Biomimetic signals ‘provide better communication between prostheses and brain’

Amputees using a prosthetic leg were able to climb steps quicker and concentrate on other things while walking thanks to a new approach to communication between the device and the brain, researchers have said.

Designed to highlight the benefits of naturally inspired, biomimetic stimulation for the next generation of ‘neuroprosthetics’, the tests were carried out by Professor Stanisa Raspopovic and his team at ETH Zurich in Switzerland. The team used a prosthetic device connected to the sciatic nerve in the test subject's thigh via implanted electrodes, developed by a team under Professor Raspopovic several years ago.

Neuroprostheses are electro-mechanical devices that are connected to the nervous system. As yet they are unable to provide natural communication with the brain, the researchers said, and the electrical pulses used often evoke artificial, unpleasant sensations, similar to a feeling of tingles over the skin.

This ‘paraesthesia’ could be caused by overstimulation of the nervous system, so the Zurich team proposed that neuroprostheses should transmit biomimetic signals that are easier for the brain to understand.

To generate these signals, Natalija Katic – a doctoral student in Raspopovic’s research group – developed a computer model called FootSim. The model is based on data collected by collaborators in Canada, who recorded the activity of receptors in the sole of the foot while touching different points on the feet of volunteers with a vibrating rod.

The model simulates the dynamic behaviour of large numbers of receptors in the sole of the foot and generates the neural signals that shoot up the nerves in the leg towards the brain – from the moment the heel strikes the ground and the weight of the body starts to shift forward to the outside of the foot, until the toes push off the ground ready for the next step.

“Thanks to this model, we can see how sensory receptors from the sole, and the connected nerves, behave during walking or running, which is experimentally impossible to measure,” Katic said.

To assess how closely the biomimetic signals calculated by the model corresponded to the signals emitted by real neurons, Giacomo Valle – a postdoc in Raspopovic’s research group – worked with colleagues in Germany, Serbia and Russia on experiments with cats, whose nervous systems process movement in a similar way to humans.

The experiments, carried out in accordance with European Union guidelines, showed that the pattern of activity recorded in the spinal cord when researchers pressed the bottom of a cat’s paw resembled the patterns elicited in the spinal cord when the researchers stimulated the leg nerve with biomimetic signals.

In human clinical trials with leg amputees, users were able to climb steps faster, and also made fewer mistakes in a task that required them to climb the same steps while spelling words backwards.

“Biomimetic neurostimulation allows subjects to concentrate on other things while walking,” Raspopovic said. “We concluded that this type of stimulation is more naturally processed and less taxing on the brain.”

The same approach could also be used in other aids and devices, the researchers said, including spinal implants and electrodes for brain stimulation.

“We need to learn the language of the nervous system,” Raspopovic said. “Then we’ll be able to communicate with the brain in ways it really understands.”