Interpretación de polimeros

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Review

Design and applications of interpenetrating polymer network hydrogels.

A review

Ecaterina Stela Dragan

Petru Poni Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41 A, 700487 Iasi, Romania

h i g h l i g h t s

Design concepts and applications of interpenetrating polymer networks hydrogels are reviewed.Influence of the second network on properties of IPN hydrogels is discussed.Deswelling and mechanical properties of PNIPPAm are improved in IPN hydrogels.IPN hydrogels are recommended as efficient sorbents for heavy metal ions.IPN cryogels perform better than conventional hydrogels in dye removal.

a r t i c l e i n f o

Article history:

Received 18 November 2013Received in revised form 16 January 2014Accepted 20 January 2014Available online xxxx

Keywords:

HydrogelsInterpenetrating polymer networkDyesHeavy metalsSorption

a b s t r a c t

Interpenetrating polymer networks (IPN) hydrogels have gained great attention in the last decades,mainly due to their biomedical applications. This review aims to give an overview of the recent designconcepts of IPN hydrogels and their applications in controlled drug delivery, and separation processes.In the first part, the main strategies for the synthesis of semi-IPN and full-IPN hydrogels, their relevantproperties, and biomedical applications are presented based on the nature of the networks, the main cat-egories selected being: IPN hydrogels based on polysaccharides (chitosan, alginate, starch, and otherpolysaccharides), protein based IPN hydrogels, and IPN hydrogels based only on synthetic polymers.The influence of the second network on the stimuli responsiveness of the smart IPN hydrogels is dis-cussed based on the most recent publications in the field. In the second part, an overview of the mostspecific applications of IPN hydrogels in separation processes is critically presented. Factors which con-trol the separations of dyes and heavy metal ions by semi-IPN and full-IPN as novel sorbents are dis-cussed based on the recently published articles and own results. A special concern is given to themacroporous IPN composite cryogels, which are very attractive materials for separation processes beingendowed also with a high reusability.

2014 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.cej.2014.01.065

1385-8947/2014 Elsevier B.V. All rights reserved.

Abbreviations:AAm, acrylamide; AAm-g-HEC, acrylamide grafted on hydroxyethylcellulose; AAPBA, 3-acrylamidophenylboronic acid; Alg, alginate; AMPS, 2-acrylamido-2-methyl-1-propansulfonic acid; BAAm, N,N0-methylenebisacrylamide; CMC, carboxymethyl cellulose; CS, chitosan; DS, diclofenac sodium; DMAEM, 2-dimethylaminoethylmethacrylate; DSC, differential scanning calorimetry; Dx, dextran; DxS, dextran sulfate; ECH, epichlorohydrin; EWC, equilibrium water content; GA, glutaraldehyde; GE,gelatine; HA, hyaluronic acid; HEMA, 2-hydroxyethyl methacrylate; IA, itaconic acid; IEP, isoelectric point; IIH, ion imprinted hydrogel; IPN, interpenetrating polymernetwork; LCST, lower critical solution temperature; MB, Methylene Blue; MO, methyl orange; MV, methyl violet; NaPAA, poly(sodium acrylate); NIPAAm, N-isopropylacrylamide; NVF, N-vinylformamide; PA, anionically modified potato starch; PAA, poly(acrylic acid); PAAm, poly(acrylamide); PAN, poly(acrylonitrile); PASP,poly(aspartic acid); PDADMAC, poly(diallyldimethylammonium chloride); PDMAEM, poly(N,N-dimethylaminoethyl methacrylate); PDMC, poly(methacryloyloxyethylam-monium chloride); PEG, poly(ethylene glycol); PEG-DA, poly(ethylene glycol) diacrylate; PEI, poly(ethyleneimine); PFO, pseudo-first-order; PMAA, poly(methacrylic acid);PMAAm, poly(methacrylamide); PS, potato starch; PSO, pseudo-second-order; PVA, poly(vinyl alcohol); PVP, poly(vinylpirrolidone); RB, rhodamine B; SA, sodium alginate;SEM, scanning electron microscopy; SF, silk fibroin; SPH, super-porous hydrogel; SReq, equilibrium swelling ratio; VPTT, volume phase transition temperature. Tel.: +40 232 217454.

E-mail address:[email protected]

Chemical Engineering Journal xxx (2014) xxxxxx

Contents lists available at ScienceDirect

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Please cite this article in press as: E.S. Dragan, Design and applications of interpenetrating polymer network hydrogels. A review, Chem. Eng. J. (2014),http://dx.doi.org/10.1016/j.cej.2014.01.065


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Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002. Design, characterization, and biomedical applications of IPN hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.1. Polysaccharide based IPN hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.1.1. Chitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.1.2. Alginate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.1.3. Starch and derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

2.1.4. Other polysaccharides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.2. Protein based IPN hydrogels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 002.3. IPN hydrogels based only on synthetic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3. Separations mediated by IPN hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.1. Characterization of sorption properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.2. Sorption of dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.3. Sorption of heavy metal ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3.3.1. IPN hydrogels based on biopolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.3.2. IPN hydrogels based on synthetic polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 003.3.3. Ion imprinted IPN hydrogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

3.4. Desorption and reusability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 004. Summary of the benefits of semi-IPN compared to single-network hydrogels, and of the influence of the second network on the properties of IPN

hydrogels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 005. Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

1. Introduction

Hydrogels are three-dimensional, hydrophilic, polymeric net-works capable to retain large amounts of water, or biological fluids,characterized by a soft and rubbery consistence, being thus similarwith living tissues [1,2]. Hydrogels may be chemically stable orreversible (physical gels) stabilized by molecular entanglements,and/or secondary forces including ionic, H-bonding or hydrophobicinteractions, these hydrogels being nonhomogeneous[1,2]. Exam-ples of reversible hydrogels are ionotropic hydrogels formed by

the interaction between a polyelectrolyte and an oppositelycharged multivalent ion, and the polyelectrolyte complexes (com-plex coacervates) formed by the interaction between two oppo-sitely charged polyelectrolytes. Physical gels can be disintegratedby changes in the environment conditions such as ionic strength,pH, and temperature. Physical hydrogels have numerous biomedi-cal applications in drug delivery, wound dressing, tissue engineer-ing and so on. Covalently cross-linked networks form permanent orchemical gels [1]. Smart hydrogels are able to significantlychange their volume/shape in response to small alterations of cer-tain parameters of the environment. Responsive hydrogels havenumerous applications, the most of them being focused on biolog-ical and therapeutic demands[35], and sensing applications[6].However, single-network hydrogels have weak mechanical proper-

ties and slow response at swelling. To enhance the mechanicalstrength and swelling/deswelling response, multicomponent net-works as interpenetrating polymer networks (IPNs) have beendesigned.

IPNs arealloys of cross-linked polymers,at least oneof them beingsynthesized and/or cross-linked within the immediate presence of theother,without any covalent bonds betweenthem, which cannotbe sep-arated unless chemical bonds are broken [79]. The combination of thepolymers must effectively produce an advanced multicomponent poly-meric system, with a new profile[10]. According to the chemistry ofpreparation, IPN hydrogels can be classified in: (i) simultaneous IPN,when the precursors of both networks aremixed andthetwonetworksare synthesized at the same time by independent, noninterfering routssuch as chain and stepwise polymerization[7,9,11](Fig. 1a), and (ii)

sequential IPN, typically performed by swelling of a single-networkhydrogel into a solution containing the mixture of monomer, initiator

and activator, with or without a cross-linker (Fig. 1b). If a cross-linkeris present, fully-IPN result, while in the absence of a cross-linker, a net-work having linear polymers embedded within the first network isformed (semi-IPN)[7,8,12,13].

When a linear polymer, either synthetic or biopolymer, is en-trapped in a matrix, forming thus a semi-IPN hydrogel, fully-IPNcan be prepared after that by a selective cross-linking of the linearpolymer chains[1416](Fig. 1c).

By theirstructure, IPNhydrogels canbe classifiedin:(i)IPNs, formedby two networks ideally juxtaposed, with many entanglements and

physical interactionsbetweenthem; (ii)homo-IPNs,which are a specialcase of IPN, where the two polymers which form the independent net-works have the same structure; (iii) semi- or pseudo-IPNs, in whichonecomponent hasa linearinsteadofa networkstructure. MechanicallyenhancedIPNhydrogels as double networks, promoted by Gong et al.,have attracted attention by their potential for biomaterials, mainly as areplacement of natural cartilage[1719]. The particular feature of thisnew type of IPN hydrogels, characterized by high resistance to wearand high fracture strength, consists of the preparation first of a denselycross-linkedionic hydrogel, the second network being a neutral looselycross-linked network [17,18].

This review aims to give an overview on the preparation andapplications of semi- and fully-IPN hydrogels based on the most re-cent publications in the field. In the first part, the main synthesis

strategies of IPN hydrogels, their relevant properties and biomedi-cal applications will be presented. In the second part, an overviewon the most specific applications of the IPN hydrogels in separationprocesses will be given.

2. Design, characterization, and biomedical applications of IPN

hydrogels

A wide variety of hydrophilic polymers or their precursors havebeen used to synthesize hydrogels, the main classes consisting ofnatural polymers and their derivatives (polysaccharides and pro-teins), and synthetic polymers containing hydrophilic functionalgroups such as COOH, OH, CONH2, SO3H, amines and R4N

+,

and ether[1]. By the combination of polymers coming from thesetwo classes, IPN composite hydrogels can be prepared by the three

2 E.S. Dragan/ Chemical Engineering Journal xxx (2014) xxxxxx



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routes presented inFig. 1. The most often encountered combina-

tions of polymers used to prepare IPN composite hydrogels aresummarized inFig. 2.

In the next subsections, each group of IPN hydrogels will be pre-sented based on the literature information and our own investiga-tions in the field.

