The development of lightweight, mechanically flexible, and stretchable fiber-based electronics is highly sought after for a wide array of applications in e-textiles and e-skins. These wearable electronic devices demand cost-effective manufacturing processes, high reliability, multifunctionality, and long-term operational stability. Current limitations in existing technologies often involve trade-offs between flexibility, sensitivity, and manufacturing scalability. The integration of multiple sensing modalities onto a single platform is also a significant challenge. This research aims to address these limitations by introducing a novel approach to fabricating wearable electronics using three-dimensional (3D) inorganic nanofiber networks (FNs). The use of FNs offers several potential advantages, including high surface area for enhanced sensing, inherent flexibility due to the fibrous structure, and the potential for scalability through appropriate manufacturing techniques. The specific research question focuses on evaluating the feasibility of creating flexible and stretchable metal oxide nanofiber networks for multimodal sensing applications, and assessing the performance characteristics of the resulting devices in terms of sensitivity, response time, and long-term stability. The study's significance lies in its potential to contribute to the advancement of wearable electronics, opening possibilities for new applications in healthcare monitoring, human-computer interaction, and environmental sensing. The successful development of such devices could lead to improved diagnostics, personalized healthcare, and enhanced environmental monitoring capabilities.

Existing literature extensively covers various aspects of flexible and stretchable electronics, highlighting the challenges and progress in different material systems and fabrication techniques. Organic bioelectronics, leveraging the compatibility of organic materials with biological systems, offer promising avenues for wearable applications. However, organic materials often suffer from limitations in terms of stability and performance. Inorganic materials, such as metal oxides, provide enhanced stability and performance, but integrating them into flexible and stretchable devices poses considerable challenges due to their inherent brittleness. Recent advances in nanomaterials have explored the use of nanowires, nanotubes, and other nanostructures to overcome the limitations of traditional bulk materials, improving flexibility and enhancing sensing capabilities. Various fabrication methods, including printing, electrospinning, and spray coating have been explored for creating flexible electronic devices. While each method offers advantages and disadvantages, the choice is usually dependent on the materials used, desired device architecture, and scalability requirements. Previous research has demonstrated the fabrication of flexible thin-film transistors (TFTs) and other electronic devices using metal oxide nanomaterials, but the integration of multiple sensing modalities onto a single flexible platform remains an area requiring further development. The challenge of creating truly stretchable devices capable of withstanding large deformations without compromising their functionality remains a critical hurdle.

The research employed a blow-spinning technique to fabricate 3D inorganic nanofiber networks (FNs) from metal oxide precursors. This method involves the use of a polymer solution containing the desired metal salts, which is then blown into fibers onto a substrate. The selection of metal oxide materials included Indium-Gallium-Zinc Oxide (IGZO), Copper Oxide (CuO), Indium-Tin Oxide (ITO), and Copper (Cu). These materials were chosen for their diverse electronic properties: IGZO and ITO for semiconducting behavior, CuO for semiconducting/insulating, and Cu for conducting, providing flexibility in designing devices with different functionalities. After the blow-spinning process, the films were subjected to thermal annealing under optimized conditions to convert the precursors into the desired metal oxides and improve their conductivity. The annealing temperature and atmosphere were carefully controlled to obtain optimal crystal structure and desired properties for each metal oxide. The fiber density within the FNs was adjusted by varying the blow-spinning time. The resulting metal oxide FNs were then integrated with elastomeric substrates, such as styrene-ethylene-butylene-styrene (SEBS) to enhance their stretchability. The integration of the FNs with the elastomer involved contact transfer, carefully moving the FN film from its original substrate to the SEBS substrate. Finite element analysis (FEA) was performed to study the mechanical behavior of the FNs under strain. For device fabrication, different types of metal oxide FNs were used as active materials in TFTs and resistors. Thin-film transistors (TFTs) based on IGZO FNs were fabricated using conventional fabrication techniques, including metal evaporation for electrodes and gate dielectric deposition. Stretchable resistors were fabricated by depositing the metal oxide FNs onto elastomeric substrates with appropriate electrodes. The choice of electrodes varied depending on the specific application and desired stretchability, using materials such as PEDOT:PSS, Cr/Au, or Cu wires. The devices' performance was thoroughly characterized through several electrical and sensing measurements. Electrical characterization included I-V curves, transfer curves for TFTs, and resistance measurements under different strain conditions. Sensing performance was evaluated using various stimuli, such as NO2 gas, UV light, temperature variations, pressure, humidity, and human breath. The sensing experiments involved exposing the devices to controlled environments and monitoring the changes in their electrical signals. The response time, recovery time, and sensitivity were analyzed for each sensing modality. Finally, a fully integrated e-skin platform was created by monolithically integrating IGZO, CuO, and ITO FNs onto a single SEBS substrate. This integrated system was tested for its ability to simultaneously detect multiple stimuli.

