A comprehensive review of 4D-printed thermo-responsive hydrogel-based smart actuators for solar steam generation: Advanced design, modeling, manufacturing, and finite element analysis

The worldwide request for clean water and renewable energy is growing rapidly due to the rising population, changing ways of life, expanding economies, and increased utilization of natural resources. One way researchers from multiple disciplines are striving to meet these demands is to develop a direct solar steam generation (DSSG) system providing steam interrelated with the water-energy conversion process. To maximize steam generation, various systems have been developed based on the water supply path and efficient photothermal conversion structures. However, evaporative systems are vulnerable to salt generation and antifouling/antimicrobial problems, which can cause irreparable damage. To overcome these problems, recent research has been focused on thermo-responsive shape-morphing hydrogel-based DSSG systems. Although reversible actuators and materials for biomedical, soft robotics, tissue engineering, and other applications have been discussed in several reviews, no comprehensive insight has been provided on thermo-responsive actuators for DSSG. The aim of this review is to address these points while providing a comprehensive insight into thermo-responsive actuators. This is achieved by covering new and existing design and modeling strategies for hydrogel actuators with shape-morphing properties, including material modeling and numerical analysis, along with uncovering their working mechanism and production through 4D printing and evaporation dynamics.