A patient performs various everyday tasks with a sensory control module integrated with his prosthetic arm.

Achieving consistent and steady feedback is often difficult for prosthetic users.

Prosthetic arms that use nerve stimulation have sensors in the fingertips, so that when the user comes in contact with something, an electrical signal on the skin corresponds to the amount of pressure the arm exerts.

However, this often does not give the user reliable feedback and the electrodes connected to the skin can begin to peel off, causing a buildup of electrical current on the area that remains attached over time.

Sweat can also impede the connection between the electrode and the skin, causing the user to feel less or possibly no feedback at all.

A team from the University of Illinois has created a new control algorithm that regulates the current, enabling  a prosthetic user to feel a steady sensation akin to the sense of touch, even when the electrodes begin to peel off or when sweat builds up.

“We're giving sensation back to someone who's lost their hand,” Aadeel Akhtar, an MD and PhD student in the neuroscience program and the medical scholars program at the University of Illinois, said in a statement. “The idea is that we no longer want the prosthetic hand to feel like a tool, we want it to feel like an extension of the body.

“Commercial prosthetics don't have good sensory feedback,” he added. “This is a step toward getting reliable sensory feedback to users of prosthetics.”

The new prosthetic includes a controller that monitors the feedback the patient is experiencing and automatically adjusts the current level so that the user can feel steady feedback—even when sweating or when the electrodes are 75 percent peeled off.

The researchers tested the technology using two volunteers with a test where the electrodes are progressively peeled back.

The researchers found that the control module reduced the electrical current so the users felt steady feedback without shocks. They also had the volunteers perform a series of everyday tasks—like climbing stairs, hammering a nail onto a board and running on an elliptical machine— that could cause the loss of sensation due to sweat.

“What we found is that when we didn't use our controller, the users couldn't feel the sensation anymore by the end of the activity. However, when we had the control algorithm on, after the activity they said they could still feel the sensation just fine,” Akhtar said. “Although we don't know yet the exact breakdown of costs, our goal is to have it be completely covered by insurance at no out-of-pocket costs to users.”

The researchers are now hoping to develop a miniaturized module that can provide the electrical feedback that fits inside a prosthetic arm rather than attaching to the outside. They also plan to do more extensive patient testing with a larger group of participants.

“Once we get a miniaturized stimulator, we plan on doing more patient testing where they can take it home for an extended period of time and we can evaluate how it feels as they perform activities of daily living,” Akhtar said. “We want our users to be able to reliably feel and hold things as delicate as a child's hand.

“This is a step toward making a prosthetic hand that becomes an extension of the body rather than just being another tool.”

The study was published in Science Robotics.