This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 11191--11193 11191

Cite this: Chem. Commun.,2013,49, 11191

Designing functionalizable hydrogels throughthiol–epoxy coupling chemistry†

Nergiz Cengiz,ab Jingyi Rao,b Amitav Sanyal*a and Anzar Khan*b

A novel and modular strategy has been developed for the pre-

paration of reactive and functionalized hydrogels. In this strategy,

thiol–epoxy coupling chemistry was employed for the formation

of a hydrophilic network. The hydroxyl groups, generated during

the coupling process, were then engaged in anchoring a fluores-

cent probe to the hydrogel scaffold.

Hydrogels are three-dimensional polymer networks that exhibitbiocompatibility, tissue-like elasticity, water uptake, and smallmolecular transport properties. Due to these attributes, theyhave been subjects of intense research investigations for arange of biomedical applications including tissue engineeringand drug delivery.1 Therefore, development of new syntheticmethods that can yield reactive and functionalizable cross-linked polymeric materials in a facile manner and with structuraluniformity is of considerable interest.2 In particular, a variety of‘click’ reactions such as azide–alkyne dipolar cycloadditions,Diels–Alder cycloadditions, thiol–ene reactions among manyothers have been used to fabricate hydrogels.2 To this end, ourattention is drawn towards the thiol–epoxy coupling reaction.3

This chemistry is demonstrated to be an efficient and simplereaction that can operate under ambient conditions. An addedadvantage of this process is that a reactive hydroxyl group isproduced upon completion of the coupling reaction. Thissecondary hydroxyl group, although less reactive than a primaryhydroxyl group, can be converted into an ester moiety uponreaction with an acid group.3 Therefore, in the present context,this sequence of events could be employed for the preparationof a functionalized hydrogel scaffold in two simple steps. Thethiol–epoxy coupling chemistry between commercially availableand appropriately chosen precursors would give rise to a

hydrophilic network carrying the hydroxyl groups. Transforma-tion of the hydroxyl units into ester functionalities wouldfurnish the targeted functionalized hydrogel.

To test the feasibility of this concept, initially, crosslinkingbetween pentaerythritol tetrakis(3-mercaptopropionate) (PETMP),1, and diglycidyl ether terminated poly(ethylene glycol) (PEG)(Mn = 1 kDa), 2, was carried out in the presence of tetrabutylammonium fluoride (TBAF) (Scheme 1). TBAF was chosen as acatalyst due to its solubility in various organic solvents and thepresent gel formulation, and its known capacity to catalyze thethiol–epoxy coupling reaction.4 Typically, 30–60 minutes ofreaction time was required at 70 1C for the complete solidifica-tion of material with conversions between 88 and 91% (Fig. 1).The solid materials were thoroughly washed with ethanol andwater and dried. IR spectroscopy demonstrated that the SHstretch present at 2570 cm�1 for the precursor 1 disappearedafter the gelation reaction (Fig. S1, ESI†). Moreover, a broadsignal, centered at 3470 cm�1, could be observed due to theformation of the hydroxyl groups. The ester stretch at 1730 cm�1

remained unperturbed after the gelation process indicatingstability of the ester functionalities towards the TBAF-catalyzedthiol–epoxy reaction.

To study the swelling behavior and the extent of wateruptake by the gels, the dried materials were swollen in waterand studied by gravimetric analysis until they reached equili-brium (Fig. 2). This study showed that the water uptake capacityof the gels was about 400% (Table 1). These gel samplesexhibited a compressive modulus of about 47 kPa. Therefore,to improve the mechanical properties, commercially availablediglycidyl ether-terminated poly(dimethylsiloxane) (PDMS) (Mn =0.8 kDa), 3, with the ability to be covalently crosslinked to thehydrogel structure due to its terminal epoxy groups (Fig. 3),was added to the gel formulation.5 PDMS is known for itsoptical transparency, biocompatibility, and high flexibility.Most importantly, it has already been demonstrated that addi-tion of PDMS to PEG-based hydrogels can increase their mecha-nical performance.6 Indeed, addition of PDMS-diglycidyl ether3 to the gel system produced hydrogels with significantlyenhanced mechanical properties (Table 1). For example, 20,

a Department of Chemistry, Bogazici University, Bebek 34342, Istanbul, Turkey.

E-mail: amitav.sanyal@boun.edu.tr; Fax: +90 212 287 2467;

Tel: +90 212 359 7613b Department of Materials, Swiss Federal Institute of Technology (ETH),

CH-8093 Zurich, Switzerland. E-mail: anzar.khan@mat.ethz.ch;

Fax: +41 44 633 1390; Tel: +41 44 633 6474

† Electronic supplementary information (ESI) available: Experimental procedures.See DOI: 10.1039/c3cc45859h

Received 31st July 2013,Accepted 9th October 2013

DOI: 10.1039/c3cc45859h

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11192 Chem. Commun., 2013, 49, 11191--11193 This journal is c The Royal Society of Chemistry 2013

33, and 50 mol% addition of 3, relative to the PEG content, inthe formulations resulted in gels with moduli of 72, 157, and204 kPa, respectively. PDMS, however, is a hydrophobic polymer.Hence, an increase in the hydrophobic content of the hydro-philic PEG network led to a decrease in the water uptake capacityof the hydrogel materials. The improvement in the mechanicalproperties of the hydrogels with an increase in the PDMScontent, therefore, may also be a mere reflection of the reductionin the extent of the equilibrium swelling of the materials.