2.1. Polysaccharide based IPN hydrogels

2.1.1. Chitosan

Chitosan (CS), the linear cationic polysaccharide composed ofb-(1? 4)-2-amino-2-deoxy-D-glucopyranose and b-(1? 4)-2-acetamido-2-deoxy- D-glucopyranose units, randomly distributedalong the polymer chain, has attracted numerous scientists dueto its outstanding biological properties like biodegradability,biocompatibility, and antibacterial activity. By the high contentof amino and hydroxyl functional groups, CS has also drawn atten-tion as a biosorbent showing high potential for the adsorption ofproteins, dyes, and metal ions[20,21].

Numerous investigations were performed to prepare IPN hydro-gels composed of CS, or its derivatives, and other polysaccharides,or their derivatives, mainly in order to design novel and more effi-cient drug release systems. Thus, cellulose[22]and its derivatives[2325], or AAm-g-dextran[26]have been first blended with CSfollowed by selective cross-linking of CS with glutaraldehyde(GA). Semi-IPN hydrogels composed of cross-linked CS and en-trapped AAm-g-hydroxyethyl cellulose (AAm-g-HEC) and their en-hanced loading with diclofenac sodium (DS) have been recently

reported [23], the schematic representation of their formation

being presented inFig. 3.The encapsulation efficiency of DS in semi-IPN increased up to

83%, and the percent of in vitro release depended on the pH, thecontent of the entrapped AAm-g-HEC, the cross-linking degreeand the drug loading.

Yang et al. have recently reported preparation of novel hydro-gels composed of PEG grafted on carboxymethyl chitosan and algi-nate (Alg), and found an improvement of the protein release at pH7.4, suggesting this composite hydrogel to be promising for proteindrug delivery in the intestine[27]. As cross-linker for CS, GA is usu-ally used due to the fast formation of Schiff base between the NH2groups of CS and aldehyde groups of GA. However, GA is highlytoxic in nature and therefore, pharmaceutical scientists normallydo not recommend its use in the synthesis of IPN for the purpose

of drug delivery. Recently, a new natural cross-linking agent, gen-ipin, has been successfully used in the preparation of CS basedhydrogels[2831].

Various IPN hydrogels composed of CS and synthetic polymershave been recently designed and investigated for their biomedicalapplications, the most relevant being presented in the next threesubsections.

2.1.1.1. IPN hydrogels based on chitosan and synthetic ionic matri-

ces. Chitosan and its derivatives have been used as components inthe formation of IPN composite hydrogels with various ionic poly-mers containing carboxylic groups like poly(acrylic acid) (PAA)[32,33], copolymers of acrylic acid[3437], poly(methacrylic acid)(PMAA) [3840], poly(N-acryloylglycine)[41], or cationic centers

like quaternary ammonium groups [42] and amine groups [4345]. The synthesis of semi-IPN has been carried out either byselective cross-linking of CS in the presence of a preformedpolyelectrolyte [33,38,43]or by the synthesis of the cross-linkedpolyelectrolyte in the presence of CS [32,34,35,39,40,42,44].Full-IPNs have been also prepared by the post-cross-linking of CSentrapped in a polyelectrolyte matrix[36].

In the case of the composite IPN of CS and polyelectrolytes con-taining carboxylic groups, the ionic interactions between NH3groups of CS and COO from the anionic polyelectrolyte, whichwere identified in a certain range of pH, contributed to the increaseof the mechanical properties of the gels and to the decrease of theswelling degree because they contribute to the relative increase ofthe cross-linking density of the gel [33,38,39]. However, the ionic

cross-links make the gels to be reversible responsive to variationof the solution pH and ionic strength [32,37,38]. The interest in

Fig. 1. Schematic representation of the IPN formation: (a) simultaneous strategy; (b) sequential strategy; (c) selective cross-linking of a linear polymer entrapped in semi-IPN.

Protein +

syntheticpolymer

Chitosan + otherpolysaccharides

Only

syntheticpolymers

Polysaccharide

based IPN hydrogels

Design of IPN

hydrogels

Starch +synthetic polymersAlginate +

synthetic polymers

Chitosan +synthetic polymers

Otherpolysaccharides +

synthetic polymers

Fig. 2. Possible combinations of hydrophilic polymers used to prepare IPNhydrogels.

E.S. Dragan/ Chemical Engineering Journal xxx (2014) xxxxxx 3



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the preparation of IPN composite hydrogels based on CS and poly-anions containing carboxylic groups has been motivated by thefinding of more efficient systems for the sustained release of pro-teins[34,36,40]and drugs[35].

Guo and coworkers have reported an interesting approach toobtain thermo- and pH-responsive semi-IPN polyampholytehydrogels based on carboxymethyl chitosan and poly(dimethyl-aminoethyl methacrylate) (PDMAEM)[44]. The semi-IPN hydrogelshrunk most at the isoelectric point (IEP) and swelled when pHdeviated from the IEP. In the presence of PDMAEM, which presenta lower critical solution temperature (LCST), the swelling ratio ofthe composite gel dramatically decreased between 30 and 50 C,at pH 6.8. The key advantage of this composite hydrogel is thatthe release rate of coenzyme A could be modulated as a functionof temperature, being higher at 50 C than at 37 and 25 C, at pH6.8, making the semi-IPN hydrogel of great promise in pH/temper-ature responsive drug delivery systems.

2.1.1.2. IPN hydrogels based on chitosan and synthetic nonionic

matrices. Numerous IPN composite hydrogels have been preparedby cross-linking polymerization of nonionic monomers in the pres-ence of CS, the most employed monomers being acrylamide (AAm)[4652], N-isopropylacrylamide (NIPAAm)[53,54], N,N-dimethyl-acrylamide[55], and 2-hydroxyethyl methacrylate (HEMA)[5659].Currently, the modulation of the mechanical properties and thewater content of hydrogels by the preparation of the abovementioned IPN gels are expected, one main purpose being theiruse in biomedical applications such as controlled release systems

and as scaffolds in tissue engineering. Kim et al. described an inter-esting approach for the preparation of semi-IPN composed of CSand poloxamer [60]. Their strategy consisting of photo-cross-linking the poloxamer macromer in the presence of CS coupledwith freeze-drying to obtain sponge type hydrogels. These IPNcomposite hydrogels demonstrated rapid water adsorption, highmechanical strength, and interconnected pores, which recommendthem for wound dressing application.

In our own research, we have prepared first semi-IPN hydrogelcomposed of CS as entrapped polymer in a matrix of PAAm, as con-ventional composite hydrogels[15]. Formation of full-IPN hydro-gels was performed by a selective cross-linking of CS withepichlorohydrin (ECH) (Fig. 1c), in alkaline medium, when a simul-taneous generation of anionic sites on the PAAm matrix, by the

partial hydrolysis of amide groups[61,62], occurred. The formationof IPN is schematically presented inFig. 4.

The semi-IPN and IPN hydrogels were characterized by FTIR,differential scanning calorimetry (DSC), scanning electron micros-copy (SEM), and equilibrium swelling. FTIR spectroscopy sup-ported the presence of CS and the hydrolysis of amide groups infull-IPN. The freeze-dried hydrogels displayed a porous morphol-ogy generated by the sublimation of water under freeze-dryingconditions. The swelling kinetics have been followed for bothsemi- and full-IPN, and found that the transport of water was theFickian diffusion for all the gels[15].

Other strategies for the synthesis of IPN composite hydrogelsconsist of the blending CS with preformed synthetic polymers likepoly(acrylamide) (PAAm) [46], polyacrylonitrile (PAN) [63,64],poly(ethylene glycol) (PEG) [28,65,66], poly(vinyl alcohol) (PVA)[6773], poly(vinyl pyrrolidone) (PVP) [7476], poly(dimethylsi-loxane)PEG copolymer[77]followed by the selective cross-link-ing of CS. pH and temperature responsive semi-IPN hydrogelshave been obtained by the cross-linking of CS in the presence of(PAN)[63,64].

2.1.1.3. IPN cryogels based on chitosan and synthetic matrices. Cryo-gels are 3-D macroporous polymeric gels prepared below the freez-ing point of the solvent (water in the case of hydrogels)[7882].The unique feature of cryogels consists of their interconnectedmacropores (with sizes between 1 and 100 lm), which allow rapidand non-restricted mass-transport of any solute, and even micro-particles [78,79,81]. Cryogels are endowed with a capillary net-work through which the solvent can flow by convective masstransport, a high mechanical strength and osmotic stability, which

make them adequate materials for various biomedical applicationsand bioseparations [79,82]. IPN composite cryogels for bioengi-neering applications have been recently developed by Kumar andcoworkers[79]. A characteristic of cryogels is their very fast swell-ing at equilibrium, which was found to depend on the total mono-mer concentration, the cross-linking density, the synthesistemperature, etc. As already mentioned, unlike the conventionalhydrogels, cryogels have valuable mechanical properties. Thus, Jainand Kumar[79]determined the Youngs modulus of the PAAm/CScryogels to get information about the pressure which they couldbear as scaffolds for biomaterial applications, and found that, bytheir hydrophilic nature, cryogels had a very high flexibility.

In our own investigations, we have prepared ionic compositecryogels consisting of two independently cross-linked and oppo-

sitely charged networks [14]. Semi-IPN cryogels were preparedfirst by cross-linking polymerization of AAm with BAAm in the

Fig. 3. Schematic representation of the synthesis of CS/AAm-g-HEC semi-IPN hydrogel (reproduced with permission from Ref.[23]).




