Amphibious epidermal area networks for uninterrupted wireless data and power transfer

The seamless integration of wearable sensors across the human body for applications in virtual/augmented reality, precision health, and activity monitoring necessitates robust, multi-environment compatible communication solutions. Current Body Area Networks (BANs) often rely on wired connections which are unsuitable for many applications. Wireless solutions like Bluetooth, Wi-Fi, and cellular are ineffective in humid or underwater settings due to low signal penetration. Near-field magnetic coupling offers improved path loss near the body but lacks compatibility with existing systems. This research addresses this gap by developing an epidermal BAN using metamaterials for uninterrupted wireless power and data transfer in diverse environments.

Existing BANs face limitations due to wired connections' fragility and discomfort in wearable applications, restricting their use to clinical settings. While wireless technologies offer mobility, their far-field nature compromises performance in challenging environments like underwater or humid conditions. Near-field magnetically coupled antennas show promise, but integration with current communication systems remains a challenge. Previous work has explored metamaterials for near-field EM waveguides, enabling propagation across magnetically coupled resonators with reduced sensitivity to surrounding media. This research builds upon this foundation to create a fully integrated, amphibious BAN.

The proposed BAN employs arrays of magnetically coupled resonators constructed from multilayer planar loops with engineered inductance, resistance, and capacitance. Stray capacitance between loops and the surrounding environment is considered in the model. Lossy dielectric characteristics are controlled by encapsulating loops in a thin, non-conductive insulator. The dispersion characteristics of complex resonator structures were systematically derived by cascading the transfer matrices of sub-models representing mutual inductive coupling, loop resistance, and the surrounding environment. A novel waterborne silver ink formulation, combining a liquid glue matrix, silver flakes, a plasticizer, and crosslinkers, was developed for stretchable coil fabrication. This ink exhibits superior conductivity and stretchability compared to traditional silver inks, allowing for the creation of flexible and robust resonators. The coils were encapsulated with PDMS to enhance performance in various media, and the resulting BAN's performance was experimentally validated across various conditions including dry indoor/outdoor settings and underwater environments. The system's mechanical and electrical stability were assessed through strain and bending tests. Finally, the BAN was integrated with passive NFC sensors and an NFC reader for data transmission and power harvesting. The specific absorption rate (SAR) was also simulated.

The researchers successfully demonstrated a functional BAN operating reliably in dry and underwater environments. The metamaterial design exhibited minimal sensitivity to changes in the surrounding medium's dielectric properties, ensuring consistent performance across diverse environments. The novel waterborne silver ink enabled the creation of highly stretchable and conductive coils, critical for comfortable and robust wearable applications. Experimental results showed significant path loss enhancement with increased PDMS encapsulation thickness. The system achieved a packet reception rate (PRR) exceeding 89% in long-term underwater tests, substantially outperforming Bluetooth in underwater conditions. The BAN demonstrated stable electromagnetic performance even under various joint movements, highlighting its robustness during activity monitoring. SAR simulations confirmed compliance with NFC safety standards. The system’s successful implementation of mid-range underwater wireless power and data transmission represents a significant breakthrough in wearable technology, overcoming limitations of existing solutions.

This research addresses the critical need for reliable, multi-environment BANs for various applications. The use of metamaterials and a novel silver ink addresses the limitations of existing technologies by offering reliable near-field communication in diverse and challenging environments. The demonstrated performance in underwater settings opens new possibilities for activity monitoring and health monitoring applications in aquatic environments. The compatibility with existing NFC infrastructure simplifies integration and reduces complexity, enhancing the practicality of the technology. The system's superior performance compared to Bluetooth in underwater settings confirms the efficacy of the proposed approach.

This paper presents the first amphibious, self-powered BAN compatible with existing consumer electronics. The system's successful demonstration in underwater settings opens new avenues for seamless and uninterrupted human activity and health monitoring. Future work could explore the integration with augmented/virtual reality devices and further miniaturization of components.

While the study demonstrates impressive performance, potential limitations include the range of the near-field communication, which might be restricted compared to far-field solutions. Further research could also investigate the long-term stability of the silver ink and PDMS encapsulation in extreme conditions. The current study focused on a specific activity monitoring application; exploring other applications and optimizing the system for different use cases is warranted.