ARTICLE pubs.acs.org/IECR
Semi-Interpenetrating Polymer Network Hydrogel Blend Microspheres of Gelatin and Hydroxyethyl Cellulose for Controlled Release of Theophylline Praveen B. Kajjari, Lata S. Manjeshwar,* and Tejraj M. Aminabhavi† Department of Chemistry, Karnatak University, Dharwad 580 003, India ABSTRACT: Semi-interpenetrating polymer network (semi-IPN) hydrogel blend microspheres of gelatin and hydroxyethyl cellulose were prepared by a water-in-oil (w/o) emulsion technique and used to investigate the controlled release (CR) of theophylline (THP), an antiasthamatic drug. About 74% encapsulation of THP was achieved, and the drug release profiles were analyzed in terms of gelatin and hydroxyethyl cellulose blend composition, amount of cross-linking agent, and percentage drug loading. Fourier transform infrared (FTIR) spectroscopy confirmed the formation of the IPN blend matrix, as well as chemical stability of the drug in the microsphere. The physical state of the drug in the IPN matrix as evaluated by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) remained undisturbed. The size of the microspheres varied from 98 to 144 μm as measured by laser light scattering. Scanning electron microscopy (SEM) indicated the smooth surface morphology of the microspheres. Equilibrium and dynamic swelling of the microspheres in distilled water were measured to compute the diffusion coefficient (Dv) of the drug solution through the microspheres. The in vitro cumulative release data were analyzed using an empirical equation to compute the diffusion exponent (n), whose values suggest a non-Fickian mode of transport.
1. INTRODUCTION Over the past few decades, interpenetrating polymer networks (IPNs) have been widely used, as such devices can help to control the release of bioactive molecules.15 In the area of controlled release (CR), after the development of suitable formulations, several drugs of interest have been administered through the oral route.68 In such applications, degradation of drugs in the gastrointestinal track (GIT) poses problems. To overcome this problem, natural polymers that can form hydrogels are expected to be the most effective CR devices, which would help to control the transport of drugs, depending on the nature of the medium and the wall material of the matrix.9,10 Therefore, it is necessary to develop polymeric blends in the form of IPN hydrogels that can dictate the release of drug in a controlled manner.11,12 The natural choices of polymers in this category are biopolymers such as gelatin (GE)13,14 and hydroxyethyl cellulose (HEC).15,16 Both of these polymers are water-soluble, and preparation of IPNs of these polymers is a straightforward approach, wherein polymers can be mixed in appropriate compositions and cross-linked with glutaraldehyde (GA) to give spherically shaped drug-loaded microspheres. Earlier, we reported on the preparation and characterization of microspheres of gelatin blended with carboxy methylcellulose17 and methylcellulose-grafted-acrylamide18 for the CR of ketorolac tromethamine and nifedipine, respectively. Recently, gelatin blended with chitosan IPN blend microspheres were prepared19 for the CR of theophylline (THP). In all of these studies, the dependence of the swelling ratio, encapsulation efficiency, and drug release profiles on the blend composition and cross-link density was investigated. As a further contribution, we report here on the preparation of hydrogel blend microspheres of GE with HEC useful for the CR of THP. A high viscosity and inertness toward the pH of external media because of its nonionic nature16 make HEC a suitable candidate for blending with GE to r 2011 American Chemical Society
minimize the gastrointestinal pH response. THP is a naturally occurring 1,3-dimethylxanthine alkaloid, known for the treatment of asthma; its biological half-life is 58 h, but the drug has dose-related side effects (>20 μg/mL) such as nausea, ulcers, cardiac arrhythmia, and epigastria pain.20,21 Hence, developing a CR formulation of THP would minimize the dosing frequency and side effects. In this article, we report on the preparation of microspheres of IPN blend microspheres of GE and HEC that are cross-linked with GA to encapsulate THP and their controlled release properties in acidic and alkaline media. The formulations of this study were characterized by a variety of experimental techniques to understand drugpolymer interactions, particle morphology, and drug release kinetics under acidic and alkaline conditions to recommend them for oral administration.