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presence of CS, under freezing conditions, the main parametersvaried being cross-linker ratio (X), pH of the CS solution, and CSmolar mass (CS1 and CS2 having Mv= 235 kDa, and 467 kDa,respectively). It was found that the fraction of CS trapped in thesemi-IPN cryogels increased with the increase of X, pH of CS solu-tion, and CS molar mass. To obtain fully-IPN cryogels, the CS chainstrapped in the semi-IPN cryogels were cross-linked with ECH un-der alkaline conditions, as it was already shown for the synthesisof conventional IPN hydrogels (Fig. 4). The structure of the IPNcomposite gels was confirmed by FTIR spectroscopy, the hydrolysisdegree in full-IPN being also evaluated[14]. Interior morphology ofsemi-IPN and IPN cryogels examined by SEM evidenced intercon-nected macropores with sizes in the range 3080 lm. Some SEM

images are presented inFig. 5to illustrate morphological changes,which occurred during the generation of the second network inIPN. The code of the semi-IPN hydrogels consists of semi-IPN fol-lowed by two numbers separated by dots: the first one denotesthe CS used as trapped polymer, the second one represents themole number of AAm per one mole of BAAm. The code of full-IPN consists of the term IPN followed by the same numbers likethe semi-IPN used for their preparation.

As can be seen in Fig. 5, the decrease of the cross-linker ratiofrom 1/40 to 1/60, for the same molar mass of CS (CS1), conductedto about twice larger pores (average pore size 34 lm comparedwith 75 lm), and to less compact pore walls, these being moreaccessible for the diffusion of low molecular weight species(IPN1.40 compared with IPN1.60).

Super-fast swelling characterized all semi-IPN cryogels, theequilibrium swelling state being attained in 23 s, the differenceconsisting of the equilibrium swelling ratio (SReq), which increasedwith the decrease of the cross-linker ratio[14]. The main differ-ences between IPN and semi-IPN cryogels concerning the swellingkinetics was the much higher values of theSReq(155 g/g comparedwith 33 g/g) and of the time necessary to reach the equilibriumswelling, which was about 45 s (IPN) compared with 3 s (semi-IPN). The increase of the time necessary to reach the equilibriumswelling has been explained by the presence of two networks,which respond independently to the environmental changes. Themuch higher swelling ratios of IPN cryogels were explained bythe presence of the anionic matrix, which is bearing COO groups,known for their high hydrophilicity. Both semi-IPN and IPN were

pH responsive, the SR values of semi-IPN decreasing when pH in-creased from 4 to 7, because the deprotonation of the amine

groups in CS occurred, while the IPN cryogels, having two indepen-dent networks responsive at pH, behaved completely different.Thus, the swelling feature at pH < 3 has been dominated by the cat-ionic network based on CS, the carboxylic groups being less hydro-philic at this pH. At pH > 4, the gel dramatically swelled due to theelectrostatic repulsion between COO groups.

2.1.2. Alginate

Sodium alginate (SA) is a linear polysaccharide, derived fromsea algae composed of 1-4-linked b-D-mannuronic acid (M) anda-L-guluronic acid (G), arranged in a blockwise fashion as homo-polymer blocks (MM, GG) or alternating blocks of M and G with

different M/G ratios[83]. It can be easily cross-linked by divalentions (for example Ca2+), which bind the guluronic residues withthe transformation in hydrogel. Due to this characteristic, SA hasbeen widely used in conditioning of fabrics, foods, and various drugdelivery systems. For the preparation of IPN composite hydrogels,SA was combined with various synthetic polymers[84]. Tempera-ture and pH responsive IPN hydrogels, composed of SA andpoly(diallyldimethylammonium chloride) (PDADMAC), have beenprepared by Kim et al. by a sequential strategy [10]. The pH-responsiveness of the IPN hydrogels in the pH range 26 showedthat the swelling ratios increased with increasing pH value, havinga maximum at pH 4, but decreased in the range 46. When the car-boxylic acid groups are below pKavalues, they are in the form ofCOOH. As the pH of the solution increased, the COOH were ion-

ized to COO

, and the resulting electrostatic repulsion causes thehydrogels to swell. On the other hand, in this range of pH, theCOO in Alg and the ammonium sites in PDADMAC coexisted,and formed polyelectrolyte complexes resulting in a decrease ofthe swelling ratio of the IPN hydrogels.

Various IPN composite hydrogels, composed of SA and syntheticpolymers containing carboxylic groups, with novel properties likesuper-porous [16], electrical sensitivity [85], drug controlled re-lease [86], multi-responsive [87,88] have been designed. Super-porous IPN composite hydrogels were prepared by Yin et al. [16]through sequential cross-linking, by fast cross-linking polymeriza-tion of AAm and sodium acrylate in the presence of SA as en-trapped polymer and sodium bicarbonate as blowing agent. CaCl2was applied to cross-link the SA chains in semi-IPN gels. Owing

to their high porosity, the IPN hydrogels thus prepared had a fastswelling and a high swelling ratio, this being affected by the exter-

APS

TEMED

PAAm network

CS chains

PAAm network

CS network

ECH/2 M NaOH

24 h/22oC

Fig. 4. Schematic representation of the formation of IPN hydrogels composed of CS and PAAm.




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nal pH and ionic strength. The IPN hydrogels had a high mechani-

cal strength and a good biocompatibility.Alg based IPN have gained widespread interest for industrial

applications as actuators or muscles close materials. To realizesuch materials, polymer gels with fast electric response and a highmechanical strength were required. IPN composite hydrogelsbased on PMAA and SA, showed a significant and quick bendingwhen subjected to an electric field, in HCl solution, and thereforethe authors assumed that this hydrogel could be useful for artificialorgan components, such as muscle-like contractile structure, sen-sors, and electric current modulated drug delivery systems[85].

It is known that PNIPAAm, one of the most widely studied ther-mo-responsive polymers, has a slow response rate at the tempera-ture changes. Synthesis of multi-responsive IPN compositehydrogels, based on SA and PNIPAAm, constitutes one of the strat-

egies adopted by numerous groups to increase the porosity of thegels and thus to achieve gels with a faster response rate as requiredfor drug release systems. Both semi-IPN [8894] and full-IPN[95,96]have been investigated. The pH/temperature sensitive re-lease of indomethacin from semi-IPN hydrogel beads composed ofCa-alginate and previously synthesized PNIPAAm has been re-ported by Shi et al. [97]. A drastic change in the drug release wasachieved by alternating the pH of the buffer solution between 2and 7. The drug release was higher at 37 C than at 25 C andshowed that the Ca-alginate/PNIPAAm beads had potential as effec-tive pH/temperature responsive delivery system of bioactiveagents. The pulsatile swelling/deswelling behavior of semi-IPNhydrogels composed of cross-linked PNIPAAm and linear SA re-vealed that theprocess wasrepeatable, by alternating both temper-

ature and pH, their mechanical strength making them suitable forstimuli-responsive drug release systems [93]. An interesting ap-

proach for the synthesis of semi-IPN SA/PNIPAAm hydrogels with

an enhanced deswelling rate compared to pure PNIPAAm consistsof thein situ generation of magnetic iron oxidesby oxidationof ironcations coordinated to the Alg network[91]. It was demonstratedthat the in situ synthesis of the iron oxide nanoparticles preventedtheir diffusion out of the semi-IPN, and that the porosity of the gelsincreased because a partial hydrolysis of the Alg chains occurred.

An interesting strategy for the preparation of IPN SA/PNIPAAmrecently reported consists of the preparation first of the ionicallycross-linked SA beads, which were than soaked in the solution ofNIPAAm, cross-liker, initator followed by cross-linking polymeriza-tion of NIPAA at a temperature above the LCST of PNIPAAm [95].The obtained composite beads changed their transparency in re-sponse to the change of temperature but kept their original shapeand size. The dynamic temperature cycling revealed the repeatabil-

ity of the thermoresponsivity, a hysteresis, characteristic to thecoil/globule transition of PNIPAAm, being observed.

Synthesis of IPN composite hydrogels based on SA and othersynthetic polymers like PAAm[98], PEG[99], PVA[100], and PVP[101]and their swelling properties and applications for controlledrelease of bioactive agents were also recently reported. In situformed IPN hydrogels based on a physical network of calciumAlg interpenetrated with a chemical cross-linked network basedon dextran derivatized with HEMA were prepared and evaluatedfor the protein release, mechanical characteristics and biocompat-ibility[102].

2.1.3. Starch and derivatives

As the second abundant polysaccharide after cellulose, starch

offers an interesting set of characteristics, including biodegradabil-ity, biocompatibility, and bioactivity. Native starch granules are

Semi-IPN1.40 Semi-IPN1.60

IPN1.40 IPN1.60

Fig. 5. SEM images of semi-IPN composite cryogels composed of CS entrapped in PAAm matrix with two cross-linker ratios (up), and of the full-IPN resulted by the selectivecross-linking of CS with ECH (down).




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water insoluble, containing two major components: (1) amylose,2030% of the starch granules, which consists of linear chains ofa-(1-4-linked-D-glucose) units, and (2) amylopectin, which con-sists of branched chains ofa-(1-4-linked-D-glucose) units inter-linked by a-(1-6-linked-D-glucose) linkages, in proportion of 7080%. Various modifications of starch were developed to improveits hydrophilic character [103,104]. Native and modified starches

have been used as raw materials in the preparation of biodegrad-able hydrogels for biomedical applications [105,106]. For manyapplications, multicomponent hydrogels as semi-IPN or IPN showimproved mechanical properties, faster response rate and diffusionof solutes[107110]. The swelling/diffusion properties such as ini-tial swelling rate, swelling rate constant,SReq, mechanism of waterdiffusion of the semi-IPN hydrogels prepared by the cross-linkingcopolymerization of AAm and sodium methacrylate in thepresence of starch have been investigated by Keshavara Murthyet al.[107]. The high EWC of these composite hydrogels recom-mends them as novel biomaterials in biomedical/pharmaceuticaltechnology or as moisture maintenance materials in agriculturefields.

