otto Bock has developed a hand incorporating multiple electrodes which can drive wrist flexing and rotation.
While Scottish company Touch Bionics builds hands which use software to control individual finger movement, so that the hand can clasp around objects.
The surgical rewiring of nerves in an amputee can also offer a great deal, enabling those with no arm at all, for example, to drive bionic arms with elbow and hand movement.
But there are problems. Sweat on the skin or any movement in the prosthetic can disrupt the signal to the bionic limb. The prosthetics can also rub against the skin and cause discomfort and sores.
The next generation of bionics will try to overcome these problems and offer some sensory feedback to the user.
Researchers in Britain have already developed the Intraosseous Transcutaneous Amputation Prosthesis (ITAP), a rod screwed into the bone of an amputee onto which prosthetics can be fitted directly and securely, be they hands, legs or fingers.
The rod means higher loads can be carried than with traditional prosthetics which fit over the stump of an amputee like a glove. It also avoids friction between the prosthetic and the skin.
Scientists such as Prof James Fawcett, of the Cambridge Centre for Brain Repair, are meanwhile developing neural interfaces whereby prosthetics will communicate wirelessly with implants fitted directly into the nerve fibres in the stump.
“People have produced very sophisticated prosthetics which will do very sophisticated things, but in almost every case the thing that people are struggling with is to link it up successfully to the nervous system,” he says.
“A lot of soldiers who have lost limbs apparently have given up using these devices and gone back to a simple hook, which at least is reliable.
“The device we’re producing is for recording sensory impulses in a nerve and gets inserted into the limb nerve itself.”
Once the device is inserted into the nerve, nerve fibres grow through it. Nerve signals associated with particular movements are then selected, and these signals transmitted wirelessly to a receiver in the prosthetic.
So far the device has been tested in mice and rats for up to 12 months. While the researchers do have some concerns that scarring within the device could strangle nerve fibres and disrupt signals, no such problems have been detected to date, says Prof Fawcett.
“We have a programme which will develop a prototype interface in about three years’ time and that will then be taken forward through the legislature for human safety and toxicity trials,” he predicts.
Researchers in Italy are also working on wiring bionics to the peripheral nerve system, and have already conducted trials in which electrodes temporarily connected to the nerves were used to drive an unattached prosthetic hand.
Elsewhere, researchers are looking to make more responsive prosthetics with many looking to flexible electronics or “prosthetic skin” to do the job.
“We’re looking into putting electronics onto surfaces that can be deformed, flexed but also stretched like a rubber band,” says Stephanie Lacour of Switzerland’s Ecole Polytechnique Federale de Lausanne.
“The idea with the prosthetic skin would be to have some kind of a glove like a latex glove which we could fit around the current prosthetic limb but that would be full of electronic sensor function that would mimic the sense of touch we have in human skin.”
Eventually, such sensors might feed information back to the brain via neural interface devices, but in the meantime there are other options.
Otto Bock are working on simpler devices whereby electronic sensors on a prosthetic detect information about objects and temperature which is fed back to the user via vibrations or pressure applied to the adjacent skin.
The surgical rerouting of sensory nerves in the stump (or chest muscles where an entire arm is replaced by a prosthetic) could enhance the effect, by creating areas of skin which feel what fingers would once have felt.
A new generation of bionics could also enhance the lives of individuals who are paralysed from the neck downwards or who have conditions like Locked-In Syndrome.
Last year, a paralysed US man made headlines after temporary electrodes placed on his brain were used to control a remote prosthetic hand which he used to stroke his girlfriend’s hand.
Back in Switzerland, researchers are testing a thought-controlled wheelchair which uses electrodes placed on the skin in a skullcap to drive the chair.
Prof Fawcett says such machines will be “a very exciting technology for the future” but says there are big problems to overcome.
“The issue with electrodes which record from the brain is bandwidth. You can transmit very little information and it’s slow.
“The electrodes also have to be very localised so you can only record from one bit of the brain and at the moment the electrodes are very unreliable and tend to produce inflammation and this stops the electrodes working.
“The other issue is that the electronics which you have to add are very complicated and you have to attach large structures to these skulls.”
The above story is reprinted from materials provided by BBC NEWS (Neil Bowdler).
Photo credits: National Geographic