Published: November 28, 2011

r 2011 American Chemical Society 13742 dx.doi.org/10.1021/ie200758b | Ind. Eng. Chem. Res. 2011, 50, 13742–13751

ARTICLE

pubs.acs.org/IECR

Design of Molecular Imprinted Hydrogels for Controlled Release ofCisplatin: Evaluation of Network Density of HydrogelsBaljit Singh,* N. Chauhan, and Vikrant Sharma

Department of Chemistry, Himachal Pradesh University, Shimla 171005, India

ABSTRACT: In this era of new technologies, there is an ongoing interest in enhancing the efficiency of polymer-based drug deliverysystems, andmolecular imprinting technique has been suggested as one step in this direction. Therefore attempts have beenmade todesign molecular imprinted polymers (MIPs) of HEMA andMAAc for the slow release of cisplatin. The characterization of networkstructure of hydrogels has been studied by FTIR, swelling studies, and determining the various structural parameters of hydrogelsuch as polymer volume fraction in the swollen state (ϕ2,s), Flory�Huggins interaction parameter (χ1), molecular weight of thepolymer chain between two neighboring cross-links (M

c), cross-link density(F), and the corresponding mesh size (ξ) by swelling

equilibriummethod. Effect of the cross-linker concentration on the network parameters has been studied along with influence of thenetwork density on drug entrapment and release of drug from the hydrogel.

1. INTRODUCTION

Unprecedented developments in genomics and molecularbiology today offer a plethora of new drugs. For all these excitingnew drug candidates, it is necessary to develop suitable dosageforms or drug delivery systems to allow their effective, safe, andreliable application to the patient. Hence, the improvement indrug therapy is a consequence of not only the design of new drugmolecules, but also the development of suitable drug deliverysystems. Polymer-based drug delivery devices can provide deliv-ery of drugs in a controlled and sustained manner and canmaintain the ideal pharmacokinetic profile of the drugs. In thisera of new technologies, there is an ongoing interest in enhancingthe efficiency of polymer-based drug delivery systems, and themolecular imprinting technique has been suggested as one step inthis direction.

Molecular imprinting is a novel and fast evolving technique thathas various biomedical applications,1�3 and recently its applicationin controlled drug delivery systems (DDS) has been reported.4,5 Incontrolled drug delivery devices themolecular imprinted polymers(MIP) are synthesized by the copolymerization of functionalmonomers and a cross-linker in the presence of drug or templatemolecules in the reaction system.6 The ability of imprintedpolymers to bind a template molecule with high affinity lends totheir application as excipient for sustained drug delivery. To createa rate attenuating excipient for a transdermal controlled releasedevice, methacrylic acid (MAAc) has been used as a functionalmonomer to prepare MIPs for propanolol, a β-blocker.7 Thepermeation of propranolol was slower from the MIP devices thanfrom nonimprinted devices. Various therapeutic agents such astetracycline,8 theophylline,9 ciprofloxacin,10 timolol,11 and nor-floxacin,12 have been released through MIP approach. Theophyl-line-reloaded particles were able to sustain drug release in pH 7.0phosphate buffer for several hours, especially those loaded withlow amounts of theophylline (0.1�2.0 mg/g).13,14 Molecularimprinted DDS have also been explored to deliver colon cancerdrugs, viz sulfasalazine (prodrug used in the diseases of thecolon),15 5-fluorouracil,16 and methotrexate.17

Metal-based drugs exhibit powerful anticancer, antitumor,antidiabetic, anti-inflammatory, antibacterial, antiviral, and anti-parasitic properties.18 Cisplatin is a widely used metal-basedantineoplastic drug that exhibits therapeutic activity againstseveral solid tumors19,20 along with testicular cancer, ovariancancer, lymphoma, and glioma.21,22 It is also one of the mosteffective anticancer agents against gynecological and gastro-intestinal cancers.9,23 Its entrapment in polymeric implants hasreduced systemic toxicity and increased activity.24,25

Both poly(acrylic acid) and poly(methyl methacrylate) havebeen used to develop MIP devices (hydrogels) because of theirbiocompatibility.26 Acrylic polymers have shown high in vivotolerance in rats after subcutaneous implantation for up to 24days.27 In addition, the carboxylic acid groups in poly(acrylic acid)can form hydrogen bonds with mucin, a glycoprotein secretedlocally that coats the mucosal surfaces.26 Hydrogels developedfrom these functional polymers have mucoadhesive and bio-compatible properties. The hydrogels prepared from the ionicmonomers swell/deswell quickly in response to change intheir external environment. These changes can be induced bychanging the surrounding pH, temperature, ionic strength,and electro stimulus.28,29 The response of hydrogels to pHmake them suitable candidates for site-specific delivery ofdrugs to the colon.

