3D Printed Hydrogel Flexible Sensors: Fabrication Techniques, Sensing Mechanisms, and Application Advances

Flexible sensors, as an emerging development in the field of sensors, have advantages that are incomparable to those of traditional rigid sensors. Although recognized as ideal materials for flexible sensors due to their intrinsic softness, biocompatibility, and stimuli-responsiveness, hydrogels face fabrication challenges in achieving complex structures and integrated functionalities. The advent of 3D printing technology has successfully addressed the limitations inherent in conventional fabrication approaches, thereby offering robust technical underpinnings for the advancement and innovation of flexible sensors. This paper first introduces the 3D printing techniques employed in the fabrication of hydrogel-based flexible sensors, followed by an analysis of their underlying sensing mechanisms and examines the application studies of 3D printed hydrogel-based flexible sensors across various domains, including strain, pressure, pH, temperature, and biosensing. Finally, the review summarizes the current achievements, discusses persistent challenges, and provides perspectives on future directions for material innovation, printing technology advancement, and broader application scenarios of 3D printed hydrogel flexible sensors.