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Synthesis and Characterizations ofMacroporous Poly(acrylamide-2-acrylamido-2-methyl-1-propanesulfonicacid) Hydrogels for In Vitro Drug Releaseof Ranitidine HydrochlorideK. Varaprasad a , N. Narayana Reddy a , S. Ravindra a , K. Vimala a &K. Mohana Raju aa Synthetic Polymer Laboratory, Department of Polymer Science &Technology, Sri Krishnadevaraya University, Anantapur, IndiaVersion of record first published: 09 Jun 2011.

To cite this article: K. Varaprasad , N. Narayana Reddy , S. Ravindra , K. Vimala & K. Mohana Raju(2011): Synthesis and Characterizations of Macroporous Poly(acrylamide-2-acrylamido-2-methyl-1-propanesulfonic acid) Hydrogels for In Vitro Drug Release of Ranitidine Hydrochloride, InternationalJournal of Polymeric Materials and Polymeric Biomaterials, 60:7, 490-503

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Synthesis andCharacterizations ofMacroporousPoly(acrylamide-2-acrylamido-2-methyl-1-propanesulfonic acid)Hydrogels for In Vitro DrugRelease of RanitidineHydrochloride

K. Varaprasad, N. Narayana Reddy, S. Ravindra, K. Vimala,

and K. Mohana Raju

Synthetic Polymer Laboratory, Department of Polymer Science & Technology,Sri Krishnadevaraya University, Anantapur, India

Macroporous hydrogels were prepared with acrylamide (AM) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) as well as with anhydrous dextrose (AD) asporogen by crosslinking with N,N1-methylenebisacrylamide (MBA). The chemicalstructure of hydrogels is characterized by Fourier transform infrared (FTIR) spec-troscopy. Morphological studies done by scanning electron microscopy (SEM) showedthe macroporous nature of the hydrogels. Swelling studies of hydrogels were done indistilled water, in aqueous NaCl solution and in different pH solutions. In addition,drug release studies of selected macroporous hydrogels (DAMPS1, DAMPS4,DAMPSM1 and DAMPSM3) are also investigated.

Received 28 June 2010; accepted 10 October 2010.Address correspondence to K. Mohana Raju, Synthetic Polymer Laboratory,Department of Polymer Science & Technology, Sri Krishnadevaraya University,Anantapur-515055, AP, India. E-mail: mohanarajuk@yahoo.in

International Journal of Polymeric Materials, 60:490–503, 2011

Copyright # Taylor & Francis Group, LLC

ISSN: 0091-4037 print=1563-535X online

DOI: 10.1080/00914037.2010.531816

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Keywords 2-acrylamido-2-methyl-l-propanesulfonic acid, anhydrous dextrose,macroporous hydrogels

INTRODUCTION

Generally, macroporous hydrogels (MPH) are post response materials when

compared to normal hydrogels, because MPH have pores. The pores of macro-

porous hydrogels hold an enormous amount of water when allowed to remain

in a water reservoir for a long period of time. Because of their unique proper-

ties these hydrogels are used in a number of applications [1] such as in cellular

and tissue engineering [2,3] and drug delivery [4].

The swelling properties of MPHs mainly depend on the composition of

co-monomers and the porosity of the hydrogel. These hydrogels release the

drug entrapped inside the pores through a mechanism dependent on drug dif-

fusion coefficient [5]. MPHs are prepared by various methods such as phase

separation [6,7], cryogelation technique [8] and other methods [9]. However,

we have chosen the free radical polymerization process in the presence of sol-

uble substances such as dextrose anhydrous, and sucrose. These materials

should not react with other reactants (vinyl monomers, crosslinkers, initiators,

activators and other polymers) in the polymerization system and which are

washed out from the hydrogel after polymerization [10,11]. Superabsorbent

hydrogels such as macroporous poly(acrylamide-co-sodium methacrylate) can

be obtained by use of the crosslinking polymerization method [12]. The rapid

de-swelling of porous poly(N-isopropylacrylamide) hydrogels prepared using

a free radical polymerization [13], Jia-Ming Chern and his coworkers reported

preparation and swelling characterization of poly(N-isopropylacrylamide)-

based porous hydrogels [14] and also Hideaki Tokuyama et al. reported prep-

aration of porous poly(N-Isopropylacrylamide) gel [15].

This report describes the development and the characterization of a series of

poly(acrylamide-2-acrylamido-2-methyl-1-propanesulfonic acid) P(AM-AMPS)

hydrogel. The effects of AMPS, MBA ratio, pH and different concentrations of

NaCl on the hydrogels swelling properties were investigated. Additionally,

release profiles of ranitidine hydrochloride (RH) drug from test hydrogels were

studied in simulated gastric media pH (2.2) at 37�C.

