Sensor technology extends range of movement for prosthetic arms

Robotic prosthetic arms allow amputees to perform a particular set of actions by twitching the remaining muscles in their severed limb.

But the range of movements possible with existing robotic arms is severely limited, leading up to half of amputees using the devices to abandon them out of sheer frustration.

Now researchers at Imperial College London have developed sensor technology for use with robotic arms that should ultimately make it possible to carry out a far greater range of actions using the prosthetics.

The technology, published in the journal Nature Biomedical Engineering, is designed to detect signals from motor neurons - nerve cells in the spinal cord that control muscles via fibres known as axons - rather than the damaged muscle in the shoulder or arm.

This allows more signals to be detected by the sensors, meaning more commands could be programmed into the prosthetic limb, according to Dr Dario Farina at Imperial College London, who carried out much of the research while at the University Medical Centre Gottingen.

“We use muscle tissue to connect the nerves that are severed after amputation, so surgically these nerves are redirected into muscle tissue,” said Farina.

Six volunteers underwent a surgical procedure at the Medical University of Vienna in which parts of their Peripheral Nervous System connected with hand and arm movements were re-routed to healthy muscles in their body.

This allowed the team to clearly detect the electrical signals sent from the spinal motor neurons, said Farina. “We establish an interface with the spinal cord, using the muscles to amplify the nerve activity,” he said.

The researchers then decoded some of the information in these electrical signals and interpreted them in computer models.

“We build a virtual arm, for which we estimate the forces at every joint, and then we translate those forces into the command of the robotic limb,” Farina said.

The researchers encoded specific motor neuron signals as commands into the design of the prosthetic, and then connected a sensor patch onto the muscle that had been operated on during the re-routing procedure.

This sensor patch was connected to the prosthetic, enabling the amputees to control the device by thinking about specific arm and hand manoeuvres.

The researchers now plan to move on from laboratory tests to full clinical trials.