Researchers have built a prosthetic hand that, with the help of artificial intelligence, can act a lot more like a natural one.
The key is to have the hand recognize when the user wants to do something, then share control of the motions needed to complete the task.
The approach, which combined AI with special sensors, helped four people missing a hand simulate drinking from a cup, says Marshall Trout, a researcher at the University of Utah and the study's lead author.
When the sensors and AI were helping, the participants could "very reliably" grasp a cup and pretend to take a sip, Trout says. But without this shared control of the bionic hand, he says, they "crushed it or dropped it every single time."
The success, described in the journal Nature Communications, is notable because "the ability to exert grasp force is one of the things we really struggle with in prosthetics right now," says John Downey, an assistant professor at the University of Chicago, who was not involved in the research.
Problems like that cause many amputees to grow frustrated with their bionic hands and stop using them, he says.
A helping hand
The latest bionic hands have motors that allow them to swivel, move individual fingers, and manipulate objects. They can also detect electrical signals coming from the muscles that are used to control those actions.
But as bionic hands have become more capable, they have also become more difficult for users to control, Trout says.
"The person has to sit there and really focus on what they're doing," he says, "which is really not how an intact hand behaves."
A natural hand, for example, requires very little cognitive effort to carry out routine tasks like reaching for an object or tying a shoelace. That's because once a person puts the task in motion, most of the work is done by specialized circuits in the brain and spine that take over.
These circuits allow many tasks to be accomplished efficiently and automatically. Our conscious mind only intervenes if, say, a shoelace breaks, or an object is moved unexpectedly.
So Trout and a team of scientists set out to make a smart prosthetic that would act more like a person's own hand.
"I just know where my coffee cup is, and my hand will just naturally squeeze and make contact with it," he says. "That's what we wanted to recreate with this system."
The team turned to AI to take on some of these subconscious functions. This meant detecting not just the signal coming from a muscle, but the intention behind it.
For example, the AI control system learned to detect the tiniest twitch in a muscle that flexes the hand.
"That's when the machine controller kicks on, saying, 'Oh, I'm trying to grasp something, I'm not just sitting still,'" Trout says.
To make the approach work, the scientists modified a bionic hand by adding proximity and pressure sensors. That allows the AI system to gauge the distance to an object and assess its shape.
Meanwhile, the pressure sensors on the fingertips tell the user how firmly their prosthetic hand is holding the object.
Sharing control
The idea of sharing control of a bionic hand addresses a reaction many people have when they use a prosthetic with superhuman abilities, says Jacob George, a professor at the University of Utah and director of the Utah NeuroRobotics Lab.
"You can make a robotic hand that can do tasks better than a human user," he says. "But when you actually give that to someone, they don't like it."
That's because the device feels foreign and out of their control, he says.
John Downey says that one reason we feel connected to our own hands is that they are controlled jointly by our thoughts and by reflexes in the brain stem and spinal cord.
That means the thinking part of our brain doesn't have to worry about the details of every motion.
"All of our motor control involves reflexes that are subconscious," Downey says, "so providing robotic imitations of those reflex loops is going to be important."
George says the smart bionic hand solves for that issue.
"The machine is doing something and the human is doing something, and we're combining those two together," he says.
That's a critical step toward creating prosthetic limbs that feel like an extension of the person's own body.
"Ultimately, when you create an embodied robotic hand, it becomes a part of that user's experience, it becomes a part of themselves and not just a tool," George says.
Even the most advanced bionic hands still need some help from a human brain, Downey says.
For example, a person can use the same natural hand to gently thread a needle, then firmly lift up a child.
"The dynamic range on that is far beyond what robots typically handle," Downey says.
That is likely to change, as bionic limbs become increasingly versatile and capable. What won't change, scientists say, is humans' desire to retain a sense of control over their artificial appendages.
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