ARTICLE pubs.acs.org/IECR
Design of Molecular Imprinted Hydrogels for Controlled Release of Cisplatin: Evaluation of Network Density of Hydrogels Baljit 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 delivery systems, and molecular imprinting technique has been suggested as one step in this direction. Therefore attempts have been made to design molecular imprinted polymers (MIPs) of HEMA and MAAc for the slow release of cisplatin. The characterization of network structure of hydrogels has been studied by FTIR, swelling studies, and determining the various structural parameters of hydrogel such as polymer volume fraction in the swollen state (ϕ2,s), FloryHuggins interaction parameter (χ1), molecular 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. Effect of the cross-linker concentration on the network parameters has been studied along with influence of the network density on drug entrapment and release of drug from the hydrogel.
1. INTRODUCTION Unprecedented developments in genomics and molecular biology today offer a plethora of new drugs. For all these exciting new drug candidates, it is necessary to develop suitable dosage forms or drug delivery systems to allow their effective, safe, and reliable application to the patient. Hence, the improvement in drug therapy is a consequence of not only the design of new drug molecules, but also the development of suitable drug delivery systems. Polymer-based drug delivery devices can provide delivery of drugs in a controlled and sustained manner and can maintain the ideal pharmacokinetic profile of the drugs. In this era of new technologies, there is an ongoing interest in enhancing the efficiency of polymer-based drug delivery systems, and the molecular imprinting technique has been suggested as one step in this direction. Molecular imprinting is a novel and fast evolving technique that has various biomedical applications,13 and recently its application in controlled drug delivery systems (DDS) has been reported.4,5 In controlled drug delivery devices the molecular imprinted polymers (MIP) are synthesized by the copolymerization of functional monomers and a cross-linker in the presence of drug or template molecules in the reaction system.6 The ability of imprinted polymers to bind a template molecule with high affinity lends to their application as excipient for sustained drug delivery. To create a rate attenuating excipient for a transdermal controlled release device, methacrylic acid (MAAc) has been used as a functional monomer to prepare MIPs for propanolol, a β-blocker.7 The permeation of propranolol was slower from the MIP devices than from nonimprinted devices. Various therapeutic agents such as tetracycline,8 theophylline,9 ciprofloxacin,10 timolol,11 and norfloxacin,12 have been released through MIP approach. Theophylline-reloaded particles were able to sustain drug release in pH 7.0 phosphate buffer for several hours, especially those loaded with low amounts of theophylline (0.12.0 mg/g).13,14 Molecular imprinted DDS have also been explored to deliver colon cancer drugs, viz sulfasalazine (prodrug used in the diseases of the colon),15 5-fluorouracil,16 and methotrexate.17 r 2011 American Chemical Society
Metal-based drugs exhibit powerful anticancer, antitumor, antidiabetic, anti-inflammatory, antibacterial, antiviral, and antiparasitic properties.18 Cisplatin is a widely used metal-based antineoplastic drug that exhibits therapeutic activity against several solid tumors19,20 along with testicular cancer, ovarian cancer, lymphoma, and glioma.21,22 It is also one of the most effective anticancer agents against gynecological and gastrointestinal cancers.9,23 Its entrapment in polymeric implants has reduced systemic toxicity and increased activity.24,25 Both poly(acrylic acid) and poly(methyl methacrylate) have been used to develop MIP devices (hydrogels) because of their biocompatibility.26 Acrylic polymers have shown high in vivo tolerance in rats after subcutaneous implantation for up to 24 days.27 In addition, the carboxylic acid groups in poly(acrylic acid) can form hydrogen bonds with mucin, a glycoprotein secreted locally that coats the mucosal surfaces.26 Hydrogels developed from these functional polymers have mucoadhesive and biocompatible properties. The hydrogels prepared from the ionic monomers swell/deswell quickly in response to change in their external environment. These changes can be induced by changing the surrounding pH, temperature, ionic strength, and electro stimulus.28,29 The response of hydrogels to pH make them suitable candidates for site-specific delivery of drugs to the colon. Various biomedical applications of hydrogels are due to their cross-linked structure which is determined by various network parameters. These network parameters enable them to encapsulate the drug molecules effectively and to release them in a controlled manner for extended periods of time.30 The rate of drug diffusion from drug-loaded hydrogel can be tailored by evaluating various network parameters such as the polymer volume fraction in the swollen state (ϕ2,s), molecular weight of the polymer chain between two neighboring cross-links (M c), ̅ Received: April 9, 2011 Accepted: November 11, 2011 Revised: November 4, 2011 Published: November 28, 2011 13742
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Industrial & Engineering Chemistry Research Scheme 1. Reaction Showing the Formation of MIPs
cross-link density (F), and the corresponding mesh size (ξ).31 The drug loading capability and mechanical strength of hydrogels increased with increase in cross-link density (F) of hydrogels.32,33 Knowledge of network parameters is proved to be an important tool for molecular imprinting. Hart and co-workers have reported dependence of both selectivity and absolute capacity of cross-linked hydrogels on cross-link density of polymers.34 In view of technological significance of molecular imprinting polymers in drug delivery, the present study is an attempt to synthesize 2-hydroxyethylmetacrylate (HEMA)- and methacrylic acid (MAAc)-based hydrogels imprinted with model drug cisplatin (1 mg/mL). For the synthesis of these hydrogels N,N0 methylenebisacrylamide (NN-MBA) has been used as crosslinker, ammonium persulfate (APS) as initiator, and N,N,N0 ,N0 tetramethylethylenediamine (TEMED) as accelerator. Both molecular-imprinted polymers (MIPs) and nonimprinted polymers (NIPs) have been synthesized and have been used to study the swelling and in vitro release dynamics of the drug.
