Flexible electronic devices, particularly pressure sensors, are gaining traction for applications in health monitoring, robotics, and human-machine interfaces. Piezoresistive pressure sensors, which detect resistance changes under pressure, offer advantages like high sensitivity and simple structure. High sensitivity and a wide linear pressure range are crucial for practical applications. Microstructures like pyramids, microdomes, and micropillars are explored to enhance sensitivity. Polydimethylsiloxane (PDMS) is a common substrate due to its flexibility and biocompatibility. However, traditional fabrication methods (lithography, etching) are complex, expensive, and require cleanroom environments. 3D printing offers a cost-effective and simpler alternative. Previous studies using direct ink writing (DIW) 3D printing to create concentric circle patterns (CCPs) showed sensitivities of 2.08 and 2.4 kPa⁻¹ in low-pressure regions. However, DIW's scalability is limited. Fused deposition modeling (FDM) offers advantages like simplicity, low cost, fast printing speed, and easily controllable parameters. A key challenge with FDM is the inherent roughness of its layer-by-layer printing. This research leverages the unique properties of FDM to create a high-sensitivity CCP-based pressure sensor for health monitoring applications using PDMS and PEDOT:PSS. The goal is to develop a simple, low-cost fabrication process that yields a high sensitivity pressure sensor suitable for detecting physiological signals.

The literature review highlights the growing interest in flexible pressure sensors for various applications, particularly in health monitoring. Different types of pressure sensors (piezoresistive, capacitive, piezoelectric, triboelectric) are discussed, focusing on the advantages of piezoresistive sensors: high sensitivity, simple structure, and ease of readout. The importance of microstructure design in enhancing sensitivity is emphasized, with examples of existing microstructures such as pyramids, microdomes, and micropillars. Existing fabrication methods are critically examined, with their limitations in terms of complexity, cost, and cleanroom requirements. The use of 3D printing as a promising alternative for fabrication is reviewed, noting previous attempts to mimic human fingerprints using direct ink writing (DIW), along with their limitations in terms of scalability. The study then focuses on the potential of Fused Deposition Modeling (FDM) 3D printing despite its typical limitations to produce a superior pressure sensor.

This study proposes a novel approach to fabricate a CCP-based flexible pressure sensor using an FDM-type 3D printer. The process involves two main steps: (1) fabrication of the CCP surface PLA plane and (2) fabrication of the pressure sensor using the PLA plane. In step one, a cone-shaped model is designed using CAD software and 3D-printed using PLA filament. The FDM process inherently creates a rough surface on the printed cone. A novel compression method is employed. The 3D-printed cone is heated above the glass transition temperature of PLA (120 °C for 10s) and compressed using a 2-kg weight for 50 seconds to flatten it into a 2D plane with a CCP surface. The optimal conditions for the compression method are investigated, with compression times greater than 50s resulting in damaged microstructures. In step two, PDMS is poured onto the PLA plane mold, cured, and then peeled off. Oxygen plasma treatment is performed on the PDMS to improve hydrophilicity for better PEDOT:PSS coating. PEDOT:PSS is drop-cast onto the PDMS surface to create the conductive active layer. The process involves careful control of parameters such as printing layer height (PLH) to control the size and sensitivity of the sensor. Four different PLHs (0.1, 0.12, 0.14, and 0.16 mm) are used to fabricate sensors with varying CCP microstructures. The width of each CCP is theoretically calculated and experimentally verified. Optical and SEM images are utilized to characterize the fabricated CCP structures. Finally, the fabricated pressure sensor is tested and characterized to assess its sensitivity, linear pressure range, response time, recovery time, and durability.

The fabricated pressure sensor demonstrates high sensitivity. The sensitivity increases with increasing PLH. The sensor with a 0.16 mm PLH shows an outstanding sensitivity of 160 kPa⁻¹, which corresponds to a linear pressure range of 0–0.577 kPa with a good linearity of R² = 0.978. This is significantly higher than sensitivities reported in previous studies using similar CCP structures. The sensor exhibits stable and repeatable operation under various pressures and shows good durability, maintaining its performance over 4000 loading/unloading cycles at 6.56 kPa. The response time is 114 ms, and the recovery time is 192 ms. The theoretical calculation of CCP width based on PLH (w = √3h) shows good agreement with experimental measurements. Successful application in detecting various physiological signals, including wrist pulse during exercise and rest, swallowing activity, and speech, demonstrates the potential of the developed sensor for health monitoring.

The high sensitivity and wide linear pressure range achieved in this study demonstrate the effectiveness of the proposed fabrication method. The use of the novel compression method effectively transforms the 3D-printed cone into a 2D plane with well-defined CCP microstructures, enabling superior sensitivity compared to direct 2D printing. The use of PEDOT:PSS as a conductive material provides a cost-effective and flexible alternative to other conducting materials. The successful demonstration of health monitoring applications highlights the potential for translating this technology into wearable healthcare devices. The results suggest that the proposed fabrication method offers a promising approach to creating high-performance, flexible pressure sensors for various applications, surpassing the limitations of existing fabrication techniques in terms of cost, complexity, and cleanroom requirements.

This study successfully demonstrated a simple, cost-effective fabrication method for a high-sensitivity flexible pressure sensor using FDM 3D printing and a novel compression technique. The resulting sensor shows excellent performance characteristics, including high sensitivity (160 kPa⁻¹ at 0.16 mm PLH), a wide linear pressure range, good stability, and durability. Successful real-time detection of various physiological signals highlights its potential for health monitoring applications. Future research could explore optimization of CCP design for even higher sensitivity, investigation of other conductive materials, and integration with other sensing modalities for more comprehensive health monitoring.

The study primarily focuses on the sensor's performance under specific conditions. Further testing is needed to assess its performance under a broader range of temperatures, humidities, and loading conditions. The long-term stability and reliability of the sensor under continuous use also need to be evaluated. The current design might be challenging to scale up to mass production, though the fabrication method holds inherent scalability advantages over DIW techniques. Further refinement of the compression process may also yield further improvements in microstructural control and reproducibility.