2. EXPERIMENTAL SECTION 2.1. Materials. Theophylline was purchased from Loba Chemicals, Mumbai, India. Gelatin, high-viscosity-grade hydroxyethyl cellulose, analytical-reagent-grade glutaraldehyde solution 25% (v/v), n-hexane, and light liquid paraffin were all purchased from S.D. Fine Chemicals, Mumbai, India. Span-80 was purchased from Loba Chemicals, Mumbai, India. All other chemicals were used without further purification. 2.2. Preparation of Semi-IPN Hydrogel Microspheres. Semi-IPN hydrogel blend microspheres of GE and HEC were prepared by a water-in-oil (w/o) emulsion cross-linking method following the procedure proposed earlier.17 Briefly, 20 mL of 14% (w/v) Received: March 15, 2011 Accepted: May 13, 2011 Revised: May 12, 2011 Published: May 13, 2011 7833
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polymer solution was prepared by dissolving varying amounts of GE and HEC in deionized water at 37 C and stirring until a homogeneous solution was obtained. A required amount of THP (5, 10, and 15 mass %, dry mass of the polymer) was then dissolved into the polymer mixture, and the solution was added slowly to light liquid paraffin oil (100 mL) containing 1% (w/v) Span-80 with constant stirring at 400 rpm using a Eurostat high-speed stirrer (IKA Labortechnik, Staufen, Germany) for 10 min. To this w/o emulsion was slowly added a required amount of GA as a cross-linking agent, and the mixture was stirred further for 2 h. The hardened microspheres were separated by filtration and washed with n-hexane to remove the oil. Finally, the microspheres were washed with 50 mL of 0.1 M glycine solution to mask the unreacted GA. The microspheres were air-dried at 40 C for 24 h and stored in a desiccator before use. The assigned codes listed in Table 1 refer to different formulations. 2.3. Drug Content. The theophylline content of the microspheres was estimated in distilled water. Microspheres of known weight (10 mg) were ground to a powder using an agate mortar, extracted for 1 h in 50 mL of deionized water, and sonicated using a probe sonicator (UP400S, Hielscher Ultrasonics GmbH, Teltow, Germany) for 30 min. The solution was centrifuged to remove the polymeric debris and washed twice to completely extract the drug. The clear supernatant solution was analyzed by UV spectrophotometer (Secomam, model Anthelie, Ales, France) at the λmax value of 272 nm. The percentage encapsulation efficiency (EE) was calculated as ! actual drug loading encapsulation efficiency ð%Þ ¼ 100 theoretical drug loading
2.4. Fourier Transform Infrared (FTIR) Spectral Measurements. FTIR spectra were recorded on a Nicolet (Milwaukee, WI) Impact 410 spectrophotometer to confirm the cross-linking reaction, as well as the chemical stability of THP in the microspheres. FTIR spectra of pristine GE, pristine HEC, placebo microspheres, THP-loaded microspheres, and plain THP were obtained by crushing the samples with KBr powder to obtain pellets by applying 600 kg/cm 2 pressure. Spectral scanning was done in the range between 4000 and 500 cm1 . 2.5. Differential Scanning Calorimetry (DSC). DSC (Rheometric Scientific, Surrey, U.K.) was performed on placebo and THP-loaded microspheres, as well as pristine THP. Samples were heated from 10 to 400 C at a heating rate of 10 C/min in a nitrogen atmosphere (flow rate of 20 mL/min). 2.6. X-ray Diffraction (XRD). XRD data were collected for pristine THP, placebo microspheres, and THP-loaded microspheres to determine the crystallinity of THP after encapsulation. Scanning was done up to a 2θ angle of 43 using a Bruker model D8 Advance X-ray diffractometer. 