gels Review Functional Stimuli-Responsive Gels: Hydrogels and Microgels Coro Echeverria 1,* ID , Susete N. Fernandes 2 ID , Maria H. Godinho 2, João Paulo Borges 2 ID and Paula I. P. Soares 2,* 1 Instituto de Ciencia y Tecnología de Polímeros, ICTP-CSIC, Calle Juan de la Cierva 3, Madrid 28006, Spain 2 I3N/CENIMAT, Department of Materials Science, Faculty of Science and Technology, Universidade NOVA de Lisboa, Campus de Caparica, Caparica 2829-516, Portugal; [email protected] (S.N.F.); [email protected] (M.H.G.); [email protected] (J.P.B.) * Correspondence: [email protected] (C.E.); [email protected] (P.I.P.S.); Tel.: +351-212948564 (P.I.P.S.) Received: 4 May 2018; Accepted: 8 June 2018; Published: 12 June 2018 Abstract: One strategy that has gained much attention in the last decades is the understanding and further mimicking of structures and behaviours found in nature, as inspiration to develop materials with additional functionalities. This review presents recent advances in stimuli-responsive gels with emphasis on functional hydrogels and microgels. The first part of the review highlights the high impact of stimuli-responsive hydrogels in materials science. From macro to micro scale, the review also collects the most recent studies on the preparation of hybrid polymeric microgels composed of a nanoparticle (able to respond to external stimuli), encapsulated or grown into a stimuli-responsive matrix (microgel). This combination gave rise to interesting multi-responsive functional microgels and paved a new path for the preparation of multi-stimuli “smart” systems. Finally, special attention is focused on a new generation of functional stimuli-responsive polymer hydrogels able to self-shape (shape-memory) and/or self-repair. This last functionality could be considered as the closing loop for smart polymeric gels. Keywords: functional gels; hydrogels; microgels; hybrid microgels; stimuli-responsive; shape memory hydrogels; self-healing gels 1. Introduction Gels have pervaded our everyday life in a variety of forms. The wet soft solids that we encounter in the form of commercial products such as soap, shampoo, toothpaste, hair gel and other cosmetics, as well as contact lenses and gel pens, etc., are all gels derived from polymeric compounds. Polymer gels have been known for centuries and used for application in fields as diverse as food, medicine, materials science, cosmetics, pharmacology, and sanitation, among others. In general, gels are viscoelastic solid-like materials comprised of an elastic cross-linked network and a solvent, which is the major component. The solid-like appearance of a gel is a result of the entrapment and adhesion of the liquid in the large surface area solid three-dimensional (3D) matrix [1]. Pierre-Gilles de Gennes during his Nobel lecture in 1991 recognized the place of gels among the broad category of “soft matter”. Focusing on the phenomenological characteristics, polymer gels are 3D networks swollen by a large amount of solvent. Solid-like gels are characterized by: (i) the absence of an equilibrium modulus; (ii) the presence of a storage modulus, G’(u), which exhibits a pronounced plateau extending to times at least of the order of seconds; and (iii) the presence of a loss modulus, G”(u), which is considerably smaller than the storage modulus in the plateau region. The presence of liquid-like behavior on molecular length scales combined with solid-like macroscopic properties Gels 2018, 4, 54; doi:10.3390/gels4020054 www.mdpi.com/journal/gels Gels 2018, 4, x FOR PEER REVIEW 2 of 36 Gels 2018, 4, 54 2 of 37 Gels 2018, 4, x FOR PEER REVIEW 2 of 36 makes them very unique systems. Polymer gels are classified as chemical and physical gels, makes them very unique systems. Polymer gels are classified as chemical and physical gels, makesdepending them on very the unique nature systems. of crosslinks Polymer (Figure gels 1) are [2]. classified as chemical and physical gels, depending depending on the nature of crosslinks (Figure 1) [2]. on the nature of crosslinks (Figure1)[2]. Figure 1. Schematic representation of polymer gels formed by chemical crosslinks (left image) or Figure 1. Schematic representation of polymer gels formed by chemical crosslinks (left image) or Figurephysical 1. crosslinks Schematic ( rightrepresentation image) [2]. of polymer gels formed by chemical crosslinks (left image) or physical crosslinks (right image) [2]. physical crosslinks (right image) [2]. 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