An alternative strategy to improve the mechanical strengthof the materials is by increasing the crosslinking density.This can be achieved by increasing the concentration of the

precursors in the gel formulation.7 Application of this strategyresulted in a significant increase in the compressive modulusof the hydrogels (Table 1). For example, increasing the PEGconcentration from 27 to 31% (corresponding increase in thePETMP concentration from 6.6% to 7.8%) could increase themodulus of the material at equilibrium swelling from 204 kPato 367 kPa. We assume that higher reaction concentrationsmost likely produce materials with relatively few structuraldefects and more comprehensive formation of the networkpoints. At present, however, no experimental proof could beprovided to support this hypothesis.

Therefore, by combining the aforementioned two concepts—addition of PDMS and an increase in the PEG concentration—materials with a wide range of mechanical strengths (28–367 kPa)could be obtained.

To further increase the water uptake capacity of the hydrogels,PEG 2 (1 kDa) was replaced with PEG 4 (2 kDa) (Fig. 3). A longerreaction time (60 min) was required for obtaining a freestandinghydrogel. This increase in the chain length of the hydrophilic gelcomponent resulted in a sharp increase in the degree of materialswelling (Fig. 4). For example, gels from precursors 2 and 4exhibited a water uptake capacity of 400% and 1500%, respectively.

Scheme 1 Schematic illustration of hydrogel synthesis and functionalization.

Fig. 1 Digital picture of a typical thiol–epoxy hydrogel sample.

Fig. 2 Water uptake by the hydrogels containing 0 (solid line), 20 (dashed line),33 (dotted line), and 50 mol% (dash dotted line) of PDMS 3.

Table 1 Chemical composition and properties of hydrogelsa

EntryPEG concentrationb

(wt%)PDMS (3)c

(mol%)H2Ouptake (%)

Compressivemodulus (kPa)

1d 27 (PEG 2) 0 400 472 27 (PEG 2) 20 275 723 27 (PEG 2) 33 180 1574 27 (PEG 2) 50 110 2045 31 (PEG 2) 0 500 286 31 (PEG 2) 20 235 1507 31 (PEG 2) 33 176 1968 31 (PEG 2) 50 118 3679d 60 (PEG 4) 0 1500 4

a Functional group equivalence thiol : epoxy : TBAF�3H2O 1 : 1 : 0.25,temp = 70 1C, time = 30 min. b Thiol concentration also increasescorrespondingly. c PDMS (3) (mol%) = molPDMS/(molPDMS + molPEG) �100. d Gelation time of 60 minutes was required for samplesolidification.

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Additionally, these gels were fragile in comparison to onesobtained using shorter PEG chains.

Finally, to examine the availability of the hydroxyl groups forfurther gel functionalization purposes, esterification of freeze-dried and tetrahydrofuran (THF)-swollen gels with 1-pyrene-carboxylic acid was carried out at room temperature for aperiod of 12 hours. After this process the gels were washedthoroughly with dimethyl sulfoxide (DMSO), water, methanol,and THF to remove any physically absorbed materials and thenanalysed using fluorescence emission microscopy. As can beseen in Fig. 5, pyrene functionalized gels exhibited blue fluores-cence upon excitation at a wavelength of 365 nm. Moreover,IR spectroscopy showed significant reduction in the intensity ofthe OH stretches at 3470 cm�1 after the gel functionalizationindicating consumption of the hydroxyl groups during theesterification reaction (Fig. S1, ESI†). A new signal could belocated at 710 cm�1 belonging to the CQC aromatic stretchesof the pyrene functionality. In addition to these, a new esterband appeared at 1710 cm�1 after the esterification reaction.

These results demonstrated that the hydrogels could be func-tionalized with a fluorophore through an esterification reactionunder mild conditions.

In conclusion, thiol–epoxy coupling chemistry is a viablesynthetic tool for the preparation of PEG-based hydrogel materials.An added benefit of this chemistry is that it produces a reactivehydroxyl group during the gelation process. This reactive groupcan be used as an anchoring site to install a desired functionalgroup in the hydrogel scaffold through an esterification reac-tion. Therefore, functionalized hydrogels can be obtained in asimple two-step process starting from commercially availableprecursors. In addition, the developed strategy is modular andallows for systematic variation in the mechanical as well aswater uptake properties of the hydrogels. In essence, this reportestablishes a novel, facile, and modular method for the pre-paration of hydrogel materials with tunable chemical composi-tion and properties.

Financial support from Tubitak (BIDEB 2214) for NC isgratefully acknowledged. AK thanks Prof. Schluter (ETH-Z) forconstant support. Thanks are due to Dr Kirill Feldman (ETH-Z)for carrying out the mechanical testing experiments.

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5 In these cases, the epoxy : thiol ratio includes the epoxide units in thePDMS component.

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Fig. 4 Water uptake by the hydrogels containing 2 (dashed line) and 4 (solid line).

Fig. 5 Optical fluorescence microscopy images (bright field = left, upon excita-tion at 365 nm = right) of the functionalized gel.

Fig. 3 Chemical structures of polymers 3 and 4.

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