Battery-free skin-conformal wearable system can measure electrocardiogram signals

 

A research team led by Prof. Jerald Yoo from the Department of Electrical and Computer Engineering at Seoul National University (SNU) has developed a skin-conformal wearable health care system, "SkinECG," capable of measuring electrocardiogram (ECG) signals without a battery. By combining energy harvesting with human body–coupled power transfer, the study presents a new solution to one of the most critical challenges in wearable devices: power supply.

The findings are published in Science Advances.

Wearable health care systems are emerging as next-generation medical technologies that enable real-time monitoring of physiological signals through body-worn sensors, allowing early detection of disease-related abnormalities.

A representative example is the electrocardiography (ECG) sensor, which measures electrical signals generated by the heart and is essential for detecting cardiovascular conditions such as arrhythmia.

However, a major obstacle remains in the commercialization and long-term use of wearable devices: the battery. Batteries increase device size and weight, reducing wearability, and once depleted, they can interrupt the collection of physiological signals. In addition, the need for periodic charging and replacement poses inconvenience for users and makes continuous long-term health monitoring difficult.

To overcome this challenge, researchers have explored energy harvesting technologies that convert ambient energy sources—such as light, heat, and motion—into electricity. However, a fundamental limitation remains: the optimal location to harvest energy is often not the same as the best location to measure physiological signals.

For example, ECG sensors are typically placed on the chest, while solar cells operate most efficiently on body areas exposed to sunlight, such as the arms or legs. This creates what the research team calls a "location mismatch": the best place to harvest energy is often different from the best place to measure biosignals .

Solving the energy location mismatch

To overcome this location mismatch, Prof. Yoo's team proposed a novel power supply architecture that wirelessly delivers energy from one or more body-worn energy-harvesting devices to a remotely located ECG sensor.

The developed SkinECG system consists of a skin-adherent hydrocolloid patch integrating a flexible circuit board and a custom-designed semiconductor chip for ECG sensing, along with a multi-source wireless power supply network that transmits energy harvested from multiple devices to the sensor. This technology is termed an Orthogonal Energy Harvesting Network (O-EHN) in the study.

One or more energy-harvesting units convert ambient energy into electrical power, which is then wirelessly delivered to a chest-mounted ECG sensor via body-coupled power transfer. By transmitting power at orthogonal frequencies, the harvesters reduce interference between power signals, allowing their number and placement to be flexibly adjusted while maintaining stable power delivery to the ECG sensor.

Guiding wireless power along skin

Conventional wireless power transfer methods rely on electromagnetic radiation through the air. Near the human body, however, electromagnetic waves are often absorbed or scattered, leading to a significant drop in power-transfer efficiency.

To overcome this limitation, Prof. Yoo's team took a different approach: instead of radiating power over a distance, they guided the power along the surface of the skin.

Using body-coupled power transfer, the team wirelessly delivered power from on-body energy harvesters directly to the ECG sensor without wires. To prevent interference among multiple power sources, each harvester was assigned a distinct frequency channel, ensuring stable power reception at the sensor.

The system was also designed with human safety in mind. The power coupled to the body was kept at a level comparable to everyday exposure from ambient electromagnetic environments, ensuring safety for human use.

Toward broader battery-free health monitoring

The researchers demonstrated that the ECG sensor could be powered solely through energy harvesting, without batteries or wired connections. The system also reduces constraints on the number and placement of energy harvesters and is compatible with existing commercial energy harvesting technologies, indicating strong potential for expansion into a wide range of wearable health care devices.

This technology can be applied not only to ECG monitoring but also to long-term measurement of other physiological signals, such as electromyography (EMG) and electroencephalography (EEG). Furthermore, it holds promise as a foundational solution for addressing power supply challenges in both wearable electronics and implantable medical devices.

Prof. Jerald Yoo explained, "In wearable health care systems, there has been a fundamental limitation in that the optimal locations for harvesting ambient energy and for measuring physiological signals are different. This study resolves that issue by enabling wireless power transfer along the surface of the human body."

He added, "By limiting the power coupled to the human body to levels comparable to those encountered in daily life, we designed the system with human safety in mind while demonstrating stable power delivery to ECG sensors without bulky batteries.

"This approach can be extended to multimodal digital health care platforms capable of operating various biosignal sensors such as EMG and EEG, as well as to broader wearable power supply technologies."

This research was conducted as an international collaboration led by Prof. Yoo's team at Seoul National University, with participation from the University of Tokyo and the National University of Singapore. The team expects the results to mark an important step toward solving power-supply challenges for next-generation wearable health care systems.