2.7. Scanning Electron Microscopy (SEM). SEM images were obtained for GE and HEC semi-IPN blend microspheres that were cross-linked with 5 mL of GA and loaded with 5% (w/w) THP. Microspheres were sputtered with gold coating to make them conducting and placed on a copper stub. Scanning was done using JEOL model 6390 LA instrument (Tokyo, Japan). 2.8. Particle Size Measurements. Particle sizes and size distributions were measured using a Mastersizer instrument (model MS-2000, Malvern Instruments, Malvern, U.K.). The sizes of completely dried microspheres of different formulations were measured using a dry sample adapter. Completely dried microspheres were placed on the sample tray with an inbuilt vacuum under the compressed air system, which was used to suspend the particles. The volume mean diameter was recorded, and the results are included in Table 2. 2.9. Swelling Studies. Equilibrium water uptake of the GAcross-linked GE and HEC semi-IPN blend microspheres loaded with THP was determined by measuring the extent of swelling of the matrix after immersion in water. To ensure complete equilibration, samples were swollen for 24 h, excess surfaceadhered liquid drops were removed by blotting, and the swollen microspheres were weighed to an accuracy of (0.01 mg on an electronic microbalance (Mettler, AT 120, Griefensee, Switzerland). The microspheres were dried in an oven at 60 C for 5 h until no change in the dried mass of the sample
ð1Þ
Table 1. Formulation Codes and Different Ingredients Used to Prepare Microspheres formulation code
GE (% w/w)
HEC (% w/w)
THP (% w/w)
F0 F1
GA (mL)
100
0
5
5
90
10
5
5
F2
80
20
5
5
F3
70
30
5
5
F4
80
20
5
2.5
F5 F6
80 90
20 10
5 10
7.5 5
F7
90
10
15
5
Table 2. Results of Percentage Encapsulation Efficiency, Volume Mean Particle Size, Percentage Equilibrium Water Uptake, and n Values from eq 6 along with Correlation Coefficients, r, for All Formulations eq 6 volume mean particle size (μm)
equilibrium water uptake (%)
n
r
58
98
420
0.48
0.991
65
122
432
0.47
0.999
F2
70
135
454
0.47
0.975
F3 F4
71 73
141 144
478 502
0.39 0.37
0.994 0.984
F5
68
117
438
0.49
0.997
F6
68
129
449
0.36
0.996
F7
72
138
460
0.26
0.905
formulation code
EE (%)
F0 F1
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D¥. Experiments were performed in triplicate, but average values are considered in all calculations and graphical displays. 2.10. In Vitro Release Studies. Dissolution studies on semiIPN blend microspheres equivalent to 10 mg of THP were performed in a USP apparatus-I dissolution tester (Dissotest, LabIndia, Mumbai, India). Drug release was monitored at pH 1.2 initially for 2 h, and the sample was then dissolved in a phosphate buffer solution of pH 7.4 until completion of the drug release. The microspheres were placed back in the basket that was immersed in 500 mL of dissolution medium maintained at 37 C and at 100-rpm stirring speed. Five-milliliter aliquots were withdrawn at different time intervals and filtered through a 0.45mm filter. The dissolution medium was replenished with another 5 mL of fresh dissolution media to maintain the sink conditions. The concentration of THP was determined by UV spectrophotometer (Secomam, model Anthelie, Ales, France) at λmax = 272 nm.
Figure 1. FTIR spectra of (A) GE, (B) HEC, (C) placebo microspheres, (D) THP-loaded microspheres, and (E) pristine THP.
