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Functional Stimuli-Responsive Gels: Hydrogels and Microgels

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Functional Stimuli-Responsive Gels: Hydrogels and Microgels 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]. Considering the current needs in the field of materials science, the strategy that has gained attentionConsidering in the last the decades current is the needs understanding in the fieldfield and ofof materialsma furtherterials mimicking science,science, the of structures strategystrategy that and hashasbehaviours gainedgained attentionfound in innature the last in decadesorder to isdevelop the understanding materials with and additionalfurther mimicking functionalities. of structures These and materials behaviours will foundfoundhave the inin nature natureability in into order orderrespond to to develop developto several materials materials stimuli with with and additional additionalcould functionalities.be usedfunctionalities. for biomedical These These materials purposes materials will or have will in thehaveeveryday ability the abilitylife to respond applications. to respond to several In thisto stimuliseveral context, and stimuli the could so -calledand be usedcould “smart” for be biomedical orused responsive for purposesbiomedical polymers or inpurposes are everyday becoming or life in applications.everydayincreasingly life important. Inapplications. this context, This In the kindthis so-called context, of polymer “smart”the so is-called ab orle responsiveto “smart” transform or polymers responsiveexternal arestimuli polymers becoming into shapeare increasingly becoming change important.increasinglyor movement This important. by kind taking of polymerThis advantage kind is of able polymerof tothe transform surroun is able dingexternalto transform environment. stimuli external into The shape stimuli interest change into in shape or the movement stimuli-change byorresponsive takingmovement advantage polymers by taking of has the continuedadvantage surrounding overof environment.the many surroun decadesding The and environment. interest it is still in the a hot stimuli-responsiveThe topic interest in materials in the polymers stimuli-science. hasresponsiveIndeed, continued researchers’ polymers over many interesthas continued decades in the and abilityover it many is that still decadesthis a hot kind topic and of in gelsit materials is stillpresent a hot science. when topic subjected in Indeed, materials researchers’ to externalscience. interestIndeed,stimuli can inresearchers’ the be abilityseen ininterest that the thisliterature in kind the ability ofsince gels thatthe present earlythis kind when1990’s of subjected (Figuregels present 2a) to externaland when is transversalsubjected stimuli can to to external beseveral seen instimuliscientific the literature can subjects be seen since (Figure in thethe 2b). earlyliterature In 1990’sthis sinceregard, (Figure the polymer early2a) and 1990’s gels is transversalhave (Figure a special 2a)to and severalplace is transversal in scientificthe field toof subjects severalstimuli (Figurescientificresponsive2b). subjects Insystems, this (Figure regard, and have polymer2b). attractedIn this gels regard, havemuch a polymerattent specialion placegels due haveto in their the a fieldspecial composition, of stimuliplace in versatility, responsive the field of and systems, stimuli wide andresponsivevariety have of attractedpotential systems, much applications,and have attention attracted in due particular tomuch their attent in composition, theion biomedical due to versatility, their field. composition, However, and wide versatility, we variety can also of and potential find wide its applications,varietyuse within of potential fields in particular as applications,catalysis, in themechanical, biomedicalin particular chemical field. in the However, or biomedical optical we sensors, canfield. also orHowever, actuators, find its usewe among can within also others. fields find its as catalysis,use within mechanical, fields as catalysis, chemical mechanical, or optical sensors,chemical or or actuators, optical sensors, among or others. actuators, among others. FigureFigure 2. (a)( aNumber) Number of publications of publications per year per yearand (b and) publications (b) publications by subject by subjectfeaturing featuring stimuli- stimuli-responsiveFigureresponsive 2. ( agel.) Number Databased gel. Databasedof publications used for used the per for bibliographic theyear bibliographic and ( banalysis:) publications analysis: Scopus Scopusby® Elsevier,subject® Elsevier, featuringAmsterdam, Amsterdam, stimuli- The TheresponsiveNetherlands Netherlands gel. consulted Databased consulted on the onused 2nd the for 2ndof theMay of Maybibliographic 2018. 2018. analysis: Scopus® Elsevier, Amsterdam, The Netherlands consulted on the 2nd of May 2018. TheThe naturenature of of the the stimulus stimulus can can be classifiedbe classified as physical, as physical, chemical chemical or biological or biological [3]. Physical [3]. Physical stimuli The nature of the stimulus can be classified as physical, chemical or biological [3]. 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