Marin Simeonova,
Bistra Kostovab and
Elena Vassileva*a
aLaboratory on Structure and Properties of Polymers, Faculty of Chemistry and Pharmacy, University of Sofia, 1, J. Bourchier blvd., 1164 Sofia, Bulgaria. E-mail: evassileva@chem.uni-sofia.bg
bDepartment of Pharmaceutical Technology and Biopharmaceutics, Faculty of Pharmacy, Medical University of Sofia, 2, Dunav Str., 1000, Sofia, Bulgaria
First published on 30th June 2016
Novel interpenetrating polymer networks (IPN) of poly(methacrylic acid) (PMAA) and polyacrylamide (PAAm) were synthesized and characterized in terms of their swelling ability, microhardness and morphology. The potential of these new polymeric materials as a sustained delivery system for cationic drug was revealed. The study demonstrates that the IPN's composition is a powerful tool to control the IPN's structure and properties and hence their performance as a new polymeric system for sustained drug delivery.
Along with the above listed advantages, hydrogels suffer from several drawbacks regarding their drug delivery applications, namely: (i) they are brittle, which is a problem when used as drug eluting implants; (ii) they are not the best delivery system for hydrophobic drugs because the amount of the loaded drug and its homogeneous distribution are limited; (iii) the high water content and large pore sizes of hydrogels often result in rapid drug release.
These drawbacks could be overcome by utilizing the concept of interpenetrating polymer networks (IPN).2 This is one of the approaches usually taken to improve polymer hydrogels' mechanical strength and stability. IPN possess entangled structure, which could be easily tuned, preventing in this way the fast drug release and exerting control over it. IPN could be produced either as bulk materials or as micro and nanogels depending on the envisaged application. Thus, they provide a versatile platform for drug delivery applications.
Recently we have demonstrated the potential of IPN based on poly(acrylic acid) (PAA) and polyacrylamide (PAAm) as sustained drug delivery system for a cationic drug (verapamil hydrochloride, VPM).3 We have outlined two main factors governing the drug delivery behavior of this system: (i) the IPN's functionality as well as (ii) IPN's network density. The acidic functionality of the PAA component was expected to control the interaction with the cationic drug (VPM), while the IPN's specific structure (phase separation at nano level) influenced the diffusion of the drug within the polymer network. Both factors were controlled simply through varying the IPN's composition.
Poly(methacrylic acid) (PMAA) has very similar structure however different properties as compared to PAA. The extra methyl group in PMAA results into a pH induced conformational transition in aqueous solutions at relatively low degrees of ionization.4 At low pH, PMAA chains are shrunk into compact coils and highly compact clusters from the PMAA chains, connected between themselves5 are formed. These hydrophobic clusters are formed via strong attractive hydrophobic forces between the PMAA backbone methyl groups (i.e. short-range interactions), which take place at low pH in addition to the hydrogen bonding between COOH groups.6 Upon a pH increase, the PMAA chains transform into expanded random coils thus resembling the well-known globule-to-coil transition in proteins. When pH increases above the pKa of PMAA (∼5.5), the repulsions between carboxylate anions start to dominate and the hydrophobic attractions are completely broken down, i.e. the chains expand to a water-swollen structure9 in an abrupt mode. PAA and PMAA also differ in their acid strength, the pKa of PMAA being higher by 1 pH unit from the pKa of PAA, i.e. PAA is 10 times stronger acid as compared to PMAA. Moreover, PMAA is known to be more biocompatible as compared to PAA.
All these peculiar properties of PMAA as compared to PAA provoked our interest towards developing IPN of PMAA and PAAm, a novel material which combines acidic functionality (imparted by PMAA) with strong propensity for hydrogen bonds formation imparted by both components, PMAA and PAAm. In this way the newly developed polymer material was designed to be an ideal candidate for the sustained drug delivery of a cationic drug. As model cationic drug we choose VPM. VPM is applied for treatment of angina pectoris and mild to moderate systemic hypertension.7 When used for antihypertensive therapy it must be taken several times daily. Thus due to its pharmacokinetics and physicochemical properties, it is a good candidate for developing controlled release formulations.
