The Human Touch

GATES WORKS TO RESTORE HAND FUNCTION IN AMPUTEES BY DREW MOSER

Human hands are amazing. They help us define by touch whether things are hot or cold, hard or soft. They enable us to grab and hold onto objects. We need them to do so many everyday activities.

For the past eight years, Dr. Deanna Gates, associate professor of Movement Science, and colleagues from Michigan Medicine, Biomedical Engineering, and Mechanical Engineering have been working to perfect advanced prosthetic devices that can give amputees more of their hand functionality back.

According to Gates, there are currently no viable options that give patients the intuitive control they need for everyday living. Most prosthetic hands are controlled by triggers, which can either come from co-contraction of muscles, gesture control (fast movements of the arm), or EMG pattern recognition (machine learning that deciphers surface muscle activity). Gates and her colleagues are looking to change that.

Gates’s Rehabilitation Biomechanics Laboratory team is incorporating brain signals into a prosthesis with its own suite of sensors that returns some sense of touch to an individual.

“There is this definite need to enhance the control [patients] have and make it more reliable and intuitive. That’s what we’re trying to achieve when we access these signals,” Gates said.

Gates is working with researchers in plastic surgery to measure signals from severed peripheral nerves in the amputated limb. Surgeons create regenerative peripheral nerve interfaces (RPNI) by attaching a small piece of muscle from another part of the body to the end of the nerve.

“This prevents a neuroma [painful growth on the end of a nerve] from growing and reduces phantom limb pain,” explained Gates. “If you try to put an electrode right in the nerve itself, it’s hard to differentiate neural signals from the noise. The muscle amplifies those signals such that we can recognize what the user intended to do.”

A second surgery places small electrode wires into the muscle. The wires come out of the skin and are attached to a connector, which attaches to a computer that controls the hand.

“We’re trying to achieve something that is going to be intuitive for [patients]. We’re trying to capture the signals that would’ve gone to that muscle to cause that action. And if we can measure that and then predict what action they wanted to do, then the hope is that it will replicate what the body did,” Gates said.

Once the surgery is complete, Gates and her lab staff begin using tasks and evaluations to assess the individual’s hand functionality and the cognitive effort associated with their daily living activities.

The lab uses motion capture technology to quantify movement compensations, or how bodies create new movement patterns to achieve functional motor skills when a normal movement pattern is unavailable, which can cause musculoskeletal and coordination issues. According to Gates, nearly 50% of amputees have pain in their intact limb because they start doing everything with that limb, or use compensatory strategies that

move their torso in order to get their hand to interact with things.

Michael Gonzalez, a graduate student research assistant in Gates’s lab, focuses on sensation and how sensory feedback plays a role in the prosthesis’s control loop. His main goal is to figure out how sensation is different for prosthesis users versus individuals using an anatomical limb. He studies the differences in how individuals pick up, grab, and touch objects and uses those measures to improve the way sensation and sensory feedback are given to individuals with prostheses.

“By stimulating their peripheral nerves, we can make them have some sensation in that area they used to have a limb in,” he said. “That is always a fun experience to see their eyes open wide and begin describing where the sensation is being felt.”

Karen Sussex makes coffee using a prosthetic hand prototype. Scott Soderberg/ Michigan Photography.

Christina Lee, another graduate student research assistant in the lab, helps design the functional tasks that assess the subject’s movements and determine how much compensation is being made. For example, Lee has participants make a cup of coffee in order to measure multiple hand grips. Using a Keurig coffee machine, Lee monitors how well the patient pours water into the Keurig, inserts the coffee pod, closes the lid, turns the machine on, and pours a packet of sugar into the mug.

The test also examines the patient’s body posture changes when wrist motion or multiple grips are added to a task.

“We can then provide a quantitative assessment of the functional improvement,” Gates said. “There are a lot of things we can look at from a hardware side of how accurate this is, but the functional benefit is really what is going to take it into the future.” Karen Sussex, from Jackson, Michigan, has been participating in Gates’s research for the last three years. Five years ago, she had part of her right hand amputated following an illness. After surgery, she wasn’t able to move her hand because the tendons in her arm were destroyed. She saw Dr. Paul Cederna, the Robert Oneal professor of plastic surgery and chief of plastic surgery at Michigan Medicine, for the amputation and RPNI surgeries.

It wasn’t until the follow-up surgery that she heard about Gates’s research.

Sussex said it’s easier to complete tasks with the prosthesis in Gates’s lab than with her current bodypowered one. She said her body-powered unit fatigues her left arm and shoulder and limits what she can do.

“When I wear it at home, I can only stabilize my pots and pans and pick things up off the floor,” Sussex explained. “I would not want to carry a glass, cup, bowl, or plate of food across the room because I would drop it and have a mess to clean up.”

Alex Vaskov, a postdoctoral research fellow in biomedical engineering, is developing ways to make the new prosthetic control approach portable. He places a receiver in a backpack worn by the user that sends signals to a wireless receiver in the prosthesis socket.

“Ideally for everyday use, you would want a fully implanted system that would send signals wirelessly to the prosthesis so you don’t have to worry about regularly cleaning and rebandaging the exit sites,” Vaskov explained.

Sussex tested the portable system in June 2022 by carrying a bag from Panera Bread back to the lab. “I was also able to open a door with this prosthesis, which I cannot do with my body-powered one,” she added.

The next step is to create a small, unobtrusive version of the wireless device that could be placed under a bandage and feature the same wireless circuitry as an implant. If things go well, the team would then test an implanted system.

According to Gates, the goal is to have the wearable version ready for at-home trials in the next five to eight years.

“I think this is amazing,” Sussex said. “I feel like I’m doing a lot for not only me but for people that would come through this after me. In the future, a lot of people will be able to use this type of prosthesis more freely because of what the other participants and I are doing here.” n