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Hydroxyethylmethacrylate (HEMA) and methacrylic acid (MAAc) were obtained from Merck-Schuchardt, Germany, Ammonium persulfate (APS) and N,N0 -methylenebisacrylamide (NN-MBA) were obtained from S.D. Fine, Mumbai, India, and were used as received. The N,N, N0 ,N0 -tetramethylethylenediamine (TEMED) was obtained from Merck-Schuchardt, Germany. Cisplatin was obtained from Alkem Laboratory Limited, Mumbai, India. 2.2. Synthesis of poly(HEMA-cl-MAAc) Hydrogels. Synthesis of molecular imprinted polymers (MIPs) was carried out with 4.38 102 mol/L of APS, 7.68 101 mol/L of HEMA, 11.62 101 mol/L of MAAc, 3.89 102 and 6.49 102 mol/L of NN-MBA, and 1.72 101 mol/L of TEMED in
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aqueous solution of model drug (cisplatin) at 37 °C temperature for half an hour. The polymers were washed with distilled water, then dried at 37 °C in the oven until constant weight was obtained and were named poly(HEMA-cl-MAAc) thereafter. Synthesis of nonimprinted polymers (NIPs) was carried out without drug under the similar conditions. The MIPs and NIPs were synthesized with two different concentrations of the cross-linker (3.89 102 and 6.49 102 mol/L) to observe the effect of cross-linker on the template entrapment and thereafter on the release. The NIP and MIP prepared with two different cross-linker concentrations (3.89 102 and 6.49 102 mol/L) were designated as NIPs-3 and NIPs-5 and MIPs-3 and MIPs-5, respectively. All the reactions were carried out in triplicate. The reaction for formation of MIPs is shown in Scheme1. 2.3. Characterization. Polymers were characterized by FTIR spectroscopy, swelling studies,35 and various structural parameters of hydrogel such as polymer volume fraction in the swollen state (ϕ2,s), FloryHuggins interaction parameter (χ1), molecular weight of the polymer chain between two neighboring crosslinks (M c), cross-link density (F), and the corresponding mesh size (ξ) ̅by swelling equilibrium method. FTIR spectra of MIPs and NIPs polymers were recorded in KBr pellets on Nicolet 5700FTIR THERMO. Swelling of the polymers was carried out in distilled water by gravimetric method.35 The equilibrium swelling was taken after 24 h. 2.4. Release Dynamics of Drug from poly(HEMA-clMAAc). 2.4.1. Preparation Calibration Curves. In this procedure, the absorbance of a number of standard solutions of the reference substance at concentrations encompassing the sample concentrations was measured on a UV visible spectrophotometer (Cary 100 Bio, Varian) and a calibration graph was constructed. The concentration of the drug in the sample solution was read from the graph as the concentration corresponding to the absorbance of the solution. The calibration graph of cisplatin was made to determine the amount of cisplatin release from the drug loaded MIPs 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 the procedure mentioned in Experimental Section 2.2, and NIPs were 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 a definite volume of releasing medium at 37 °C. The amount of drug released was assayed spectrophotometrically after each 30 min. The absorbance of the solution was measured at wavelength 300 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 an oven, and reloading of the drug into MIPs and NIPs was carried out by swelling equilibrium method. Reloading of MIPs and NIPs was carried out with same concentration of the drug (1 mg/ mL). The hydrogels were allowed to swell in the drug solution for 24 h at 37° and than dried to obtain the release device. Swelling kinetics of both MIPs and NIPs and release dynamics of drug from drug-loaded MIPs and NIPs was studied in distilled water. 2.5. Mechanism of Swelling and Drug Release from Polymer Matrix. Swelling of polymers has been classified into three types of diffusion mechanisms, on the basis of relative rate of diffusion of water into polymer matrix and rate of polymer chain relaxation.3639 The values of diffusion exponent n and diffusion coefficients have been evaluated (by using eqs 14) for the 13743
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Figure 1. FTIR spectra of MIPs of poly(HEMA-cl-MAAc) hydrogels.
swelling of the polymers and for the release of the drug from the polymer. Mt ¼ kt n ð1Þ M∞ 0:5 Mt Di t ¼4 M∞ πl 2 DA ¼
0:049l 2 t 1=2
# " Mt 8 ð π2 DL tÞ ¼ 1 exp π2 M∞ l2
ð2Þ ð3Þ ð4Þ
where Mt/M∞ is the fractional release of drug in time t, k is the constant characteristic of the drugpolymer system, and n is the diffusion exponent characteristic of the release mechanism. Mt and M∞ is drug released at time t and at equilibrium, respectively, Di, DA, and DL are the initial, average, and late diffusion coefficients, 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 crosslinked structure of the prepared hydrogels was determined by studying the swelling of cylindrical polymer in distilled water. The density of polymer was calculated by measuring the radius and height of dry cylindrical polymer along with the weight of same. The samples were placed in distilled water, allowed to swell to equilibrium at certain temperature, and weighed after 24 h. The volume fraction (ϕ) of polymer in swollen state was calculated by the method used by Aithal and co-workers.40 According to which ϕ can be calculated by using eq 5. 1 dp w∞ wo ϕ¼ ð5Þ þ 1 ds wo 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 and cross-linker concentration on different network parameters we have taken swelling of hydrogels prepared with 3.89 and 6.49 102 mol/L of NN-MBA in distilled water at different (300.15, 310.15, and 320.15 K) temperatures. The detailed discussion on various network parameters is presented in a later section. 2.7. Statistical Analysis. All swelling and drug release studies were performed in triplicate and the mean, the standard deviation (sd), and the 95% confidence interval (CI) were calculated for the diffusion coefficients for each triplicate. Statistical significance was determined at p e 0.05 for all data analysis.
