Better Prosthetics Through Magnets

For many patients with limb loss, the remaining arm muscles still function as if the hand remained. A research team at the BioRobotics Institute at Sant’Anna School of Advanced Studies in Pisa, Italy, has taken advantage of this ghost function to develop a myokinetic trans-radial prosthesis using magnets embedded in arm muscles to control the prosthetic hand. 
 
Different muscles moved independently or in coordination to create specific hand movements, which were mapped to program the prosthetic arm. By embedding magnets in the arm muscles, the myokinetic hand captured muscle movement more accurately, improving dexterity and functionality. 
 
Modern myoelectric prostheses relied on surface electrodes to measure muscle movement at the skin’s surface. Both systems required power to operate and measure muscle signals, relying on batteries and regular charging. By implanting magnets instead of another type of electric device, the design eliminated the need for embedded power sources, avoiding wires and reducing the risk of infection or other complications. 
 
“Myoelectric prosthetics captured surface signals, which lacked selectivity and produced noise from skin electrode contact. We aimed for more selective signals by capturing them directly from inside the muscle,” explained Marta Gherardini, a researcher at the BioRobotics Institute.
 

Six-week trial

Before the system could be tried on a human subject, researchers overcame several hurdles. They developed myokinetic technology, built the prosthetic, and programmed real-time adaptive software. They also found a participant with enough muscle length for the device to read contractions and sufficient arm muscle memory to create movement for the phantom hand. 
 
After securing medical authorization for the procedure, the research team implanted six cylindrical, two-millimeter biocompatible magnets in the arm of Daniel, a trans-radial amputee who lost his hand in 2022, using minimally invasive surgery. They placed two magnets each, thirty millimeters apart in the muscle fascia of three target muscles: the flexor carpi ulnaris (wrist control), extensor digitorum (finger flexion), and flexor pollicis longus (thumb flexion). 
 
The 90-minute surgery, performed with local anesthetic and sedation, used regular ultrasound imaging to ensure accurate magnet placement.  
 
Researchers conducted tests on the participant, including objective measures like task completion time and error rates, alongside subjective assessments based on the participant’s experience with the prosthetic. They evaluated how intuitive control felt, how it functioned in real time, and how much mental effort each movement required. 
 
One major challenge stemmed from the limited movement of the implanted magnets, placed beneath the muscle fascia, the thin connective tissue around the arm muscles, instead of deeper within the muscle. This shallow placement resulted in smaller, less perceptible movements, reducing the prosthetic's ability to recognize multiple functions during tasks and limiting dexterity. The team addressed this by implementing a differential measurement strategy to increase sensitivity to muscle movement. In future trials, researchers aim to implant the magnets deeper into the muscle to increase functionality. 
 
Elbow movements unexpectedly impacted magnet pairs due to nearby muscle deformations or socket pressure from bending or extending the elbow. The extensor digitorum muscle, responsible for finger flexion, faced the most disruption from socket pressure during elbow movement. The team managed the effects of elbow-induced movement on control by implementing an enable/disable switch based on elbow speed.
 
Repeated use warped the arm socket, which affected sensor readings and forced the team to retrain the pattern recognition model. Variations in wear and arm swelling likely shifted sensor positions relative to the magnets, disrupting the algorithm. The team retrained the software, developed strategies to make the algorithm more robust, and addressed sensor-to-magnet shifts during repeated movements to ensure consistent outputs.
 

Phantom pain

Although phantom limb pain wasn’t a primary focus, researchers observed that the magnets and prosthesis neither increased nor decreased the pain, leaving it unaffected overall.  
 
“The next step involved long-term chronic implants, keeping the magnets in place over extended periods, and testing this interface with more patients. We aimed to gather more data, demonstrate its functionality over time, and explore its full potential,” Gherardini said. The team’s ultimate goal involved bringing the myokinetic prosthetic to clinics for amputee patients.

Watch Our Video: A Veteran’s Quest to Harness the Power of Prosthetics
 
First-generation prosthetics used a mechanical pulley system with cables connected via a body harness to shoulder or back muscles to control the device. Although they didn’t require power and improved functionality over no prosthesis, they lacked dexterity and left much room for improvement. Recent advancements produced myoelectric prostheses, which recorded muscle movement at the skin’s surface using electromyography (EMG) signals to operate the device. 
 
Researchers aimed to compare the myokinetic performance to myoelectric solutions. 
 
“We found the myokinetic prosthesis intuitive and easy for the patient to control, with functionality comparable to myoelectric prosthetics. Our goal remained to create something even better,” Gherardini said.
 
The six-week trial demonstrated that myokinetic prostheses offer intuitive control and performance. The research team successfully implanted magnets to improve signal accuracy and functionality, creating a more natural prosthetic experience. Though the trial focused on arm prostheses, the team plans to explore long-term use, additional patients, and other applications, including leg prosthetics and exoskeletons. This promising research paves the way for advancements in prosthetics and rehabilitation technology, potentially transforming lives by enhancing mobility and independence for amputees and individuals with motor disorders.
 
Nicole Imeson is a technology writer in Calgary, Alta.