gels Review Thiol-Mediated Chemoselective Strategies for In Situ Formation of Hydrogels Jing Su Department of Chemistry, Northeastern Illinois University, Chicago, IL 60625, USA; [email protected]; Tel.: +1-773-442-5773 Received: 31 July 2018; Accepted: 31 August 2018; Published: 2 September 2018 Abstract: Hydrogels are three-dimensional networks composed of hydrated polymer chains and have been a material of choice for many biomedical applications such as drug delivery, biosensing, and tissue engineering due to their unique biocompatibility, tunable physical characteristics, flexible methods of synthesis, and range of constituents. In many cases, methods for crosslinking polymer precursors to form hydrogels would benefit from being highly selective in order to avoid cross-reactivity with components of biological systems leading to adverse effects. Crosslinking reactions involving the thiol group (SH) offer unique opportunities to construct hydrogel materials of diverse properties under mild conditions. This article reviews and comments on thiol-mediated chemoselective and biocompatible strategies for crosslinking natural and synthetic macromolecules to form injectable hydrogels for applications in drug delivery and cell encapsulation. Keywords: hydrogel; chemical crosslinking; chemoselective reaction; thiols 1. Introduction Hydrogels are a class of highly hydrated materials with three-dimensional (3D) networks composed of hydrophilic polymers, which are either synthetic or natural in origin [1]. The structural integrity of hydrogels depends on the crosslinks formed between polymer chains via various physical interactions and chemical bonds. Because they have mechanical properties similar to the extracellular matrix in native tissues, hydrogels have been widely employed as implantable medical devices such as contact lenses and biosensors [2–5], surgical adhesives [6,7], immunoisolating capsules for tissue transplantations [8,9], scaffolds for tissue regeneration [10–12], and materials for drug delivery [13,14]. In particular, in situ forming hydrogels have been very attractive since they allow the delivery of polymer precursors in combination with cells and soluble drugs in aqueous solutions through injection, resulting in the formation of 3D functional hydrogel networks at desired locations [15,16]. Tremendous natural and synthetic materials have been developed for the in situ formation of physical hydrogels by noncovalent electrostatic attraction, hydrogen bonding, and hydrophobic interactions [15,17]. Many of these, however, need to be initiated by changes in pH, temperature, or ionic concentration, such as pH-sensitive leucine-zipper protein assembly [18], thermosensitive collagen gelation [19], Alginate-Ca2+ crosslinking [20], and peptide amphiphile assembly [21–23]. These environmental triggers are not always physiologically relevant or biocompatible, and can be irreversibly detrimental to encapsulated cells and macromolecule drugs. It is also difficult to reproducibly control these conditions in clinical settings. In addition, physically crosslinked hydrogels do not have sufficient mechanical strength and structural stability against environmental changes or even hydrodynamic shearing. On the other hand, the crosslinking of polymers through covalent chemical bond formation in physiological conditions can produce robust hydrogel networks bearing tunable mechanical strength and stability in a much greater range. Hydrogels formed in situ through chemical crosslinking alone or through a hybrid of physical and chemical crosslinking have been shown Gels 2018, 4, 72; doi:10.3390/gels4030072 www.mdpi.com/journal/gels Gels 2018, 4, 72 2 of 22 Gels 2018, 4, x FOR PEER REVIEW 2 of 22 tobeen meet shown the needsto meet of the many needs different of many biomedical different applications,biomedical applications, from artificial from load-bearing artificial load-bearing connective tissue,connective to 3D tissue, tissue to scaffolds, 3D tissue to scaffolds, controlled to delivery controlled of therapeuticsdelivery of therapeutics [24,25]. [24,25]. In order to develop chemically crosslinked hydrogels to achieve a desired biomedical function, the right polymer precursors, crosslinking methods, and degradation properties of formed hydrogel are all essential. A good understanding of the biological system of interest is required to evaluate the interactions between the system and the applied polymerpolymer precursors,precursors, crosslinkingcrosslinking catalyst/initiators,catalyst/initiators, any possiblepossible product product released released from from the crosslinkingthe crosslinking reaction, reaction, and degradation and degradation products fromproducts hydrogels. from Inhydrogels. many cases, In many methods cases, of