Meander Coil++: A Body-scale Wireless Power Transmission Using Safe-to-body and Energy-efficient Transmitter Coil

Title of the Paper

Meander Coil++: A Body-scale Wireless Power Transmission Using Safe-to-body and Energy-efficient Transmitter Coil

Paper Information

• Research Domain: Wireless energy transmission for wearable devices
• Keywords: Wireless power transmission, liquid metal, safe electromagnetic exposure, woven coil, body-scale

Research Background and Issues

• Problems and Challenges:

  1. Wearable devices require frequent charging, which affects their convenience for long-term use.
  2. Conventional textile-based wireless charging coils suffer from high resistance, leading to low transmission efficiency (1%-3%) and potential safety issues due to strong electromagnetic interference with the human body.
  3. Some existing high-efficiency coils (e.g., copper wires) are too rigid for textile integration, compromising the wearability and comfort of clothing.
  4. Although liquid metal has good conductivity, previous studies need to improve its durability and transmission performance in practical applications.

• Significance of the Research: Inefficient energy management and safety limitations for the human body severely hinder the adoption of wearable computing and personal health devices. This study aims to address these issues, enabling seamless integration of devices into daily life.

• Motivation and Related Work: The authors combined the latest advancements in energy harvesting and wireless power transmission technologies, leveraging liquid metal and modern weaving techniques to address problems such as low charging efficiency, poor wearability, and safety concerns caused by traditional coils.

Solution

• Methods and Innovations:

  1. Proposed a coil architecture named Meander Coil++:
     • Utilization of liquid metal: High conductivity, low loss, stretchable;
     • Patterned coil design: Balances energy efficiency while reducing coil radiation interference for safety considerations.
  2. Adopted Distributed Capacitor Arrangement (DCA) to mitigate the capacitive coupling effect between the coil and the human body at high frequencies.
  3. The overall system design is based on magnetic resonance coupling principles (WPT-MRC) to enhance loose coupling transmission efficiency between the coil and devices.

• Implementation Steps and Key Technologies:

  • TX Coil Component: Liquid metal (e.g., eGaIn injected into silicone tubes) is used for the coil, combined with hand-soldered chip capacitors and durable textile techniques to maintain structural flexibility.
  • Power Module Design: Includes a stable AC power source, adjustable capacitor arrays, and an RX coil detection module.
  • RX Receiving Devices: Designed multiple RX coils adaptable to various application scenarios, supporting devices such as smartwatches and sensors.

• Innovative Contributions:

  • Applied low-loss liquid metal to the field of textile electronics.
  • Combined distributed capacitors to significantly reduce electromagnetic radiation exposure while improving energy utilization efficiency.

Research Outcomes

• Specific Results:

  1. High-efficiency Power Supply: Achieved 25% DC-to-DC efficiency to deliver several watts of power to multiple fixed/dynamic devices such as smartwatches and phones, far exceeding the capabilities of existing textile-based wireless transmission (which only transmits tens of milliwatts).
  2. Safety Assurance: Complies with the International Commission on Non-Ionizing Radiation Protection (ICNIRP) standards for electromagnetic exposure to the human body.
  3. Durability:
     • Withstands approximately 1,000 stretches and 30-40 washes without significant conductivity or loss issues.
     • Estimated average lifespan of several years.
  4. Good Wearability: The TX system is lightweight (total weight approximately 133g), can be integrated into clothing, and is comfortable to wear.

• Advantages Compared to Existing Solutions:

  • Energy Efficiency Improvement: The application of low-loss liquid metal overcomes the high resistance issues of traditional textile-based wireless charging solutions, enhancing energy transmission efficiency.
  • Human-friendly Design: The new coil design suppresses electromagnetic exposure to the human body, ensuring safe close-contact usage.
  • Enhanced Flexibility: Supports a wide range of human movements without concerns about fixed positioning (invisible wiring).

• Experimental and Evaluation Results:

  • Maximum transmission efficiency reached 41% (AC-to-AC), maintaining high efficiency while the user is standing.
  • Actions such as receiver displacement or user bending slightly affect efficiency, but dynamic capacitor adjustments can partially mitigate these effects.

• Limitations and Future Directions:

  • Flexibility Constraints: The production of liquid metal coil systems still requires significant manual operations, making large-scale manufacturing challenging.
  • Insufficient Long-term Durability Verification: Liquid metal needs to remain above 15.7°C to stay in liquid form, requiring avoidance of extreme conditions that could affect device lifespan.
  • Lack of User Studies: Large-scale user testing is needed to comprehensively evaluate coil transmission performance across different body types and clothing styles.
  • Improvement in Manufacturing Processes: Optimization of chip installation and batch packaging technologies is especially needed.

Conclusion

The proposed Meander Coil++ combines liquid metal technology with human-centered design to provide a novel solution for long-term wearable and scalable wireless power transmission. The study highlights the potential of this new technology in enhancing efficiency, improving comfort, and ensuring safe usage, pointing the way toward sustainable applications in everyday smart clothing.