Amphoteric semi-IPN composite hydrogels have been preparedby the graft copolymerization of AA onto cationic starch in thepresence of either PDADMAC[109]or poly(methacryloyloxyethy-lammonium chloride) (PDMC)[110]. The existence of salt linkagesbetween the carboxyl groups in cationic starch-g-AA network andquaternary ammonium groups on PDMC chains has been testifiedby FT-IR spectroscopy. The swelling studies showed a high swellingcapacity in distilled water and outstanding pH-sensitivity of thesemi-IPN hydrogels. It was also found that the hydrogels contain-ing more PDMC were not sensitive in the basic medium, whichendowed the hydrogel with potential application in agriculture.

In our own research, native potato starch (PS) or anionicallymodified PS (PA) have been entrapped in a PAAm matrix, both con-ventional semi-IPN hydrogels [111,112] and semi-IPN cryogelsbeing prepared[113115]. Anionically modified PS has been pre-pared by the alkaline hydrolysis of the nitrile groups in PS-g-PAN

[111]. The properties of the gels have been further modulated bycontrolled hydrolysis under alkaline conditions (0.5 M NaOH, 4 hat 25 C). Formation of semi-IPN hydrogel and the main functionalgroups after the controlled hydrolysis can be seen in Fig. 6.

To evaluate the sorption mechanism of water in the conven-tional hydrogels, the swelling data were analyzed by the empirical

equation of Frenson and Peppas[116], swelling being controlled bya Fickian diffusion when PS was used as entrapped polymer(n< 0.45), and by an Anomalous transport (0.45 3. Therapid response rate to the external stimuli of the smart hydrogelsis the most essential function for their applications, and thereforevarious methods have been used to increase the response kinetics.

The study of the deswelling/reswelling kinetics of the compositegels, having a low cross-linker ratio (1/80), in ethanol/pure water,and 1 M NaCl/pure water showed a higher responsivity of theanionic gels prepared with the initial monomer concentration, Co,of 3% compared to the gels having Co= 5%, and this support theirpotential as smart materials[114].

APS

TEMED

4 h, 25 oC

0.5 M NaOH

Fig. 6. Schematic representation of the PAAm/PA semi-IPN hydrogel formation and of the controlled hydrolysis (with permission from Ref.[115]).




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2.1.4. Other polysaccharides

Many other polysaccharides or their derivatives have been usedin the preparation of semi-IPN or IPN composite hydrogels, themost employed being: cellulose [117], carboxymethylcellulose(CMC) [118123], hyaluronic acid (HA) [124128], kappa-karra-

geenan[129131], xantan[132], guar gum[133], chondroitin sul-fate[124,134], etc. Semi-IPN hydrogels composed of cross-linkedPAA and entrapped CMC have been prepared by Bajpai and Mishra,the network parameters (average molecular weight between cross-links, crosslink density) being evaluated from the water sorptioncapacity[118]. The release profiles of the entrapped drug (tetracy-cline) were investigated as a function of the IPN composition[119].The hydrophilic character of CMC was further increased by graftingNaAA in the presence of the linear PVP and a cross-linker, novelsuperabsorbent semi-IPN composite hydrogels being thus pre-pared by Wang et al.[122]. The gels structure, the network param-eters and the pH responsiveness have been evaluated as a functionof the gel composition. The incorporation of PVP improved theswelling capabilities, swelling kinetics and salt-resistant properties

of the composite hydrogel, which recommend them as potentialcandidate for water-manageable materials or drug deliverysystems.

Hyaluronic acid (HA), frequently mentioned as hyaluronan ow-ing to its existence as polyanion, is a linear polysaccharide of highmolecular weight consisting of two alternating disaccharide unitsof b-1,4-D-glucuronic acid and b-1,3-N-acetyl-D-glucosamine[125]. HA is the most commonly exploited natural polysaccharidein scaffold assembly for tissue engineering and as component forimplant materials. HA is very hydrophilic and influences severalcellular functions such as migration, adhesion and proliferation,contributing to the regulation of water balance, behaving like a lu-bricant by protecting the articular cartilage surface, acting as ascavenger molecule for free radicals[124]. HA-based materials of-

fer excellent biocompatibility, biodegradability, and versatility inproducing materials for tissue engineering scaffolds. One drawback

of unmodified HA is the low stability of the resulting construct be-cause of its high water solubility, and therefore some strategieshave been developed to get stable constructs like semi-IPN hydro-gels composed of HA and synthetic polymers[124]or HEMA deriv-atized dextran [128]. Semi-IPN with rapid response rate at

temperature composed of kappa-carrageenan and a matrix ofpoly(N,N-diethylacrylamide) were recently reported[129,130].

Our own investigation on the synthesis and characterization ofsemi-IPN composite hydrogels composed of PAAm as a matrix andeither dextran (Dx) [135,136]or dextran sulfate (DxS) [137,138]were focused on: (1) the preparation of macroporous semi-IPNcomposite hydrogels with super-fast responsiveness, and (2) thecharacterization of the novel composites by porosity, morphology,swelling behavior, and rheology as a function of the synthesisparameters such as cross-linking, monomer concentration, andsynthesis temperature. The characteristics of semi-IPNs compositehydrogels were compared with those of the cross-linked PAAmwithout polysaccharide. The gel preparation temperature and thepresence of Dx or DxS were found to be the key factors determin-

ing the porous structure of the networks. Thus, the interior net-work structures of the semi-IPNs prepared at 18C (cryogels)exhibited a heterogeneous morphology consisting of pores withsizes around 100lm, while those formed at +5C or +25 Cshowed pores with sizes around 3 lm. The swelling ratios of thecomposite hydrogels were higher than those found for the PAAmgels, irrespective of the gel preparation temperature. Moreover,by conducting the cross-linking polymerization reaction at18C, semi-IPNs with super-fast responsive rate have been ob-tained [135,136]. It was found that the stability of DxS into thecomposite hydrogels increased with the decrease of the synthesistemperature and with the increase of the cross-linker ratio, thelowest percentage of DxS being released from the compositehydrogels obtained at 18C and having a cross-linker ratio of

1/40 (Fig. 8, up, left). The dry state porosity of the composite gelswas much higher in the case of cryogels (gel preparation

PAAm/PS60.5 PAAm/PA60.5 PAAm/PA80.5

PAAm/PS60.5H PAAm/PA60.5H PAAm/PA80.5H

Fig. 7. SEM images of semi-IPN composite cryogels composed of PS (left) and PA (middle and right) entrapped in PAAm matrix, before (up), and after (down) the controlledhydrolysis.




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temperature of18C) than in the case of conventional hydrogels(gel preparation temperature of 5 and 25 C), being less influencedby the cross-linker ratio (Fig. 8up, right).

The interior morphology of the semi-IPN cryogels is illustratedby the SEM image included inFig. 8(down, left), interconnectedpores with sizes of about 120 lm being visible. The uniaxial com-pression measurements performed on equilibrium swollen PAAm/

DxS composite hydrogels showed that hydrogels highly stableagainst the mechanical forces were obtained by conducting thecross-linking copolymerization at subzero temperature (Fig. 8down, right).

2.2. Protein based IPN hydrogels

Various proteins were used in the preparation of IPN compositehydrogels in combination with either synthetic polymers or withgelatin (GE), the protein derived from collagen. The main goal inthis case was to enhance the blood biocompatibility of the semi-IPN hydrogels[139], and biological activity of synthetic polymers[140,141]or, as recently reported, to increase the structural stabil-ity of the GE nanofibers[142]. In the last case, polyethyleneglycol

diacrylate (PEG-DA) was used as a cross-linker because, unlike theother cross-linkers which interact directly with gelatin, PEG-DAundergoes free radical cross-linking polymerization with noreaction with the functional groups of GE, creating a matrix whichenhanced the structural stability of the semi-IPN scaffold inaqueous solutions. Macroporous IPN composite hydrogels havebeen prepared by Jain et al. by cryogelation technique, when thecross-linking polymerization of acrylonitrile in the presence ofGE and cross-linking of GE with GA simultaneously occurred[139].

Silk fibroin (SF), another biopolymer intensively used in thepreparation of IPN hydrogels, is a fibrous protein of silk fiber andconsists of heavy (350 kDa) and light (25 kDa) chain polypeptides,connected by a disulfide link[143]. The regenerated fiber has been

considered as candidate for biomaterials owing to its goodmechanical strength in the wet state, biocompatibility for thegrowth of cells, and high resistance against enzymatic degradation.As it was already mentioned, there are numerous studies to reducethe limitations of hydrogels composed of PNIPAAm, such as thelack of biocompatibility, deswelling rate, and mechanical proper-ties. PNIPAAm hydrogels have a very slow deswelling rate due to

the formation of a skin layer, which interrupts the release of inter-nal water molecules in the deswelling process (skin effect). Synthe-sis of multicomponent materials like IPNs of PNIPAAm and SF wasdeveloped by Gil and Hudson [143] to improve the deswellingkinetics by suppressing the skin layer formation. IPN compositehydrogels composed of SF and other synthetic polymers[144,145], by their good mechanical properties, are of interest intissue engineering and regenerative medicine.

Silk sericin is a water soluble globular protein derived from silk-worm, which has been used by Wu et al. in the preparation of IPNhydrogels with PNIPAAm [146] and PMMA [147], the last onebeing a fast pH-responsive hydrogel. Other protein based IPNhydrogels are composed of fibrin and PEG [148], fibrin and HA[149], soy protein and PNIPAAm [150,151], collagen and HA[152], PAAm and poly(c-glutamic acid)[153].

