Advances in the Development of Hydrogel-Based Adhesion Layers for Wearable Health Monitors: Focusing on Biocompatibility, Conductivity, and Mechanical Strength

High adhesion between the sensor and the skin is crucial for the accurate detection of signals from the human body while maintaining mechanical and electrical properties, including self-recovery and reliability. However, ensuring all of these properties in the adhesion layer simultaneously is challenging. Without conformal contact between the skin and the sensor, real-time monitoring is hampered by a high signal-to-noise ratio. In this review, we highlight the importance of the adhesion layer for real-time healthcare monitoring, focusing on reduced noise and the absence of irritation or side effects for long-term usage. The types and characteristics of hydrogels are summarized, detailing the significance of conductive hydrogels and the role of tough hydrogels with robust mechanical properties. Various types of adhesion layers with hydrogels and various patterns and shapes are also discussed as well as wet and dry electrodes, along with their advantages and limitations. Furthermore, by introducing research on double-sided skin-sensitive conductive hydrogel-based adhesive layers that ensure adhesion to the skin and can be utilized as sensors, this review aims to present the potential advancements in hydrogel-based sensors. By balancing the electrical and mechanical properties of conductive hydrogels and ensuring excellent adhesive layers, these hydrogels have the potential to advance research related to various biosensors. Moreover, due to their sustained adhesion, they can expand sensing applications into emerging biofields, such as stress detection, which has been challenging to measure. Therefore, this review introduces the characteristics and applications of hydrogels that can be utilized as wearable sensing and adhesive layers.