Smart Hydrogel-Based Mechanical Metamaterials: A Review

Mechanical metamaterials have garnered significant attention due to their extraordinary mechanical properties derived from structural design. This displays their huge potential for emerging material applications. Although hydrogels present challenges in the fabrication of mechanical metamaterials due to their intrinsic high water content, their inclusion is desirable because of their unique responsive properties. This distinctive responsiveness and their extraordinary mechanical characteristics opens up exciting possibilities for smart applications in fields such as biomedical engineering, soft robotics, and sensors. In the past five years, advancements in hydrogel fabrication techniques such as additive manufacturing and multimaterial approaches have enabled the design of more complex and customizable hydrogel ranging from macro- to nanostructures. This review offers recent insights into hydrogel materials as mechanical metamaterials, highlighting their development in diverse applications by harnessing hydrogel characteristics. It discusses various mechanical metamaterial characteristics, including negative hydration expansion, auxetic behavior, geometric transformation, and programmable mechanical properties, while also addressing current challenges and charting future prospects in the field.