A research team at the University of Michigan has developed a method for more intuitive control of prosthetic hands using implanted electrodes. This innovation addresses the challenges faced by individuals with upper-limb amputations, enhancing their ability to perform daily activities. Traditional prosthetic limbs often rely on surface electrodes that can suffer from poor signal quality, limiting their effectiveness.
Progressing Beyond Surface Electrodes
Conventional upper-limb prostheses utilize surface electrodes placed on the skin to capture electrical activity from underlying muscles. These electromyography (EMG) signals are then processed to control finger and wrist movements. However, surface electrodes face various limitations, including inconsistent positioning, changes in limb volume, and interference from sweat and movement artifacts.
In contrast, implanted electrodes are surgically attached to muscles, allowing for deeper signal capture. This method results in higher signal-to-noise ratios, providing a more reliable control mechanism for prosthetic devices. Techniques such as regenerative peripheral nerve interface (RPNI) surgery enable these electrodes to connect to muscles that may be absent after amputation, thus facilitating better control over the prosthetic limb.
According to Cynthia Chestek, a senior author of the study published in the Journal of Neural Engineering, RPNI grafts not only improve control but also benefit the nerve endings. “They provide a target for nerve endings that prevent the formation of painful neuromas, which may help reduce phantom limb pain,” she stated. Future advancements could allow for the implantation of electrodes and a wireless transmitter during the initial amputation surgery, eliminating the need for additional procedures.
Experimental Findings and Real-World Applications
The research team conducted experiments involving two individuals with forearm amputations. Each participant had EMG electrodes implanted into RPNIs and muscles in their residual limbs. During the trials, they controlled a virtual hand and wrist by mimicking movements displayed on a screen. The team recorded EMG signals from both the implanted electrodes and surface electrodes designed to improve skin contact.
In one notable experiment, the participants demonstrated superior control using implanted electrodes compared to traditional surface electrodes. For instance, participant one achieved an average classification accuracy of 82.1% with implanted electrodes, while the accuracy for surface electrodes varied between 58.2% and 77.1%. Participant two showed similar trends, with accuracies of 91.2% for implanted electrodes compared to 67.1% for dry-domed surface electrodes.
The study also examined the participants’ ability to perform tasks in more dynamic settings. While movement generally reduced classification accuracy, the implanted electrodes maintained higher stability. This consistency in performance is attributed to the higher EMG signal amplitudes and reduced variability in signal quality during movements.
To illustrate practical applications, participant one completed the “Coffee Task,” which involved a series of movements required to prepare a cup of coffee. Using the iLimb Quantum myoelectric prosthetic hand, the participant successfully completed the task with implanted electrodes on all three attempts. In contrast, when using surface electrodes, they reached the maximum allowed time of 150 seconds in two out of three attempts.
Further analysis revealed that controlling wrist movements alongside hand functions significantly reduced compensatory body movements. Without wrist rotation, the participant had to lean their upper body excessively to pour sugar into the cup. Enabling wrist rotation minimized this adjustment, underscoring the importance of integrated joint control for prosthetic users.
Chestek noted the implications of these findings, stating that previous studies indicated large body movements were necessary when using prostheses without active wrist functionality. “Fortunately, the implantable electrodes provide highly specific and high-amplitude signals, allowing us to incorporate wrist movement without compromising the ability to classify different grasps,” she said.
The researchers aim to expand their work to include continuous movement control for all joints of the hand, although they caution that such advancements will take time. This breakthrough in prosthetic technology marks a significant step forward in enhancing the quality of life for individuals with upper-limb amputations.
