Amphibious epidermal area networks for uninterrupted wireless data and power transfer

The study addresses the challenge of reliable, seamless power and data transfer for wearable body area networks (BANs) across diverse environments, including dry, humid, and underwater settings. Conventional wired BANs suffer from robustness issues, user discomfort, and poor scalability, particularly in moist or underwater conditions. Standard far-field wireless technologies (Bluetooth, Wi‑Fi, cellular) experience severe attenuation in humid/underwater environments due to limited penetration at UHF bands. Capacitive body-coupled and far-field radiative approaches also underperform near the body. Near-field magnetically coupled solutions offer improved path loss near the body but lack compatibility with existing consumer systems. The purpose of this work is to realize a skin-integrated, amphibious BAN that provides uninterrupted wireless power and data transfer using magneto-inductive (MI) metamaterial waveguides operating at 13.56 MHz (NFC band), compatible with consumer electronics, and robust to environmental changes, enabling applications in VR/AR, precision health, and activity monitoring.

Background literature highlights the limitations of wired BANs and far-field wireless protocols near the body and underwater, and prior advances in near-field and metamaterial-enabled body-centric communications. Prior studies have shown improved path loss for near-field magnetic coupling compared to capacitive and far-field methods but with limited compatibility to consumer platforms. Metamaterials can act as near-field EM waveguides via chains of magnetically coupled resonators, offering engineered spectral and mechanical properties with reduced sensitivity to environmental permittivity. Existing wearable materials and inks (e.g., silver-based inks) face stretchability and conductivity trade-offs. Underwater wireless communications commonly rely on acoustics; EM approaches at high frequencies face severe losses, motivating low-frequency magnetic induction and MI waveguides. This work builds on metamaterial textiles and MI waveguides to create an epidermal MI-BAN compatible with NFC.

Design and theory: The BAN employs arrays of magnetically coupled resonators (multilayer planar loops) with self-inductance L and resistance R, tuned by an effective capacitance C to resonate at 13.56 MHz. Stray capacitances to background and skin and corresponding losses (G and B) are mitigated by encapsulating the loops in a thin nonconductive insulator, forming MI metamaterials with reduced sensitivity to surrounding permittivity and conductivity. Neighboring resonators are inductively coupled with mutual inductance M = kL, with slight geometric overlap (spacing d) to sustain coupling. Dispersion and propagation along linear arrays are modeled by cascading sub-model ABCD matrices (T_M ⊗ T_R ⊗ T_C) to form the unit cell transfer matrix; the propagation constant γ = β + jα is derived from unit-cell eigenvalues and used to predict S-parameter transmission. Comparative modeling against far-field propagation (Bluetooth/Wi‑Fi bands) assesses environmental sensitivity. Fabrication: A stretchable, waterborne silver-flake conductive ink is formulated using PVAc (Glue-All) as matrix, glycerol triacetate as plasticizer, and borax crosslinkers, enabling low-temperature curing and biocompatibility. Patterns are created via laser-cut vinyl stencils on PDMS membranes; ink is applied, stencils removed, and multilayer loops closed by overlapping ends with additional ink, then post-cured. Surface-mount tuning capacitors are added. Coils are encapsulated with PDMS (optimized 0.5 mm thickness) to insulate from lossy media and provide waterproofing. Characterization: Transmission S21 of 90 cm arrays is measured under varying PDMS thicknesses (0.25–0.75 mm) and backgrounds (tissue, saline up to 35 g/L NaCl, water) using a vector network analyzer. Mechanical tests include strain-to-failure, resonance stability under normal strain up to 15%, extreme stretchability up to 120% strain, and cyclic bending (0–180°) for up to 10,000 cycles while monitoring resonance (S11). Passband bandwidth engineering is achieved by setting coil overlap (~20 mm, ~15% of coil length) to exceed 84 kHz to support NFC ASK data rates and accommodate bending-induced shifts. Arrays include serpentine structures at joints for flexibility. System integration: The epidermal BAN is assembled by placing encapsulated rectangular coils along the skin, with serpentine coils across joints, adhered by transparent medical film. Passive NFC sensors (TI RF430FRL153H) with integrated loop antennas and external resistive strain gauges are encapsulated for underwater use and powered via the MI-BAN. An NFC reader with a small battery serves as the active master node, powering sensors and receiving data; software-based time-domain multiple access provides sequential interrogation without additional multiplexing hardware. Consumer NFC-enabled smartphones can act as readers. Experiments: Arm activity monitoring is performed during light swimming in an outdoor pool. The reader collects data at an overall 14 Hz refresh (≈4.6 Hz per sensor), with calibration mapping raw ADC values to joint angles. Packet reception ratio (PRR) is computed as successfully received packets per requested packets per refresh across nodes. Underwater robustness is benchmarked against Bluetooth by comparing PRR and RSSI when transitioning from dry to submerged conditions. SAR is simulated per NFC constraints (max H-field 10.5 mA), and compliance assessed against standards. Long-term underwater PRR stability is also recorded. Additional method details include transfer matrix derivations for magnetic and conducted coupling, silver ink synthesis steps, sensor encapsulation procedures, and Bluetooth RSSI measurement protocols.