was observed. The percentage equilibrium water uptake was calculated as Ms Md equilibrium water uptake ð%Þ ¼ 100 ð2Þ Md where Ms and Md are the masses of the swollen and dry microspheres, respectively. Dynamic swelling of the microspheres was performed according to the method reported by Robert et al.22 The dried microspheres were placed on observation plates equipped with a liquid container and were examined under an optical microscope. The initial diameter, D0, was recorded, and the container was filled with deionized water. The increase in swelling diameter, Dt, due to water transport into the microspheres was measured as a function of time (at ambient temperature, i.e., 25 C) until the microspheres reached complete equilibrium, giving the diameter
3. RESULTS AND DISCUSSION In this research, semi-IPN hydrogel blend microspheres of GE and HEC were prepared using GA as a cross-linker and were employed to investigate the CR of THP. During the preparation of the microspheres, a trace amount of GA from the microspheres was removed by repeated washings with glycine solution (0.1 M) to convert the amine group of glycine to an imine bond by reaction with the aldehydic group of the unreacted GA that was deactivated. The absence of unreacted GA was further confirmed by Brady’s test,15 indicating no traces of GA in the formulations, thus confirming their safety for in vivo applications. 3.1. Encapsulation Efficiency (EE). The EE values of the microspheres show a dependence on the blend ratio of the semiIPN, the amount of GA used as a cross-linking agent, and the extent of drug loading. In the present formulations, 58% EE was observed for GE microspheres, but for the blended formulations, the EE values ranged from 68% to 74% (see Table 2). For microspheres containing 10% (F1), 20% (F2), and 30% (F3) (w/w) HEC in the blend, the calculated EE values are 65%, 70%. and 71%, respectively. Thus, the percentage EE values increased with increasing amount of HEC, because the excess amount of HEC present in the blend increased the viscosity of the blend polymer solution. However, highly viscous solutions might not allow smaller THP particles to move easily, and hence, more of the drug particles would be entrapped inside the blend matrix instead of leaching out of the matrix during the formulation step. The extent of cross-linking also showed a significant effect on the percentage EE; specifically, for microspheres that were crosslinked with 2.5 mL (F4), 5 mL (F2), and 7.5 mL (F5) of GA, the EE values were 73%, 70%, and 68%, respectively. This type of decreasing trend is the result of increasing cross-link density of the matrix, thereby reducing the free volume spaces of the matrix, giving lower EE values. The microspheres loaded with 5% (F1), 10% (F6), and 15% (F7) (w/w) THP exhibited percentage EE values of 65%, 68%, and 72%, respectively. These results indicate that the percentage EE values were higher for formulations incorporated with higher loadings of THP, because, at higher drug loadings, more of the drug particles were entrapped in the matrix, thereby increasing the EE values. 3.2. FTIR Analysis. Cross-linking of gelatin chains by GA was confirmed by FTIR spectroscopy, as shown in Figure 1 for pristine gelatin, pristine HEC, and placebo semi-IPN blend microspheres. In the case of pristine gelatin, a band for N—H stretching is observed at 3432 cm1, whereas that at 1538 cm1 is 7835
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Figure 3. XRD patterns of (A) placebo microspheres, (B) THP-loaded microspheres, and (C) pristine THP.
Figure 2. DSC thermograms of (A) placebo microspheres, (B) THPloaded microspheres, and (C) pristine THP.
due to the N—H bending vibration. However, the band at 1630 cm1 reflects the amide I (CdO) stretching frequency, and a part of the band at 1334 cm1 is due to the C—N bond stretching vibration. The bands at 2923 and 2856 cm1 are representative of aliphatic C—H asymmetric and symmetric stretching vibrations, respectively. The characteristic bending absorption of the methylene group is observed at 1465 cm1. The hydroxyl group stretching vibration of HEC is observed at 3412 cm1, whereas that for C—O—C bond stretching is observed at 1118 cm1. The bands at 1063 and 1023 cm1 indicate C—O stretching vibrations. The asymmetric and symmetric stretching vibrations of C—H bonds are observed at 2920 and 2874 cm1, respectively. For the placebo semi-IPN blend microspheres, all of the peaks that appeared for GE and HEC were also observed, in addition to a new peak