The increasing demand for miniaturized, high-power wearable electronics necessitates advanced thermal management solutions that do not compromise flexibility or breathability. Traditional approaches often face challenges in balancing these conflicting requirements. High thermal conductivity materials are typically rigid, while breathable materials often have low thermal conductivity. This research addresses this challenge by exploring the use of a composite material. Wearable electronics are rapidly becoming integrated into daily life, finding applications in health monitoring, sensory detection, and movement assistance. However, the comfort and functionality of these devices are directly impacted by their thermal properties, flexibility, and permeability. Electrospinning offers a method to create porous fibrous membranes, which can act as substrates or matrices for flexible devices, providing good permeability. However, these mats often exhibit low thermal conductivity. Thermoplastic polyurethane (TPU), a biocompatible, hydrophobic polymer, is frequently utilized in flexible electronics due to its elasticity and toughness. While TPU is suitable for flexibility, its low thermal conductivity hinders effective heat dissipation. High thermal conductivity materials, such as boron nitride nanosheets (BNNSs), offer potential solutions; however, their inherent rigidity presents challenges in flexible applications. This study aims to overcome this limitation by combining the advantageous properties of TPU and BNNSs to create a flexible, breathable composite with excellent thermal management capabilities.

The literature review section highlights the existing research on flexible and stretchable electronics, focusing on materials and techniques used to achieve advanced thermal management. The authors review various polymers suitable for flexible electronic devices, highlighting their advantages and disadvantages regarding thermal conductivity and flexibility. The use of electrospinning to generate porous structures for improved breathability is discussed, as is the incorporation of high thermal conductivity fillers such as graphene and carbon nanotubes to improve heat dissipation. The review also examines previous research on the use of BNNSs for thermal management, acknowledging their excellent thermal properties but also emphasizing the limitations of their rigidity in flexible applications. The authors explicitly discuss the conflicting requirements of high thermal conductivity, flexibility, and breathability and how their proposed approach addresses these challenges.

The fabrication process starts with creating patterned electrospun TPU fibrous mats using a metal grid collector. This creates a grid-like structure with open spaces between the fibers, improving breathability. Next, a mixture of BNNSs and Ecoflex, a flexible polymer, is coated onto the grids of the patterned TPU fibrous mats. Ecoflex is chosen for its elasticity, non-crystallinity, and low thermal conductivity. The coating process is controlled using a metal mask to ensure the BNNSs layer is deposited only on the grids, leaving the spaces between the fibers open for breathability. The resulting composite is characterized using several methods: Scanning Electron Microscopy (SEM) is used to examine the morphology and structure of the composite material, specifically to confirm the successful coating of BNNSs on the TPU fibers and the preservation of porosity. Air permeability is measured using a digital air permeability measuring instrument to quantify the breathability of the composite. Water vapor permeability is assessed by measuring the weight loss of water contained in a bottle covered with the composite material. The thermal conductivity of the composite is determined using a Hot Disk TPS 2500S thermal conductivity analyzer. A thermal bridge method is employed to measure the thermal conductivity of individual TPU fibers. The thermal management capabilities of the composite are evaluated by integrating it into flexible graphene-based devices (conductors and strain sensors). The performance of these devices is assessed under different bending conditions and cycles to determine the stability and efficiency of the thermal management in the presence of mechanical stress. Finally, a prototype wearable device is fabricated using inkjet-printed Ag conductors on the composite material, and its performance and temperature behavior are examined under various bending scenarios.

The study successfully fabricated a flexible, breathable composite material with significantly enhanced thermal conductivity. The patterned electrospun TPU fibrous mats provided the necessary breathability. The incorporation of BNNSs (up to 25 wt%) resulted in a remarkable 4442% increase in thermal conductivity compared to the pure patterned TPU fibrous structure. Importantly, the breathability of the composite remained relatively high, with only a minimal reduction in air permeability after coating with the BNNSs/Ecoflex layer. This combination of high thermal conductivity and maintained breathability was demonstrated through air permeability testing (27.54 mm/s before coating to 21.43 mm/s after coating) and water vapor permeability tests (showing comparable rates to the uncovered and uncoated samples). When integrated into flexible graphene-based devices, the composite exhibited exceptional thermal management capabilities. The saturating operating temperature of the device was significantly lower with the composite compared to the use of pure Ecoflex packaging, demonstrating its effectiveness in heat dissipation. The devices also showed minimal temperature fluctuations (< 0.5 °C) during over 2000 bending cycles, demonstrating the composite's excellent thermal stability under mechanical stress. The prototype wearable device fabrication showed that the integration of the composite with inkjet-printed Ag conductors allowed for creating flexible and thermally managed devices with desirable architectures. The composite displayed consistent performance during various bending and flexing tests, including simulations of cycling and elbow movement.

The successful fabrication of a flexible, breathable composite with high thermal conductivity directly addresses the critical challenges in developing advanced wearable electronics. The results demonstrate that the synergistic combination of the patterned TPU fibrous structure and the BNNSs/Ecoflex coating effectively balances the conflicting requirements of high thermal conductivity, flexibility, and permeability. The significant improvement in thermal management capabilities, as observed in the reduction of operating temperature and minimal temperature fluctuations under bending conditions, has important implications for enhancing the performance and lifespan of wearable electronics. The creation of a prototype wearable device highlights the practical feasibility of this approach. The study's findings provide a valuable blueprint for the design and fabrication of next-generation wearable electronics that are not only functional but also comfortable for extended use. Further research could explore other high-thermal-conductivity materials, alternative polymer matrices, or further optimization of the fabrication process to further enhance the performance and applicability of this technology.

This research successfully demonstrated a novel method for creating flexible, breathable composites with substantially improved thermal management capabilities. The key innovation lies in the strategic combination of patterned electrospun TPU fibrous mats and a BNNSs/Ecoflex coating. This approach achieved a significant enhancement in thermal conductivity while maintaining sufficient breathability, leading to superior thermal performance in wearable electronic devices. Future studies could focus on exploring different filler materials, optimizing the composite structure, and integrating this technology into a broader range of wearable electronic applications.

While this study presents a significant advancement in creating thermally managed flexible composites, some limitations exist. The fabrication process involves multiple steps and may require further optimization for large-scale production. The long-term stability of the composite under extreme environmental conditions needs further investigation. The study focused on specific materials (TPU, BNNSs, Ecoflex) and device architectures; more research is needed to explore the adaptability and versatility of this approach across various materials and applications.