3. RESULTS AND DISCUSSION 3.1. Fourier Transform Infrared Spectroscopy. FTIR spectra of molecularly imprinted and nonimprinted poly(HEMA-clMAAc), i.e., MIPs and NIPs, were recorded and are presented in Figures 1 and 2, respectively. FTIR spectrum of MIPs showed absorption bands at 3442.6 cm1 (OH stretching vibrations), 3002.7 and 2950.6 cm1 (CH3 and CH2 stretching vibrations), 23602550 cm1 (overtones and combinations of OH bending and CO stretching vibrations), 1718.8 cm1 (CdO stretching), 1552.3 cm1 (OH in-plane bending), 1482.3 cm1 (CH2 bending), 1263.5 cm1 (COC stretching vibration), 10751190 cm1 (CO stretching), and 751.7 cm1 (CH2 rocking vibrations). FTIR spectrum of NIPs was found to be similar to MIPs having absorption bands at nearly the same wavenumber (cm1) but with less absorbance. 3.2. Release Dynamics of Drug. To observe the effect of cross-linker on the release of drug, the MIPs (MIPs-3 and MIPs-5) and NIPs (NIP-3 and NIP-5) were synthesized with two different concentrations (3.89 102 and 6.49 102 mol/ L of NN-MBA) respectively. The MIPs were synthesized in the presence of drug and after synthesizing the MIPs the polymers were subjected to the drug release study to remove the loaded drug. The release profile of drug from the drug-imprinted poly(HEMA-cl-MAAc), i.e., MIPs is shown in Figures 35. Complete removal of drug from the MIPs occurred after 72 h. 13744
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Figure 2. FTIR spectra of NIPs of poly(HEMA-cl-MAAc) hydrogels.
Figure 3. Release dynamics of cisplatin from MIPs of poly(HEMA-clMAAc) hydrogels in distilled water at 37 °C (prepared with different [NN-MBA]). Reaction time = 30 min, reaction temperature = 37 °C, [HEMA] = 7.68 101 mol/L, [MAAc] = 11.62 101 mol/L, [APS] = 0.438 101 mol/L, and [TEMED] = 1.72 101 mol/L.
Then the polymers were dried in oven at 37 °C. Total amount of drug 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 been observed to be 1.188 and 0.711, respectively, for the MIPs-3 and MIPs-5 (Table 1). Because the release of drug occurred through the Case II and non-Fickian diffusion mechanism, respectively, itself implies that the release of drug from the polymers prepared with low cross-linker concentration was very fast. The actual purpose of this study was to remove the drug
Figure 4. Release of cisplatin from MIPs of poly(HEMA-cl-MAAc) prepared with different [NN-MBA].
from the polymer and leave the template space in the polymer for further recognition of the drug molecule. The values of the diffusion coefficients are shown in Table 1. The values of diffusion coefficients are different in the two cases. These trends may be due to the occurrence of two different release mechanisms, i.e., Case II diffusion mechanism and non-Fickian diffusion mechanism, 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 of cisplatin was carried out in both the cases (i.e., MIPs and NIPs) to observe the binding capacity of hydrogels for the template. As the molecular imprinting is a technique, producing synthetic materials containing highly specific receptor sites that have an 13745
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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 reloaded MIPs and NIPs of poly(HEMAcl-MAAc) hydrogels in distilled water at 37 °C.
Table 1. Results of Diffusion Exponent n, Gel Characteristic Constant k, and Various Diffusion Coefficients for the Release of Cisplatin from the MIPs [poly(HEMA-cl-MAAc)] diffusion coefficients (cm2/min) diffusion
gel characteristic
sample exponent n constant k 102
initial Di 104
average
late time
DA 104 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
affinity for a target molecule and MIPs can mimic the recognition and binding capabilities of the template molecule. In the present case, it has been observed that the binding affinity of the MIPs for the cisplatin is higher as compared to NIPs when reloading of drug was carried out by swelling equilibrium method by keeping both the MIPs (MIPs-3 and MIPs-5) and NIPs-3 and NIPs-5 in 1 mg/mL solution of cisplatin for 24 h at 37 °C. Then polymers were dried to obtain the release device for further study. The drug is introduced into the polymer network via imbibition, which is equilibrium partitioning after the network is formed or the drug is included 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, the MIPs and NIPs were dried at room temperature and then were used to study the swelling of the poly(HEMA-cl-MAAc) hydrogels 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 been observed higher as compared to NIPs. Total amount of water taken by both types of polymers after 24 and 48 h is shown in Figure 7. The higher sorption in case of MIP may be due to the template formed during the synthesis of MIPs, while the template molecule of drug was removed and left behind points for solvent interactions. The values of diffusion exponent n and gel characteristics constant k are presented in Table 2. Swelling in general occurred through non-Fickian diffusion mechanism.
Figure 7. Swelling of reloaded MIPs and NIPs of poly(HEMA-clMAAc) hydrogels after 24 and 48 h in distilled water at 37 °C.
The values of the diffusion coefficients are shown in Table 2. The rate of swelling in the earlier stages of swelling was higher than that in the latter stages. From the swelling studies it is also clear that hydrogels had maintained structural integrity after swelling. In general, the structure of the imprinted cavities should be stable enough to maintain the conformation in the absence of the template, but somehow flexible enough to facilitate the attainment of a fast equilibrium between the release and reuptake of the template in the cavity. This will be particularly important if the device is used as a diagnostic sensor or as a trap of toxic substances in the gastrointestinal tract or release of drug to the GIT. The mechanical properties of the polymer and the conformation of the imprinted cavities depend to a great extent on the proportion of the cross-linker. MIPs for drug delivery should be stable enough to resist enzymatic and chemical attack and mechanical stress. The device will enter into contact with biological fluids of complex composition and different pH, in which the enzymatic activity is intense. 13746
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Table 2. Results of Diffusion Exponent n, Gel Characteristic Constant k, and Various Diffusion Coefficients for the Swelling Kinetics and Release Dynamics of MIPs (Reloading) and NIPs diffusion coefficients (cm2/min) diffusion
gel characteristic
sample exponent n constant k 102
initial Di 104
average
late time
DA 104 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
MIP-3
0.772
0.90
1.62
11.30
1.32
MIP-5 NIP-3
0.788 0.602
0.87 2.99
3.16 2.44
14.33 21.52
2.49 2.52
NIP-5
0.546
4.15
2.51
25.67
2.81
release dynamics
Figure 9. Release of cisplatin from MIPs and NIPs of poly(HEMA-clMAAc) after 24 and 48 h in distilled water at 37 °C.