EXPERIMENTAL

MaterialsAcrylamide (AM), and 2-acrylamido-2-methyl-1-propanesulfonic acid

(AMPS) were purchased from Aldrich Chemicals. N,N1-methylenebisacrylamide

(MBA), ammonium persulfate (APS), N,N,N1,N1-tetramethylethylenediamine

Macroporous Hydrogels 491

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(TMEDA) and anhydrous dextrose (AD), were purchased from S.D fine chemi-

cals. Ranitidine hydrochloride was obtained from Aurobindo Pharmaceutical

Limited, Hyderabad, India. All the chemicals were used without further purifi-

cation. Throughout the experiments double-distilled water was used.

Preparation of Macroporous Hydrogels14.6 mM of AM dissolved in 2 ml of double-distilled water and 0.24 mM of

AMPS is added into the 100 ml beaker. To this reaction mixture, 4.622 mM of

AD a porogen, 1 ml of MBA (1 g=100 ml) as a crosslinker and also 1 ml of APS

(5 g=100 ml), 1 ml of TMEDA (1 g=100 ml) as an initiating pair were sequen-

tially added by stirring at 100 rpm on a magnetic stir plate. After the reaction

has been completed, the hydrogel was immersed in distilled water at 25�C for

24 h to remove the unreacted materials present in the hydrogel network.

Finally, the hydrogel was dried at 25�C for 2 d. Similarly, other hydrogels were

prepared in a similar manner. The feed compositions of the hydrogels are pre-

sented in Table I.

CharacterizationsFTIR spectrophotometer is used to identify the hydrogel formation. To rec-

ord the FTIR spectra of hydrogel, the sample was completely dried in an oven

(Baheti Enterprises, Hyderabad, India) at 40�C for 6 h. These samples were

read between 600 and 4000 cm�1 on a Bruker IFS 66 V FTIR spectrometer

(Ettlingen, Germany) using the KBr disk method. UV-vis absorption spectra

of the samples were recorded on an ELICO SL 164 Model (The Elico Co.,

Hyderabad, India). The drug loading efficiency and releasing profiles of rani-

tidine hydrochloride in the hydrogels was determined by UV-spectral

measurement at kmax 315 nm. Scanning electron microscopy (SEM; JEOL

JSM 840A, Tokyo, Japan) is employed to study the internal structure of hydro-

gels under dry conditions. The dry samples were coated with a thin layer of

palladium gold alloy.

Swelling MeasurementsThe dried hydrogels are immersed in distilled water at 25�C until the swell-

ing equilibrium is reached (almost 26 h) and they are removed and blotted with

filter paper to remove the excess water solution present on the surface and then

weighed. The immersion time and drying procedure were repeated until the

weight of swollen samples is constant. The swelling ratio, equilibrium swelling

ratio and the swelling water content in percentage is defined as follows:

Swelling ratio ðSg=gÞ ¼ ½Wt � Wd=Wd�

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Table

1:Fe

ed

compositionand

swelling

data

ofmacroporouspoly(A

M-A

MPS)

hydrogels

Hydro

gels

code

AM

(mM)

(AMPS)

mM

DA

(mM)

(MBA)

mM

(APS)

mM

(TMED

A)

mM

Swelling

ratio

(g/g)

Equilibrium

water

content

(EWC)(%

)

Feed

compositionofhydrogels

DAMPS1

14.06

0.2412

4.6227

0.0648

0.21929

0.172

32.24

96.99225

DAMPS2

14.06

0.4824

4.6227

0.0648

0.21929

0.172

50.12

98.04412

DAMPS3

14.06

0.7237

4.6227

0.0684

0.21929

0.172

70.27

98.59698

DAMPS4

14.06

1.20625

4.6227

0.0684

0.21929

0.172

81.81

98.79245

DAMPSM1

14.06

1.20625

4.6227

0.0324

0.21929

0.172

61.5

98.4

DAMPSM2

14.06

1.20625

4.6227

0.0486

0.21929

0.172

59.12

98.33676

DAMPSM3

14.06

1.20625

4.6227

0.0810

0.21929

0.172

52.04

98.11481

Hydro

gels

code

Swelling

exponent(n

)

Diffusion

coefficient

(D)cm

2s�

1

Initialsw

elling

rate

(ri)

[(g

water/g

hydro

gel)/min]

Theore

tical

equilibrium

swelling

(TSeq)

[(g

water/g

hydro

gel)]

Swelling

rate

constant(k

s)

