Using the laptops and cellphones in today’s world is solved by using modern prosthetic devices that connect signals from the brain via surface-mounted sensors and detectors. New sensor technologies combined with high-performance microwiring is extending the performance and reliability of active myoelectric prosthetics. The evolution of newly developed microprocessor chip sensors offers advanced communication from the brain to the prosthetic devices. Myographic chips monitor and measure the force produced by muscles as they move from relaxed mode to contractions and can deliver intensity and muscle signal speed. The surface-mounted electromyographic sensors with isolated microwiring receive analog signals sent from the brain to be captured and converted to digital signals. They are then connected to newly adapted prosthetic devices that enhance the life and capability of amputee patients.

New sensor technologies enable the development of prosthetic devices that enhance the life of amputee patients. (Credit: AdobeStock) The National Institutes of Health supports additional development of various EMG sensor devices and systems. (Credit: NIH) Future surface EMGs. (Credit: AdobeStock)

Keys to a quality human-to-machine interface systems are extensive but exacting and quite well defined. Depending upon each circumstance, options for directing prosthetic movements include using body-power, using electric assisted power, and for simpler applications, using passive or pneumatic devices. The prostheses method selected should optimize the prosthetic device utilization for patients and their lifestyles. For a number of years, industry has focused on deep electromyography (EMG) for collecting muscular signaling from areas inside the forearm for use with the prosthetic hand. Today, however, surface level electromyograms, (sEMGs) , have significantly improved data-acquisition capabilities, are noninvasive, and have evolved to eclipse the surgical-related prosthesis process.

Improving the Human-Machine Interface

The data-processing system consists of an electrode interface, a signal conditioning unit, and a power source designed into a small sealed device. Getting a reliable and useful signal from the sensors that drive the machine instruments requires several steps. Two to three sensor pads are carefully placed externally on the arm at that point determined to exhibit strong muscular signal activity. Sensor placement must be near the tendon bulge entering the muscle to collect best signals. This ideal position of electrode placement is between the innervation zone, (or motor unit), and where the muscle tissue is attached to the tendons.

When the brain directs muscle activity, the sensor pads detect minute analog electrical signals that result within the muscular system. Those signals are filtered at both the high and low frequencies to remove electrical noise and to isolate the signals from potential power supply interference. In some cases, a variable resistor is employed to act as a digital potentiometer to help control signal gain stability, as signals sometimes bounce up and down during excited use. The cleaned signals are then rectified to offer a digital signal that exhibits definable shifts in voltage in typical range of 5–12 V. In some cases, an amplifier is used to set the signal levels and gain to optimize wiring impedance and drive mechanisms in the prosthetic unit. By design, most prosthetic motor/driver systems operate and provide more precision in responding to low-level digital stepping signal technology.

Specialized wire and cable designs are required to protect and isolate the minute digital signals of the EMGs from power wiring the motors. (Credit: AdobeStock)

The complexity of this human-to-machine interface increases significantly the need increases to drive process controls for multiple degrees-of-freedom in the prosthetic device. Fortunately, these sEMGs are improving and offering opportunities for use beyond the older intramuscular EMG prosthesis method. Surface-style electrodes easily form a reliable electrochemical state between the detection surface and the skin of the body so that current can flow into the electrodes. Because sensor design and signal collection remains the key element in the challenge, skin preparation is critical, specifically in the area of the sensor. One needs to ensure good data acquisition and clean signal transfer to the amplifiers. Prior to attachment, skin and hair must be scrubbed to reduce epidermis buildup and then dried thoroughly before application of the sensors.

To this end, neurological signal detection electrodes are being tested in a number of formats. Both dry and gel surface sensors have been studied. Gel electrodes use an electrolytic paste of silicone imbedded with silver chloride. This increases signal conductivity and prevents oxidation of the metal to skin interface. When clean, the electrical resistance is low, and the conduction is strong enough to block outside and surface-generated signal noise as well. Dry electrode sensors often use small pre-amplifier modules with multiple collection dots and don’t use gels between skin and device. Though electronically better, dry electrode sensors are more vulnerable to shock and vibration, and even sweat can challenge circuit stability.

