Wireless battery-free body sensor networks using near-field-enabled clothing

Wearable sensors offer continuous health monitoring outside clinical settings, improving diagnosis and treatment. Advances have produced various skin-mounted sensors measuring temperature, electrical activity, and other physiological parameters. Simultaneous multi-location recording enhances utility and reliability, but interconnecting multiple sensors is challenging. Current clinical systems use wires, restricting movement and limiting use outside clinics. Wireless technologies offer freedom, but radio-based methods like Bluetooth and Wi-Fi require individual sensor power sources (batteries or energy harvesters), hindering conformability and comfort, and requiring replacements. Near-field communication (NFC) is an alternative: battery-free, low-cost, and secure, but it's limited to a few centimeters sensor-reader separation, restricting application during unconstrained activity. While some studies have demonstrated multiple battery-free sensors, they often rely on large readers incompatible with upright activity or require individually powered readout circuits. This research demonstrates continuous physiological monitoring using a battery-free body sensor network via near-field-enabled clothing, establishing wireless power and data connectivity across the body. Conductive threads and computer-controlled embroidery integrate ordinary clothing with near-field-responsive inductor patterns, wirelessly connecting skin-mounted sensors to a reader up to a meter away. Unlike previous textile integrations of NFC, this approach is entirely fabric-based, robust, and compatible with unmodified NFC-enabled devices. The system is demonstrated for spinal posture monitoring and continuous temperature and gait measurement during exercise.

The paper extensively reviews existing wearable sensor technologies and their limitations. It highlights the challenges associated with wired systems, including limited mobility and impracticality outside clinical settings. Radio-based wireless solutions are also discussed, emphasizing the drawbacks of requiring individual power sources for each sensor, which adds bulk and complexity. The authors specifically address the advantages and limitations of Near Field Communication (NFC) technology, noting its potential for battery-free, secure operation but acknowledging its short operational range as a key limitation. Previous research on battery-free sensor networks for temperature/pressure mapping and neonatal monitoring is critically examined, outlining their limitations in terms of scalability and applicability to dynamic movement. The review sets the stage for the proposed novel approach of using near-field-enabled clothing to overcome the range limitations of NFC-based sensor networks.

The study involved several key methodological components: 1. **Near-field-enabled clothing design and fabrication:** The researchers designed and fabricated clothing incorporating conductive thread embroidered patterns that act as near-field relays. These relays enhance the communication range of NFC systems. Computer-controlled embroidery was used to create the patterns on standard textiles, ensuring robustness and suitability for daily wear. The conductive thread was selected for its flexibility and resistance to wear and tear. The design process integrated circuit modeling and full-wave simulations to optimize power transfer efficiency (η) as a function of various parameters such as relay length, inductor radius, turn number, and wire gap. This optimization ensured reliable wireless power and data transfer between the reader and distant sensors. 2. **Electromagnetic simulations:** Full-wave simulations using CST Microwave Studio were employed to model the electromagnetic behavior of the near-field relays. These simulations allowed the researchers to optimize the design of the conductive thread patterns to maximize power transfer efficiency and operational range. 3. **Experimental validation:** The functionality of the near-field relays was validated experimentally using a smartphone-sized NFC antenna and NFC sensors. Measurements of power transfer efficiency were conducted to confirm the extended range capabilities of the system under various conditions, including varying distances, orientations, and numbers of sensor nodes. The experimental results showed good agreement with the simulation predictions. The system's robustness to deformation and wetting was also tested, showing minimal performance degradation under realistic wear conditions. 4. **Sensor node development:** Custom resistive elastomeric strain gauges were integrated with commercial NFC chipsets to create battery-free strain sensors for posture monitoring. These sensors were designed for skin conformability and flexibility and demonstrated accurate wireless strain measurements comparable to wired readings. Similarly, battery-free temperature sensors were developed and calibrated against wired thermocouples. 5. **Multi-node operation and data acquisition:** The researchers investigated the multi-node capabilities of their system using parallel and series network configurations for the near-field relays. The system's ability to simultaneously readout multiple sensors was validated using standard NFC multiplexing protocols and a commercial NFC reader supporting anti-collision protocols. Data rates and maximum operational distances were measured to assess the system's performance with increasing numbers of sensor nodes. 6. **Application demonstrations:** The near-field-enabled clothing and sensor network were demonstrated for two applications: multi-node spinal posture monitoring and continuous exercise monitoring. For spinal posture monitoring, battery-free strain sensors were placed along the spine, and their data were wirelessly acquired using the clothing-integrated relays. For exercise monitoring, temperature and strain sensors were integrated to measure axillary temperature and knee strain during running on a treadmill, respectively. Results were compared to reference measurements from a motion capture system and a gyroscope. The methods section describes the numerical simulations, fabrication of near-field enabled clothing, wireless measurements, power transfer efficiency calculation, and all the experimental details of the two applications with sufficient details.