[(g

hydro

gel/g

water)/min]

Swelling

data

ofporousP(A

M-A

MPS)

hydrogels

DAMPS1

0.99791

2.04482

0.02759

36.24502

0.003046

DAMPS2

0.66662

2.29724

0.01791

55.83473

0.000948

DAMPS3

0.68249

2.56446

0.01222

81.83306

0.000493

DAMPS4

0.98504

3.7445

0.00981

101.9368

0.000301

DAMPSM1

0.81135

2.7836

0.01511

66.18134

0.000673

DAMPSM2

0.66518

2.46387

0.0158

63.29114

0.000797

DAMPSM3

0.7728

2.64043

0.0178

56.17978

0.000893

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Equilibrium swelling ratio ðSeqÞ ¼ ½Ws � Wd=Ws�

Swelling % ¼ ½ðWs � WdÞ = Wd� � 100

where Wt, Ws and Wd are the weights of the swollen state of the gels at time t, at

equilibrium and dry gel, respectively.

Swelling and Diffusion Characteristics EvaluationThe hydrogels swelling and diffusion kinetic parameters, such as initial

swelling rate (ri), swelling rate constant (ks), and diffusion coefficient (D) were

evaluated by employing various equations reported elsewhere [16–22]. All

these parameters were calculated by using dynamic swelling experiments of

hydrogels.

Drug LoadingRanitidine hydrochloride (RH) hydrophilic drug is loaded into the hydro-

gels by an absorption method. The 50 mg of hydrogels were allowed to swell

in 10 ml of RH solution (5 mg=10 ml distilled water) for 24 h at 25�C. The load-

ing efficiency of RH in the hydrogels is determined spectophotometrically [23].

The drug-loaded hydrogel is placed in 50 ml of buffer solution (pH 2.2) and

stirred vigorously for 3 d to extract the drug from the hydrogels. The solution

is filtered and assayed by UV spectrophotometer at fixed k max value of

315 nm. The results of % drug loading and encapsulation efficiency were

calculated using following equations.

% Drug loading ¼ ½weight of drug in the hydrogel

weight of hydrogel� � 100

Table 2: Drug encapsulation efficiency and release kinetics parameter ofmacroporous hydrogels data at pH 7.4

% of Encapsulation efficiency and release kinetics parameters of differentformulations at 25�C

Hydrogels code % Encapsulation efficiency n k R2

DAMPS1 71.69 0.4219 1.3587 0.9776DAMPS4 83.81 0.2703 1.6214 0.9895DAMPSM1 63.70 0.3989 1.4327 0.9956DAMPSM3 19.60 0.4831 1.2204 0.9726

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% Encapsulation efficiency ¼ ½%actual loading

% theoretical loading� � 100

Release of Ranitidine Hydrochloride (RH)In order to study the release pattern of RH from the loaded hydrogels, a

known weight of RH-loaded hydrogel is placed in 100 ml of 2.2 pH phosphate

buffer at 37�C. The released amount of RH is determined at different time

intervals by recording the absorbance of the release medium by using UV-vis

spectrophotometer ELICO SL 164 Model (The Elico Co., Hyderabad, India).

The recorded absorbance is then related to the amount of RH released using

a calibration plot (0.12–5% concentration). The absorption of the solutions of

RH is measured at kmax 315 nm.

RESULTS AND DISCUSSION

Effect of Monomer on Swelling and DiffusionCharacteristics of HydrogelsThe incorporation of AMPS, a hydrophilic monomer, into the macromole-

cular network is expected to enhance the water uptake capacity as well as

the drug uptake capacity of the hydrogel, which, in turn, will facilitate release

of the entrapped drug. The concentration of AMPS is varied from

0.24–1.206 mM to study the effect of AMPS content on the swelling rations

of the hydrogels (Figure 1). With increasing the amount of AMPS, the swelling

ratios of the hydrogels are found to increase. The behavior is caused by hydro-

philic properties of the AMPS, which is derived from hydrophilic groups (-SO3

H) in its structure. The swelling ratios of the hydrogels increase from

32.34–81.81 g=g with increasing AMPS content. The higher the AMPS content

in the hydrogels the higher the content of hydrophilic groups (Acid groups)

[24]. Figure 2 shows the dry and swollen hydrogel photographs of DAMPS3

hydrogel.