The aim of the present study was to synthesize and study novel IPN from PMAA and PAAm and to reveal the potential of these new materials as drug delivery system for a cationic drug, VPM. The effect of the substitution of PAA with PMAA in the IPN was evaluated in terms of IPN–VPM interaction as well as its influence on the IPN's sustained drug delivery performance.
The chemical formula of both monomers as well as of the drug, VPM, are presented in Fig. S1.†
The 2nd PAAm network was in situ synthesized into the 1st SN PMAA. To this purpose seven dry SNs PMAA were transferred into five aqueous solutions with different AAm concentrations (0.5 M, 1 M, 2 M, 3 M, 4 M, 5 M, 6 M). The solutions contained also 0.1 mol% PPS and 0.1 mol% MBAA (both % relatively to AAm). The SNs PMAA were left to swell for 48 h until constant weight was reached. The in situ crosslinking polymerization of AAm in the SN PMAA took place at 60 °C for 6 hours. In this way seven IPN with various PMAA/PAAm ratios were obtained (Table 1). Each of the newly synthesized IPN PMAA/PAAm was washed with distilled water to completely remove traces from non-reacted chemicals (the wastewaters were checked by UV). The non-reacted AAm quantity was determined in order to obtain the exact IPN' composition. To this purpose the UV method was applied as described elsewhere.8 The AAm conversion to PAAm was determined to be 99% ± 1%.
Sample | PMAA | IPMA1 | IPMA2 | IPMA3 | IPMA4 | IPMA5 | IPMA6 | IPMA7 | PAAm |
φPMAA | 1 | 0.86 | 0.71 | 0.52 | 0.39 | 0.30 | 0.26 | 0.20 | 0 |
The weight fraction of PAA in the final IPN was determined using the equation:
φPMAA = mPMAA/(mPMAA + mPAAm) | (1) |
For sake of comparison one SN PMAA and one SN PAAm were obtained. All SNs were used as referent samples (Table 1).
The VPM entrapment efficiency (EE) was calculated using the formula:
EE [%] = (moVPM − munloaded VPM) × 100/moVPM | (2) |
ESR = (mswollen − mdry)/mdry | (3) |
HV = 1.854 × P/d2 | (4) |
When comparing the ESRs of the IPN only, a trend of an ESR decrease is clearly seen as the PMAA content increases, i.e. the φPMAA increase results into an ESR decrease. IPMA7, the IPN with the lowest PMAA content (φPMAA = 0.2), shows the highest ESR value. This observation is exactly the opposite when compared to the IPN of PAA and PAAm studied previously.3 There, the ESR increased with increasing the polyacid (PAA) content in IPN. The observed difference in the ESR dependence on the IPN composition when comparing PMAA/PAAm and PAA/PAAm IPN could be due to the following factors:
• PMAA is more hydrophobic and shows lower ability to swell in water as compared to the neat PAA especially at low and medium pH, i.e. at low dissociation degree (water is bad solvent for PMAA in non-dissociated state9). When comparing both acidic SNs, PMAA and PAA, obtained at the same monomer, initiator and crosslinking agent concentrations, temperatures, polymerization time, etc., the ESR of the neat PAA is ∼7,3 while for the neat PMAA it is ∼5 (Fig. 1).
• PMAA chains form highly compact hydrophobic clusters connected between themselves by short extended parts of the PMAA chains at low pH.5 The clusters are formed via strong attractive hydrophobic forces between the PMAA backbone methyl groups (i.e. short-range interactions) and they additionally reduce the swelling ability of PMAA regions in the PMAA/PAAm IPN.
