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

Electrochemical detection methods offer high sensitivity and specificity for detecting biological substances, crucial for medical diagnosis and health monitoring. Wearable electrochemical biosensors are gaining prominence due to the increasing demand for continuous physiological monitoring, especially for chronic diseases like diabetes. Existing wearable systems often utilize flexible substrates like SEBS, PI, PDMS, and PTFE, but these materials can lack sufficient air permeability, potentially causing skin irritation. Fabrics offer a more comfortable and breathable alternative. However, fabricating complex circuits directly onto fabric using conventional conductive inks presents challenges due to the difficulty of creating multilayer interconnections. This research introduces a novel electrochemical sensing system on fabric using Galinstan, a room-temperature liquid metal with excellent mobility and conductivity, overcoming these limitations. The use of Galinstan allows for a simple and fast fabrication process using a template method with polymethacrylate (PMA) glue to enhance adhesion without compromising the fabric's air permeability. The double-layered circuit integrates signal acquisition and processing, enabling real-time data transmission to a laptop for analysis. The system's functionality is verified through potassium ferricyanide tests and glucose detection in artificial sweat.

The introduction extensively reviews existing literature on wearable electrochemical biosensors. It highlights the advantages and disadvantages of various flexible substrates used in previous studies, emphasizing the limitations of poor air permeability and the challenges of fabricating complex circuits on fabrics using conventional conductive inks. The review sets the stage for the proposed liquid metal-based system by showcasing the need for a more comfortable, breathable, and easily manufacturable wearable biosensor.

The study developed a double-layered liquid metal-based electrochemical sensing system on fabric. The system comprises a flexible fabric circuit using Galinstan (68.5% Ga, 21.5% In, and 10% Sn) as the conductor, a replaceable electrode for electrochemical detection, and control software on a laptop. The fabric circuit is fabricated using a template method with PMA glue to enhance the adhesion of the liquid metal to the cotton fabric. The double-layered design integrates signal acquisition and processing modules. The detection module includes a power supply module, potentiostat module, I/V conversion module, amplification and filtering module, and a digital-to-analog conversion module. A LabVIEW-based software controls the circuit, collects data, and displays the sensor response curve in real time. The system's performance was characterized through resistance measurements of the liquid metal wires over time, microscopic imaging (inverted microscope and environmental scanning electron microscopy), and contact resistance measurements of electronic components and liquid metal wires. The functionality was validated using potassium ferricyanide tests and glucose detection in artificial sweat. The manufacturing process involves printing PMA glue and liquid metal onto the fabric using templates to create the upper and lower circuits, followed by connecting the two layers with liquid metal and integrating electronic components.

The fabricated liquid metal-based circuit on fabric showed minimal variation in printed wire width compared to the design template (maximum error 5.16%). Resistance measurements demonstrated the time stability of the liquid metal wires, with only slight increases in resistance over 16 months. The resistance of the liquid metal wires was significantly lower than the minimum resistance in the circuit design, suggesting that the wire resistance has negligible effect on the overall circuit performance. Microscopic images confirmed the quality of the printed liquid metal patterns. Contact resistance between electronic components and liquid metal wires remained low even after six months. The system successfully detected glucose in artificial sweat at the millimolar level, proving its functionality as an electrochemical sensor. The fabrication method was simple, user-friendly, and fast, while maintaining good air permeability of the fabric.

The results demonstrate the successful fabrication and validation of a flexible, air-permeable, and comfortable electrochemical sensing system on fabric using liquid metal. The double-layered design successfully miniaturizes the system and integrates signal acquisition and processing. The system's ability to detect glucose in sweat at the millimolar level is significant for potential applications in wearable health monitoring and point-of-care testing. The simple and scalable fabrication process makes it a promising candidate for mass production. While the slight increase in resistance over time was observed, it did not significantly affect the overall circuit performance. The findings address the challenges of creating flexible and comfortable wearable electrochemical sensors, paving the way for advancements in personal health monitoring technologies.

This research successfully demonstrated a novel double-layered liquid metal-based electrochemical sensing system on fabric for wearable glucose detection. The system offers advantages in terms of miniaturization, flexibility, air permeability, and ease of fabrication. Future research could focus on exploring other biomolecules and improving long-term stability. Integrating advanced signal processing algorithms and wireless communication could enhance the system's capabilities for real-world applications.

The study primarily focused on glucose detection in artificial sweat. Further research is needed to validate the system's performance in real sweat samples, considering the complex composition of human sweat. The long-term stability of the liquid metal circuit under various environmental conditions requires further investigation. The current system uses commercially available electrodes; exploring the development of integrated, miniaturized electrodes could further improve the system's portability and performance.