Various biomedical applications of hydrogels are due to theircross-linked structure which is determined by various networkparameters. These network parameters enable them to encapsu-late the drug molecules effectively and to release them in acontrolled manner for extended periods of time.30 The rate ofdrug diffusion from drug-loaded hydrogel can be tailored byevaluating various network parameters such as the polymervolume fraction in the swollen state (ϕ2,s), molecular weight ofthe polymer chain between two neighboring cross-links (M

c),

Received: April 9, 2011Accepted: November 11, 2011Revised: November 4, 2011

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cross-link density (F), and the corresponding mesh size (ξ).31

The drug loading capability and mechanical strength of hydro-gels increased with increase in cross-link density (F) of hy-drogels.32,33 Knowledge of network parameters is proved to be animportant tool for molecular imprinting. Hart and co-workershave reported dependence of both selectivity and absolutecapacity of cross-linked hydrogels on cross-link density ofpolymers.34

In view of technological significance of molecular imprintingpolymers in drug delivery, the present study is an attempt tosynthesize 2-hydroxyethylmetacrylate (HEMA)- and methacrylicacid (MAAc)-based hydrogels imprinted with model drug cispla-tin (1 mg/mL). For the synthesis of these hydrogels N,N0-methylenebisacrylamide (NN-MBA) has been used as cross-linker, ammonium persulfate (APS) as initiator, and N,N,N0,N0tetramethylethylenediamine (TEMED) as accelerator. Both mo-lecular-imprinted polymers (MIPs) and nonimprinted polymers(NIPs) have been synthesized and have been used to study theswelling and in vitro release dynamics of the drug.

2. EXPERIMENTAL SECTION

2.1. Materials and Methods. Hydroxyethylmethacrylate(HEMA) and methacrylic acid (MAAc) were obtained fromMerck-Schuchardt, Germany, Ammonium persulfate (APS) andN,N0-methylenebisacrylamide (NN-MBA) were obtained fromS.D. Fine, Mumbai, India, and were used as received. The N,N,N0,N0-tetramethylethylenediamine (TEMED) was obtainedfrom Merck-Schuchardt, Germany. Cisplatin was obtained fromAlkem Laboratory Limited, Mumbai, India.2.2. Synthesis of poly(HEMA-cl-MAAc) Hydrogels. Synthe-

sis of molecular imprinted polymers (MIPs) was carried outwith 4.38� 10�2 mol/L of APS, 7.68� 10�1 mol/L of HEMA,11.62 � 10�1 mol/L of MAAc, 3.89 � 10�2 and 6.49 � 10�2

mol/L of NN-MBA, and 1.72 � 10�1 mol/L of TEMED in

aqueous solution of model drug (cisplatin) at 37 �C tempera-ture for half an hour. The polymers were washed with distilledwater, then dried at 37 �C in the oven until constant weight wasobtained and were named poly(HEMA-cl-MAAc) thereafter.Synthesis of nonimprinted polymers (NIPs) was carried outwithout drug under the similar conditions. The MIPs and NIPswere synthesized with two different concentrations of thecross-linker (3.89 � 10�2 and 6.49 � 10�2 mol/L) to observethe effect of cross-linker on the template entrapment andthereafter on the release. The NIP and MIP prepared withtwo different cross-linker concentrations (3.89 � 10�2 and6.49 � 10�2 mol/L) were designated as NIPs-3 and NIPs-5and MIPs-3 and MIPs-5, respectively. All the reactions werecarried out in triplicate. The reaction for formation of MIPs isshown in Scheme1.2.3. Characterization. Polymers were characterized by FTIR

spectroscopy, swelling studies,35 and various structural para-meters of hydrogel such as polymer volume fraction in the swollenstate (ϕ2,s), Flory�Huggins interaction parameter (χ1), molec-ular weight of the polymer chain between two neighboring cross-links (M

c), cross-link density (F), and the corresponding mesh

size (ξ) by swelling equilibrium method. FTIR spectra of MIPsand NIPs polymers were recorded in KBr pellets on Nicolet5700FTIR THERMO. Swelling of the polymers was carriedout in distilled water by gravimetric method.35 The equilibriumswelling was taken after 24 h.2.4. Release Dynamics of Drug from poly(HEMA-cl-

MAAc). 2.4.1. Preparation Calibration Curves. In this procedure,the absorbance of a number of standard solutions of the referencesubstance at concentrations encompassing the sample concen-trations was measured on a UV visible spectrophotometer (Cary100 Bio, Varian) and a calibration graph was constructed. Theconcentration of the drug in the sample solution was read fromthe graph as the concentration corresponding to the absorbanceof the solution. The calibration graph of cisplatin was made todetermine the amount of cisplatin release from the drug loadedMIPs and NIPs at wavelength 300 nm.2.4.2. Drug Loading to the MIPs. The loading of a drug into

