Artificial Nerve System Gives Prostheses Sense of Touch

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Researchers at Stanford University and Seoul National University, Korea, have developed an artificial sensory nerve system that can activate the twitch reflex in a cockroach as well as identify letters in the Braille alphabet. The work, published May 31 in Science, is a step toward creating artificial skin and restoring sensation to people who use prosthetic limbs, said Zhenan Bao, PhD, a professor of chemical engineering at Stanford and one of the senior authors.

"We take skin for granted, but it's a complex sensing, signaling, and decision-making system," Bao said. "This artificial sensory nerve system is a step toward making skin-like sensory neural networks for all sorts of applications."

The paper describes how the research team constructed an artificial sensory nerve circuit that could be embedded in a future skin-like covering for neuroprosthetic devices. This rudimentary artificial nerve circuit integrates three previously described components. The first is a touch sensor that can detect even minuscule forces. This sensor sends signals through the second component—a flexible electronic neuron. The touch sensor and electronic neuron are improved versions of inventions previously reported by Bao's research lab. Sensory signals from these components stimulate the third component, an artificial synaptic transistor modeled after human synapses. The synaptic transistor was created by Tae-Woo Lee, PhD, of Seoul National University, who spent his sabbatical year in Bao's lab at Stanford to initiate the collaborative work.

"Biological synapses can relay signals, and also store information to make simple decisions," said Lee, a second senior author on the paper. "The synaptic transistor performs these functions in the artificial nerve circuit."

Lee used a knee reflex as an example of how more advanced artificial nerve circuits might one day be part of an artificial skin that would give prosthetic devices senses and reflexes.

The researchers say artificial nerve technology remains in its infancy. For instance, creating artificial skin coverings for prosthetic devices will require new devices to detect heat and other sensations, the ability to embed them into flexible circuits, and then a way to interface them to the brain. Though it will be some time before the work reaches that level of complexity, in the Science paper, the group describes how the electronic neuron delivered signals to the synaptic transistor, which was engineered in such a way that it learned to recognize and react to sensory inputs based on the intensity and frequency of low-power signals, just like a biological synapse.

The group members tested the ability of the system to generate reflexes and sense touch.

In one test, researchers hooked up their artificial nerve to a cockroach's leg and applied small increments of pressure to their touch sensor. The electronic neuron converted the sensor signal into digital signals and relayed them through the synaptic transistor, causing the leg to twitch as the pressure on the touch sensor increased or decreased.

They also showed that the artificial nerve could detect various touch sensations. In one experiment the artificial nerve was able to differentiate Braille letters. In another, the researchers rolled a cylinder over the sensor in different directions and it accurately detected the direction of the motion.

To read more about Bao's research efforts into restoring the lost sense of touch with stretchable, electronically-sensitive synthetic materials, visit Skin-like Prosthetic Coverings Closer To Reality With Stanford Breakthrough.

Editor's Note: This story was adapted from materials provided by Stanford University