Figure 8. In vitro release dynamics of cisplatin from reloaded MIPs and NIPs of poly(HEMA-cl-MAAc) in distilled water at 37 °C.
Figure 10. Release rate curves for release of cisplatin from reloaded MIPs and NIPs of poly(HEMA-cl-MAAc) hydrogels in distilled water at 37 °C.
3.4.2. Release Dynamics of the Drug. The release profile of the drug from per gram of the MIPs and NIPs is presented in Figures 810. It has been observed from the figures that the amount of drug released from the MIP-3 was higher as compared to that of the NIPs-3. Drug from the MIPs has been released in a controlled 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 h from the MIPs-3, MIPs-5, NIPs-3, and NIPs-5, respectively (Figure 9). Higher release of drug from the MIPs may be due to the higher swelling and higher loading of the drug in MIPs. The higher loading is due to the reason that the number of binding sites (i.e., template sites) was higher in the MIPs initially loaded with cisplatin. Initially, there were no template sites available in the NIPs and hence, these polymers showed the lower entrapment of the drug per gram of the gel. The release of drug was more in the case of MIPs prepared with lower amount of cross-linker. Here it is pertinent to mention that the release of
water-soluble drug, entrapped in a hydrogel, occur only after water penetrates the network to swell the polymer and dissolve the drug, followed by diffusion along the aqueous pathways to the surface of the device. The release of drug is closely related to the swelling characteristics of the hydrogels, which, in turn, is a key function of chemical architecture of the hydrogels. The polymers prepared with higher concentration will form the networks of higher cross-linking density which exerts its effect on swelling and drug release. The release of drug occurred through nonFickian diffusion mechanism. In non-Fickian diffusion mechanism, both diffusion of the drug molecules from the polymers and relaxation times of polymer chains are comparable. The values of diffusion coefficient for the release of drug from these polymers are presented in Table 2. It has been observed from the table that the values obtained for the diffusion coefficients in the earlier stages were higher than those in the later stages. This may be due to the concentration gradient and swelling of polymer matrix. 13747
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Table 3. Release Kinetic Parameters of MIPs and NIPs of poly(HEMA-cl-MAAc) Hydrogels maximum amount
constant of the
initial release correlation
Table 4. Volume Fraction of Hydrogels at Different Temperatures and [NN-MBA] temperature
[NN-MBA]
(K)
102 mol/L
wo (g)
w∞ (g)
fraction (ϕ)
of drug released kinetic of drug release rate ro 102 coefficient
volume
sample
Cmax (mg L‑1)
krel 108 (s‑n)
(mg L‑1s‑1)
(R)
MIP-3
1382.313
162.086
309.713
0.93949
300.15 310.15
3.89 3.89
1.246 1.203
3.583 3.709
0.30834 0.28573
MIP-5
1153.685
207.559
276.258
0.98107
320.15
3.89
1.308
4.206
0.27253
NIP-3
763.359
944.261
550.237
0.99465
300.15
6.49
1.108
3.144
0.32604
NIP-5
564.972
1617.314
516.236
0.99578
310.15
6.49
1.168
3.414
0.31545
320.15
6.49
1.214
3.684
0.30255
From the values of diffusion coefficients it is clear that the rate of release of drug in both the cases (MIP and NIPs) was higher in the earlier stages than the latter stages. But, the rate of release of drug 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 was used and the plots of t/Ct versus t (min) are presented in Figure 10. t ¼ α þ βt Ct
ð6Þ
Here, Ct is the amount of drug released at time t, β = 1/Cmax is the inverse 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 the constant of the kinetic of drug release.41 The kinetic parameters calculated from the slope and intercept of straight lines in Figure 10 presented in Table 3. It is clear from this table that MIP-3 has released maximum amount of drug and NIP-3 has released drug at highest initial release rate as compare to other hydrogels. NIP-5 shows 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 the polymer matrix is a function of the extent of cross-linking which defines the three-dimensional network structures of polymer. The network structure is defined by several parameters, i.e., the number of cross-links, their functionality and distribution, network defects (dangling chains and loops), and entanglements.42 The most important parameters used to characterize network structure are the polymer volume fraction in the swollen state (ϕ), molecular weight of the polymer chain between two neighboring cross-links (M c), cross-link ̅ Due to the density (F), and the corresponding mesh size (ξ). random nature of the polymerization process, only average values of M c can be calculated which is a measure of the degree ̅ of cross-linking of the polymer, regardless of the nature (physical or chemical) of cross-linking and has been studied by a number of techniques (the equilibrium swelling theory and the rubber elasticity theory).43 However, one of the most popular techniques to calculate M c is to study the swelling of polymer in a ̅ case we have taken the 24 h swelling of solvent.44 In the present cylindrical hydrogels of pHEMA/MAAc prepared with different [NN-MBA], in distilled water at different temperatures. When the polymer is placed in a solvent, it swells until the osmotic forces that help to dissolve the polymer are balanced by the elastic forces due to the stretched segments of the polymer chains. These elastic retractive forces are inversely proportional to M c. Thus, the molar mass between two junction points in the ̅ network polymer becomes rigid and exhibits limited swelling. When M c is large, the network is more elastic and swells rapidly if brought ̅ in contact with a compatible liquid.