The key findings of this research demonstrate the successful development and validation of a novel wireless, battery-free body sensor network using near-field-enabled clothing. Specific findings include: 1. **Extended range NFC:** The near-field relays integrated into the clothing significantly extended the operational range of NFC from a few centimeters to over a meter, enabling wireless power and data transfer between a reader and multiple sensors placed at various locations on the body. This was experimentally validated with power transfer efficiencies exceeding 6% over a 1-meter relay within 1.5 cm proximity. 2. **Fabric-based design:** The near-field relays were entirely fabric-based, eliminating fragile silicon components and improving the robustness and durability of the system for daily wear. The conductive thread demonstrated high resilience to bending and immersion in hot water, maintaining consistent performance over extended periods. 3. **Multi-node sensing:** The system successfully demonstrated simultaneous readout of multiple sensors in both parallel and series configurations. While series interconnection offered higher efficiency, the parallel configuration provided greater robustness to individual inductor failures. The study showed that with six terminals, the transfer efficiency to each sensor was sufficient for reliable communication. 4. **Scalability analysis:** A systematic study was conducted to assess the scaling of system performance with an increasing number of sensor nodes. While maintaining constant reader output power, the study showed that the maximum sensor readout distance and the data rate decreased as the number of sensors increased, indicating a trade-off between scalability and performance. Total reader power consumption remained constant. 5. **Robustness to deformation and wetting:** The near-field relays exhibited high robustness to deformation and wetting, showing minimal performance degradation when subjected to bending, curving, or water immersion. This makes the system practical for use in various environments and conditions. 6. **Spinal posture monitoring:** The system's ability to perform continuous real-time multi-node spinal posture monitoring was demonstrated, enabling the differentiation of neck, lower back, and whole spine movements. The wirelessly acquired strain measurements showed excellent agreement with data obtained from a camera-based motion capture system. 7. **Continuous exercise monitoring:** The system demonstrated continuous real-time monitoring of axillary temperature and gait during untethered running on a treadmill. Axillary temperature measurements showed expected increases during acclimatization and exercise, and gait measurements based on knee strain showed strong correlation with gyroscope-based angular velocity measurements. The system reliably monitored these parameters for an extended duration, even with perspiration and movement.

This research successfully addresses the limitations of existing wireless sensor technologies by demonstrating a robust, scalable, and practical battery-free body sensor network using near-field-enabled clothing. The extended range capability of the near-field relays, coupled with the flexible and durable fabric-based design, overcomes the short-range limitation of conventional NFC and enables continuous monitoring in unconstrained environments. The successful demonstration of spinal posture monitoring and exercise monitoring highlights the potential applications of this technology in diverse fields such as healthcare, athletics, and human-computer interaction. The findings contribute significantly to the advancement of wearable sensor technology by providing a solution for multi-node wireless sensing with improved comfort, reliability, and scalability. The ability to integrate various sensing modalities and the system's robustness to real-world conditions make this technology particularly promising for long-term, continuous health monitoring applications.

This paper presents a significant advancement in wearable sensor technology by introducing near-field-enabled clothing for creating battery-free, wireless body sensor networks. The system successfully extends the operational range of NFC, enabling continuous monitoring of physiological signals from multiple locations on the body. The fabric-based design enhances durability and comfort, and the system's robustness to deformation and wetting makes it suitable for various applications. Future research could focus on integrating additional sensing modalities, enhancing data rates and multiplexing capabilities, and exploring advanced materials to further improve the system's performance and scalability.

While the study demonstrates the potential of near-field-enabled clothing for creating battery-free sensor networks, several limitations should be considered. The power transfer efficiency decreases with an increasing number of sensor nodes, limiting scalability. The system's performance is sensitive to the alignment between the sensors and the inductive patterns in the clothing, potentially affecting the accuracy and reliability of measurements. Further research is needed to validate the long-term reliability of the sensor nodes and to explore user experience improvements, such as aesthetic modifications to the clothing design.