Magnetic hydrogel-based flexible actuators: A comprehensive review on design, properties, and applications

Development of magnetic hydrogel-based actuators is a thriving research frontier in contemporary advanced untethered flexible actuators. Recent development in magnetic hydrogels has witnessed a growing interest in unique flexible magnetic actuators. Unlike the soft actuators operated with a tethered power supply, magnetic actuators provide spatiotemporally controlled manipulation in a non-invasive and remote manner. In this review, we provide the state-of-the-art progress in magnetic hydrogel-based flexible actuators, particularly highlighting creative fabrications, underlying mechanisms, and emerging applications. The performance regulation, mechanical reinforcing mechanism, and magnetization of magnetic hydrogels are introduced, together with their emerging applications in forms of artificial fingers, muscles, catheters, cilia/micropillars, microgrippers, microvalves, micromotors, microrobots, and microtransformers. Finally, concluding remarks with perspectives for future development, challenges, and opportunities in this exciting field are also enlightened. This review will guide readers in understanding the fabrication of magnetic actuators, through the comprehensive knowledge of architecture design, functional tailoring, and application exploration.