To calculate the M c values, the FloryRehner40,45 equation in ̅ following form was used. 1 M c ¼ dP vm, 1 ϕ1=3 lnð1 ϕÞ þ ϕ þ χϕ2 ð7Þ The polymer volume fraction in the swollen state is a measure of the amount of fluid imbibed and retained by the hydrogel. The volume fraction, ϕ, of the polymer in the swollen state was calculated by using eq 5 and vm,1 is the molar volume of the swelling agent (18.1 cm3/mol for water). FloryHuggins interaction parameter, χ, given by ZΔW1,2/RT (where Z is lattice coordinaion number, ΔW1,2 is interaction energy per mole), is the energy change (in units of RT) that occurs when a mole of solvent molecules is removed from the pure solvent (where ϕ = 0) and is immersed in an infinite amount of pure polymer (where ϕ = 1). Because of the approximate nature of the lattice theory, χ is found to depend on the concentration of the solution and decreases with an increase in the polymersolvent interaction.46 That means the higher is the value of χ1, the weaker is the interaction between polymer and water, and the stronger is the interaction between hydrophobic groups.47,48 According to its definition, χ depends inversely on the temperature. χ is generally positive, with values at 25 °C and at infinite dilution being near 0.5. Positive value of χ is means that the dissolution of a polymeric solute in a solvent is generally an endothermic process (as ΔHmix = RTχn1ϕ, where n1 is number of molecules of solvent).43 FloryHuggins interaction parameter (χ) can be calculated experimentally from the temperature coefficient40,45 of volume fraction (dϕ/dT). Thus, from FloryRehner model we get " 1 #1 h i dϕ χ ¼ ϕð1 ϕÞ1 þ Nlnð1 ϕÞ þ Nϕ 2ϕ ϕ2 N ϕ2 T 1 dT
ð8Þ 1
where N = ((ϕ )/(3) (2)/(3))(ϕ (2)/(3)ϕ) , and dϕ/dT is the slope obtained by plotting the volume fraction data versus temperature (K). For this purpose we have taken 24 h swelling of polymers prepared with different [NN-MBA], at 300.15, 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-link density.49 To further analyze the swelling behavior of these hydrogels in aqueous medium, the cross-link density, F, was calculated from eq 9.50,51 2/3
F¼
dp 1 ¼ Mc νðM c Þ
1/3
ð9Þ
Here, v = 1/dp is the specific volume of polymer. Cross-link density is influenced by ratio of cross-linker, functionality of 13748
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Table 5. Network Parameters of poly(HEMA-cl-MAAc) Hydrogels [NN-MBA]
density dp
temp T (K)
102 mol/L
(g/cm3)
volume fraction (ϕ)
dϕ/dT
N
χ
300.15
3.89
1.192
0.30831
0.00179
1.09472
310.15
3.89
1.192
0.28573
0.00179
1.11513
320.15
3.89
1.192
0.27253
0.00179
300.15
6.49
1.121
0.32604
310.15
6.49
1.121
320.15
6.49
1.121
mesh size, F 104 (mol/cm3)
ξ (Å)
0.57014
Mc (g/mol) ̅ 2383.406
5.00125
49.397
0.56405
3017.093
3.95082
57.004
1.12836
0.56185
3567.111
3.34164
62.968
0.00117
1.08038
0.52152
1066.036
10.51559
32.426
0.31545
0.00117
1.08879
0.52295
1201.050
9.33350
34.800
0.30255
0.00117
1.09970
0.52341
1381.455
8.11463
37.845
cross-linker, radiation time for cross-linking, and molecular weight of polymer chain segments. A higher cross-link density leads to a higher retractive force of the swollen network and thus to a smaller degree of swelling. Mesh size, ξ, defines the space between macromolecular chains in a cross-linked network and characterized by the correlation length between two adjacent cross-links. Mesh size is an important factor for determining mechanical strength, degradability, and diffusivity of the releasing molecule. Most hydrogels used in biomedical applications have mesh sizes ranging from 5 to 100 nm in their swollen state. The linear mesh size of polymer networks was calculated by using eq 10 as discussed by Gudeman and Peppas.52 1=2 2M ̅ c 1=3 ξ¼ϕ l Cn ð10Þ Mr Here, Cn is the characteristic ratio determined as a weighted average 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 the average repeating unit calculated as a weighted average between the values of 130.14 (for HEMA) and 86.09 (for MAAc), and l is the carboncarbon bond length (1.54 Å). The values of different network parameters are presented in Table 5. 3.5.1. Effect of Temperature on Network Structure. Temperature of the swelling medium has exerted affect on the swelling of polymeric networks,55 which in turn affect the network parameters of polymer. To study the effect of temperature on different network parameters, swelling studies of hydrogels in distilled water at different temperatures (300.15, 310.15, and 320.15 K) was carried out. From the swelling studies of hydrogels, it is observed that volume fraction of polymer in swollen state, ϕ, decreased with increase in temperature of swelling medium (Table 4). With increase in temperature of swelling medium, the rate of diffusion of solvent molecules into the polymeric networks increases, which results in increase in volume fraction of solvent and hence decrease in volume fraction of polymer in swollen gel. The M c values of polymer increase with increase in temperature due to̅ more swelling of hydrogels at high temperature, which increased the chain length between two cross-links and hence increased the M c value. Kulkarni and coworkers45 have observed increase in M c̅ values of sodium alginate ̅ the same reason crossbeads with increasing temperature. For link density (F) has decreased as we increase the temperature of swelling medium. The mesh size, ξ (Å), of polymer networks increased from 32.426 to 37.845 Å (prepared with 6.49 102 mol/L of NN-MBA) and from 49.397 to 62.968 Å (prepared with 3.89 102 mol/L of NN-MBA) as the temperature of swelling 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 in less swelling of hydrogels and small release of loaded drug occurred . 