Design Strategies and Roles of Hydrogels for Sustainable Energy Conversion and Harvesting from Natural and Biological Environments.

The growing demand for sustainable energy has spurred interest in harvesting ambient sources, such as solar radiation, mechanical vibrations, water flow, and temperature gradients, and biological activities, such as motion and respiration. In this context, hydrogels have emerged as promising materials bridging natural and physiological energy environments. Known for their polymer networks and biocompatibility, hydrogels are widely used across bioengineering, biomedicine, and agriculture. Beyond these applications, hydrogels are also gaining attention in environmental and energy-related technologies, including solar-driven desalination, catalysis, and energy generation and storage. Their appeal lies in unique physicochemical properties, stimuli-responsiveness, tunable interfacial chemistry, environmental benignity, and efficient mass and heat transfer while maintaining mechanical compatibility with hybrid or soft-hard systems. Despite these promising attributes, few reviews focus on the role of hydrogels in energy harvesting. This review addresses that gap by examining hydrogel-based technologies driven by environmental stimuli and emphasizing their unique contributions to energy conversion. It offers insights into design strategies and recent advancements in functional hydrogels, highlighting opportunities and challenges in this field. As hydrogel-based energy harvesting evolves, innovative design, deeper mechanistic understanding, and interdisciplinary integration are needed to unlock its potential.