Hydrogel sensors for biomedical electronics

Hydrogels and their sensors are emerging as key players in biomedical electronics, offering flexible, programmable structures and functions. Hydrogel-based sensors with stress–strain and electrophysiological sensing capabilities have consistently attracted interest. However, non-implantable hydrogel sensors often face challenges such as low sensitivity and slow response times, hindering their ability to detect subtle movements and causing delays in signal transmission. On the other hand, implantable hydrogel sensors, known for their biocompatibility and ionic conductivity, are better suited for long-term in vivo monitoring. Yet, their interface with biological tissues can lead to high impedance, adversely affecting signal quality and reducing sensor effectiveness. Furthermore, certain implantable hydrogel sensors may have limitations in light transmission properties, posing challenges in co-transmitting photoelectric signals for various applications. To address these challenges, researchers have been focusing on optimizing the mechanical and electrical properties of hydrogels. This involves modifications in precursor materials and structural designs, leading to the integration of multiple functionalities and significant performance improvements in hydrogel sensors. This systematic review covers existing studies on hydrogel sensors for biomedical electronics, from basic functionalities to more complex ones, and includes both non-implantable and implantable devices. Additionally, it provides insights into the future design principles and potential application scenarios for the next generation of multifunctional hydrogel sensors.