3.5.2. Effect of Cross-Linker on Network Structure. Crosslinkers are molecules with at least two reactive functional groups that allow the formation of bridges between polymeric chains. The concentration and functionality of cross-linker has affected the network structure of hydrogels.56 The effect of cross-linker on network structure was studied by preparing hydrogels with 3.89 and 6.49 102 mol/L of NN-MBA and taking 24 h swelling in distilled water at 310.15 K. The values of ϕ and F were increased, whereas the values of M c and ξ were decreased with ̅ Similar trends have been increase in cross-linker concentration. observed for each parameter at 300.15 and 320.15 K for both NN-MBA variations (Table 5). Li and co-workers50 have reported decrease in M c values 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-linker concentration, there will be increase in number of cross-links between polymeric chains, resulting in decrease in chain length of polymer between two cross-links and hence decrease the M c value and increase cross-link density of polymer. The smaller̅ value of ξ (Å) at high [NN-MBA] shows high extent of crosslinks between polymeric chains which increased the cross-link density of polymer, reduced the mobility of polymer chains and hence 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 is directly linked with the swelling which is affected by the crosslink density and mesh size of polymeric networks. Both the drug loading and release are affected by change in these network parameters.30 In the present study, the drug entrapment is decreased 4.46% in MIPs and 4.61% in NIPs on increasing [NN-MBA] from 3.89 to 6.49 102 mol/L during the polymerization reaction. This may be due to the reason that with increase in [NN-MBA], the cross-link density (F) increased (from 3.95082 to 9.33350 104 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 of hydrogel is less with large mesh size which facilitates the diffusion of drug molecules into the polymeric networks and results in high entrapment of drug as compare to hydrogel prepared with high cross-linker concentration.57 The total drug release from drug reloaded MIPs and drug loaded NIPs is decreased (form 7.91 ( 0.68 to 6.06 ( 0.39 mg/g of gel) and (from 5.05 ( 0.14 to 4.33 ( 0.80 mg/g of gel), respectively, with increase in cross-linker from 3.89 to 6.49 102 mol/L (Figure 9). In both cases the drug release was less at high [NN-MBA] due to higher cross-link density and smaller mesh size. The cisplatin interacts with MIPs formed with HEMA and MAAc (the polymers of clinical importance)11,37,58,59 13749
dx.doi.org/10.1021/ie200758b |Ind. Eng. Chem. Res. 2011, 50, 13742–13751
Industrial & Engineering Chemistry Research through supramolecular interactions. It is reported that cisplatin could be present in hydrogels in free form and in the form of complex. The complex may have one or two chloride ions replaced with COOH groups. However it is also reported that its release from the hydrogels occurs in free native form.60 The release of drug from the drug loaded hydrogels occurred through non-Fickian diffusion mechanism. In this mechanism both diffusion of the drug molecules from the polymers and relaxation times of polymer chains are comparable. The values of the diffusion exponent are supporting the fact that there exist drug polymer interactions.
4. CONCLUSIONS It is concluded from the foregoing discussion that the concentration of the cross-linker during the synthesis of MIPs can play an important role in defining their network structure and their drug loading and release efficiency. Increase in [NN-MBA] results in less swelling and slow release of loaded drug from both MIPs and NIPs of poly(HEMA-cl-MAAc) hydrogels. MIPs show more drug entrapment as compared to corresponding NIPs. Network parameters of the hydrogels are affected by the [NNMBA] which can further affect the swelling kinetics and release dynamics of the drug from the hydrogels. The values of ϕ and F were increased whereas the values of M c and ξ were decreased with increase in cross-linker concentration. In both MIPs and NIPs cases the drug release was less at high [NN-MBA] due to higher cross-link density and smaller mesh size. Because of the supramolecular interactions, MIP can be used for the development of the biomimic drug delivery devices. However, additional research should be carried out to obtain information about its behavior in vivo environments. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: +(91)1772830944. Fax: +(91)1772633014. E-mail:
[email protected].
’ REFERENCES (1) Mosbach, K.; Ramstrom, O. The emerging technique of molecular imprinting and its future impact on biotechnology. Biotechnology 1996, 14, 163. (2) Wulff, G. Molecular imprinting in cross-linked materials with the aid of molecular templates. A way towards artificial antibodies. Angew. Chem., Int. Ed. Engl. 1995, 34, 1812. (3) Shea, K. J. Molecular imprinting of synthetic network polymers: The de novo synthesis of macromolecular binding and catalytic sites. Trends. Polym. Sci. 1994, 2, 166. (4) Alwarez-Lorenzo, C.; Concheiro, A. Molecularly imprinted polymers for drug delivery. J. Chromatogr. 2004, B 804, 231. (5) Hilt, J. Z.; Byrne, M. E. Configurational biomimesis in drug delivery: Molecular imprinting of biologically significant molecules. Adv. Drug. Deliv. Rev. 2004, 56, 1599. (6) Ye, L.; Cormack, P. A. G.; Mosbach, K. Molecular imprinting on microgel spheres. Anal. Chim. Acta 2001, 435, 187. (7) Allender, C. J.; Richardson, C.; Woodhouse, B.; Heard, C. M.; Brain, K. R. Pharmaceutical applications for molecularly imprinted polymers. Int. J. Pharm. 2000, 195, 39. (8) Cai, W.; Gupta, R. B. Molecular-imprinted polymers selective for tetracycline binding. Sep. Purif. Technol. 2004, 35, 215. (9) Quick, D. J.; Macdonald, K. K.; Anseth, K. S. Delivering DNA from photocrosslinked, surface eroding polyanhydrides. J. Contr. Rel 2004, 97, 333.
