Prosthetics of the Future Could Provide a Sense of Touch

Researchers have developed an artificial "skin" that can be added to prosthetics to re-create a sense of touch. Bao Research Group, Stanford University

When it comes to prostheses, the days of metal-hook hands and wooden legs are long over. But while robotic limbs have changed the lives of millions of amputees, the high-tech appendages still don't compare with the real thing, mainly because prosthetics are purely functional. They allow a person to walk, pick up items and carry out other daily tasks—but without a sense of touch.

A team of scientists at Stanford University and the Palo Alto Research Center are working to change this. The group has developed a special "skin" that can be added to artificial limbs that may allow a person with a prosthetic hand to actually feel a handshake. The researchers reported their work in a paper published Friday in Science.

The human sense of touch is complex. It begins in the thousands of receptors called mechanoreceptors located in the skin. Pressure on the skin's surface produces electrical pulses that are then transmitted to the brain. The amount of pressure on the skin's surface determines the number of electrical pulses produced. Heavy pressure sets off more electrical pulses than light pressure.

The artificial mechanoreceptors mimic this process. To make the prosthetic "skin," the scientists combined a pressure sensor with a flexible circuit layer that produces electrical pulses. The sensors are made of flexible pyramid-shaped nanotubes that funnel electric currents to the circuit layer. The circuit layer, made of plastic, is created from ink-jet printing. When pressure increases, the nanotubes compress and electricity flows through the sensors and the circuit layer. This produces electrical pulses, and the number of pulses is determined by the level of pressure—just as mechanoreceptors work in real skin.

The researchers tested their design by connecting a prosthetic hand fitted with their artificial "skin" to a slice of brain from a mouse. For this study's purpose, the researchers decided to measure the skin's reaction to light. The scientists used neurons in the brain tissue that typically involves touch, but they genetically engineered the neurons so they would also react to light. This approach served as a "proof of concept" that the skin would also be able to work with these rapid-firing neurons to simulate a sense of touch, once the rest of the central nervous system, such as peripheral nerves, is involved.

The scientists found that light stimulation to the skin produced neuronal activity. The researchers say this indicates the man-made material would potentially be able to transmit physical sensation as well. "If the brain is following the light pulses, that means the brain understood the information," explains Zhenan Bao, a professor of chemical engineering at Stanford and the lead author on the paper.

"The next step we want is to do such experiments in live animals, and increase the complexity to add other sensing functionalities," says Bao. That could mean creating skin that can feel temperature or texture rather than just pressure. "But how humans will actually react and how the signal will be perceived—that requires experiments in the future."