Hydrogels have been generated and explored for use in various applications. The main objective of this comprehensive review is to collate the fundamental concepts of hydrogels, and elaborate on knowledge gaps, and to provide a perspective on the future directions. This review includes details of constituent molecules (monomers, cross‐linkers, composite materials, etc.) and the methods used to prepare polymer networks. Moreover, the review highlights modifications of hydrogels that introduce new properties or enhance the existing features to suit the desired applications and challenges of synthetic polymer hydrogels. The other important topics covered in this review are the synthesis and applications of 3D printed hydrogels, nanocomposite hydrogels, injectable hydrogels, and self‐healing hydrogels.

Inorganic semiconductor nanostructures coupled to flexible substrates such as natural and synthetic fibrous materials have been studied for a wide range of potential applications that include wearable electronics, protective textiles, portable and flexible photovoltaic and solar cell devices. Here, we report the fabrication of ZnO/ZnS core-shell nanorod arrays on a cotton platform via a simple, low-temperature hydrothermal growth technique. ZnO nanorods were converted to ZnO/ZnS core-shell nanorod arrays through a mild sulfidation process with sodium sulfide. TEM, XRD, and XPS were used to characterize the nanorods to reveal a highly crystalline ZnO core with a polycrystalline ZnS shell. Photoluminescence measurements demonstrate a remarkably large UV emission for the ZnO/ZnS nanorod arrays on cotton. Such materials are anticipated to be useful for wearable, portable electronic devices and as protective textiles.