Are ultrasonic sensors the better alternative to electrodes in prosthetics?

20th November 2023
Sheryl Miles

In the field of biomedical engineering, particularly in prosthetics, a shift from traditional electrode-based systems to more advanced technologies like ultrasonic sensors is gaining traction.

Both electrode technology and ultrasonic sensors have their own unique advantages and challenges, each of which can shape the future of prosthetic development.

Understanding electrode-based prosthetics

Electrode-based prosthetic devices have been the cornerstone of myoelectric prosthetics for years. These systems use electrodes to detect electrical activity generated by muscle contractions in the residual limb. This activity is then translated into movements of the prosthetic limb.

Advantages of electrode-based systems

Simplicity and reliability: electrode-based systems are relatively simple in design and have a proven track record of reliability.

Ease of use: users typically find these systems easier to learn and adapt to, primarily because they mimic natural muscle movements.

Disadvantages of electrode-based systems

Limited control: these systems offer limited control and may struggle to replicate more complex or subtle limb movements.

Interference and maintenance: electrode sensitivity can be affected by factors like sweat, skin conditions, and the need for regular maintenance.

The rise of ultrasonic sensors in prosthetics

Ultrasonic sensors, a more recent innovation, use high-frequency sound waves to detect muscle movements. This technology, as seen in projects like those by the Fraunhofer Institute, offers a more nuanced approach to prosthetic control.

Advantages of ultrasonic sensors

Enhanced precision: ultrasonic sensors can detect minute muscle movements, allowing for more precise control of the prosthetic limb.

Greater range of motion: they can interpret a wider range of motions, providing the potential for more natural and fluid limb movements.

Non-invasive and comfortable: unlike some electrode systems, ultrasonic sensors are non-invasive and can be more comfortable for long-term wear.

Speaking to Electronic Specifier, Dr Marc Fournelle, Head of Sensors and Actuators at Fraunhofer IBMT, commented: "The main advantage related to the use of ultrasound is the spatial resolution we get (obviously when comparing to intramuscular electrodes, there is the advantage of being non-invasive, but this is the case for surface electrodes as well). The ultrasound signal is time-resolved, which means by knowing the speed-of-sound in tissue, we know from which depth in tissue a given signal is coming from. This is not the case in EMG signal, where there is no time of flight information. Furthermore, we easily can get deep-tissue information with high SNR since penetration depths of several cm is not particularly challenging in ultrasound (as we see in every day´s clinical sonography)."

Disadvantages of ultrasonic sensors

Complexity and cost: the technology is more complex and potentially costlier than traditional electrode systems.

Learning curve: users may face a steeper learning curve in adapting to the sensitivity and range of controls offered by ultrasonic sensors.

Comparative studies and examples

Studies, such as those published in journals accessible through PubMed and IEEE Xplore, have shown that ultrasonic sensors can offer improved functionality in tasks requiring fine motor skills. For example, a user performing delicate tasks like typing or playing a musical instrument may find an ultrasonic sensor-based prosthetic more responsive.

On how he perceives overcoming the challenges of user training and adapting this technology for everyday use, compared to traditional electrode-based systems, Fournelle commented: "Our project partners at Imperial College have already shown that a very good performance (recognition of 3 degrees of freedom) can be achieved with a short training phase in the range of 1–2min. Another challenge relates to acoustic coupling, where we used ultrasound gel in our research. But we are working on 'dry coupling', which means have reliable acoustic contact over longer durations without the need for using ultrasound gel. So it might be slightly more demanding in terms of calibration / training than electrode based setups, but at the same time provide a better precision."

The future of prosthetic development

The choice between ultrasonic sensors and electrodes largely depends on the specific needs and abilities of the user, as well as the intended use of the prosthetic limb.

While ultrasonic sensors offer exciting possibilities, electrode-based systems continue to be a reliable and accessible option for many users.

As the field of biomedical engineering advances, the debate between ultrasonic sensors and electrodes in prosthetics is likely to continue. Each technology brings its strengths and challenges, shaping the future of prosthetic design and user experience.

Ultimately, the goal remains the same: to improve the quality of life for the individuals using these devices.

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