2.3. IPN hydrogels based only on synthetic polymers

The large variety of this category of IPN composite hydrogels isexplained by the numerous possibilities to combine synthetic poly-mers, which structures could be practically designed according tothe imagination of the investigator. Unlike the biopolymers, whichstructures depend on many natural factors, the structure of syn-thetic polymers can be designed and reproduced whenever theyare requested. In a first attempt to group these IPN hydrogels,the following two main classes are proposed:

Fig. 8. Percentage removal of DxS from semi-IPN PAAm/DxS composite hydrogels after 48 h immersion in water (pH = 5.5) at 25 C, as a function of the gel preparationtemperature (up, left); dry state porosity of semi-IPN PAAm/DxS composite hydrogels as a function of the gel preparation temperature, at two cross-linking ratios (up, right);

SEM image of semi-IPN PAAm/DxS composite cryogel at a cross-linking ratio of 1/40, Mag 100 (down, left); comparative behavior under uniaxial compression of semi-IPNPAAm/DxS hydrogels prepared at 20 C (A) and 18 C (B) (down, right).




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IPN hydrogels based on nonionic synthetic polymers, wherePHEMA, PEG, PAAm, PVA represent the most employed poly-mers in the preparation of semi-IPN or IPN composite hydro-gels, mainly for biomedical applications and separationprocesses[154164].

Ionic IPN hydrogels, such as: anionic [165179], cationic[180185], and anionic/cationic[186191]. Polyampholyte IPN

hydrogels based exclusively on synthetic polymers have beenonly seldom reported[192,193].

A particular type of IPN hydrogels based only on synthetic poly-mers, is that of homo-IPN [155,158], comprehensively discussed byChirila et al. taking as a model homo-IPN based on HEMA[155].

The majority of the IPN composite hydrogels based on syn-thetic polymers are responsive at two [169,171,172,194,195] orthree external stimuli [173,178,181]. The second network or justthe entrapped linear polymer in semi-IPN could alter the respon-sivity of the gels. Thus, it was observed that the responsiveness(deswelling/reswelling kinetics) of the semi-IPN hydrogels ismuch faster than that of the single-network hydrogels both inthe case of homo-semi-IPN [158] and hetero-semi-IPN[164,172,180], excepting the poly(styrenesulfonic acid sodiumsalt) entrapped into the PNIPAM matrix, which had an oppositeeffect, an irreversible collapse of semi-IPN being reported, i.e.the composite gel did not restore their volume by reswellingafter shrinking [170]. Furthermore, semi-IPN hydrogels havingtemperature responsive swelling properties have been generatedby entrapping PVP in a P(HEMA/IA) matrix, the conventional(HEMA/IA) matrix having no thermosensitivity [194].

To increase the drug loading capacity of hydrogels for hydro-phobic drugs, Huang et al. have prepared a multiple-responsivesemi-IPN hydrogel based on b-cyclodextrin-ECH (b-CD-ECH) en-trapped in a matrix of poly(3-acrylamidophenylboronic acid-co-2-dimethylaminoethyl methacrylate) [P(AAPBA-co-DMAEM)][181]. The drug release was slower than in the conventional P(AAP-BA-co-DMAEM) hydrogel due to the presence of b-CD, and was

influenced by pH, temperature, ionic strength and the glucose con-centration. The method used for the synthesis of IPN could havealso an influence on the swelling kinetics and drug release, a higherthermosensitivity being observed for sequential than for simulta-neous semi-IPN, at least in one case [176].

IPN hydrogels having anionic and cationic groups attached todifferent chains (networks) are stabilized not only by covalentbonds but also by ionic bonds, which contribute to the increaseof their mechanical strength and to the pH and ionic strengthreversible responsiveness [189,191]. At a certain ratio betweenthe opposite charges they could form polyion complexes, whichare an interesting class of functional materials with various appli-cations as biomaterials, drug delivery systems, dumping materi-als, etc. An interesting approach to design ionic IPN hydrogels

having a controlled ratio between cationic and anionic groupswas proposed by Ajiro et al. who prepared IPN of PAA and pro-tected poly(vinylamine), the primary amine groups available inIPN being generated by the selective hydrolyzes of N-vinylforma-mide (NVF) in NVF-co-N-vinylacetamide[186], the ratio betweenanionic and cationic groups being controlled by the content inPAA. IPN hydrogels endowed with enhanced mechanical proper-ties were prepared by a sequential technique incorporating a sec-ond polymer network (PAN) inside a super-porous hydrogel(SPH) [196]. Mechanical properties including compressivestrength and elasticity were significantly improved up to 50-foldtimes as compared with a control SPH. The incorporation of thesecond polymer network did not decrease the fast swelling ofSPHs due to the initial interconnected pore structure.

3. Separations mediated by IPN hydrogels

3.1. Characterization of sorption properties

To evaluate the sorption capacity for ionic species, the behaviorof various IPN hydrogels in the presence of ionic dyes or heavy me-tal ions has been evaluated, usually in batch mode. From the deter-

mination of the residual concentration of the dye or metal ion as afunction of various parameters, the adsorption or binding capacity,q, expressed by Eq.(1), and the removal efficiency, R, %, expressedby Eq.(2)can be calculated.

qe Co CeV

W ; mg=g 1

R Co Ce=Co 100 2

where Coand Ceare the initial and final concentrations, respectively,Vis the volume of the aqueous phase (L), and Wis the amount of thedried composite gel (g).

The main factors which influence the sorption process are: pH,sorbent dosage, temperature and the initial concentration of sol-ute. The interference with other ionic species should be also con-sidered. Sorption mechanism and its potential rate-controllingsteps, which help in the evaluation of the sorbent quality, areexamined by fitting various kinetic models on the experimentaldata, the most employed being: pseudo-first-order (PFO) kineticmodel[197], pseudo-second-order (PSO) kinetic model[198], We-ber and Morris kinetic model [199], and Elovichs kinetic model[200,201]. Plotting the sorbed amount of solute on the solid surface(mg/g) versus the equilibrium concentration of solute in solution(mg/L), at a constant temperature, gives an experimental isotherm.The experimental binding isotherms are described by theoreticalisotherms, such as: Langmuir[202], Freundlich[203], Sips or Lang-muirFreundlich [204,205], DubininRadushkevich [201], andTempkin[206,207].

The thermodynamic parameters of sorption (DGo, DSo and DHo)

are examined by performing sorption experiments at various tem-peratures[201,205]. The relation between the Langmuir constant,KL, or the equilibrium constant of adsorption, KC, and the free en-ergy of adsorption (DGo) is given by Eq.(3):

DGoads RTln KC 3

Eq.(4)allows evaluating the thermodynamic parameters of theadsorption process by plotting lnKCversus 1/T, according to theVant Hoff equation:

lnKCDSoadsR

DHoadsRT

4

The feasibility of sorption is indicated by the sign ofDGo, thenegative values of DGo indicating the sorption is spontaneous

and thermodynamically favorable. The sign of DHo indicates ifthe process is endo- or exothermic and gives information aboutthe sorption mechanism.

3.2. Sorption of dyes

Water pollution with dyes is becoming a huge environmentalproblem due to the large variety of dyes used in textile, paper, plas-tics, and cosmetic industries, which discharge a large amount ofeffluents including dyes. Majority of the dyes are toxic, carcino-genic, and nonbiodegradable. Conventional methods like biologicaltreatment, coagulation/flocculation, chemical precipitation, sol-vent extraction, membrane filtration, and oxidation, employedfor the dye removal from industrial waste waters, are not always




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effective. Adsorption is considered an effective and economicalmethod to remove dyes even at high concentrations, having someadvantages such as flexibility in the selection of the adequate sor-bent and operation, and the production of effluents suitable to bereused [20,21,208,209]. Therefore, the interest focused on findingnovel sorbents with high adsorption capacities, fast adsorption/desorption rate, and easy separation and regeneration strongly in-

creased last decade [210,211]. In this context, multicomponenthydrogels as semi-IPN or IPN incorporating either synthetic poly-mers [175,212216] or polymers coming from bioresources[15,111,112,217222] are gaining more and more interest forapplication in the preparation of hydrogel-based sorbents.

Semi-IPN and IPN based on PVA and poly(AA-co-HEMA) withvarious compositions were prepared by Mandal et al. and theirsorption capacity for rhodamine B (RB) and methyl violet (MV)from dilute aqueous solution have been investigated [214]. Itwas found that the sorption capacity of both dyes at pH 7 increasedwith the increase of the copolymer content in the gel, and the sorp-tion of MV was always higher than that of RB because, beside thecationic groups, RB contains a carboxylic group, which at pH 7 isalso ionized and repels the similar groups of hydrogel. The dyeadsorption of both dyes was lower on IPN than on semi-IPN, situ-ation attributed to the tighter network structure of the IPN. Suchhydrogels could be effective in the removal of traces of dyes fromwater. The influence of the dye concentration in the feed solutionon the binding mechanism of cationic dyes (MV and basic fuchsin)on the semi-IPN composed of CMC and P(AAm-co-HEMA) (PAMHE-MA) as a matrix has been deeply investigated by Bhattacharyyaand Ray [222]. All the hydrogels showed chemical sorption atlow concentration of the dye, and physical sorption at high concen-trations. Solpan et al. prepared SA/PAAm semi-IPN conventionalhydrogels containing 3 wt.% SA entrapped in PAAm matrix, and fol-lowed their usability in the removal of some textile dyes[220]. Forall cationic dyes, S type adsorption isotherms were found, theadsorption capacities at pH 7 and 25 C being in the order:MV > methylene blue (MB) > Safranine-O > Magenta.

Considerable interest has been lately focused on the macropo-rous hydrogels, characterized by a faster response rate at smallchanges of the external stimuli than the conventional hydrogels.Ionic multicomponent cryogels, having enhanced mechanical andchemical resistance, have been tested as novel sorbents in the sep-aration processes of small ionic species and in bioseparations[14,82,223,224].Table 1summarizes some of the recently reportedresults on the sorption of dyes, either on conventional IPN hydro-gels or on IPN cryogels.