- MI-BAN robustness across environments: The magnetic nature of coupling renders transmission largely insensitive to lossy media conductivity; the encapsulated MI channel maintains a stable passband from dry to underwater conditions. - Insulation optimization: PDMS encapsulation of 0.5 mm improves path loss by 4.4 dB/m relative to 0.25 mm; increasing from 0.5 to 0.75 mm yields a smaller 1.1 dB/m improvement, leading to selection of 0.5 mm as optimal for balance of flexibility and performance. - Mechanical performance: Encapsulated resonators withstand up to 120% strain without mechanical failure; resonance is hysteresis-free under up to 15% normal strain; bending reliability shows stable central frequency over 10,000 full 0–180° bending cycles. - Bandwidth engineering: Approximately 20 mm horizontal overlap (~15% of coil length) between neighboring coils achieves bandwidth >84 kHz, sufficient for NFC ASK data rates and to cover bending-induced detuning. - System operation: Battery-free passive sensors (≈3 g each) are powered and read via the MI-BAN; reader refresh rate is 14 Hz overall (~4.6 Hz per sensor) with software TDMA. - Underwater communication: During gradient wetting and full submersion at 10 Hz refresh, the MI-BAN exhibits ~23% packet loss; long-term underwater operation maintains PRR >89%. - Comparison to Bluetooth: Bluetooth RSSI drops instantaneously upon submerging the beacon, indicating failed communication, whereas the MI-BAN continues to operate with manageable packet loss, underscoring superior robustness underwater. - Safety: Simulated SAR under NFC field limits yields 8.1 mW/kg averaged over muscle volume, well below the 400 mW/kg industry standard at ~1.3 MHz.

The results demonstrate that a skin-integrated MI metamaterial network at 13.56 MHz enables continuous power and data transfer across a wide range of environmental conditions, including underwater, addressing a key limitation of conventional far-field wireless protocols near the human body in lossy media. The insensitivity of magneto-inductive coupling to surrounding permittivity and conductivity, combined with thin PDMS encapsulation, preserves the transmission band and reduces path loss across environments. Engineered coil overlap ensures sufficient bandwidth for NFC ASK data rates and tolerance to motion-induced detuning, while serpentine geometries at joints maintain mechanical compliance without sacrificing RF performance. The system’s compatibility with off-the-shelf NFC readers (including smartphones) enables practical deployment without specialized hardware. Underwater tests validate reliable operation (PRR typically >89% long-term), with acceptable packet loss during transitions, outperforming Bluetooth whose far-field link fails underwater. The mechanical durability (120% strain tolerance, 10,000-cycle bending stability) and low SAR support safe, comfortable, and reliable long-term wear. Collectively, these findings confirm that MI-BANs can deliver uninterrupted, battery-free sensing and communication for activity monitoring and other wearable applications in challenging environments where traditional wireless solutions fail.

This work introduces an amphibious epidermal MI-BAN that provides uninterrupted wireless power and data transfer across dry, humid, and underwater environments, while remaining compatible with consumer NFC electronics. Key contributions include: (1) an encapsulated magneto-inductive metamaterial waveguide on skin with engineered bandwidth and low environmental sensitivity; (2) a biocompatible, waterborne stretchable silver ink and fabrication process supporting highly flexible, waterproof resonators; (3) demonstration of battery-free sensing with robust performance underwater and during motion; and (4) validation of safety and mechanical durability. Future research directions include scaling to larger multi-node networks and higher aggregate data rates, integrating with AR/VR platforms and textile-based routes, optimizing routing and scheduling for dynamic activities, further reducing packet loss during rapid wetting/submersion transitions, and expanding sensor modalities for comprehensive physiological and biomechanical monitoring.

- Underwater transitions cause noticeable packet loss (~23% at 10 Hz during gradient wetting/full submersion), indicating room for improving robustness during rapid environmental changes. - Data rate is constrained by the NFC-band operation and required passband (>84 kHz), which may limit high-throughput applications. - Mechanical failure occurs in not fully encapsulated structures (exposed ink) at ~25% strain, emphasizing the need for proper encapsulation. - Experiments were not randomized; sample size was not predetermined; investigators were not blinded, which may limit generalizability of performance metrics. - RF performance above 2.5 GHz may degrade in the presence of sweat or nearby metallic objects; while the system operates at 13.56 MHz, environmental metals and user conditions could still influence performance and require consideration in deployments.