MIPs was carried out during synthesis of the hydrogels by theprocedure mentioned in Experimental Section 2.2, and NIPswere synthesized without drug.2.4.3. Drug Release from MIPs. In vitro release studies of the

drug were carried out by placing dried and loaded sample in adefinite volume of releasing medium at 37 �C. The amount ofdrug released was assayed spectrophotometrically after each30min. The absorbance of the solutionwasmeasured at wavelength300 nm each case.2.4.4. Drug Reloading to the MIPs and NIPs. After removal of

template from the MIPs, the polymers were dried at 37 �C in anoven, and reloading of the drug into MIPs and NIPs was carriedout by swelling equilibrium method. Reloading of MIPs andNIPs was carried out with same concentration of the drug (1mg/mL). The hydrogels were allowed to swell in the drug solution for24 h at 37� and than dried to obtain the release device. Swellingkinetics of both MIPs and NIPs and release dynamics of drugfrom drug-loaded MIPs and NIPs was studied in distilled water.2.5. Mechanism of Swelling and Drug Release from Poly-

mer Matrix. Swelling of polymers has been classified into threetypes of diffusion mechanisms, on the basis of relative rate ofdiffusion of water into polymer matrix and rate of polymer chainrelaxation.36�39 The values of diffusion exponent n and diffusioncoefficients have been evaluated (by using eqs 1�4) for the

Scheme 1. Reaction Showing the Formation of MIPs

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swelling of the polymers and for the release of the drug from thepolymer.

Mt

M∞¼ ktn ð1Þ

Mt

M∞¼ 4

Dit

πl 2

� �0:5

ð2Þ

DA ¼ 0:049l 2

t1=2ð3Þ

Mt

M∞¼ 1� 8

π2

� �exp

ð � π2DLtÞl 2

" #ð4Þ

where Mt/M∞ is the fractional release of drug in time t, k is theconstant characteristic of the drug�polymer system, and n is thediffusion exponent characteristic of the release mechanism. Mt

andM∞ is drug released at time t and at equilibrium, respectively,Di, DA, and DL are the initial, average, and late diffusioncoefficients, respectively, and l is the thickness of the sample.t1/2 is the time required for 50% release of drug.2.6. Determination of Network Parameters. The cross-

linked structure of the prepared hydrogels was determined bystudying the swelling of cylindrical polymer in distilled water.The density of polymer was calculated by measuring the radiusand height of dry cylindrical polymer along with the weight ofsame. The samples were placed in distilled water, allowed to swellto equilibrium at certain temperature, and weighed after 24 h.The volume fraction (ϕ) of polymer in swollen state wascalculated by the method used by Aithal and co-workers.40

According to which ϕ can be calculated by using eq 5.

ϕ ¼ dpds

� �w∞ � wo

wo

� �þ 1

� ��1

ð5Þ

where dp and ds are densities of polymer and solvent, respectively,and w∞ and wo are, respectively, the weight of polymer before

and after 24 h swelling. To study the effect of temperature andcross-linker concentration on different network parameters wehave taken swelling of hydrogels prepared with 3.89 and 6.49 �10�2 mol/L of NN-MBA in distilled water at different (300.15,310.15, and 320.15 K) temperatures. The detailed discussion onvarious network parameters is presented in a later section.2.7. Statistical Analysis. All swelling and drug release studies

were performed in triplicate and themean, the standard deviation(sd), and the 95% confidence interval (CI) were calculated forthe diffusion coefficients for each triplicate. Statistical signifi-cance was determined at p e 0.05 for all data analysis.

3. RESULTS AND DISCUSSION

3.1. Fourier Transform Infrared Spectroscopy. FTIR spectraof molecularly imprinted and nonimprinted poly(HEMA-cl-MAAc), i.e., MIPs and NIPs, were recorded and are presented inFigures 1 and 2, respectively. FTIR spectrum of MIPs showedabsorption bands at 3442.6 cm�1 (�OH stretching vibrations),3002.7 and 2950.6 cm�1 (�CH3 and�CH2 stretching vibrations),2360�2550 cm�1 (overtones and combinations of OH bendingand C�O stretching vibrations), 1718.8 cm�1 (CdO stretching),1552.3 cm�1 (OH in-plane bending), 1482.3 cm�1 (CH2bending),1263.5 cm�1 (C�O�C stretching vibration), 1075�1190 cm�1

(C�O stretching), and 751.7 cm�1 (CH2 rocking vibrations).FTIR spectrum of NIPs was found to be similar to MIPs havingabsorption bands at nearly the same wavenumber (cm�1) but withless absorbance.3.2. Release Dynamics of Drug. To observe the effect

of cross-linker on the release of drug, the MIPs (MIPs-3 andMIPs-5) and NIPs (NIP-3 and NIP-5) were synthesized withtwo different concentrations (3.89� 10�2 and 6.49� 10�2 mol/L of NN-MBA) respectively. The MIPs were synthesized in thepresence of drug and after synthesizing the MIPs the polymerswere subjected to the drug release study to remove the loadeddrug. The release profile of drug from the drug-imprinted poly-(HEMA-cl-MAAc), i.e., MIPs is shown in Figures 3�5. Com-plete removal of drug from the MIPs occurred after 72 h.