at 1647 cm1, which indicates the stretching vibrations of the imine group (CdN) of the Schiff base formed from the reaction of the aldehydic functional group of GA and the amino group of GE. This band further confirms the formation of the semi-IPN structure due to the crosslinking between GE and GA. The chemical stability of THP in the semi-IPN matrix was confirmed by FTIR spectroscopy, as shown by the spectra of placebo microspheres, THP-loaded microspheres, and pristine THP displayed in Figure 1. In case of THP, a broad band at 3428 cm1 is due to N—H stretching vibrations, whereas those at 3066, 2981, 2929, and 2829 cm1 are attributed to both aromatic and aliphatic C—H stretching vibrations. A band at
1716 cm1 corresponds to the imide group stretching of the heterocyclic ring, and a sharp peak at 1672 cm1 is due to tertiary amide group stretching vibrations. A band at 1569 cm1 represents N—H bending vibrations, but that at 1245 cm1 is due to C—N stretching vibrations. In the case of THP-loaded semi-IPN blend microspheres, the bands that were present initially in THP also appeared, but the bands of THP were not prominent in the drugloaded microspheres because of the merging of the bands of THP in the broad bands of the polymer matrix, suggesting the chemical stability of THP after encapsulation within the polymer matrix. 3.3. Differential Scanning Calorimetric Study. DSC thermograms of placebo semi-IPN blend microspheres, THP-loaded semi-IPN blend microspheres, and pristine THP are displayed in Figure 2. Pristine THP shows a sharp peak at 273 C, representing its melting point. In the case of the placebo semi-IPN blend microspheres, three endothermic transitions are observed at 66, 197, and 321 C, where the broad endothermic peak at 66 C is due to the loss of moisture and the peak at 321 C corresponds to the degradation of GE chains.23 The phase transition occurring at 197 C might be the result of interactions between the polymer chains. Thermograms of THP-loaded microspheres exhibit all of the peaks that are present in the thermograms of placebo semi-IPN blend microspheres, but no peak is observed that corresponds to THP, indicating the molecular-level dispersion of THP in the polymer matrix. 3.4. X-ray Diffraction (XRD). X-ray diffractograms of placebo semi-IPN blend microspheres, THP-loaded microspheres, and pristine THP presented in Figure 3 exhibit characteristic intense peaks at 2θ = 12.5 and 25.4, respectively, representing the crystalline nature of the THP, but these peaks are not observed in the THP-loaded microspheres, and only the peaks observed in the placebo polymer matrix are seen. The XRD peaks depend on crystal size; in the present study, for all of the THP-loaded matrices, characteristic peaks of THP are not observed because THP is distributed uniformly in the polymer matrix, and hence, the crystal size is beyond the limit of detection, which indicates that THP is dispersed molecularly into the polymer matrix and that no crystals are present in the THP-loaded formulations. 3.5. Scanning Electron Microscopy. Typical SEM images of THP-loaded semi-IPN blend microspheres at 100, 500, and 7836
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Figure 5. Particle size distribution curve for GEHEC semi-IPN blend microspheres (F2).
Figure 4. SEM images of THP-loaded microspheres.
1500 magnifications, shown in Figure 4, suggest spherical shapes with smooth surfaces. 3.6. Particle Size Analysis. The volume mean diameters of all formulations are presented in Table 2. A typical size distribution curve for formulation F2 [80% (w/w) GE/20% (w/w) HEC/5% (w/w) THP cross-linked with 5 mL of GA] is shown Figure 5. The sizes of the microspheres of all the formulations fall in the range of 98144 μm, depending on the amount of HEC in the semi-IPN blend matrix, the concentration of the cross-linking agent, and the extent of THP loading. With increasing HEC content in the blend (F0, F1, F2, and F3), the particle size increased from 98 to 141 μm, because the presence of HEC enhanced the viscosity of the blend matrix, thus facilitating the formation of larger emulsions, producing larger particle sizes. The particle size decreased with increasing extent of cross-linking (F4, F2, and F5). For microspheres containing 80% (w/w) GE, 20% (w/w) HEC, and 5% (w/w) THP, along with increasing amounts of GA from 2.5 to 7.5 mL, the particle size decreased from
Figure 6. Plots of Dt/D0 versus time t: effects of (A) extent of crosslinking, (B) drug loading, and (C) amount of HEC in the blend.
