A robust near-field body area network based on coaxially-shielded textile metamaterial

Abstract

A body area network involving wearable sensors distributed around the human body can continuously monitor physiological signals, finding applications in personal healthcare and athletic evaluation. Existing solutions for near-field body area networks, while facilitating reliable and secure interconnection among battery-free sensors, face challenges including limited spectral stability against external interference. Here we demonstrate a textile metamaterial featuring a coaxially-shielded internal structure designed to mitigate interference from extraneous loadings. The metamaterial can be patterned onto clothing to form a scalable, customizable network, enabling communication between near-field reading devices and battery-free sensing nodes placed within the network. Proof of concept demonstration shows the metamaterial's robustness against mechanical deformation and exposure to lossy, conductive saline solutions, underscoring its potential applications in wet environments, particularly in athletic activities involving water or significant perspiration, offering insights for the future development of radio frequency components for a robust body area network at a system level.

Fig. 1. Coaxially-shielded textile metamaterial enabled robust near-field body area network (BAN).

a Concept of near-field BAN-integrated smart clothing, capable of interconnecting multiple sensing nodes while remaining robust against rainy environment and water. b Digital embroidery process and the sectional view of the open slit inside the coaxially-shielded metamaterial unit cell. c A 5-unit metamaterial array being used to simultaneously power three LED nodes and a wireless sensing node through an NFC-enabled smartphone. Simulated surface current density on the associated conductor surfaces (dimension is not to scale) inside a metamaterial unit cell (d) and the integrated surface current profile along the metamaterial conductor (e). Inset: Cross-sectional view of the coaxial cable where the current is integrated along the azimuthal direction. Gray arrows denote the electric field confinement. Simulated side view (longer side) of the electric field $∣E∣$ (f) and magnetic field $∣H∣$ (g) distribution in the vicinity of a copper spiral resonator and a coaxially-shielded resonator, sharing the same dimension and tuned to 13.56 MHz. Electric field (h) and magnetic field (i) plotted along the white dashed lines in (f, g). j The metamaterial’s reflection coefficient S₁₁ remains stable after mechanical deformations.

Fig. 2. Electromagnetic characterizations of the body area network (BAN).

a Inter-unit cell coupling coefficient k as a function of the inter-unit cell overlap α. b Measured transmission coefficient S₂₁ of BANs with different α and numbers of unit cells N. The NFC load modulation requires an NFC passband that can be used to leverage an optimal α value. c Measured transmission coefficient and the simulated magnetic field strength along a 12-unit inline array. d Simulated magnetic field $∣H∣$ at 13.56 MHz along two arrays, showing robust signal transmission. The 12-unit inline array demonstrates spectral stability after machine washing, being drenched, or entirely immersed in DI water when configured straight (e), with multiple bends (f), and 720° winded (g). Insets: Pictures of the textile metamaterial array being deformed while powering three battery-free LED nodes. h The BAN is insensitive to loss induced by conductive matter, covering the NFC passband when submerged in saline solutions. Inset: Measured transmission coefficient at 13.56 MHz with increasing salt concentration.

Fig. 3. Increased stretchability with the stretchable joint unit.

a Picture of the stretchable joint unit that adopts an alternative internal topology. b Equivalent circuit diagram of the joint unit. L and R represent the inductance and series resistance of the inner conductor on each side of the serpentine link, $C_G$ represents the capacitance formed at the gap of the outer cuts, $C_1$ and $C_2$ represent the large structural capacitance formed between the outer conductor and the inner conductor at each side of the outer cuts. c Measured reflection coefficient S₁₁ of the joint unit under different rotating angle θ. d–f Simulated surface current density on the associated conductor surfaces (dimension is not to scale) for the working mode (d) and the higher second mode (e), and the integrated surface current profile along the conductor of the joint unit (f). g, h Simulated magnetic field $∣H∣$ at 13.56 MHz showing weakened transmission when bending an array without a joint unit (g), in contrast to robust signal transmission passing a bent joint unit (h). i Measured transmission coefficient S₂₁ at 13.56 MHz of two 12-unit metamaterial arrays (with and without replacing the sixth and seventh units with a joint unit) as a function of the degree of bending. Insets: Pictures showing improved stretchability at shoulder and knee joints.

Fig. 4. Construction of the body area network (BAN) using near-field communication (NFC) transponders with coaxially-shielded antennas.

a Simplified equivalent circuit diagram of an NFC tag integrated with a coaxially-shielded antenna, the current distribution at the antenna’s outer cut and equivalent circuit diagram of the antenna. L and R represent the inductance and series resistance of the inner conductor, $C_G$ represents the capacitance formed at the gap of the outer cut, $C_1$ and $C_2$ represent the structural capacitance formed between the outer conductor and the inner conductor at each side of the outer cut. $C_T$ is the tuning capacitor. Measured quality factor preservation $Q_{Loaded}/Q_{Unloaded}$ (b) and self-capacitance variations $\Delta C/C_{Unloaded}$ (c) of the two antennas when affixed to skin (1), wetted by sweat (2) and submerged in water (3). d Measured reflection coefficient S₁₁ of the two antennas. Data transfer through NFC tag with conductive thread antenna is interrupted during the submersion in water (e) while the coaxially-shielded antenna promotes continuous data readout (f). g Proof of concept demonstration of multi-node signal monitoring through textile metamaterial BAN-integrated clothing. The sensor data is simultaneously recorded by three battery-free wireless sensing nodes. Calibrations of the thermistor using a wired thermocouple (h) and the strain sensor using a multimeter (i).