Leaf-meridian bio-inspired nanofibrous electronics with uniform distributed microgrid and 3D multi-level structure for wearable applications

Flexible electronic devices are crucial for next-generation technologies, including health monitoring, human-machine interfaces, and soft robotics. Wearable sensors, in particular, are gaining traction due to their ability to translate physical stimuli into electronic signals. While advancements have been made in creating high-performance, comfortable, and durable wearable sensors using materials like gels, composite PDMS elastomers, and modified conductive Ecoflex, challenges remain in achieving optimal breathability, moisture permeability, lightness, skin-friendliness, and multifunctionality. Electrospinning nanofibers offer a promising platform due to their flexibility, high surface area, and diverse morphologies. Their large-scale production and ability to incorporate biocompatible matrices make them ideal for direct interfacing with biological tissues. However, improving the sensitivity and linear range of nanofiber-based sensors remains a challenge. Novel structural designs, inspired by natural interfaces, such as pyramids, cylinders, hemispheres, and hierarchical structures, have shown promise in enhancing sensor performance. Microstructures provide high compressibility, enabling deformation even under low stimuli and reducing the negative impact of viscoelasticity and hysteresis. Despite these advances, effective strategies for improving the sensing performance of nanofibrous membrane-based pressure sensors are limited. Most approaches focus on surface modification of individual nanofibers, which often suffers from drawbacks like poor substrate selectivity, high cost, and low service life. This research proposes a novel strategy to address these limitations by creating a multifunctional nanofibrous membrane with a specific spatial multi-level structure and grid-like morphology.

The introduction thoroughly reviews existing literature on wearable sensors and their limitations. It highlights the use of various materials, such as gels, PDMS-based elastomers, and modified Ecoflex, along with their respective advantages and drawbacks. The advantages of electrospun nanofibers are discussed, emphasizing their flexibility, high surface area, and biocompatibility. The review also covers different approaches to improving sensor performance through structural design, referencing examples like pyramid, cylinder, and hemisphere structures. Furthermore, the limitations of existing surface modification techniques are outlined, motivating the need for a novel, cost-effective, and universally applicable strategy.

The researchers fabricated a multifunctional nanofibrous membrane using a combination of electrospinning and in-situ polymerization. A metal mesh template was employed to create the grid-like microstructure and spatial multi-level structure. The process began with the electrospinning of a carboxylated carbon nanotube (CCNT)-doped polyurethane (PU) nanofiber membrane (PCN). Citric acid was added as a complexing agent to facilitate subsequent modifications. Plasma treatment was used to enhance the hydrophilicity of the PCN membrane, preparing it for the next steps. The membrane was then immersed in a ferric chloride (FeCl3) solution to facilitate the adsorption of Fe3+ ions. This was followed by in-situ polymerization of 3,4-ethylenedioxythiophene (EDOT) on the PCN-Fe3+ surface to create a conductive PEDOT layer (PPCN). Finally, ultrasonication was used to anchor CCNTs onto the PPCN surface, resulting in the final TPPCN membrane. The fabrication process was carefully characterized using various techniques like SEM, TEM, EDS mapping, XRD, and FTIR to confirm the morphology, structure, and composition of the materials at each stage. Electrospinning parameters like voltage, flow rate, and distance were precisely controlled. The effects of varying parameters like polymerization time and ultrasonication time on the final product were also investigated and optimized.

The TPPCN membrane exhibited a unique multi-level structure with a grid-like morphology, confirmed through SEM and TEM imaging. The incorporation of CCNTs and PEDOT significantly enhanced the conductivity of the membrane. The pressure sensing capabilities of the TPPCN membrane were outstanding, showing a high gauge factor (GF) of 5.13 kPa⁻¹ in the 0–1.0 kPa range and 0.41 kPa⁻¹ in the 1.0–8.0 kPa range, along with an ultralow detection limit of 1 Pa and fast response/recovery times of 80 ms and 120 ms, respectively. These results were significantly better than those for a control sample without the grid-like structure. Finite element analysis (FEA) simulations confirmed the effectiveness of the grid-like structure in achieving homogeneous pressure distribution, thereby enhancing sensitivity. The TPPCN membrane also demonstrated successful applications as a touch sensor, activating LEDs upon touch, and as a wearable heater, reaching temperatures up to 70 °C with 2V input within 60s. Furthermore, the membrane was shown to function effectively as a triboelectric nanogenerator (TENG), generating a voltage of 78V and a current of 0.45 µA. The device’s reliability was established by its consistent performance over 1000 loading/unloading cycles and in various experimental scenarios involving different pressure levels and frequencies.

The superior performance of the TPPCN membrane-based pressure sensor is attributed to the synergistic effects of its unique multi-level structure and grid-like morphology. The FEA simulations revealed the critical role of these structural features in creating homogeneous pressure distribution, leading to enhanced sensitivity, fast response, and wide detection range. The results highlight the potential of bio-inspired designs in developing high-performance flexible electronics. The multifunctionality of the TPPCN membrane, demonstrated through its applications as a heater and TENG, further strengthens its potential for use in versatile wearable devices. The study's findings contribute significantly to the field of flexible and wearable electronics by providing a novel fabrication strategy for high-performance, multifunctional sensors.

This study successfully demonstrated a novel fabrication method for a multifunctional nanofibrous membrane based on a bio-inspired leaf-meridian structure. The resulting TPPCN membrane showcased exceptional pressure sensing performance, with high sensitivity, rapid response/recovery, and low detection limits. Its multifunctionality as a heater and TENG expands its potential for wearable applications. This work contributes significantly to the development of advanced wearable electronics by providing a scalable and effective fabrication strategy.

While the study demonstrates the significant potential of the TPPCN membrane, some limitations should be acknowledged. The long-term stability and durability of the membrane under various environmental conditions require further investigation. The scalability of the fabrication process for mass production could also be further optimized. Finally, in-vivo testing and comprehensive biocompatibility studies are needed to assess its suitability for long-term biomedical applications.