144 to 117 μm, probably because of the increased cross-link density and rigid nature of the network structure that might be caused by 7837
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Table 3. Transport Data of Water through Matrix Formulations eq 4
eq 5
Dv 106 (cm2/s)
n
r
1.58
0.47
0.997
1.69
0.46
0.971
2.35
2.10
0.44
0.964
F3
2.64
3.63
0.40
0.989
F4
2.57
3.21
0.38
0.955
F5
2.11
1.86
0.46
0.906
F6
2.21
2.16
0.34
0.973
F7
2.43
3.11
0.25
0.953
formulation code
equilibrium normalized diameter (D¥/D0)
F0
1.7
F1
2.0
F2
shrinkage of the matrix, thereby reducing the sizes. The effect of the drug loading on the particle size was observed by maintaining a constant blend ratio between GE and HEC and a constant extent of cross-linking. For formulations F1, F6, and F7 with increasing amounts of THP in the microspheres, particle size also increased, because of the occupation by the drug particles of the free volume spaces of the matrix, thereby hindering the inward shrinkage of the polymer matrix. 3.7. Swelling Studies. The percentage equilibrium water uptake data of the cross-linked semi-IPN hydrogel blend microspheres presented in Table 2 show the influence of the polymer blend composition, cross-link density, and drug loading, but the cross-link density depends on the concentration of cross-linking agent used during the formulation. In the present study, different amounts of GA were used during the formulation of semi-IPN blend hydrogel microspheres of GE (80% w/w) with HEC (20% w/w) containing 5% (w/w) THP. By increasing the concentration of GA from 2.5 to 5 and 7.5 mL, the equilibrium water uptake values decreased systematically from 502% to 454% and 438%, respectively. Such a reduction in water uptake is due to the formation of a rigid network structure in the presence of higher concentrations of GA in the matrix. The percentage drug loading also has an effect on the percentage water uptake capacity of the matrices. As the THP loading was increased from 5% to 15% (F1, F6, and F7), the equilibrium water uptake also increased from 431% to 460%. When THP-loaded microspheres were in contact with water, because of matrix swelling, the water-soluble THP might have diffused out of the matrix. On the other hand, formulations that contained higher amounts of HEC showed higher swelling. For instance, formulations F1 [10% (w/w) HEC], F2 [20% (w/w) HEC], and F3 [30% (w/w) HEC] exhibited equilibrium water uptake values of 432%, 454%, and 478%, respectively compared to 420% observed in case of F0 (i.e., plain GE matrix). This could be due to the increased hydrophilic nature of HEC, which would tend to increase the extent of matrix swelling due to higher water uptake. Dynamic swelling studies were performed by monitoring the changes in the diameter of the microspheres as a function of time, Dt, using an optical microscope. Figure 6A displays a plot of normalized diameter, Dt/D0, as a function of time t for formulations containing different amounts of GA, from which it is evident that the normalized diameter shows a decrease with increasing concentration of GA, because of the rigid network structure formed at higher concentrations of GA. Similar profiles were also observed for microspheres loaded with different amounts of THP, as depicted in Figure 6B, which shows that, with increasing THP loading, the normalized diameter also
increased with time and attained higher equilibrium saturation. Figure 6C displays a plot of Dt/D0 versus t for different amounts of HEC contained in the blend, for which the normalized diameter can be seen to increase with increasing HEC content of the matrix because of the higher hydrophilic nature of HEC compared to GE; overall, this enhances the water transport rate and increases the water uptake capacity of the microspheres. Dimensional changes of the microspheres due to swelling (i.e., volume change, ΔVt, with time calculated from the initial volume, V0) were analyzed to compute the diffusion coefficient, Dv, of drug solutions using the equation17 2 3 ΔV¥ 1=2 4 6 ΔVt 7 Dv 6 V0 7 7 ¼6 t 1=2 ð3Þ 4 D0 5 π ΔV0 Rearranging eq 3, Dv was calculated as V 0 D0 Dv ¼ ð1:7733 slopeÞ 4ΔV¥
ð4Þ
The results for equilibrium swollen diameters (D¥ normalized to the original diameter, D0) and diffusion coefficients presented in Table 3 show that the diffusion coefficients were higher for microspheres with lower extents of cross-linking and higher drug loadings, but for those microspheres containing higher HEC contents, the diffusion coefficients were also higher. The water uptake values of the microspheres depend on the extent of hydrodynamic free volume, as well as the availability of hydrophilic functional groups. Thus, as the amount of GA increased, the extent of cross-linking also increased, leading to a reduction in hydrodynamic free volume and thereby showing the effect of the crosslink density of the matrix on the molecular transport of the drug solution through the polymer matrix. The dynamic swelling data of all the formulations were fitted to the empirical equation22 Dt =D¥ ¼ kt n
ð5Þ
From this equation, the rate constant k and exponent n, a parameter representing the type of transport mode, were computed by a least-squares analysis at the 95% confidence limit. The computed values of the correlation coefficients, r, and the n values are included in Table 3. It is observed that the n values range from 0.23 to 0.44, suggesting a non-Fickian transport mode of THP through the matrices of this study. 3.8. In Vitro Release Studies. In vitro release experiments were performed in acidic and alkaline media to understand the 7838
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Figure 7. (A) Effect of HEC content on in vitro release profiles of F0 (0% w/w), F1 (10% w/w), F2 (20% w/w), and F3 (30% w/w) formulations. (B) Effect of extent of cross-linking on in vitro release profiles for formulations F4 (2.5 mL), F2 (5 mL), and F5 (7.5 mL). (C) Effect of percentage drug loading on in vitro release profiles of F1 (10% w/w), F6 (20% w/w), and F7 (30% w/w) formulations.