ARTICLE
(10) Liu, P.; Liu, L.; Zhang, L.; Jiang, N.; Liu, Z.; Wang, Y. Synthesis and characterization of molecularly imprinted polymers for recognition of ciprofloxacin. Front. Chem. China 2008, 3, 378. (11) Hiratani, H.; Lorenzo, C. A. The nature of backbone monomers determines the performance of imprinted soft contact lenses as timolol drug delivery systems. Biomaterials 2004, 25, 1105. (12) Alvarez-Lorenzo, C.; Yanez, F.; Barreiro-Iglesia, R.; Concheiro, A. Imprinted soft contact lenses as norfloxacin delivery systems. J. Contr. Rel. 2006, 113, 236. (13) Norell, M. C.; Andersson, H. S.; Nicholls, I. A. Theophylline molecularly imprinted polymer dissociation kinetics: A novel sustained release drug dosage mechanism. J. Mol. Recog. 1998, 11, 98. (14) Vlatakis, G.; Andersson, L. I.; Muller, R.; Mosbach, K. Drug assay using antibody mimics made by molecular imprinting. Nature 1993, 361, 645. (15) Puoci, F.; Iemma, E.; Muzzalupo, R.; Spizzirri, U. G.; Trombino, S.; Cassano, R.; Picci, N. Spherical molecularly imprinted polymers (SMIPs) via a novel precipitation polymerization in the controlled delivery of sulfasalazine. Macromol. Biosci. 2004, 4, 22. (16) Puoci, F.; Iemma, F.; Cirillo, G.; Picci, N.; Matricardi, P.; Alhaique, F. Molecularly imprinted polymers for 5-fluorouracil release in biological fluids. Molecules 2007, 12, 805. (17) Curcio, M.; Parisi, O. I.; Cirillo, G.; Spizzirri, U. G.; Puoci, F.; Iemma, F.; Picci, N. Selective recognition of methotrexate by molecularly imprinted polymers. e-Polym. 2009, 78, 1. (18) Gielen, M., Tiekink, E. R. T, Eds. Metallotherapeutic Drugs & Metalbased Diagnostic Agents: The Use of Metals in Medicine; John Wiley & Sons Ltd.: England, 2005. (19) Giese, B.; McNaughton, D. Interaction of anticancer drug cisplatin with guanine: Density functional theory and surface enhanced raman spectroscopy study. Biopolymers 2003, 72, 472. (20) Reedijk, J. New clues for platinum anti tumor chemistry: Kinetically controlled metal binding to DNA. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 3611. (21) Ekborn, A.; Laurell, G.; Johnstrom, P.; Wallin, I.; Eksborg, S.; Ehrsson, H. d-Methionine and cisplatin ototoxicity in the guinea pig: d-Methionine influences cisplatin pharmacokinetics. Hear. Res. 2002, 165, 53. (22) Decatris, M. P.; Sundar, S.; O’Byrne, K. J. Platinum-based chemotherapy in metastatic breast cancer: Current status. Cancer Treat. Rev. 2004, 30, 53. (23) Schechter, B.; Arnon, R.; Wilchek, M. Polymers in drugdelivery-immunotargeting of carrier-supported cisplatinum complexes. React. Polym. 1995, 25, 167. (24) Iga, K.; Hamaguchi, N.; Igari, Y.; Ogawa, Y.; Toguchi, H.; Shimamoto, T. Increased tumor cisplatin levels in heated tumors in mice after administration of thermosensitive, large unilamellar vesicles encapsulating cisplatin. J. Pharm. Sci. 1991, 80, 522. (25) Yapp, D. T. T.; Lloyd, D. K.; Zhu, J.; Lehnert, S. M. Cisplatin delivery by biodegradable polymer implant is superior to systemic delivery by osmotic pump or IP injection in tumor bearing mice. AntiCancer Drugs 1998, 9, 791. (26) Park, H.; Robinson, J. R. Mechanisms of mucoadhesion of poly(acrylic acid) hydrogels. Pharm. Res. 1987, 4, 457. (27) Fournier, E.; Passirani, C.; Montero-Menei, C. N.; Benoit, J. P. Biocompatibility of implantable synthetic polymeric drug carriers: Focus on brain biocompatibility. Biomaterials 2003, 24, 3311. (28) Singh, B.; Chauhan, G. S.; Sharma, D. K.; Kant, A.; Gupta, I.; Chauhan, N. The release dynamics of model drugs from the psyllium and N-hydroxymethylacrylamide based hydrogels. Int. J. Pharm. 2006, 325, 15. (29) Singh, B.; Chauhan, G. S.; Sharma, D. K.; Chauhan, N. The release dynamics of salicylic acid and tetracycline hydrochloride from the psyllium and polyacrylamide based hydrogels (II). Carbohydr. Polym. 2007, 6, 559. (30) Ende, M. T.; Hariharan, D.; Peppas, N. A. Factors influencing drug and protein transport and release from ionic hydrogels. React. Polym. 1995, 25, 127. 13750
dx.doi.org/10.1021/ie200758b |Ind. Eng. Chem. Res. 2011, 50, 13742–13751
Industrial & Engineering Chemistry Research (31) Ende, M. T.; Peppas, N. A. Transport of ionizable drugs and proteins in crosslinked poly(acrylic acid) and poly(acrylic acid-co-2hydroxyethyl methacrylate) hydrogels. II. Diffusion and release studies. J. Contr. Rel. 1997, 48, 47. (32) Varshosaz, J.; Koopaie, N. Cross-linked poly(vinyl alcohol) hydrogel: Study of swelling and drug release behaviour. Iran. Polym. J. 2002, 11, 123. (33) Kumar, A.; Pandey, M.; Koshy, M. K.; Saraf, S. A. Synthesis of fast swelling superporous hydrogel: Effect of concentration of crosslinker and acdisol on swelling ratio and mechanical strength. Int. J. Drug Deliv. 2010, 2, 135. (34) Hart, B. R.; Shea, K. J. Molecular imprinting for the recognition of N-terminal histidine peptides in aqueous solution. Macromolecules 2002, 35, 6192. (35) Singh, B. Psyllium as therapeutic and drug delivery agent. Int. J. Pharm. 