A double-layered liquid metal-based electrochemical sensing system on fabric as a wearable detector for glucose in sweat

The work addresses the need for comfortable, flexible, and conformable wearable electrochemical biosensing systems for health monitoring and medical applications. Conventional PCB-based systems lack air permeability and conformability, while many flexible substrates (SEBS, PI, PDMS, PTFE) still hinder skin breathability. Fabrics offer skin-friendliness and air permeability but complex multilayer interconnections are hard to realize with conventional conductive inks. The study proposes using a room-temperature liquid metal (Galinstan) patterned directly on fabric to realize a compact, double-layer electrochemical sensing system capable of detecting glucose in sweat, aiming to combine miniaturization, flexibility, air permeability, and reliable electrochemical performance.

Prior efforts in wearable electrochemical biosensing include integrated systems for glucose monitoring and insulin delivery using microneedles coupled with flexible electronics; battery-free, wireless flexible smart wound dressings monitoring temperature, pH, and uric acid with controlled drug delivery; mass-produced laser-engraved multimodal sensors integrated with skin patches for continuous monitoring of vital signs and metabolites; and battery-free platforms powered by flexible triboelectric nanogenerators to operate multiplexed sweat biosensors with wireless transmission. Despite advances, many flexible substrates restrict skin breathability, and fabric-based sensors—though comfortable and air-permeable—face challenges fabricating complex multilayer interconnections using conductive inks such as nano-copper, nano-silver, nano-gold, and graphite. Liquid metals have emerged as promising flexible conductors due to high mobility, conductivity, self-healing, and extensibility, but typical patterning methods can be complex or require material pretreatments that reduce fluidity or conductivity. The present work leverages unmodified liquid metal with PMA-assisted adhesion to address these challenges on fabric.

System architecture: The wearable electrochemical sensing system comprises (i) a flexible fabric circuit implementing the detection electronics, (ii) a replaceable electrochemical electrode, and (iii) LabVIEW-based control and data acquisition software on a laptop. The detection electronics include modules for power (REF3318 reference, AMS1117 regulator providing 3.3 V and 1.8 V), a potentiostat (precision amplifier AD8572 maintaining CE and RE potentials via feedback), I/V conversion, amplification and filtering to measure nanoamp–microamp currents and suppress interference, and an ADC/DAC interface using a microcontroller (STM32f042f4p6) for digitization and transmission to the laptop. Fabrication: The circuit is implemented on 100% cotton fabric using unmodified Galinstan (68.5% Ga, 21.5% In, 10% Sn) as the conductor. A template printing method is employed. Polymethacrylate (PMA) glue is first printed only where traces are designed to enhance adhesion via interactions (hydrogen bonding) between the gallium oxide skin and PMA, preserving overall fabric breathability elsewhere. Liquid metal is then patterned directly along the glued regions to form upper and lower circuit layers. Vias at intersections are created by pushpin drilling, and interlayer electrical connections are made by applying liquid metal through the holes. Electronic components are attached to the patterned traces, and the two layers are aligned and bonded to form a compact double-layer circuit. This approach avoids complex equipment (e.g., vacuum suction, spray guns, 3D printers) or liquid metal pretreatments/doping. Software: A LabVIEW GUI controls the detection sequence, sets potentials (0–3 V range), starts/stops acquisition, displays real-time voltage/current, resets the system, and saves data/plots. Characterization and tests: Patterning fidelity is evaluated by comparing printed linewidths to template designs across 12 traces. Electrical stability is assessed by measuring trace resistances at Month 0 and Month 16. Contact resistances between component pins and liquid metal traces are measured at Month 0 and Month 6 on randomly selected pins of ten components. Microscopic and SEM imaging, including cross-sections and contact angle assessments, examine pattern quality, adhesion, and surface oxidation. Electrochemical functionality is verified using potassium ferricyanide. Application feasibility is demonstrated by detecting glucose in artificial sweat at millimolar concentrations.

- Patterning accuracy: Across 12 liquid metal traces, printed linewidths closely matched design values, with maximum error 5.16% (Line 12) and minimum error 0.63% (Line 1), indicating reliable PMA-assisted patterning on fabric. - Baseline resistances (Month 0): Line 2 (length 53.706 mm, width 0.508 mm) showed 1.46 ± 0.10 Ω; Line 6 (length 12.475 mm, width 0.508 mm) showed 0.68 ± 0.07 Ω. Lines with 0.381 mm width had resistances: Line 8 = 0.82 ± 0.04 Ω, Line 11 = 0.74 ± 0.08 Ω, Line 12 = 0.88 ± 0.19 Ω. - Aging (Month 16): All trace resistances increased slightly after 16 months; the largest reported resistance was for Line 6 at 1.76 ± 0.12 Ω. Despite this, trace resistances remained negligible compared to circuit resistors (most in kΩ; minimum designed resistor 33 Ω), indicating minimal impact on overall circuit performance. - Contact resistance stability: Component–trace contact resistances were initially 0.1–0.2 Ω (Month 0) and increased by Month 6 but remained low (<2 Ω). The maximum measured value occurred at pin 1 of chip U1. - Physical quality: Inverted microscopy showed straight patterns with neat edges. SEM imaging indicated good adhesion in PMA-treated regions and suggested increased surface roughness/oxide after long-term storage. - System function: The double-layer liquid metal fabric circuit supported a potentiostat-based detection module with real-time laptop visualization via LabVIEW. Electrochemical tests with potassium ferricyanide verified functionality. The system detected glucose in artificial sweat at millimolar concentrations, demonstrating feasibility for wearable sweat glucose monitoring.

The study demonstrates that unmodified Galinstan can be reliably patterned onto fabric using a simple PMA-assisted template printing approach to create multilayer, interconnected circuits with high fidelity and low resistance. The resulting double-layer architecture enables miniaturization while maintaining the intrinsic flexibility, conformability, and air permeability of fabric—key requirements for wearable comfort and continuous monitoring. Electrical measurements over months indicate that although trace and contact resistances increase slightly due to oxidation/corrosion, these changes remain small relative to the circuit’s designed resistances and do not materially affect function. Microscopy and SEM corroborate pattern quality and adhesion. Electrochemical validation with ferricyanide and successful detection of glucose in artificial sweat at millimolar levels show that the platform can perform practical biosensing tasks. Collectively, the findings address the challenge of building breathable, fabric-based, multilayer electrochemical systems and highlight their relevance for wearable health monitoring and point-of-care applications.

The work introduces a miniaturized, double-layer liquid metal electrochemical sensing system fabricated directly on breathable fabric using a simple template method with PMA-assisted adhesion. The system integrates a potentiostat, I/V conversion, amplification/filtering, and microcontroller-based ADC/DAC into a flexible platform with LabVIEW control, achieving reliable patterning, low resistance, and time-stable performance. Electrochemical testing verifies functionality, and glucose detection in artificial sweat at millimolar levels demonstrates application potential for wearable health monitoring.

- Aging effects: Trace resistances increased slightly over 16 months, likely due to oxidation/corrosion, and component–trace contact resistances rose over 6 months (though remaining <2 Ω). These aging effects may influence long-term performance and should be managed in prolonged deployments. - Scope of sensing validation: While potassium ferricyanide tests and millimolar-level glucose detection in artificial sweat demonstrate feasibility, detailed analytical figures of merit (e.g., limit of detection, selectivity in complex matrices, on-body performance) are not provided in the available text.