The swelling and diffusion kinetic parameters, such as initial swelling rate

(ri, g water=g hydrogel=min) (0.012–0.0098), swelling rate constant (ks, g gel=g

water=min) (0.00304–0.0003), theoretical equilibrium swelling (Tseq g water=g

hydrogel) (36.245–101.936) (Figure 1(b)), swelling exponent(n) (0.66–0.99)

(Figure 1(c)) equilibrium water content (EWC%, 96.992 to 98.792), and dif-

fusion coefficient (cm2 s�1) (2.044–3.744) (Figure 1(d)) were calculated from

the dynamic swelling ratio values for the hydrogels prepared with different

amounts of AMPS. The swelling exponent (n) results (0.666–0.997) indicates

that the swelling transport mechanism is non-Fickian in nature. These values

are presented in Table I.

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Effect of Crosslinker on Swelling and DiffusionCharacteristics of HydrogelsThe concentration of MBA is varied from 0.032 m �0.081 mM to study the

effect of MBA content on the swelling ratios of the hydrogels (Figure 3). The

Figure 2: Poly (AM-AMPS) hydrogel (DAMPS3) photographs a) dry hydrogel and b) swollenhydrogel.

Figure 1: AMPS varied hydrogels a) swelling behavior of hydrogels b) swelling rate curves ofhydrogels c) diffusion curves of hydrogels, and d) diffusion coefficient curves of hydrogels.

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results indicate that the swelling ratio gradually decreases with increasing

the amount of the crosslinker [25,26]. The crosslinker is sufficient to reduce

the macromolecular chain mobility and the swelling ratios of the hydrogel

are lower. The above results are consistent with the influence of crosslinker

content on the average pore size and the morphology of hydrogels. Similar per-

spectives have been reported [27]. The swelling and diffusion kinetic para-

meters were calculated from the dynamic swelling ratio values for the

hydrogels prepared with different amounts of MBA. These values are pre-

sented in Table I. The swelling exponent (n) results (0.665–0.811) indicate that

the swelling transport mechanism is non- Fickian in nature.

FTIR SpectroscopyHydrogel chemical structure is confirmed by FTIR spectroscopy. FTIR

spectra of hydrogel (DAMPS3) were shown in Figure 4. According to

Figure 4, IR spectroscopy of the synthesized poly(AM-AMPS) porous hydrogels

showed strong absorbing peaks at 3423.8 cm�1, 2922 cm�1, 2852.6 cm�1,

1664.1 cm�1, 1456.4 cm�1, 1352.7 cm�1, 1125.9 cm�1 and 641.6 cm�1, verifying

Figure 3: MBA varied hydrogels a) swelling behavior of hydrogels b) swelling rate curves ofhydrogels c) diffusion curves of hydrogels, and d) diffusion coefficient curves of hydrogels.

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the formation of hydrogel. Here 3423.8 cm�1 indicated N-H stretching unit of

acrylamide, 2922 cm�1 and 2852.6 cm�1 indicated -CH2 stretching. The sec-

ondary amide carbonyl group is indicated by a band observed at 1664.1 cm�1.

This peak can be confirmed to carboxylate group of AMPS [28], the carboxylate

group symmetric stretching vibrations are indicated by a band observed at

1456.4 cm�1. The S=O asymmetric stretch occurs at 1352.7 cm�1 and C-N

stretching vibration occurs at 1125.9 cm�1 for the hydrogel unit [29]. The

C-S stretching occurs at 641.6 cm�1 (AMPS Unit) [29].

SEM ImagesThe SEM (Figure 5) images reveal the network structures of the porous

hydrogels. The images shows the different between the drug-loaded hydrogels

and also clearly explain the distribution of Ranitidine hydrochloride drug

particles on the surface of the hydrogel.

Effect of Salt Solution on the Swelling Behavior ofHydrogelsThe effect of the NaCl concentration in aqueous solution on the equilib-

rium swelling is also studied for the hydrogels. The swelling ratio of the

hydrogels in saline solution is significantly reduced when compared with

the values measured in distilled water. This phenomenon, is commonly

observed in the swelling of ionic hydrogels [30]. Figure 6(a) illustrates the

relationship between swelling and salt solution concentration (0.0, 0.1, 0.5,

1.0 and 5 mol=L) at 25�C. It shows that the change of salt concentration

Figure 4: FTIR spectra of the poly(AM-AMPS) with dextrose anhydrous.

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higher than about 1.0 mol=L has no appreciable influence on the swelling ratio

of the hydrogel.

Effect of Swelling Medium pHTo investigate the swelling behavior of the hydrogels at various pH solu-

tions, the hydrogels are swollen in different buffer solutions of pH 2, 3, 5, 7,

11 and 12 at 37�C. As shown in Figure 6(b) the swelling ratios of the hydrogels

decrease with increasing pH values. In all the hydrogels, maximum swelling is

Figure 5: SEM images of the hydrogels A) DAMPS4, B) DAMPS1 drug-loaded hydrogel,C) DAMPS4 drug-loaded hydrogel.

