Hydrogel machines

As polymer networks infiltrated with water, hydrogels constitute the major components of the human body; and hydrogels have been widely used in applications that closely interact with biological organisms, such as tissue engineering, drug delivery, and biological research. More recently, owing to their superior softness, wetness, responsiveness, biocompatibility, and bioactivity, hydrogels are being intensively investigated for versatile functions in devices and machines including sensors, actuators, coatings, optics, electronics, and water harvesters. A nascent field named hydrogel machines rapidly evolves, exploiting hydrogels as key components for devices and machines. While there are reviews on individual categories of hydrogel machines in literature, a comprehensive discussion on various categories of hydrogel machines that systematically correlate hydrogels’ properties and machines’ functions is still missing in the field. This review is aimed to provide such a panoramic overview. We first classify various hydrogel machines into a number of categories according to their applications. For each category, we discuss (i) the working principles of the hydrogel machines, (ii) the specific properties of hydrogels that enable the key functions of the machines, and (iii) challenges faced by hydrogel machines and recent developments to address them. The field of hydrogel machines will not only translate fundamental understanding of hydrogels into new applications, but also shift the paradigm in machine design by integrating hydrogels that can potentially minimize physical and physiological mismatches with biological organisms.