Pressure sensors are increasingly vital components in wearable technology, electronic skins, and human-machine interfaces due to their low cost, flexibility, and high integration potential. Four main types exist: capacitive, piezoresistive, piezoelectric, and triboelectric. Piezoresistive sensors are particularly attractive for their low energy consumption and ease of signal acquisition. Recent research focuses on micro/nanostructured designs to boost sensitivity. Tapering geometries, resembling spines or bristles, are particularly effective, promoting signal transduction while offering mechanical protection. Prior work with sea urchin-like structures using ZnO, metal nanoparticles, and hollow carbon spheres demonstrated high sensitivity, but only within narrow pressure ranges. This limitation arises from the inherent low conductivity of semiconductors and the difficulty in achieving significant current changes over a wide pressure range. High sensitivity requires low initial current and substantial changes in output current under pressure. The semiconductor/conductor interface piezoresistive effect, along with the use of heterojunctions, which are known to enhance charge transport and reduce interfacial resistance in devices like LEDs and photodetectors, offers a potential solution for achieving both high sensitivity and wide pressure range.

The authors review several existing pressure sensor designs focusing on piezoresistive sensors and micro/nanostructured approaches to enhance sensitivity. Studies using sea urchin-like structures of ZnO, metal nanoparticles, and carbon spheres are highlighted, showing that while high sensitivity is achievable, these sensors typically operate within limited pressure ranges. The use of heterojunctions in improving the performance of various devices is also reviewed, indicating their potential for enhancing the performance of pressure sensors. The existing literature demonstrates a need for a sensor that can achieve both high sensitivity and a wide pressure range simultaneously.

This study proposes a pressure sensor based on a ternary nanocomposite, Fe₂O₃/C@SnO₂, synthesized through a hydrothermal method. Acetylene black carbon acts as a conductive carrier, encapsulating Fe₂O₃ particles and filling the gaps between Fe₂O₃ needles. SnO₂ nanoparticles partially adhere to Fe₂O₃ needles, forming Fe₂O₃/SnO₂ heterostructures, and partially disperse within the carbon layer, creating SnO₂@C structures. The resulting composite is then applied to a melamine sponge substrate to create the pressure sensor. The fabrication process is detailed, including material synthesis, composite formation, sponge impregnation, and electrode connection. Characterization techniques employed include X-ray diffraction (XRD) to analyze crystal structures, scanning electron microscopy (SEM), transmission electron microscopy (TEM), elemental mapping, and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) for microstructure analysis, and X-ray photoelectron spectroscopy (XPS) for compositional analysis. Pressure sensing properties are evaluated using a custom-built system combining a universal testing machine and a digital source meter. Sensitivity, response time, and stability are quantified through various tests, including cyclic loading-unloading, tests under different pressures and with varying loads (paper, rice, coins, car weight), and assessments under different humidity levels.

The fabricated Fe₂O₃/C@SnO₂ pressure sensor exhibits remarkable performance. XRD confirms the presence of the desired components, while SEM, TEM, and STEM images show the sea urchin-like morphology of Fe₂O₃ and the distribution of carbon and SnO₂. XPS verifies the composition of the ternary nanocomposite. Sensitivity measurements reveal three distinct regions: a high sensitivity of 680 kPa⁻¹ below 10 kPa, 98 kPa⁻¹ between 10 and 50 kPa, and 35 kPa⁻¹ between 50 and 150 kPa. The response time is exceptionally fast (10 ms). The sensor demonstrates excellent reproducibility, maintaining stable performance over 3500 loading-unloading cycles at 110 kPa. Tests with small weights (paper, rice grains) show high resolution, able to distinguish pressure differences caused by these light objects. High pressure tests with coins and even a car demonstrated that the sensor maintained its high sensitivity over this ultra-broad pressure range. The sensor's performance is not affected by humidity and the area and thickness of the sponge. Finally, the device successfully monitors various human physiological activities like wrist pulse, speech, facial muscle movements (occlusion), fist clenching, finger bending, and walking, demonstrating its potential for applications in wearable health monitoring and human-machine interfaces.

The exceptional performance of the Fe₂O₃/C@SnO₂ pressure sensor is attributed to the synergistic effects of the three distinct structures within the ternary nanocomposite. The sea urchin-like Fe₂O₃ provides high sensitivity and mechanical robustness, while the carbon enhances conductivity, expanding the sensing range to high pressures. The Fe₂O₃/SnO₂ heterostructure further boosts sensitivity by promoting electron transfer. The results strongly support the hypothesis that combining a unique morphology with a ternary composite structure enables a significant improvement in both sensitivity and pressure range compared to sensors based on individual materials or simpler composites. The stability of the sensor is enhanced by the protective sea urchin-like structure and the ability of the carbon and SnO₂ to fill gaps between the Fe₂O₃ needles. The wide range of applications demonstrated emphasizes the sensor's versatility and potential impact on various fields.

This research successfully demonstrates a high-performance pressure sensor based on a novel Fe₂O₃/C@SnO₂ ternary nanocomposite. The sensor exhibits unprecedented sensitivity, a wide pressure range, and excellent stability. Its effectiveness in monitoring various human physiological activities showcases its significant potential in wearable electronics and healthcare applications. Future research could explore different materials or composite ratios to further optimize sensor performance and investigate the integration of this sensor into more complex wearable systems.

While the study demonstrates excellent performance, the use of a melamine sponge as the substrate might introduce some limitations. The sponge's inherent flexibility and porosity could influence the sensor's response time and long-term stability under extreme conditions. Furthermore, although the humidity tests showed no significant effect, further investigation across a wider range of environmental conditions could provide a more comprehensive understanding of the sensor's robustness. The study focuses on specific applications and further research could explore its performance in other scenarios.