Macroporous Hydrogels 499

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observed at pH 2 and minimum at pH 12. The effect of pH on the swelling of

the AMPS hydrogels is explained on the basis of protonation of the amino

groups of AMPS. In the acidic medium, the protonation of the amino groups

leads to repulsion in the polymer chains, thus allowing more water in the

hydrogel network. At higher pH, deprotonation of the animo groups takes

place and repulsion in polymer chains is reduced.

In Vitro Release StudyPolyacrylamide-based hydrogels have received considerable attention

because of their use in many applications [31–33]. Among them porous hydro-

gels are most important in controlled-release applications, which received

greater attention in recent years as effective drug delivery devices in biomedi-

cal engineering [34]. Drug absorption from the gastrointestinal tract is a

complex procedure and is subject to many variables. In addition, porous hydro-

gels have also been developed for prolonging the residence time of delivery sys-

tems in the gastrointestinal tract. In this study, our intention is to develop

porosity in the copolymeric hydrogels of AM-AMPS by incorporating dextrose,

thereby the prolonged delivery of the drug can be achieved.

The loading efficiency of ranitidine hydrochloride (RH) is found to be high-

est in DAMPS4 (85.43), DAMPSM1 (68.7), DAMPSM4 (53.56) and DAMPS1

(49.69). This can be explained by the higher hydrophilic content of hydrogel

uptake with the higher amount of drug. This is due to the hydrophilic nature

(acid groups) of the AMPS monomer. But with MBA, varied hydrogels lower

the crosslink density of hydrogel uptake with a higher amount of drug. This

is due to the basis of the crosslink density of hydrogel (Figure 3).

However, drug-loading efficiency depends on the swelling behavior of

hydrogels [23]. It can be explained by Figure 7(a). The entrapments of drug

Figure 6: Swelling profiles of different hydrogels in (a) different concentrations of NaCl2solution, (b) different buffer solutions.

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particles as clumps in hydrogels can be known by the SEM image of

drug-loaded hydrogel (Figure 5(B, C)).

Figure 7(b) shows the cumulative release profiles of RH from hydroges in

phosphate buffers of pH 2.2 at 37�C as a function of time. The percentage

cumulative release (R) of RH drug from the selected hydrogels is calculated

using the following equation:

R ¼ Mt=Mo � 100;

where Mt is the amount of drug released at time t and Mo is the initial loaded

drug amount. Figure 7(b) shows the model drug RH release studies and

released profiles of the drug showed that more than �13.45–35.36% of the

loaded drug is released in the first 30 min at a buffer solution pH 2.2, and

the rest of the drug was released slowly. Also this result indicated that the

release of RH from the hydrogel was clearly influenced by their hydrogel

strength and also RH was released from the polymeric hydrogel in a controlled

rate over 20 h.

In order to specifically understand the release mechanism, the drug

released from the hydrogel is fitted to the following equations proposed by

Ritger and Peppas (1987) [31,35].

Mt=M1 ¼ ktn;

where Mt=M1 is the fractional of drug released at time t, k is a constant

related to the properties of the drug delivery systems, and n is the release

exponent, indicative of the mechanism of release. When n< 0.5 the release

is Fickian diffusion mechanism [36] and when 0.5<n< 1, the release mech-

anism is non-Fickian diffusion, and at n¼ 1 there is continuous zero-order

release, where the system will be relation controlled [37]. The values of the

Figure 7: (a) Drug loading efficiency of hydrogel and (b) drug release profiles of hydrogels.

Macroporous Hydrogels 501

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exponent of release n are calculated according to the above method. The

release rate characteristics of drug from hydrogels are presented in Table I.

Table values 0.27 to 0.48 clearly indicate that the hydrogels follows Fickan

diffusion-controlled release mechanism [35].

CONCLUSIONS

This study demonstrates the preparation of porous hydrogels in a free radical

polymerization process. The morphological and chemical structures of the

hydrogels were confirmed using SEM and FTIR analyses. The swelling

and deswelling kinetic parameters suggest that the hydrogels’ network is

non-Fickian in nature. The studies on the swelling ratio of hydrogels reveal

their sensitive response to environmental pH values change. The effects of dif-

ferent parameters on RH drug release behavior of hydrogels were studied. The

results of drug release kinetics for the hydrogels indicated a Fickian type

transport (at pH 2.2). All the results indicate that this hydrogel may serve

as a potential device for the delivery of drugs in which the primary target is

the gastrointestinal tract.

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