In our research, the sorption of two ionic dyes, the anionic dyeDirect Blue 1 (DB1) and the cationic dye MB on the semi- and full-

IPN hydrogels based on PAAm and CS was investigated on bothconventional hydrogels[15]and cryogels[14]. It was found thatsemi-IPN hydrogels preferentially sorbed DB1, owing to the posi-tively charged groups from CS, while the full-IPN sorbed a muchhigher amount of MB than semi-IPN, the time required to achievethe equilibrium sorption of the dye being about 40 min, for bothtypes of gels. The strong interaction of IPN with cationic dye has

been attributed to the presence of anionic charges, COO

. The con-trolling mechanism of adsorption was investigated by fitting threekinetic models on the experimental data: the PFO kinetic model[197], the PSO kinetic model[198], and the intra-particle diffusionmodel by Weber and Morris[199]. It was found that the theoreticalqe,calcvalues estimated by PFO kinetic model were very close to theexperimental values, for both semi-IPN and full-IPN, and this sup-ported physisorption as the main controlling mechanism of sorp-tion. Information about the boundary layer diffusion effect wereobtained from the Weber and Morris plots, the highest value ofthe diffusion rate constant, Ci, being found for the highest amountof the dye adsorbed[15]. Fully-IPN PAAm/CS cryogels with a MBsorption capacity, estimated by fitting Sips model, of755.5 mg MB/g gel have been also prepared [14]. The fully-IPNcryogels showed excellent properties in separation of MB fromits mixture with methyl orange (MO).

Semi-IPN composite cryogels having PA as entrapped polymerin a matrix of PAAm, endowed with enhanced sorption of MB fromaqueous solutions, have been recently reported [112,115]. Thesorption capacity has further increased by a controlled hydrolysisof PAAm matrix, owing to the increase of the density of activeCOO groups. The shape of the sorption isotherm changed froman isotherm of L type, before hydrolysis, to an H type isotherm,after hydrolysis, which supports a very high affinity of the hydro-lyzed composite gel for the cationic dye. The sorption kinetic hasbeen better described by the PFO kinetic model and this showedthe overall adsorption process appearing to be controlled by phys-isorption owing to the electrostatic attraction between cationic dyeand COO groups of cryogel.

3.3. Sorption of heavy metal ions

Among the water pollutants, heavy metal ions are consideredthe most dangerous due to their non-biodegradability, high toxic-ity and carcinogenic effects. Various methods have been developedfor removal of heavy metals, based on physical, chemical, electri-cal, thermal and biological principles. As for the removal of dyes,adsorption is preferred owing to its wide range of applicability, fea-sibility and flexibility. In this context, a large variety of sorbentsbased on the renewable resources have been lately reported in

Table 1

Maximum equilibrium sorption capacity of IPN hydrogels for ionic dyes.

Sorbent Dye Sorbent dosage, g/L pH T, C qmax, mg/g Reference

Anionic dyes

Semi-IPN CS/(AAm-PEG macromer) Methyl orange 0.6 25 185.24 [221]Semi-IPN (NaAA-co-HEMA/MBA)/SA Congo Red 1 7 25 172 [219]Semi-IPN (AA-co-HEMA/MBA)/SA Congo Red 1 7 25 149.68 [219]Semi-IPN CS/(AAm-PEG macromer) Acid Red 18 0.6 25 342.54 [221]

Cationic dyes

Cryogel IPN PAAm/CS Methylene Blue 1 6.5 25 750 [14]Cryogel semi-IPN PAAm/PA Methylene Blue 1 6.5 25 667.7 [115]Semi-IPN Alg/PASP Methylene Blue 2 25 600700 [217]Semi-IPN Alg/PASP Malachite Green 2 25 300350 [217]Semi-IPN (AA-co-HEMA/MBA)/SA Methyl violet 1 7 25 126.18 [219]Semi-IPN PAMHEMA/CMC Methyl violet, 500 mg/L in feed dye 1 7 25 613.8 [222]Semi-IPN PAMHEMA/CMC Basic fuchsin, 500 mg/L in feed dye 1 7 25 920 [222]Cryogel semi-IPN PAAm/DxS Methylene Blue 0.67 5.5 25 18.76 [224]

Semi-IPN AA/AM/n-BA/amylose Crystal Violet 0.2 7.4 25 35.09 [225]




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literature[226230]. The specific applications of IPN hydrogels forthe removal of heavy metal ions are presented in the nextsubsections.

3.3.1. IPN hydrogels based on biopolymers

IPN hydrogels based on biopolymers have lately found their de-served place [231239]. Chitosan [235,239], alginate [234,236],

starch[238], cellulose derivatives[232], and other polysaccharides[233] constitute partners in the preparation of semi- or full-IPNhydrogels with promising sorption properties for heavy metal ions,some results being summarized inTable 2.

Chauhan and Mahajan have followed the sorption of metalions (Fe2+, Cu2+ and Cr6+) from aqueous solution on semi-IPNcomposite hydrogels prepared by the incorporation of cellulosederivatives in a matrix of poly(methacrylamide) (PMAAm) andobserved that the sorption capacity and the sorption mechanismof the metal ions were strongly influenced by the functionalgroups of the gel [232]. As can be seen in Table 2, the sorptioncapacity of Fe2+ and Cu2+ dramatically increased after the trans-formation of the amide groups of the matrix into COO groups,by partial hydrolysis. Before the hydrolysis, the metal ion uptake

was more a sorption process, while after hydrolysis, chelationand ion exchange are the main driving forces for the metal up-take. On the other side, no sorption of Cr 6+ ions was found afterthe partial hydrolysis, this being explained by the large size of theion in hydrated state. Wang et al. have recently reported prepa-ration of semi-IPN hydrogels composed of CS-g-PAA as a matrixand GE as entrapped protein with very high sorption capacitiesfor Pb2+ [235]and Cu2+ [237], the equilibrium of sorption beingattained in about 15 min. The equilibrium adsorption isothermwas fitted by the non-linear form of the Langmuir isotherm,and the sorption kinetic was well described by the PSO kineticmodel indicating the chemisorption by chelation of metal ionsas the mechanism of sorption. The presence of GE chains en-hanced the mechanical strength of semi-IPN composite hydrogels

and contributed to the increase of the maximum sorption of me-tal ions (261.08 mg Cu2+/g gel, and up to 736.95 mg Pb2+/g gel),and to the increase of the sorbent reusability. Semi-IPN compos-ite hydrogels composed on SA-g-PAA as a matrix and PVP and GEas entrapped chains and their sorption capacity for Ni2+, Cu2+,Zn2+, and Cd2+ have been reported by Wang et al., the maximumequilibrium adsorption capacity, in non-competitive conditions,

being the highest for Cu

2+

(3.22 mmol/g), and the lowest forCd2+ (2.91 mmol/) [236] (Table 2). The authors have fitted theexperimental isotherms of sorption with three isotherm models(Langmuir, Freundlich and Dubinin-Radushkevich), and foundthe Langmuir isotherm suitable to describe the adsorption pro-cess, supporting the formation of a monolayer of metal ions inthe composite hydrogel. As shown in Fig. 9, the electrostaticinteraction and ion exchange between the COO groups fromSA and the grafted PAA, on the one side, and the cationic metalions, on the other side, facilitated the penetration of heavy metalions into the composite hydrogel, in the first step, the chelationof heavy metal ions with the functional groups of the compositegel (NH2, COOH, C@O), and a chemical adsorption process oc-curred in the second step[236].

The adsorption mechanism has been supported also by thesorption kinetics, which have been perfect fitted by the PSO kineticmodel.

Our own research was focused on the investigation of theequilibrium sorption capacity for Cu2+, Cd2+, Ni2+, and Zn2+ of thesemi-IPN cryogels based on PAAm as a matrix and PA as entrappedpolymer [238]. The experimental data obtained in batch modehave been analyzed by four isotherm models: Langmuir, Freundlich,Sips and Temkin. A comparison of the linear and non-linear regres-sion fitting of these isotherms has been performed because, some-times, the linearization of a model function has a negative effect onthe ability of a model to fit the experimental data [201,207,222].Based on the non-linear regression method it was found thatthe Sips isotherm fitted the best the experimental data with a

Table 2

Maximum equilibrium sorption capacity of IPN hydrogels for heavy metal ions.

Sorbent Metal ion Sorbent dosage, g/L pH T, C qmax, mmol/g Reference

Semi-IPN PMAAm/CPa Fe2+ 10 25 0.178 [232]Hydrolyzed semi-IPN PMAAm/CPb Fe2+ 10 25 0.358 [232]Semi-IPN PMAAm/HPCc Cu2+ 20 25 0.2 [232]Hydrolyzed semi-IPN PMAAm/HPCb Cu2+ 20 25 2 [232]Semi-IPN SA-g-PAA/PVP/GE Ni2+ 2 5 30 3.158 [236]Semi-IPN SA-g-PAA/PVP/GE Cu2+ 2 5 30 3.221 [236]Semi-IPN SA-g-PAA/PVP/GE Zn2+ 2 5 30 3.035 [236]Semi-IPN SA-g-PAA/PVP/GE Cd2+ 2 5 30 2.913 [236]Semi-IPN CS-g-PAA/GE Cu2+ 2 5.75 30 4.81 [237]Sequential IPN poly(PEGDA)/PMAA Pb2+ 1 5 25 0.55 [240]

Sequential IPN poly(PEGDA)/PMAA Cu

2+

1 5 25 0.36 [240]Semi-IPN poly(PEGDA)/PMAA Cd2+ 1 5 25 0.33 [240]

a CP cellulose phosphate.b Partial saponification by immersion in 0.5 M NaOH for 48 h at 25 C.c HPC hydroxypropyl cellulose.