release of THP from the GE and HEC blend matrix. To investigate the effect of the HEC content of the blend matrix on the drug release rate, the percentage cumulative release data as a function of time for formulations F0F3 are compared in Figure 7A, from which it is observed that percentage cumulative release increased from F0 to F3; that is, the release rate increased with increasing HEC content of the blend matrix, because of the more hydrophilic nature of the matrix at higher concentrations of the hydrophilic HEC, leading to higher swelling and higher release rates. In our earlier study,17 a GE and NaCMC blend matrix formulation containing 20% (w/w) NaCMC released ∼80% of the drug within 10 h, but in the present study, the blend matrix with 20% (w/w) HEC successfully extended the release of THP for up to about 24 h. The presence of carboxylate ion in NaCMC was responsible for swelling the matrix rapidly at alkaline pH, but on the other hand, the nonionic nature of
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HEC did not respond to the pH of the external medium,16 resulting in better control of the release of THP from the matrix. The percentage cumulative release versus time curves for varying amounts of GA (i.e., 2.5, 5.0, and 7.5 mL) at a fixed blend composition [80% (w/w) GE/20% (w/w) HEC] and a fixed amount of drug [5% (w/w) THP] are displayed in Figure 7B. It can be seen that the THP release is substantially faster and higher at lower amounts of GA (i.e., 2.5 mL), but the release follows a slower trend at higher amounts of GA (i.e., 7.5 mL). At higher amounts of GA, the cross-link density of the matrix would be higher, making the matrix more rigid because of the contraction of microvoids, hindering the movement of THP particles through the polymer matrix, and thus decreasing its percentage cumulative release. Figure 7C shows the release profiles of GEHEC semi-IPN blend microspheres containing different amounts of THP. The results suggest that, at higher THP loadings [15% (w/w)], the release rates are higher than those for formulations containing smaller amounts of THP. However, a prolonged release was observed for formulations containing lower amounts of THP. The release rate was found to be substantially slower at lower amounts of THP in the matrix; this might be due to the availability of extra free void spaces through which fewer drug molecules will transport. The in vitro drug release rates were correlated with the results for the diffusion coefficients (see Table 3). The diffusion coefficients were higher for formulations containing higher amounts of HEC. A similar trend was observed in the THP release profiles: formulations containing higher amounts of HEC showed much higher release rates than those containing lower amounts of HEC. Also, cross-linking agent exhibited a good correlation between the diffusion coefficient and the drug release characteristics. It was observed that microspheres that were cross-linked with higher amounts of GA showed lower diffusion coefficients and lower release rates, but those microspheres that were cross-linked with lower amounts of GA showed higher diffusion coefficients along with higher release rates. These observations suggest a correlation between diffusion coefficients and release rates of the microspheres loaded with different amounts of THP. To establish a relationship between the drug release rates and molecular transport parameters, we fitted the release data to the empirical equation24 Mt =M¥ ¼ kt n
ð6Þ
where Mt/M¥ represents the fraction of drug released at time t and k and n have the same meanings as discussed for eq 5. Using the least-squares procedure, we estimated the values of n for all formulations at the 95% confidence limit, and these values are given in Table 2 along with the correlation coefficient, r. n values ranging from 0.25 to 0.47 for the present microspheres indicate a non-Fickian mode of drug transport.24
4. CONCLUSIONS Semi-IPN hydrogel blend microspheres of GE with HEC have been prepared by an emulsion cross-linking method and investigated for the controlled release of THP. An encapsulation of THP of up to 74% was achieved. FTIR spectroscopy confirmed the formation of the blend matrix in addition to the chemical stability of THP in the blend matrix. DSC and XRD of the microspheres confirmed the molecular-level uniform distribution of THP within the polymer matrix. SEM analysis and particle 7839
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Industrial & Engineering Chemistry Research size measurements suggested a spherical nature and smooth surfaces of the microspheres, with average diameters ranging from 98 to 144 μm. The swelling kinetics was found to depend on the extent of cross-linking, as well as the amount of HEC in the IPN blend matrix. THP was released in a controlled manner through the developed formulations for up to 24 h. Values of exponent n ranging from 0.25 to 0.47 suggest their dependence on the extent of cross-linking, as well as the blend ratio. Overall, the in vitro release kinetics followed a non-Fickian trend.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ91 836 2215286. Fax: þ91 836 2771275. E-mail:
[email protected]. Present Addresses †
CSIR Emeritus Scientist, SET’s College of Pharmacy, Dharwad 580 002, India.