Figure 1. FTIR spectra of MIPs of poly(HEMA-cl-MAAc) hydrogels.

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Then the polymers were dried in oven at 37 �C. Total amount ofdrug released from MIPs-3 and MIPs-5 has been observed 14.38( 0.41 mg/g of gel and 16.44 ( 0.43 mg/g of gel, respectively(Figure 4). The values of diffusion exponent n have beenobserved to be 1.188 and 0.711, respectively, for the MIPs-3and MIPs-5 (Table 1). Because the release of drug occurredthrough the Case II and non-Fickian diffusion mechanism,respectively, itself implies that the release of drug from thepolymers prepared with low cross-linker concentration was veryfast. The actual purpose of this study was to remove the drug

from the polymer and leave the template space in the polymer forfurther recognition of the drug molecule. The values of thediffusion coefficients are shown in Table 1. The values ofdiffusion coefficients are different in the two cases. These trendsmay be due to the occurrence of two different release mechan-isms, i.e., Case II diffusion mechanism and non-Fickian diffusionmechanism, respectively, in MIP-3 and MIP-5.3.3. Reloading of the Drug. After release of drug from these

MIPs, the polymers were dried at 37 �C in the oven. Reloading ofcisplatin was carried out in both the cases (i.e., MIPs and NIPs)to observe the binding capacity of hydrogels for the template. Asthe molecular imprinting is a technique, producing syntheticmaterials containing highly specific receptor sites that have an

Figure 2. FTIR spectra of NIPs of poly(HEMA-cl-MAAc) hydrogels.

Figure 3. Release dynamics of cisplatin from MIPs of poly(HEMA-cl-MAAc) hydrogels in distilled water at 37 �C (prepared with different[NN-MBA]). Reaction time = 30 min, reaction temperature = 37 �C,[HEMA] = 7.68� 10�1 mol/L, [MAAc] = 11.62� 10�1 mol/L, [APS]= 0.438 � 10�1 mol/L, and [TEMED] = 1.72 � 10�1 mol/L.

Figure 4. Release of cisplatin from MIPs of poly(HEMA-cl-MAAc)prepared with different [NN-MBA].

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affinity for a target molecule andMIPs canmimic the recognitionand binding capabilities of the template molecule. In the presentcase, it has been observed that the binding affinity of theMIPs forthe cisplatin is higher as compared to NIPs when reloading ofdrug was carried out by swelling equilibrium method by keepingboth the MIPs (MIPs-3 and MIPs-5) and NIPs-3 and NIPs-5 in1 mg/mL solution of cisplatin for 24 h at 37 �C. Then polymerswere dried to obtain the release device for further study. The drugis introduced into the polymer network via imbibition, which isequilibrium partitioning after the network is formed or the drug isincluded during the polymerization of the network.3.4. Swelling and Release Dynamics of the Drug from the

MIPs and NIPs after Reloading. After reloading of the drug, theMIPs and NIPs were dried at room temperature and then wereused to study the swelling of the poly(HEMA-cl-MAAc) hydro-gels and release dynamics of the drug from these hydrogels.3.4.1. Swelling Kinetics. The swelling of the MIPs and NIPs is

presented in Figure 6. The swelling of the MIPs has beenobserved higher as compared to NIPs. Total amount of watertaken by both types of polymers after 24 and 48 h is shown inFigure 7. The higher sorption in case of MIP may be due to thetemplate formed during the synthesis of MIPs, while thetemplate molecule of drug was removed and left behind pointsfor solvent interactions. The values of diffusion exponent n andgel characteristics constant k are presented in Table 2. Swellingin general occurred through non-Fickian diffusion mechanism.

The values of the diffusion coefficients are shown in Table 2. Therate of swelling in the earlier stages of swelling was higher than thatin the latter stages. From the swelling studies it is also clear thathydrogels had maintained structural integrity after swelling. Ingeneral, the structure of the imprinted cavities should be stableenough to maintain the conformation in the absence of thetemplate, but somehow flexible enough to facilitate the attain-ment of a fast equilibrium between the release and reuptake ofthe template in the cavity. This will be particularly important ifthe device is used as a diagnostic sensor or as a trap of toxicsubstances in the gastrointestinal tract or release of drug to theGIT. The mechanical properties of the polymer and the con-formation of the imprinted cavities depend to a great extent onthe proportion of the cross-linker. MIPs for drug delivery shouldbe stable enough to resist enzymatic and chemical attack andmechanical stress. The device will enter into contact withbiological fluids of complex composition and different pH, inwhich the enzymatic activity is intense.

