Hydrogels as intelligent materials: A brief review of synthesis, properties and applications Mostafa Mahinroosta a , Zohreh Jomeh Farsangi b, * , Ali Allahverdi a , Zahra Shakoori b a School of Chemical Engineering, Iran University of Science and Technology, Tehran, Iran b School of Advanced Technologies in Medicine, Department of Medical Nanotechnology, Tehran University of Medical Sciences, Tehran, Iran article info Article history: Received 22 December 2017 Received in revised form 12 February 2018 Accepted 15 February 2018 Keywords: Hydrogels Intelligent hydrogels Crosslinking Polymeric network abstract Hydrogels are a unique class of three dimensional cross-linked polymeric networks that can hold a large fraction of aqueous solvents and biological fluids within their structures. Nowadays, hydrogels have attracted a growing interest of many scientists in different fields of research. Intelligent hydrogels have found a significant role in a wide variety of applications such as drug delivery systems, tissue engi- neering, optics, diagnostics and imaging. The purpose of this paper is to present a brief review on the basis concept of hydrogels, the description about classification, synthesis methods, stimulation situa- tions, relevant mechanisms, and applications. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction Fundamentally, intelligent or smart hydrogels are defined as three-dimensional cross-linked polymer networks which have a great ability for uptake of a lot of water or biological fluids [1,2]. Polymer chains are connected to each other through cross-linking and immersed in an aqueous solution [3,4]. Hydrogels have attracted a lot of attention because of their special properties such as swelling in aqueous medium, sensitivity to pH, temperature or other stimuli [5]. Various types of hydrogels including environ- ment-responsive, self-healing, self-assembled conductive, shape- memory, and supermolecular hydrogels have been developed in recent years and their features have been studied and tested in a wide range of applications. Over the last few years, there has been a great interest in the development of self-healing hydrogels, which have extensive variety of applications from soft robotics to tissue engineering [6]. The development of these types of hydrogels originates from the phenomena observed in organisms. When a microstructure is damaged, some chemical reactions take place through certain mechanisms in which biological materials repair themselves and restore their original function. By combining three- dimensional hydrogels and a self-healing function, novel intelligent hydrogels can be prepared. Self-healing hydrogels can be divided into two categories: autonomic and non-autonomic self-healing. The former does not need a stimulus to repair itself. The damage controls the repair. On the contrary, the latter requires stimuli such as light, magnetic field, heat,etc., to initiate the healing process [7]. As another type of smart hydrogels, the fabrication of shape- memory hydrogels has been inspired by the shape-memory effect involving an elastic deformation of a sample, which is temporarily repaired by reversible covalent or physical cross-links resulting in an impermanent shape. The shape reversibility can happen under the effect of an appropriate stimulus [8]. A detailed discussion of the fascinating world of all hydrogel structures requires an extensive and overwhelming study, which is beyond the scope of the present brief review. The main focus of this work is on various aspects of environment-responsive hydrogels. The search for the word “hydrogel” in the PubMed database reveals a growing trend in the number of the published articles, especially a rapid rise from 1984 onward. The search results are depicted in Fig. 1 . Fig. 2 illustrates a global trend in the number of published works on the hydrogel during 2008e2012. According to Fig. 2, most of work on hydrogels have been conducted in the United States and China, followed by Japan, South Korea, Germany and the United Kingdom. Unfortunately, no more up-to-date in- formation was found. Polymer network may be hydrophilic or hydrophobic [9]. Hy- drophilic components cause the hydrogel to swell, whereas * Corresponding author. E-mail address: zfarsangi@razi.tums.ac.ir (Z. Jomeh Farsangi). Contents lists available at ScienceDirect Materials Today Chemistry journal homepage: www.journals.elsevier.com/materials-today-chemistry/ https://doi.org/10.1016/j.mtchem.2018.02.004 2468-5194/© 2018 Elsevier Ltd. All rights reserved. Materials Today Chemistry 8 (2018) 42e55