Wearable strain sensors, capable of converting mechanical deformation into electrical signals, are crucial in various applications, including soft robotics, human-machine interfaces, and health monitoring. Stretchable resistive strain sensors, offering high sensitivity and simple fabrication, are particularly desirable. However, the resistance change under mechanical stimuli generates significant heat, potentially damaging device performance and reliability. This necessitates efficient thermal management. While nanocomposites of conductive materials and polymers improve thermal conductivity, the exposure of conductive materials poses risks. Encapsulation with electrically insulating materials is crucial, but commonly used materials like PDMS and epoxy have low thermal conductivity. Therefore, electrically insulating materials with high thermal conductivity are essential for packaging electronics. Additionally, for skin-attachable wearable sensors, maintaining thermal comfort is paramount, requiring materials with good thermal insulation to prevent overheating. Boron nitride (BN) is an excellent candidate for thermal dissipation due to its high aspect ratio, thermal transport properties, and electrical insulation. Electrospun fibrous membranes, with their porosity and tunable pore sizes, are ideal for thermal insulation. This paper aims to address the lack of high-performance wearable and stretchable electronics with excellent thermal management by introducing a novel strain sensor with advanced thermal management capabilities.

Existing literature highlights the growing interest in wearable and flexible/stretchable strain sensors for various applications. Resistive sensors, with their simple structure and facile fabrication, are particularly attractive. Carbon nanomaterials like graphene and carbon nanotubes (CNTs) are frequently used due to their flexibility, high surface area, and electrical conductivity. However, the significant heat generation caused by resistance change in these sensors is a major challenge, negatively impacting performance and reliability. While studies explore nanocomposites for thermal conduction, the safety concerns associated with exposed conductive materials necessitate encapsulation with electrically insulating materials possessing high thermal conductivity. Boron nitride (BN) emerges as a promising candidate for thermal management due to its high thermal conductivity and electrical insulation. Electrospinning offers a route to create porous structures with thermal insulation properties. This study builds upon existing research by integrating these components into a single, high-performance wearable strain sensor.

The fabrication process involves three layers: an electrospun thermoplastic polyurethane (TPU) fibrous membrane, a graphene nanoribbon (GNR) conductive nanonetwork, and a casted TPU-boron nitride nanosheets (BNNSs) film. First, GNRs are deposited onto the surface of the electrospun TPU fibrous membrane using vacuum filtration, creating a conductive network. Then, this GNR-functionalized membrane is combined with the casted TPU-BNNS film. Finally, copper foils are attached to the ends to complete the sensor. The loading content of BNNSs (25 wt%, 30 wt%, 35 wt%) and GNRs (25 µg cm⁻², 50 µg cm⁻², 75 µg cm⁻²) were optimized. The thermal conductivity was measured using the laser-flash technique, density by water displacement, and specific heat and thermal diffusivity by LFA 467. The thermal conductivity of electrospun TPU and GNR-TPU membranes were measured using a Thermo Labo II. Surface temperature measurements were conducted using an infrared thermograph. Electromechanical properties (I-V characteristics, dynamic cycling strains, gauge factor) were measured using a Keithley 6485 high-resistance meter system and a dynamical mechanical analyzer. Human motion monitoring was performed by attaching the sensor to knee and finger joints. Dragon boat paddlers' movements were monitored to assess the sensor's capability in detecting subtle differences in motion. Breathability was evaluated using a gas permeability test. Cytotoxicity was assessed using a CellTiter-Lumi Plus Luminescent Cell Viability Assay Kit and cell morphology analysis.

The resulting strain sensor exhibited a stretchability exceeding 300%, a gauge factor of 35.7 (at strains above 60%), and long-term durability for over 5000 cycles. The 35 wt% BNNS loading in the TPU-BNNS film resulted in a 337% increase in thermal conductivity compared to pure TPU. The sensor’s thermal conductivity was 2.42 times higher with 35 wt% BNNSs than without BNNSs. The in-situ measurement demonstrated that during continuous stretching-releasing cycles (0-100% strain), the surface temperature fluctuation was within 3.5 °C, highlighting excellent thermal stability. The interfacial thermal conductance between the TPU-BNNS film and air was calculated as 2.9 × 10⁴ W m⁻² K⁻¹. The sensor successfully monitored human motion (finger bending, knee bending), accurately capturing subtle movements and exhibiting high sensitivity. It also distinguished between standard and non-standard movements during dragon boat paddling training, revealing differences in shoulder, wrist, and elbow movements based on fatigue. The sensor demonstrated breathability due to the porous structure of the electrospun TPU membrane and biocompatibility as confirmed by cytotoxicity tests, showing no harm to human cells.

The study successfully addressed the challenge of heat generation in stretchable strain sensors by incorporating a novel thermal management strategy. The high thermal conductivity of the TPU-BNNS film and the thermal insulation of the porous electrospun TPU membrane resulted in a sensor that is both highly sensitive and thermally stable. The in-situ measurements of temperature fluctuation under repeated stretching-releasing cycles provide valuable insights into the dynamic thermal behavior of such devices. The successful monitoring of human motion, particularly the subtle differences in dragon boat paddling movements, demonstrates the sensor's potential for advanced motion analysis and personalized training applications. The biocompatibility results further expand the sensor's suitability for long-term wearable applications.

This research presents a high-performance wearable strain sensor with superior thermal management, significantly improving the stability and reliability of stretchable electronics. The sensor's excellent sensitivity, durability, breathability, and biocompatibility make it an ideal candidate for next-generation wearable devices, particularly in applications involving significant thermal stress. Future studies could explore the integration of this sensor into more complex wearable systems and further optimization of the material composition for enhanced performance.

The study primarily focused on the electromechanical and thermal properties of the sensor. Further investigation into the long-term stability of the sensor under extreme environmental conditions (e.g., high humidity, temperature variations) would be beneficial. A larger sample size of participants in the human motion monitoring and dragon boat paddling experiments would strengthen the statistical significance of the findings. The investigation of the sensor's performance in various skin types and under different physiological conditions may also provide additional insights.