Table 1. Results of Diffusion Exponent n, Gel CharacteristicConstant k, and Various Diffusion Coefficients for the Releaseof Cisplatin from the MIPs [poly(HEMA-cl-MAAc)]

diffusion coefficients (cm2/min)

sample

diffusion

exponent n

gel characteristic

constant k � 102initial

Di � 104average

DA � 104late time

DL � 104

MIP-3 1.188 0.044 0.41 25.15 1.82

MIP-5 0.711 1.19 6.25 47.53 5.21

Figure 5. Percentage release of total loaded drug from MIPs of poly-(HEMA-cl-MAAc) hydrogels prepared with different [NN-MBA].

Figure 6. Swelling kinetics of reloadedMIPs and NIPs of poly(HEMA-cl-MAAc) hydrogels in distilled water at 37 �C.

Figure 7. Swelling of reloaded MIPs and NIPs of poly(HEMA-cl-MAAc) hydrogels after 24 and 48 h in distilled water at 37 �C.

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3.4.2. Release Dynamics of the Drug.The release profile of thedrug from per gram of the MIPs and NIPs is presented inFigures 8�10. It has been observed from the figures that theamount of drug released from theMIP-3 was higher as comparedto that of the NIPs-3. Drug from the MIPs has been released in acontrolled manner. The total 7.91 ( 0.68, 6.06 ( 0.39, 5.05 (0.14, and 4.33 ( 0.80 mg/g of gel have been released after 48 hfrom the MIPs-3, MIPs-5, NIPs-3, and NIPs-5, respectively(Figure 9). Higher release of drug from the MIPs may be dueto the higher swelling and higher loading of the drug in MIPs.The higher loading is due to the reason that the number ofbinding sites (i.e., template sites) was higher in the MIPs initiallyloaded with cisplatin. Initially, there were no template sitesavailable in the NIPs and hence, these polymers showed thelower entrapment of the drug per gram of the gel. The release ofdrug was more in the case of MIPs prepared with lower amountof cross-linker. Here it is pertinent to mention that the release of

water-soluble drug, entrapped in a hydrogel, occur only afterwater penetrates the network to swell the polymer and dissolvethe drug, followed by diffusion along the aqueous pathways to thesurface of the device. The release of drug is closely related to theswelling characteristics of the hydrogels, which, in turn, is a keyfunction of chemical architecture of the hydrogels. The polymersprepared with higher concentration will form the networks ofhigher cross-linking density which exerts its effect on swellingand drug release. The release of drug occurred through non-Fickian diffusion mechanism. In non-Fickian diffusion mechan-ism, both diffusion of the drug molecules from the polymers andrelaxation times of polymer chains are comparable. The values ofdiffusion coefficient for the release of drug from these polymersare presented in Table 2. It has been observed from the table thatthe values obtained for the diffusion coefficients in the earlierstages were higher than those in the later stages. This may be dueto the concentration gradient and swelling of polymer matrix.

Table 2. Results of Diffusion Exponent n, Gel CharacteristicConstant k, and Various Diffusion Coefficients for the Swel-ling Kinetics and Release Dynamics of MIPs (Reloading) andNIPs

diffusion coefficients (cm2/min)

sample

diffusion

exponent n

gel characteristic

constant k � 102initial

Di � 104average

DA � 104late time

DL � 104

swelling kinetics

MIP-3 0.551 1.58 0.40 3.94 0.48

MIP-5 0.410 0.78 0.35 12.83 1.13

NIP-3 0.634 1.21 0.88 9.97 0.89

NIP-5 0.554 0.81 0.63 11.60 1.00

release dynamics

MIP-3 0.772 0.90 1.62 11.30 1.32

MIP-5 0.788 0.87 3.16 14.33 2.49

NIP-3 0.602 2.99 2.44 21.52 2.52

NIP-5 0.546 4.15 2.51 25.67 2.81

Figure 8. In vitro release dynamics of cisplatin from reloaded MIPs andNIPs of poly(HEMA-cl-MAAc) in distilled water at 37 �C.

Figure 9. Release of cisplatin from MIPs and NIPs of poly(HEMA-cl-MAAc) after 24 and 48 h in distilled water at 37 �C.

Figure 10. Release rate curves for release of cisplatin from reloadedMIPs and NIPs of poly(HEMA-cl-MAAc) hydrogels in distilled water at37 �C.