Implanted transducers that provide kinesthetic communications of force and vibration feedback are particularly helpful in full leg and arm prosthetics where activating multiple muscles simultaneously is necessary. (Credit: AdobeStock)

In many cases, the EMG system assists the patient in neurologically moving parts using electrically driven micro motors and or gears that can rotate wrists, open and close fingers, and pick up objects. But beyond muscular signal transmission, newer prosthetics are also employing sensory feedback to the system or the patient. Grip strength and touch as well as pressure are key elements required in mimicking the natural use of the human limb. Transducer electronics are included to offer the classic control of picking up an egg and not crushing it. Implantable myoelectric sensors (IMES) again paved the way to improve limb control and detect footing position and pressure. These implanted transducers provide great kinesthetic communications of three-dimensional feel of force and vibration feedback to the patient. They are particularly helpful in full leg and arm prosthetics where activating multiple muscles simultaneously is necessary.

Simultaneous interaction between multiple parts of the operator became a significant advance in prosthetic applications, such as hand and arm control. The skin contains biosensor chips that detect variations in capacitance and or pressure, similar to pressure sensors used in robotic equipment. Nanoscale microelectromechanical systems (MEMS), chips, or electrical capacitive sensing field-effect transistors (FETs) are used to provide haptic sense of feel to reflect the pressure, touch, and pain receptors in human skin. The skin is electronically connected to the nerves in the arm that are involved in relaying the sensations of touch and pain to the brain. This process allows patients to operate their new prosthetic hand in a fashion similar to their original hand.

The Future of EMG Sensors

The National Institutes of Health (NIH) and the National Institute of Biomedical and Bioengineering have continued to support additional development of various EMG sensor devices and systems. Two evolving technologies use skin-mountable systems and advanced electronics. Beginning with the use of pattern plated or 3D printing of conductive circuitry on polyimide thin-film sheets, there will likely be an evolutionary development of rugged, wearable thin-film circuitry mounted externally on the patient’s skin.

Sensor and motion control systems for prosthetics have continued to expand rapidly in both precision capability and in operating more extensive components. Compact hand and foot control designs are being extended to serve full leg devices and exoskeleton systems. Signal detection and data-processing systems are somewhat extensions of previous designs, but routing of directional signaling and response information quickly becomes a more detailed task because of the distance they must travel. Physical size of some prosthetic systems also requires higher voltage or current levels to operate devices like hip and knee motors. The wiring and interconnection physics of these systems can become a challenge. Specialized wire and cable designs are required to protect and isolate the minute digital signals of the EMGs from the power wiring of the motors. Electromotive interference from outside environments can confuse digital data being routed to portions of the prosthetic device. Wire and cable must remain relatively small and flexible throughout constant use and offer signal integrity when exposed to elevated temperatures, high humidity, and sudden shock. Special polarized-nano (PZN) and circular nano connectors assist in connecting wire to electronic elements within the system.

Electromyography and prosthetic device development technology has changed significantly. From research centers to medical device industry, the process is well developed and can rapidly develop customized devices for individual applications. When designing interconnections for prosthetics, one can begin by developing a detailed list of personal use, physical applications, and environmental exposure. Working with experienced electronic circuit designers and using fast-turn prototyping systems can rapidly enhance a system. By employing the use of solid-modeling software and working hand-in-glove online with designers, OEMs can develop the exact form and fit to meet their specific function. When the solid-model software appears correct, they can be realized in 3D built devices of each element, and a polymer mockup of the complete prosthetic device can be constructed. This would then allow preplanning the signal routing system the best serves a particular device system. Specialty cable and connectors can be assembled to match the needs of the device.

This article was written by Bob Stanton, Director of Technology, Omnetics Connector Corp., Minneapolis, MN. For more information, visit here .

Medical Design Briefs Magazine

This article first appeared in the January, 2021 issue of Medical Design Briefs Magazine.

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