When DARPA launched the Revolutionizing Prosthetics program in 2006, the state of upper-limb prosthetic technology was far behind lower-limb technology. Advancing upper-limb technology was judged to be a more difficult medical and engineering challenge.
Now, a 28-year-old man who has been paralyzed for more than a decade as a result of a spinal cord injury has become the first person to be able to “feel” physical sensations through a prosthetic hand directly connected to his brain, and even identify which mechanical finger is being gently touched.
This advance was made possible by sophisticated neural technologies developed under DARPA’s Revolutionizing Prosthetics project. Researchers say that this points to a future in which people living with paralyzed or missing limbs will not only be able to manipulate objects by sending signals from their brain to robotic devices, but also be able to sense precisely what those devices are touching.
“We’ve completed the circuit,” said DARPA program manager Justin Sanchez. “Prosthetic limbs that can be controlled by thoughts are showing great promise, but without feedback from signals traveling back to the brain it can be difficult to achieve the level of control needed to perform precise movements. By wiring a sense of touch from a mechanical hand directly into the brain, this work shows the potential for seamless bio-technological restoration of near-natural function.”
The clinical work involved the placement of electrode arrays onto the paralyzed volunteer’s sensory cortex—the brain region responsible for identifying tactile sensations such as pressure. In addition, the team placed arrays on the volunteer’s motor cortex, the part of the brain that directs body movements.
Wires were run from the arrays on the motor cortex to a mechanical hand developed by the Applied Physics Laboratory at Johns Hopkins University. That gave the volunteer the capacity to control the hand’s movements with his thoughts, a feat which had previously accomplished under the DARPA program by another person with similar injuries. (See Figure 1)
Going much further this time, the researchers were able to provide the volunteer a sense of touch. The “hand” contains sophisticated torque sensors that can detect when pressure is being applied to any of its fingers, and can convert those physical “sensations” into electrical signals. The team used wires to route those signals to the arrays on the volunteer’s brain.
In the very first set of tests, in which researchers gently touched each of the prosthetic hand’s fingers while the volunteer was blindfolded, he was able to report with nearly 100 percent accuracy which mechanical finger was being touched. The feeling, he reported, was as if his own hand were being touched.
“At one point, instead of pressing one finger, the team decided to press two without telling him,” said Sanchez. “He responded asking whether somebody was trying to play a trick on him. That is when we knew that the feelings he was perceiving through the robotic hand were near-natural.”
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