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From the values of diffusion coefficients it is clear that the rate ofrelease of drug in both the cases (MIP and NIPs) was higher inthe earlier stages than the latter stages. But, the rate of release ofdrug from the MIPs was slower than from the NIPs.To investigate the release kinetics parameters of cisplatin from

NIPs and MIPs of poly(HEMA-cl-MAAc) hydrogels, eq 6 wasused and the plots of t/Ct versus t (min) are presented in Figure 10.

tCt

¼ α þ βt ð6Þ

Here, Ct is the amount of drug released at time t, β = 1/Cmax is theinverse of the maximum amount of drug released, α = 1/(Cmax)

2

krel = 1/ro is the inverse of the initial release rate, and krel is theconstant of the kinetic of drug release.41 The kinetic parameterscalculated from the slope and intercept of straight lines in Figure 10presented in Table 3. It is clear from this table that MIP-3 hasreleased maximum amount of drug and NIP-3 has released drug athighest initial release rate as compare to other hydrogels. NIP-5shows highest constant of kinetics of drug release.3.5. Characterization of Network Structure. The swelling

behavior of hydrogels and release of the active agent from thepolymer matrix is a function of the extent of cross-linkingwhich defines the three-dimensional network structures ofpolymer. The network structure is defined by several para-meters, i.e., the number of cross-links, their functionality anddistribution, network defects (dangling chains and loops), andentanglements.42 The most important parameters used tocharacterize network structure are the polymer volume frac-tion in the swollen state (ϕ), molecular weight of the polymerchain between two neighboring cross-links (M

c), cross-link

density (F), and the corresponding mesh size (ξ). Due to therandom nature of the polymerization process, only averagevalues of M

ccan be calculated which is a measure of the degree

of cross-linking of the polymer, regardless of the nature (physicalor chemical) of cross-linking and has been studied by a numberof techniques (the equilibrium swelling theory and the rubberelasticity theory).43 However, one of the most popular techni-ques to calculate M

cis to study the swelling of polymer in a

solvent.44 In the present case we have taken the 24 h swelling ofcylindrical hydrogels of pHEMA/MAAc prepared with different[NN-MBA], in distilled water at different temperatures. Whenthe polymer is placed in a solvent, it swells until the osmoticforces that help to dissolve the polymer are balanced by theelastic forces due to the stretched segments of the polymerchains. These elastic retractive forces are inversely proportionaltoM

c. Thus, the molar mass between two junction points in the

network polymer becomes rigid and exhibits limited swelling.WhenM

cis large, the network is more elastic and swells rapidly if

brought in contact with a compatible liquid.

To calculate theM cvalues, the Flory�Rehner40,45 equation in

following form was used.

Mc ¼ � dPvm, 1ϕ1=3 lnð1� ϕÞ þ ϕ þ χϕ2

� ��1 ð7ÞThe polymer volume fraction in the swollen state is a measure ofthe amount of fluid imbibed and retained by the hydrogel. Thevolume fraction, ϕ, of the polymer in the swollen state wascalculated by using eq 5 and vm,1 is the molar volume of theswelling agent (18.1 cm3/mol for water).Flory�Huggins interaction parameter, χ, given by ZΔW1,2/RT

(where Z is lattice coordinaion number, ΔW1,2 is interactionenergy permole), is the energy change (in units ofRT) that occurswhen a mole of solvent molecules is removed from the puresolvent (where ϕ = 0) and is immersed in an infinite amount ofpure polymer (where ϕ = 1). Because of the approximate nature ofthe lattice theory, χ is found to depend on the concentration of thesolution and decreases with an increase in the polymer�solventinteraction.46 That means the higher is the value of χ1, the weakeris the interaction between polymer and water, and the stronger isthe interaction between hydrophobic groups.47,48 According to itsdefinition, χ depends inversely on the temperature. χ is generallypositive, with values at 25 �C and at infinite dilution being near 0.5.Positive value of χ is means that the dissolution of a polymericsolute in a solvent is generally an endothermic process (asΔHmix =RTχn1ϕ, where n1 is number of molecules of solvent).43

Flory�Huggins interaction parameter (χ) can be calculatedexperimentally from the temperature coefficient40,45 of volumefraction (dϕ/dT). Thus, from Flory�Rehner model we get

χ ¼ ϕð1� ϕÞ�1 þ Nlnð1� ϕÞ þ Nϕh i

2ϕ� ϕ2N � ϕ2T�1 dϕdT

� ��1" #�1

ð8Þwhere N = ((ϕ2/3)/(3) � (2)/(3))(ϕ1/3 � (2)/(3)ϕ)�1, anddϕ/dT is the slope obtained by plotting the volume fraction dataversus temperature (K). For this purpose we have taken 24 hswelling of polymers prepared with different [NN-MBA], at300.15, 310.15, and 320.15 K in distilled water (Table 4).The swelling behavior is strongly dependent on the number of

intermolecular junctions per unit volume, namely the cross-linkdensity.49 To further analyze the swelling behavior of thesehydrogels in aqueous medium, the cross-link density, F, wascalculated from eq 9.50,51