Fig. 9. The main mechanism for the adsorption of metal ions onto the hydrogel (reproduced with permission from Ref. [236]).




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theoretical sorption capacity of 40.72 mg Cu2+/g, 19.72 mg Cd2+/g,9.31 mg Ni2+/g, and 7.48 mg Zn2+/g.

3.3.2. IPN hydrogels based on synthetic polymers

IPN hydrogels based only on the synthetic polymers have beenalso evaluated as sorbents for heavy metal ions [11,240243]. In asemi-IPN hydrogel composed of PEG entrapped in a polyacrylate

(PAC) matrix, the sorption capacity of metal ions, reported by Tonget al., has been a function of the ratio between PAC and PEG beingmaximum at a ratio of 6, because PAC, by the presence of COO

groups, is more effective in coordination of heavy metal ions thanPEG [241]. The maximum sorption capacity found, in optimumconditions, for Ni2+, Cr3+, and Cd2+ has been: 102.34 mg/g,49.38 mg/g, and 33.41 mg/g, respectively.

Stimuli-responsive semi-IPN hydrogels composed of PNIPAAmas matrix and NaPAA as entrapped homopolymer and their sorp-tion properties for Cu2+ have been reported by Yamashita et al.[242]. The IPN hydrogels exhibited volume-phase transition behav-ior in the adsorption conditions, i.e. the IPN hydrogel adsorbed suf-ficiently Cu2+ ions below the volume phase transition temperature(VPTT) but not above the VPTT. Above the VPTT, when the gels col-lapsed, only water has been released and not metal ions, makingthese gels very convenient sorbents for applications in an intelli-gent recovery system of metal ions.

Wang et al. prepared IPN hydrogels with enhanced adsorptionproperties for heavy metal ions either simultaneous, by free radi-cal/cationic photopolymerization of 2-acrylamido-2-methyl-1-propansulfonic acid (AMPS) and triethylene glycol divynil ether(DVE-3) [11], or by the sequential strategy with poly(PEGDA),and PMAA as the two independent networks [240]. Adsorptionproperties of the IPN hydrogels for the removal of Cu(II), Cd(II),and Pb(II), compared with single networks, have been examinedin batch mode, under non-competitive conditions. The adsorptioncapacity of simultaneous IPN hydrogels increased with the increaseof AMPS content in the IPN hydrogel, for all metal ions, the opti-mum pH being 5[11]. The adsorption capacity of the sequential

IPN hydrogels increased with the increase of PMAA content inthe IPN hydrogel up to about 35%, the optimum pH being 5[240]. A synergistic complexation of metal ions with the two poly-mer chains in sequential IPN was assumed to explain the increaseof the experimental sorption capacity compared with the theoret-ical sorption capacity calculated taking into account the contribu-tion of each individual network. As Table 2 shows, the sorptioncapacity of IPN hydrogels was higher for Pb(II) than for Cu(II)and Cd(II), due to the smaller atomic radius of Pb(II). The adsorp-tion capacity of the IPN gels increased almost linearly with the in-crease of the initial concentration of metal ion, the experimentalisotherms being better described by the Freundlich isotherm, sug-gesting the adsorption was heterogeneous, in multi-layer pattern[11]. Examination of the sorption mechanism revealed a diffusion

controlled transport mechanism of metal ion sorption on thatIPN hydrogel. In general, the equilibrium of metal ion sorptionhas been established within 1015 min, i.e. very fast comparedwith the single network hydrogels[11,240,241].

Metal complexing membranes have been prepared by semi-IPNtechnique and their sorption properties for Pb2+ [244246], Hg2+,Cd2+, and Cu2+ [244,245]have been investigated. Thus, Bessbousseet al. have fabricated composite membranes based on a large vari-ety of complexing polymers, such as poly(ethyleneimine) (PEI),poly(allylamine), CS, PVPy, poly(vinylimidazole), PVP, entrappedwithin a matrix of PVA [244,245]. For the retention of individualions, the order of selectivity was: Hg2+ > Cu2+ > Pb2+ > Cd2+. Thehighest sorption capacities have been found for Hg2+, on the major-ity of the composite membranes, an interpretation of the results in

the frame of Hard and Soft Acids and Bases theory being attemptedby the authors[245]. The affinity sequence in binary mixtures has

been: Cu2+ > Hg2+ > Cd2+ > Pb2+, and in the quaternary mixtures,the selectivity has been similar with that found for individual ions.The better retention of Hg2+ by the PVA/PEI membrane was attrib-uted to the high affinity of Hg2+ for this membrane.

3.3.3. Ion imprinted IPN hydrogels

Selectivity in the heavy metal ions removal is still a challenging

concern, a real progress in the filed being performed by the ion-im-printed polymers, a concept similar with molecularly imprintedpolymers, recently reviewed by Branger et al.[247]. The imprintingprocess renders the resulting polymer able to recognize and selec-tively bind the template in the environment. Ion imprinted IPNhydrogels were also synthesized and evaluated for their capacityto selectively adsorb heavy metal ions [248250]. The number ofpublications in this field is still low, but it is expected to increasein the near future. Thus, Junyan et al. synthesized Cd(II) imprintedIPN containing epoxy resin, triethylenetetramine and cadmiummethacrylate acrylamide-BAAm by in situ sequential polymeriza-tion, which was successfully applied to the analysis of two naturalwater samples[248]. Liu et al. synthesized an IPN ion-imprintinghydrogel (IIH) via cross-linking of blended CS/PVA with ethyleneglycol diglycidyl ether using uranyl ion as template [249]. Thesorption experiments were performed in batch mode, the optimumpH was 5.06.0, and the adsorption process was well described byboth Langmuir and Freundlich isotherms. Equilibrium of sorptionwas achieved within 2 h and the maximum adsorption capacitywas 156 mg/g. The most significant results, which support theadvantage of the IIH compared to the non-imprinted hydrogel,consist of the selective adsorption of uranyl ion in a mixture withother heavy metal ions, the distribution ratio of IIH for uranyl ionbeing 6-fold greater than that of non-imprinted hydrogel, butwas almost the same for the other heavy metal ions. A novel ther-moresponsive Cu(II) ion-imprinted IPN [Cu(II)-IIH] has been re-cently reported by Wang and Liu [250]. The Cu(II)-IIH has beenprepared by free radical/cationic polymerization (simultaneousstrategy) of NIPAAm and triethylene glycol divinyl ether using

Cu(II) ion as template. The memory was fixed by shrinking of thegel above the VPTT, and was deleted by swelling below VPTT.The Cu(II)-IIH showed a stronger affinity for Cu(II) ions than forother competitor metal ions compared with the non-imprintedIPN hydrogel.

3.4. Desorption and reusability

Preservation of the sorption capacity during the consecutivesorption/desorption cycles is an important characteristic of allthe sorbents used in the removal of contaminants from the waste-waters. The majority of semi-IPN and IPN hydrogels used as sor-bents for both dyes and heavy metal ions showed a high level ofreusability, some examples being presented in this subsection.

For example, desorption of MB from the full-IPN PAAm/CS cryogelhas been performed with 0.1 M HCl, the sorbent being regeneratedwith 0.1 M NaOH. After four sorption/desorption cycles, the sorp-tion capacity for MB remained almost unchanged [14] and thissupport the high reusability of these IPN cryogels. After the con-trolled hydrolysis, the semi-IPN cryogels composed of PAAm andPA, described in Section 3.2, showed almost the same sorptioncapacity for MB after six consecutive sorption/desorption cycles(around 150 mg MB/g, at an initial concentration of the dye of157.4 mg/g, the sorbent dosage being 1 g/L)[115]. Semi-IPN hydro-gel composed of CMC entrapped in a matrix of AAm and HEMA(PAMHEMA) has been successfully reused for the sorption of basicfuchsin and MV (Table 1), their sorption capacity being almost un-changed after five repeated adsorption/desorption cycles[222].

Desorption of heavy metals was usually performed with dilutesolutions of HCl or HNO3 [235237,240,242]. Thus, desorption of




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metal ions from the sequential poly(PEGDA)/PMAA IPN hydrogels,using dilute HCl as desorption agent, showed that the desorptionratio was higher than 96%. The reuse of the gels in the second cycleof sorption showed a sorption capacity of around 93% from the ini-tial capacity for all metal ions, and this support the suitability ofthe gels for repeated applications[240]. The adsorption capacityof semi-IPN hydrogel composed of CS-g-PAA as a matrix and GE

as entrapped polymer for Pb

2+

[235]and Cu

2+

[237] slightly de-creased with increasing the number of consecutive adsorbtion/desorption cycles, the regeneration and metal ion recovery capabil-ities being improved by the presence of GE. The thermoresponsivesemi-IPN hydrogels composed of PNIPAAm as matrix and NaPAA asentrapped homopolymer have been easily recycled using 0.01 NHCl as desorption agent, in swollen state of the gel (20 C), andregenerated by alkali treatment (0.1 N NaOH) [242].

In conclusion, the IPN hydrogels are endowed with a high levelof reusability [111,219,222,240,242], which is of great practicalsignificance, this being further increased by the synthesis of hydro-gels at subzero temperatures[14,115].

