NERVE implants can help paralysed people perform simple tasks, such as
gripping a cup. But the lack of feedback means they can drop the cup or even
crush it. Now Danish scientists say they have found a way to help people grip
more effectively. They hope eventually to help restore sensation to those with
spinal injuries.
Without sensation, patients using neural prostheses have to rely upon their
eyes and experience: grip an object too forcefully and they risk muscle fatigue,
too gently and they drop it. This is one of the biggest problems with bionic
implants, says Morten Haugland at the Center for Sensory-Motor Interaction at
Aalborg University.
Haugland and his colleague Andreas Inmann have developed a portable system
that monitors bundles of nerves from the index finger of a patient to detect
overexertion and compensate by adjusting their grip. The system has been
designed to work with commercial neural prostheses such as the Freehand muscle
stimulator, which helps quadriplegics. Such systems work by electrically
stimulating particular muscles, causing them to contract and making it possible
to achieve coordinated movements. This, says Haugland, can help patients carry
out everyday tasks that would otherwise be impossible, such as eating with a
fork.
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But while such devices can help to improve a patient鈥檚 quality of life,
Haugland says they can be frustrating as well. Anyone who has tried drinking
after a mouth-numbing visit to the dentist knows how hard it is to use muscles
when there is no sensation to tell you what they are doing.
Haugland鈥檚 add-on device consists of three electrodes wrapped around a nerve
bundle implanted into the palm of the hand. Information on the strength of grip
is fed back to the muscle stimulator, located externally on a wrist cuff.
The stimulator is controlled by two buttons pressed by a head-mounted
prodder: one turns the system on and increases the power, the other decreases
the power and turns it off. There are systems that do this using artificial
sensors embedded in gloves, but it makes far more sense to use the sensors in
your own fingers, says Haugland.
Patients tend to crank the system up to full power all the time, says
Haugland. But this isn鈥檛 necessary: prodding a potato with a fork may require a
strong grip, but carrying it to your mouth does not. 鈥淥ur system makes sure you
use the right amount of force,鈥� he says.
His team鈥檚 ultimate goal is to restore a patient鈥檚 sensation, but for now
they are planning to exploit the new approach. Haugland is using similar
electrodes to help paraplegics balance when standing.
Haugland鈥檚 system uses the hand as an extension to a machine鈥攚ith
muscles instead of motors, and nerves replacing touch sensors. But this is
effectively what is going on normally when we grasp objects, says Gerald Loeb, a
biomedical engineer and expert in neural prosthetics at the University of
Southern California in Los Angeles. 鈥淲hen you grasp an object you do not think
about the details of what muscles to contract,鈥� he says.