3D-Printed Flexible Bending Sensors and the Effects of Using Different Conductive Track Geometries

Currently, humans live in a world of digital transformation, and sensors are essential for converting physical measurements to digital data. Conventional sensors often lack flexibility, leading to fragility and challenges in adaptation, particularly on complex and irregular surfaces. Flexible sensors can address these issues by providing stretchability and adaptability to their applications. Various techniques have been employed to develop flexible sensors; however, three-dimensional (3D) printing has gained popularity because of its ability to fabricate complex and integrated shapes at a low cost. Fused deposition modeling (FDM) is a widely utilized 3D-printing technique that holds significant potential for the development of flexible sensors utilizing flexible filaments, including thermoplastic polyurethanes (TPUs). TPU filaments typically consist of flexible thermoplastics that are non-conductive composites. However, electrically conductive versions can be developed by incorporating carbon-based composites, thereby enabling their use as functional sensors. This study investigated the influence of trace geometry on the electromechanical performance of 3D-printed flexible bending sensors. Three distinct trace configurations were designed and fabricated using a commercially available conductive TPU filament. These configurations included vertical strain gauge, horizontal strain gauge, and sinusoidal patterns. Three-point bending tests were conducted to characterize these sensors. The experimental results demonstrated a fundamental trade-off between the sensor sensitivity and electromechanical stability, which is mainly influenced by the geometrical configurations. The configuration of the vertical strain gauge achieved the highest sensitivity, with an average gauge factor (GF) in the range of −6.70 to −7.29. However, this vertical strain gauge resulted in a high mean hysteresis of 60.22%. In contrast, the sinusoidal design resulted in the highest linear predictability (R2 = 0.9183) and the lowest mean linearity error of 26.82%, even though it exhibited a lower sensitivity compared to the vertical strain gauge (GF in the range of −1.98 to −2.23). The horizontal gauge configuration performed relatively poorly across all metrics, except for the mean hysteresis, which was the lowest among the three designs (mean hysteresis = 26.66%). These findings suggest that the sinusoidal sensor is advisable for applications prioritizing overall performance metrics. In contrast, if sensitivity is the primary consideration, a vertical strain gauge sensor is more suitable. This study provides design guidelines for selecting optimal printing structures, especially in applications such as soft robotics, wearable electronics, and structural health monitoring, where consistent displacement tracking is required.