Advances in ionic conductive hydrogels for skin sensor applications

With the growing public interest in health monitoring, ionic conductive hydrogel-based skin sensors have garnered increasing attention due to their capability for real-time health monitoring. Their lightweight, flexible nature, biocompatibility, and transparency to skin greatly enhance the comfort and reliability of wearable devices for long-term health monitoring. Unlike electronic conductive hydrogels, ionic conductive hydrogels excel in mimicking biological ion transport, making them ideal for seamless integration with human tissue. This review provides a comprehensive discussion on ionic conductive hydrogels-based skin sensors, focusing on their conductive mechanisms, material properties, and applications. We first delve into the four primary conductive mechanisms: electrolyte-based, ionic liquids-based, polyelectrolyte-based, and hybrid model-based. The discussion then emphasizes the essential material properties of ionic conductive hydrogels, such as conductivity, mechanical robustness, adhesiveness, anti-freezing, anti-drying, self-healing, and other properties (e.g., transparency, biocompatibility, and anti-fouling), which are critical for enhancing sensing performance. This review also introduces state-of-the-art developments in ionic hydrogel-based skin sensors across various applications, including strain sensors, pressure sensors, bioelectrical sensors, temperature sensors, and chemical sensors. Finally, this work underscores the remaining challenges and prospects in this field, proposing potential solutions to illuminate future research and innovations in this promising field.