I’ve Got You Under My Skin

Electromyography (EMG) is a technique that measures the electrical activity generated by skeletal muscles during contraction and relaxation. This method typically involves the placement of surface electrodes on the skin above the targeted muscles or the insertion of needle electrodes directly into the muscle tissue. By capturing the electrical signals produced by motor neurons, EMG provides important insights into muscle function and activity.

EMG measurements offer a wealth of information about muscle activation patterns, the timing and duration of contractions, and the intensity of muscle activity. This data is particularly valuable in the field of wearable robotics, especially in the context of rehabilitation and assistive devices. Wearable robots, also known as exoskeletons, are designed to augment or support human movements, making them a promising tool in rehabilitation programs for individuals with mobility impairments.

Understanding muscle activity through EMG allows engineers and clinicians to optimize the design and control of wearable robots. By analyzing the patterns of muscle activation, developers can tailor the exoskeleton's assistance to synchronize with the user's natural movements. This personalized approach enhances the effectiveness of rehabilitation, ensuring that the device supports the wearer in a manner that complements their specific muscle activation patterns and biomechanics.

Efforts to build these types of wearable robotic systems have been hampered by the fact that existing EMG sensors lose accuracy in the presence of sweat or via changing skin conditions, like the presence of dead skin beneath the sensing surface. A clever team at the Korea Advanced Institute of Science and Technology has found a way to bypass this problem, however, which could help developers to build more useful wearable robots in the future. Rather than sitting on the surface of the skin, where accuracy can be negatively impacted by changing skin conditions, or being directly inserted into muscle tissue, which can be uncomfortable and limits the practical application areas, the team took a hybrid approach. They have created a wearable EMG sensor with microneedles that gets just below the surface of the skin, allowing it to collect high-quality signals without discomfort.

The wearable patch is composed of a soft, stretchable silicon polymer material with a conducting substrate. Attached to the substrate is an array of microneedles that can effortlessly and painlessly penetrate the skin’s surface. By penetrating just the stratum corneum — the outermost layer of the skin, which exhibits a high level of electrical resistance — the microneedles give the sensor access to precise EMG measurements that do not vary over time with changing conditions. An adhesive applied to the patch keeps it in place and allows it to move with the wearer even as they are active.

To validate their sensing system, the team applied the patch to users of a wearable robot that they had previously developed. This robot senses the user’s intentions and provides a boost when needed, like when they are lifting a heavy object. It was discovered that the patch provided accurate EMG measurements, enabling the robot to provide the exact assistance that was needed, right when it was needed.

In addition to being more accurate and stable over time, this new sensor also eliminates the need for prep work before use. The user does not need to, for example, remove dead surface skin or carefully wash the area before application. This could be another important factor in the ultimate development of a practical sensing system that can be used outside of a research lab.