F ¼ 1

νðMcÞ ¼ dpMc

ð9Þ

Here, v = 1/dp is the specific volume of polymer. Cross-linkdensity is influenced by ratio of cross-linker, functionality of

Table 3. Release Kinetic Parameters of MIPs and NIPs ofpoly(HEMA-cl-MAAc) Hydrogels

sample

maximum amount

of drug released

Cmax (mg L‑1)

constant of the

kinetic of drug release

krel � 108 (s‑n)

initial release

rate ro � 102

(mg L‑1s‑1)

correlation

coefficient

(R)

MIP-3 1382.313 162.086 309.713 0.93949

MIP-5 1153.685 207.559 276.258 0.98107

NIP-3 763.359 944.261 550.237 0.99465

NIP-5 564.972 1617.314 516.236 0.99578

Table 4. Volume Fraction of Hydrogels at DifferentTemperatures and [NN-MBA]

temperature

(K)

[NN-MBA]�102 mol/L wo (g) w∞ (g)

volume

fraction (ϕ)

300.15 3.89 1.246 3.583 0.30834

310.15 3.89 1.203 3.709 0.28573

320.15 3.89 1.308 4.206 0.27253

300.15 6.49 1.108 3.144 0.32604

310.15 6.49 1.168 3.414 0.31545

320.15 6.49 1.214 3.684 0.30255

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cross-linker, radiation time for cross-linking, and molecularweight of polymer chain segments. A higher cross-link densityleads to a higher retractive force of the swollen network and thusto a smaller degree of swelling.Mesh size, ξ, defines the space between macromolecular

chains in a cross-linked network and characterized by thecorrelation length between two adjacent cross-links. Mesh sizeis an important factor for determining mechanical strength,degradability, and diffusivity of the releasing molecule. Mosthydrogels used in biomedical applications have mesh sizesranging from 5 to 100 nm in their swollen state. The linear meshsize of polymer networks was calculated by using eq 10 asdiscussed by Gudeman and Peppas.52

ξ ¼ ϕ�1=3l Cn2Mc

Mr

� �1=2

ð10Þ

Here, Cn is the characteristic ratio determined as a weightedaverage of the values of the two polymers ((Cn,pHEMA = 6.9)51,53

and (Cn,pMAAc = 14.4)54), Mr is the molecular weight of theaverage repeating unit calculated as a weighted average betweenthe values of 130.14 (for HEMA) and 86.09 (for MAAc), and l isthe carbon�carbon bond length (1.54 Å). The values of differentnetwork parameters are presented in Table 5.3.5.1. Effect of Temperature on Network Structure. Tempera-

ture of the swelling medium has exerted affect on the swelling ofpolymeric networks,55 which in turn affect the network para-meters of polymer. To study the effect of temperature ondifferent network parameters, swelling studies of hydrogels indistilled water at different temperatures (300.15, 310.15, and320.15 K) was carried out. From the swelling studies of hydro-gels, it is observed that volume fraction of polymer in swollenstate, ϕ, decreased with increase in temperature of swellingmedium (Table 4). With increase in temperature of swellingmedium, the rate of diffusion of solvent molecules into thepolymeric networks increases, which results in increase involume fraction of solvent and hence decrease in volume fractionof polymer in swollen gel. The M

cvalues of polymer increase

with increase in temperature due tomore swelling of hydrogels athigh temperature, which increased the chain length between twocross-links and hence increased the M

cvalue. Kulkarni and co-

workers45 have observed increase inM cvalues of sodium alginate

beads with increasing temperature. For the same reason cross-link density (F) has decreased as we increase the temperature ofswelling medium. The mesh size, ξ (Å), of polymer networksincreased from 32.426 to 37.845 Å (prepared with 6.49 � 10�2

mol/L of NN-MBA) and from 49.397 to 62.968 Å (preparedwith 3.89 � 10�2 mol/L of NN-MBA) as the temperature ofswelling medium changed from 300.15 to 320.15 K (Table 5).Due to smaller mesh size (pore size) at low temperature, the

accessibility of water into polymer was prevented and resulted inless swelling of hydrogels and small release of loaded drugoccurred .3.5.2. Effect of Cross-Linker on Network Structure. Cross-

linkers are molecules with at least two reactive functional groupsthat allow the formation of bridges between polymeric chains.The concentration and functionality of cross-linker has affectedthe network structure of hydrogels.56 The effect of cross-linkeron network structure was studied by preparing hydrogels with3.89 and 6.49 � 10�2 mol/L of NN-MBA and taking 24 hswelling in distilled water at 310.15 K. The values of ϕ and Fwereincreased, whereas the values of M

cand ξ were decreased with

increase in cross-linker concentration. Similar trends have beenobserved for each parameter at 300.15 and 320.15 K for bothNN-MBA variations (Table 5). Li and co-workers50 have re-ported decrease in M