’ ACKNOWLEDGMENT P.B.K. and L.S.M. thank the University Grants Commission (UGC), New Delhi, India (KU/SHA/UGC/RFSMS/2009-10), for a fellowship to P.B.K. T.M.A. thanks the CSIR [21(0760)/ 09/EMR-II], New Delhi, India, for the award of Emeritus Scientist. ’ REFERENCES (1) Rokhade, A. P.; Shelke, N. B.; Patil, S. A.; Aminabhavi, T. M. Novel interpenetrating polymer network microspheres of chitosan and methylcellulose for controlled release of theophylline. Carbohydr. Polym. 2007, 69, 678–687. (2) Agnihotri, S. A.; Aminabhavi, T. M. Novel interpenetrating network chitosan-poly(ethylene oxide-g-acrylamide) hydrogel microspheres for the controlled release of capecitabine. Int. J. Pharm. 2006, 324, 103–115. (3) Rokhade, A. P.; Patil, S. A.; Aminabhavi, T. M. Synthesis and characterization of semi-interpenetrating polymer network microspheres of acrylamide grafted dextran and chitosan for controlled release of acyclovir. Carbohydr. Polym. 2007, 67, 605–613. (4) Sullad, A. G.; Manjeshwar, L. S.; Aminabhavi, T. M. Polymeric blend microspheres for controlled release of theophylline. J. Appl. Polym. Sci. 2010, 117, 1361–1370. (5) Bao-Lin, G.; Qing-Yu, G. Preparation and properties of a pH/ temperature-responsive carboxymethyl chitosan/poly(N-isopropylacrylamide)IPN hydrogel for oral delivery of drugs. Carbohydr. Res. 2007, 342, 2416–2422. (6) Levelle, E. C. Targeted delivery of drugs to the gastrointestinal tract. Crit. Rev. Ther. Drug Carrier Syst. 2001, 18, 341–386. (7) Ekici, S.; Saraydin, D. Interpenetrating polymer network hydrogels for potential gastrointestinal drug release. Polym. Int. 2007, 56, 1371–1377. (8) Fares, M. M.; Assaf, M. S.; Abul-Haija, M. Y. Pectin grafted poly(N-vinylpyrrolidone): Optimization and in vitro controllable theophylline drug release. J. Appl. Polym. Sci. 2010, 117, 1945–1954. (9) Sun, L.; Zhou, S.; Wang, W.; Li, X.; Wang, J.; Weng, J. Preparation and characterization of porous biodegradable microspheres used for controlled protein delivery. Colloids Surf. A: Physicochem. Eng. Aspects 2009, 345, 173–181. (10) Builders, P. F.; Kunle, O. O.; Okpaku, L. C.; Builders, M. I.; Attama, A. A.; Adikwu, M. U. Preparation and evaluation of mucinated sodium alginate microparticles for oral delivery of insulin. Eur. J. Pharm. Biopharm. 2008, 70, 777–783.
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