4. Summary of the benefits of semi-IPN compared to single-

network hydrogels, and of the influence of the second networkon the properties of IPN hydrogels

IPNs, as a particular class of polymer blends, have been devel-oped with the aim to improve at least one property of the con-stituent networks. The main advantages of IPNs are thatrelatively dense hydrogel matrices can be produced, which fea-ture stiffer and tougher mechanical properties, more widely con-trollable physical properties, and (frequently) more efficient drugloading compared to single-network hydrogels[4,7,13]. The load-ing capacity of the CS/PNIPAAm IPN with DS significantly in-creased compared to pure PNIPAAm hydrogel, the sharpthermosensitivity of the PNIPAAm phase being preserved [53].The diffusion of drugs out of hydrogel matrices has been im-proved by applying the IPN strategy of synthesis [13]. However,semi-IPNs can more effectively preserve rapid kinetic responserates to pH or temperature, the benefits of IPNs in controlleddrug delivery (modifying pore size, slowing drug release) beingmaintained [13].

The influence of the second network on the properties of IPNhydrogels should be correlated with the conditions in which thegeneration of the second network occurred. Thus, the SReq andthe time necessary to reach the equilibrium swelling have beenhigher in the case of PAAm/CS IPN cryogel compared to thesemi-IPN cryogel, the second network being generated by the CScross-linking in alkaline medium, when partial hydrolysis of PAAmoccurred [14]. The increase of the time necessary to reach theequilibrium swelling has been attributed to the presence of twooppositely charged networks, and the much higher SReq of IPN

cryogels were explained by the generation of the anionic matrix,bearing COO groups, during the formation of the secondnetwork.

There are numerous studies focused on the decrease of thelimitations of hydrogels composed of PNIPAAm, such as the lackof biocompatibility, deswelling rate, and mechanical properties.Synthesis of multi-responsive IPN composite hydrogels, basedon SA and PNIPAAm, constitutes one of the strategies adoptedby numerous groups to increase the porosity of the gels and thusto achieve gels with a faster response rate as required forapplications in the design of drug release systems. Both semi-IPN [8894,143] and full-IPN [95,96] have been investigated.Thus, the drug release from semi-IPN hydrogel beads composedof linear PNIPAAm entrapped in Ca-Alg matrix was higher at

37C than at 25 C and showed that the Ca-Alg/PNIPAAm beads

have potential as effective pH/temperature responsive drug deliv-ery system[97]. An interesting approach for the synthesis of SA/PNIPAAm semi-IPN hydrogels, with an enhanced deswelling ratecompared to pure PNIPAAm, consists of the in situ generationand preservation of magnetic iron oxides [91].

It was also found that the deswelling/reswelling kinetics ofthe semi-IPN hydrogels based on synthetic polymers was much

faster than that of the single-network hydrogels, both forhomo-semi-IPN[158]and hetero-semi-IPN [164,172,180]. Miyataet al. investigated the response to the solvent quality (acetone/water mixtures) of the PAAm single network and PAAm homo-semi-IPN hydrogel [158]. The fast responsiveness at shrinkageof the PAAm homo-semi-IPN hydrogel compared to PAAm singlenetwork has been attributed to the inherent mobility of the lin-ear polymer chains of PAAm entangled in the cross-linked PAAm,which respond much faster at the environment than the PAAmnetwork. Furthermore, the collapse of the linear PAAm chainscould accelerate the shrinkage of the network because they wereentangled in the cross-linked chains of the semi-IPN structure[158]. Generation of temperature responsive swelling propertiesin semi-IPN hydrogels by entrapping PVP in a P(HEMA/IA) matrixhas been recently reported, the single network hydrogel havingno thermosensitivity[194].

The strategy used for the synthesis of IPN could have also aninfluence on the swelling kinetics and drug release, a higher ther-mosensitivity being observed for sequential than for simultaneoussemi-IPN, because the sequential strategy allows a better control ofthe morphology and mechanical properties of the IPN compositehydrogels[176]. IPN hydrogels endowed with enhanced mechani-cal properties have been prepared by a sequential technique incor-porating a second polymer network inside a super-porous hydrogel[196]. Mechanical properties (compressive strength and elasticity)were significantly improved (up to 50-fold times) as compared to acontrol single-network hydrogel.

The sorption capacity for cationic dyes (RB and MV) onto semi-IPN and IPN hydrogels based on PVA and poly(AA-co-HEMA)

prepared by Mandal et al. was lower on IPN than on semi-IPN,situation attributed to the tighter network structure of the IPN[214]. Semi-IPN hydrogels composed of GE entrapped in CS-g-PAAmatrix showed very high sorption capacities for metal ions, andvery fast sorption kinetics (sorption equilibrium has been attainedin about 15 min) [235,237]. The presence of GE chains enhancedthe mechanical strength of semi-IPN hydrogels and contributedto the increase of the maximum sorption capacity of metal ions.An intelligent recovery system of metal ions consisting ofPNIPAAm as matrix and NaPAA as entrapped homopolymer hasbeen reported by Yamashita et al. [242]. The IPN hydrogeladsorbed Cu(II) ions below the VPTT and released water abovethe VPTT but not metal ions.

Wang et al. reported IPN hydrogels with enhanced adsorption

properties for heavy metal ions either simultaneous, by free radi-cal/cationic photopolymerization of AMPS and DVE-3 [11], or bythe sequential strategy with poly(PEGDA), and PMAA as the twoindependent networks [240]. Adsorption capacity of the IPNhydrogels for Cu(II), Cd(II), and Pb(II) has been compared with thatof single networks. The adsorption capacity of simultaneous IPNhydrogels increased with the increase of AMPS content in the IPNhydrogel, for all metal ions [11]. The adsorption capacity of thesequential IPN hydrogels increased with the increase of PMAA con-tent in the IPN hydrogel, a synergistic complexation of metal ionswith the two polymer chains being assumed to explain the in-crease of the experimental sorption capacity compared with thetheoretical sorption capacity[240]. The sorption equilibrium hasbeen attained very fast compared to the single network hydrogels

[11,240,241].




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5. Conclusions and perspectives

Interpenetrating polymer network hydrogels, as a particularcategory of composite materials, received a great attention lastdecade owing to their improved responsiveness and mechanicalproperties, which differentiate them on the single network hydro-gels. Even if the fabrication of IPN hydrogels by the simultaneous

strategy has the advantage to be time- and cost-saving, it wasfound that the sequential technique allows a better control of theproperties of the gels compared to the simultaneous strategy.The limitations of hydrogels composed of PNIPAAm, such as thelack of biocompatibility, slow deswelling rate, and poor mechanicalproperties have been reduced either by incorporating PNIPAAmlinear chains in a matrix of Alg or by entrapping polysaccharides(CS, Alg) or proteins in a network of PNIPAAm. The pulsatiledeswelling/reswelling behavior and the loading/release of drugshave been mainly improved by the preparation of semi-IPNhydrogels.

It was demonstrated that IPN hydrogels are endowed with veryfast kinetics for sorption of ionic species like dyes and heavy metalions compared with the single network hydrogels. Mechanicalproperties, swelling kinetics, and reusability of IPN hydrogels intheir applications as sorbents have been further improved by con-ducting the synthesis of the gels under the freezing temperature ofthe solvent. Ionic IPN cryogels based on synthetic polymers andbiopolymers, like CS and starch, are endowed with very fast swell-ing rate, a high sorption capacity for ionic species like dyes andheavy metal ions, fast kinetics of sorption, and a high level of reus-ability, features which recommend them as promising sorbents inthe future. Even if it is still a challenging task, the synthesis of ionimprinted IPN hydrogels constitutes a promising direction inincreasing the selectivity of this novel type of sorbents, which isexpected to receive much attention in the future.

Acknowledgement

This work was supported by CNCSIS-UEFISCSU by the projectPN-II-ID-PCE-2011-3-0300.

References

[1] A.S. Hoffman, Hydrogels for biomedical applications, Adv. Drug Deliv. Rev. 43(2002) 312.

[2] N.A. Peppas, P. Bures, W. Leobandung, H. Ichikawa, Hydrogels inpharmaceutical formulations, Eur. J. Pharm. Biopharm. 50 (2000) 2746.

[3] A.K. Bajpai, S.K. Shukla, S. Bhanu, S. Kankane, Responsive polymers incontrolled drug delivery, Prog. Polym. Sci. 33 (2008) 10881118.

[4] C.W. Peak, J.J. Wilker, G. Schmidt, A review on tough and sticky hydrogels,Colloid Polym. Sci. 291 (2013) 20312047.

[5] H.T. Peng, L. Martineau, L.P.N. Shek, Hydrogelelastomer compositebiomaterials: 3. Effects of gelatin molecular weight and type on thepreparation and physical properties of interpenetrating polymer networks,

J. Mater. Sci. Mater. Med. 19 (2008) 9971007.[6] A. Richter, G. Paschew, S. Klatt, J. Lienig, K.-F. Arndt, H.-J.P. Adler, Review on

hydrogel-based pH sensors and microsensors, Sensors 8 (2008) 561581.[7] D. Myung, D. Waters, M. Wiseman, P.E. Duhamel, J. Noolandi, C.N. Ta, C.W.

Frank, Progress in the development of interpenetrating polymer networkhydrogels, Polym. Adv. Technol. 19 (2008) 647657.

[8] L.H. Sperling, Interpenetrating polymer networks: an overview, in: D.Klempner, L.H. Sperling, L.A. Utracki (Eds.), Interpenetrating PolymerNetworks, American Chemical Society, Washington, 1994, pp. 338.

[9] L.H. Sperling, Interpenetrating polymer networks, in: Herman F. Mark (Ed.),Encyclopedia of Polymer Science and Technology, vol. 10, third ed., JohnWiley & Sons, New York, 2005, pp. 272311.

[10] S.J. Kim, S.G. Yoon, S.I. Kim, Synthesis and characterization of interpenetratingpolymer network hydr ogels composed of alginate andpoly(diallyldimethylammonium chloride), J. Appl. Polym. Sci. 91 (2004)37053709.

[11] J.J. Wang, F. Liu, Enhanced adsorption of heavy metal ions onto simultaneousinterpenetrating polymer ne

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