cvalues and increase in F values with the

increased amount of cross-linker content in GG/PAA hydrogels.This may be due to the reason that with increase in cross-linkerconcentration, there will be increase in number of cross-linksbetween polymeric chains, resulting in decrease in chain length ofpolymer between two cross-links and hence decrease the M

cvalue and increase cross-link density of polymer. The smallervalue of ξ (Å) at high [NN-MBA] shows high extent of cross-links between polymeric chains which increased the cross-linkdensity of polymer, reduced the mobility of polymer chains andhence influenced the drug release from drug loaded hydrogels.3.6. Effect of Network Density on Release of Drug. The

loading of the drug and release of drug from the hydrogels isdirectly linked with the swelling which is affected by the cross-link density and mesh size of polymeric networks. Both the drugloading and release are affected by change in these networkparameters.30 In the present study, the drug entrapment isdecreased 4.46% in MIPs and 4.61% in NIPs on increasing[NN-MBA] from 3.89 to 6.49 � 10�2 mol/L during thepolymerization reaction. This may be due to the reason thatwith increase in [NN-MBA], the cross-link density (F) increased(from 3.95082 to 9.33350 � 10�4 mol/cm3) and mesh size (ξ)decreased (57.004 to 34.800 Å) at 310.15K or 37 �C (Table 5).At lower cross-linker concentration the cross-link density ofhydrogel is less with large mesh size which facilitates the diffusionof drugmolecules into the polymeric networks and results in highentrapment of drug as compare to hydrogel prepared with highcross-linker concentration.57 The total drug release from drugreloadedMIPs and drug loadedNIPs is decreased (form 7.91( 0.68to 6.06 ( 0.39 mg/g of gel) and (from 5.05 ( 0.14 to 4.33 (0.80mg/g of gel), respectively, with increase in cross-linker from 3.89to 6.49 � 10�2 mol/L (Figure 9). In both cases the drug releasewas less at high [NN-MBA] due to higher cross-link density andsmaller mesh size. The cisplatin interacts with MIPs formed withHEMA and MAAc (the polymers of clinical importance)11,37,58,59

Table 5. Network Parameters of poly(HEMA-cl-MAAc) Hydrogels

temp T (K)

[NN-MBA] �102 mol/L

density dp(g/cm3) volume fraction (ϕ) dϕ/dT N χ M

c(g/mol) F � 104 (mol/cm3)

mesh size,

ξ (Å)

300.15 3.89 1.192 0.30831 �0.00179 �1.09472 0.57014 2383.406 5.00125 49.397

310.15 3.89 1.192 0.28573 �0.00179 �1.11513 0.56405 3017.093 3.95082 57.004

320.15 3.89 1.192 0.27253 �0.00179 �1.12836 0.56185 3567.111 3.34164 62.968

300.15 6.49 1.121 0.32604 �0.00117 �1.08038 0.52152 1066.036 10.51559 32.426

310.15 6.49 1.121 0.31545 �0.00117 �1.08879 0.52295 1201.050 9.33350 34.800

320.15 6.49 1.121 0.30255 �0.00117 �1.09970 0.52341 1381.455 8.11463 37.845

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through supramolecular interactions. It is reported that cisplatin couldbe present in hydrogels in free form and in the form of complex. Thecomplex may have one or two chloride ions replaced with�COOHgroups. However it is also reported that its release from the hydrogelsoccurs in free native form.60 The release of drug from the drug loadedhydrogels occurred through non-Fickian diffusionmechanism. In thismechanism both diffusion of the drug molecules from the polymersand relaxation times of polymer chains are comparable. The values ofthe diffusion exponent are supporting the fact that there exist drugpolymer interactions.

4. CONCLUSIONS

It is concluded from the foregoing discussion that the con-centration of the cross-linker during the synthesis of MIPs canplay an important role in defining their network structure andtheir drug loading and release efficiency. Increase in [NN-MBA]results in less swelling and slow release of loaded drug from bothMIPs and NIPs of poly(HEMA-cl-MAAc) hydrogels. MIPs showmore drug entrapment as compared to corresponding NIPs.Network parameters of the hydrogels are affected by the [NN-MBA] which can further affect the swelling kinetics and releasedynamics of the drug from the hydrogels. The values of ϕ and Fwere increased whereas the values of Mc and ξ were decreasedwith increase in cross-linker concentration. In both MIPs andNIPs cases the drug release was less at high [NN-MBA] due tohigher cross-link density and smaller mesh size. Because of thesupramolecular interactions, MIP can be used for the develop-ment of the biomimic drug delivery devices. However, additionalresearch should be carried out to obtain information about itsbehavior in vivo environments.

’AUTHOR INFORMATION

Corresponding Author*Tel.: +(91)1772830944. Fax: +(91)1772633014. E-mail:baljitsinghhpu@yahoo.com.

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