2007, 334, 1. (36) Peppas, N. A.; Gurny, R.; Doelker, E.; Buri, P. Modelling of drug diffusion through swellable systems. J. Membr. Sci. 1980, 7, 241. (37) Peppas, N. A.; Korsmeyer, R. W. Dynamically swelling hydrogels in controlled release applications. In Hydrogels in Medicines and Pharmacy, Volume III. Properties and Applications; Peppas, N. A., Ed.; CRC Press Inc: Boca Raton, FL, 1987; pp 118121. (38) Ritger, P. L.; Peppas, N. A. A simple equation for description of solute release I. Fickian and non-Fickian release from non-swellable devices in the form of slabs, spheres, cylinders or discs. J. Contr. Rel. 1987, 5, 23. (39) Ritger, P. L.; Peppas, N. A. A simple equation for description of solute release I. Fickian and non-Fickian release from swellable devices. J. Contr. Rel. 1987, 5, 37. (40) Aithal, U. S.; Aminabhavi, T. M.; Cassidy, P. E. Interactions of organic halides with a polyurethane elastomer. J. Membr. Sci. 1990, 50, 225. (41) Ekici, S.; Saraydin, D. Synthesis, characterization and evaluation of IPN hydrogels for antibiotic release. Drug Del. 2004, 11, 381. (42) Valentin, J. L.; Carretero-Gonzalez, J.; Mora-Barrantes, I.; Chasse, W.; Saalwachter, K. Uncertainties in the Determination of Cross-Link Density by Equilibrium Swelling Experiments in Natural Rubber. Macromolecules 2008, 41, 4717. (43) Flory, P. J., Ed.; Principles of Polymer Chemistry; Cornell University: Ithaca, NY, 1953. (44) Lira, L. M.; Martins, K. A.; Cordoba de Torresi, S. I. Structural parameters of polyacrylamide hydrogels obtained by the Equilibrium Swelling Theory. Eur. Polym. J. 2009, 45, 1232. (45) Kulkarni, A. R.; Soppimath, K. S.; Aminabhavi, T. M.; Dave, A. M.; Mehta, M. H. Glutaraldehyde crosslinked sodium alginate beads containing liquid pesticide for soil application. J. Contr. Rel. 2000, 63, 97. (46) Gliko-Kabir, I.; Yagen, B.; Penhasi, A.; Rubinstein, A. Low swelling, crosslinked guar and its potential use as colon-specific drug carrier. Pharm. Res. 1998, 15, 1019. (47) Zhihui, L.; Wenhui, W.; Jianquan, W.; Xin, J. Swelling behaviors, tensile properties and thermodynamic interactions in APS/HEMA copolymeric hydrogels. Front. Mater. Sci. China 2007, 1, 427. (48) Lin, Z.; Wu, W.; Wang, J.; Jin, X. Studies on swelling behaviors, mechanical properties, network parameters and thermodynamic interaction of water sorption of 2-hydroxyethyl methacrylate/novolac epoxy vinyl ester resin copolymeric hydrogels. React. Funct. Polym. 2007, 67, 789. (49) Mantovani, F.; Grassi, M.; Colombo, I.; Lapasin, R. A combination of vapor sorption and dynamic laser light scattering methods for the determination of the Flory parameter χ and the crosslink density of a powdered polymeric gel. Fluid Phase Equilib. 2000, 167, 63. (50) Li, X.; Wu, W.; Wang, J.; Duan, Y. The swelling behavior and network parameters of guar gum/poly(acrylic acid) semi-interpenetrating polymer network hydrogels. Carbohydr. Polym. 2006, 66, 473–479. (51) Jhaveri, S. J.; Hynd, M. R.; Dowell-Mesfin, N.; Turner, J. N.; Shain, W.; Ober, C. K. Release of nerve growth factor from HEMA hydrogel-coated substrates and its effect on the differentiation of neural cells. Biomacromolecules 2009, 10, 174.
ARTICLE
(52) Gudeman, L. F.; Peppas, N. A. pH-Sensitive membranes from poly(vinyl alcohol)/poly(acrylic acid) interpenetrating networks. J. Membr. Sci. 1995, 107, 239. (53) Ankareddi, I.; Brazel, C. S. Synthesis and characterization of grafted thermosensitive hydrogels for heating activated controlled release. Int. J. Pharm. 2007, 336, 241. (54) Podual, K.; Doyle, F. J.; Peppas, N. A. Preparation and dynamic response of cationic copolymer hydrogels containing glucose oxidase. Polymer 2000, 41, 397. (55) Klech, C. M.; Pari, J. H. Temperature dependence of nonfickian water transport and swelling in glassy gelatin matrices. Pharm. Res. 1989, 6, 564. (56) Atta, A. M.; Abdel-Azim, A. A. A. Effect of crosslinker functionality on swelling and network parameters of co-polymeric hydrogels. Polym. Adv. Technol. 1998, 9, 340. (57) Canal, T.; Peppas, N. A. Correlation between mesh size and equilibrium degree of swelling of polymeric networks. J. Biomed. Mater. Res. 1989, 23, 1183. (58) Jones, D. S.; Bruschi, M. L.; de Freitas, O.; Gremi~ao, M. P. D.; Lara, E. H. G.; Andrews, G. P. Rheological, mechanical and mucoadhesive properties of thermoresponsive, bioadhesive binary mixtures composed of poloxamer 407 and Carbopol 974P designed as platforms for implantable drug delivery systems for use in the oral cavity. Int. J. Pharm. 2009, 372, 49. (59) Wichterle, O. In Soft Contact Lenses; Ruben, M., Ed.; Wiley: New York, 1978; pp 35. (60) Neuse, E. W. Macromolecular metal-compounds in cancer researchConcepts and synthetic approaches. Macromol. Symp. 1994, 80, 111.
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