methods polymer of crosslinking polymer crosslinking would benefit would from benefit being from highly being selective highly selective to avoid cross-reactivityto avoid cross-reactivity and adverse and effects adverse on effects functional on functional components components of the biological of the systembiological (Figure system1). In(Figure physiologically 1). In physiologically relevant environments, relevant environments, as focused on as infocused this review, on in chemoselectivitythis review, chemoselectivity is defined as theis defined preferred as the reactivity preferred of reactivity a chemical of groupa chemical toward group another toward specific another functionality specific functionality in the presence in the ofpresence multiple of potentiallymultiple potentially reactive functionalities, reactive functionalities, especially thoseespecially existing those in existing biological in complexes.biological Thecomplexes. past two The decades past two have decades witnessed have awitnessed remarkable a remarkable advancement advancement of bio-orthogonal of bio-orthogonal chemical reactionschemical thatreactions covalently that covalently connect unnatural connect chemicalunnatural structures chemical [26structures], for example, [26], for 1,3-dipolar example, click1,3- cycloadditiondipolar click andcycloaddition Diels–Alder and cycloaddition, Diels–Alder providing cycloaddition, promising providing solutions promising to eliminate solutions interference to witheliminate biological interference systems with during biological the formation systems of polymeric during the hydrogels, formation as summarizedof polymeric in hydrogels, several recent as reviewssummarized[24,25 in,27 several]. However, recent these reviews unnatural, [24,25,27]. usually However, expensive these building unnatural, blocks usually may significantly expensive increasebuilding theblocks cost may of materials, significantly and increase this limits the thecost use of ofmaterials, bio-orthogonal and this reactions limits the for use producing of bio- hydrogelsorthogonal in reactions reality. for producing hydrogels in reality. Figure 1. In situ crosslinking of hydrogels in the presence of the biological complex including cells, Figure 1. In situ crosslinking of hydrogels in the presence of the biological complex including extracellular components, and therapeutic agents. Hydrogel networks should form upon cells, extracellular components, and therapeutic agents. Hydrogel networks should form upon chemoselective interactions between polymer precursors in order to minimize the disturbance to the chemoselective interactions between polymer precursors in order to minimize the disturbance to biological systems under study. the biological systems under study. On the other hand, polymers presenting naturally existing functionalities such as the amino groups (NH2) and thiols (SH, sulfhydryl), are still widely used in biomedical research and groups (NH2) and thiols (SH, sulfhydryl), are still widely used in biomedical research and applications becauseapplications of the because relatively of lowthe costrelatively and great low availability.cost and great For example,availability. the For natural example, polymer the chitosan natural presentspolymer aminochitosan groups; presents polypeptides amino groups; can presentpolypeptides amino can groups present through amino lysine groups residues through and lysine thiol groupsresidues at and cysteine thiol residues;groups at and cysteine synthetic residues; macromolecules and synthetic functionalized macromolecules with amine functionalized or thiol groups with areamine readily or thiol available groups from are manyreadily chemical available suppliers from many at affordable chemical prices. suppliers When at affordable these polymers prices. are When used inthese the polymers presence ofare biological used in the components, presence of a commonlybiological appliedcomponents, strategy a commonly for achieving applied chemoselectivity strategy for duringachieving hydrogel chemoselectivity crosslinking during is by hydrogel kinetic control,crosslinking in which is by exogenouskinetic control, polymer in which precursors exogenous are appliedpolymer at precursors much higher are applied concentrations at much than higher those concentrations biological components than those thatbiological are potentially components reactive, that drivingare potentially crosslinking reactive, reactions driving to crosslinking occur mainly reactions between to externally occur mainly supplied between polymers. externally supplied polymers.Compared to the amino group, the thiol group occurs at lower abundance in naturally existingCompared molecules to the and amino therefore group, bears the relativelythiol group higher