The study yielded several key findings demonstrating the efficacy of the fabricated flexible and stretchable metal oxide nanofiber networks for multimodal sensing. The blow-spinning method successfully produced high-quality metal oxide nanofibers with controlled density. IGZO FN-based TFTs exhibited exceptional performance, showing negligible degradation after 1000 bending cycles and remarkable room-temperature gas sensing capabilities, achieving a sensitivity of 33.6% ppm⁻¹ for NO2 detection. The sensitivity was found to depend on the concentration of NO2, exhibiting good linearity. Response and recovery times were significantly faster than those of conventional gas sensors. Flexible IGZO FN-based TFTs on polyimide substrates showed stable performance even under bending, with mobility and threshold voltage recovering to original values when bending was reversed. Stretchable resistors based on IGZO FNs demonstrated excellent stretchability, maintaining functionality even after 50% elongation, and showed minimal resistivity change up to 25% strain. The FEA simulations indicated that the mechanical stability of the FNs under strain is due to the random orientation of fibers within the network, allowing for stress distribution and preventing catastrophic rupture. A multifunctional IGZO FN-based resistor exhibited high sensitivity to light, gas, force, and temperature. The device showed photoresponsivity, on/off current ratio, and detectivity comparable to or exceeding other metal oxide-based photodetectors. Its ability to detect NO2 gas was notable, with superior response and recovery times. As a temperature sensor, it showed a higher temperature coefficient than previously reported temperature sensors made with IGZO films, graphene/polymer films, and P3HT/PDMS films. The IGZO FN sensor also demonstrated humidity sensing capabilities, exhibiting a clear relationship between current and relative humidity (RH). The same sensor was successfully used for breath analysis, distinguishing between different breathing rates and detecting alcohol consumption. CuO FN-based resistors showed high sensitivity to pressure, effectively detecting pressures below 4 kPa with a sensitivity of 0.04 kPa⁻¹. The response and recovery times were extremely fast, and it could even detect the slight pressure exerted by a dime. A monolithically integrated e-skin device, integrating IGZO, CuO, and ITO FNs on a single platform, demonstrated the ability to simultaneously and effectively differentiate various stimuli including solar light, temperature, strain, exhaled gas, and pressure. ITO FN-based devices showed excellent strain sensing capabilities and were used for gesture recognition, accurately identifying various hand gestures based on changes in resistance. The ITO-based device maintained stability for over a year.

The results presented in this study address the research question by demonstrating the successful fabrication and characterization of a new class of flexible and stretchable metal oxide nanofiber networks for multimodal sensing applications. The exceptional performance characteristics of the fabricated devices, including high sensitivity, fast response time, and remarkable durability under bending and stretching, demonstrate the potential of this approach to advance the development of wearable electronics. The findings highlight the importance of employing a 3D nanofiber network architecture for enhancing flexibility and stretchability while maintaining high electrical conductivity and sensing performance. The integration of multiple sensing modalities into a single platform opens up exciting possibilities for creating sophisticated wearable sensors. The superior performance characteristics of the devices in terms of sensitivity and response time compared to existing technologies confirm the effectiveness of the proposed approach. The ability to differentiate various stimuli, including gas, light, temperature, pressure, strain, and human breath, suggests potential applications in diverse areas such as healthcare monitoring, environmental sensing, and human-machine interfaces. The significance of the results extends beyond individual device performance, encompassing the broader impact on wearable electronics technology. This research offers a new strategy for creating durable and highly sensitive stretchable electronic functionality, pushing the boundaries of what is possible in wearable technology.

This research successfully demonstrated the fabrication of high-performance, flexible, and stretchable metal oxide nanofiber networks using a scalable blow-spinning technique. These FNs were integrated into various devices, including TFTs and resistors, showing excellent performance in gas sensing, light detection, temperature sensing, pressure sensing, humidity sensing, and breath analysis. The creation of a fully integrated multimodal e-skin platform capable of differentiating several stimuli simultaneously highlights the potential of this technology for advanced wearable electronics. Future research could explore other metal oxide combinations and investigate novel device architectures to further enhance performance and expand functionalities. Investigating biocompatibility for biomedical applications and exploring integration with wireless communication technologies for remote monitoring are other promising avenues for future work.

While the study demonstrates significant advancements in flexible and stretchable wearable electronics, certain limitations should be acknowledged. The long-term stability of the devices under extreme conditions (e.g., prolonged exposure to high temperatures or humidity) requires further investigation. The manufacturing process, although scalable, could benefit from further optimization to reduce costs and increase throughput. The current range of sensing modalities may be expanded to include other bio-relevant parameters. Finally, in-vivo testing and clinical trials are crucial to evaluate the true potential of these devices in real-world applications.