• The ESR dependence on IPN's composition is influenced also by the interaction between both IPN's components. The hydrogen bonds formation in the case of PMAA is possible in wider pH range, including neutral pH, as compared to the PAA because PAA has pKa ∼ 4.5 while for PMAA pKa is around 5.5.10 That means that below pH = 5.5, PMAA could form H bonds as it was reported for e.g. PMAA interaction with poly(N-isopropylacrylamide) (PNIPAAm). There hydrogen bonds were shown to be formed between the carboxylic groups of PMAA and the amide groups of PNIPAAm.10 In a similar manner, the PMAA chains could form hydrogen bonds with PAAm along with the hydrogen bonds between themselves.
All these factors working together result into the observed ESR decrease with PMAA's content increase in the IPN of PMAA/PAAm. As all these factors are governed by the IPN's composition, it could be summarized that the PMAA/PAAm ratio is a powerful tool to control the PMAA/PAAm IPN's network density and hence their ability to swell. Translated into the expected drug delivery performance of these novel IPN, the varying of their composition would be a tool to control the drug diffusion within the networks during drug loading as well as the drug diffusion out of them during the drug release.
The EE increases from 10% for IPMA1 (the IPN with the highest PMAA content) to 55% for IPMA6 and IPMA7 which have the lowest PMAA content but highest ESR among the IPN samples. The EE decreases almost linearly as PMAA content increases (regression coefficient = 0.98) which confirms that the IPN's composition defines the VPM's EE by controlling the IPN's total network density along with the enhancement of the PMAA–VPM ionic interaction.
Fig. 3 Vicker's microhardness dependence on IPN PMAA/PAAm composition (error bars are within the size of the graph points). |
In addition, SN PMAA has lower MH value as compared to the SN PAAm. Thus, the increase of the former's content and its prevailing in the IPN would decrease the IPN's MH values.
In summary, ESR dependence on IPNs' composition is a result from the interplay between three factors: (i) the hydrophilic/hydrophobic balance in the IPN varied through the PAAm/PMAA ratio; (ii) the interaction between both networks, consisting in mutual penetration and interlacing as well as (iii) the hydrogen bonds formation between the IPN components.
The same factors define the MH dependence on IPN's composition however they do not act all in one direction as it is the case with ESR. Thus, the MH dependence on IPN's composition in the case when φPMAA > 0.52 (Fig. 3) deviates from the expected dependence due to the prevailing of the softer network (PMAA).
Each one from the studied IPN samples exhibits a phase separated structure consisting in small domains from the PAAm (the 2nd network) uniformly dispersed within the PMAA matrix (the 1st network). PAAm network is the loose one (40 times lower crosslinking agent concentration was used for its preparation as compared to PMAA) and it looks brighter in the SEM images while PMAA component has significantly denser network and thus it looks darker (Fig. 4).
Increasing the 2nd to the 1st network ratio (i.e. the PAAm content), results into two types of IPN PMAA/PAAm morphology:
• For φPMAA > 0.52 the IPN's morphology is characterized by the presence of small PAAm (the 2nd network) domains evenly distributed within the PMAA matrix (the 1st network). The size of these domains increases as the PAAm's content increases ranging from below 100 nm for IPMA1 (φPMAA = 0.86), reaching 200 nm in IPMA5 (φPMAA = 0.30) (Fig. 4). The phase separation is impeded by the high density of the chemical cross-linking of the 1st network PMAA, which keeps the 2nd network (PAAm) domains size comparatively small.
• For φPMAA < 0.52 an inversion of the phases occurs which is initiated by the touching between the PAAm domains at φPMAA ∼ 0.52 and results into the formation of a network from PAAm's interconnected cylinders at lower φPMAA values, i.e. higher PAAm content.15 The critical point is IPMA3 (φPMAA = 0.52), i.e. PAAm/PMAA ∼ 1/1 weight ratio, where also the ESR and MH dependences on IPN's composition markedly change their behavior.
Thus, the morphology of the IPN is changing as a function of the IPN's composition giving rise and being related to the composition defined change in their properties. At the φPMAA < 0.52, the PAAm network starts to form interconnected cylinders which could be related to the MH increase in the same region as the “harder” component PAAm starts to prevail and to form its own “matrix”. On the contrary, when the PMAA prevails (φPMAA > 0.52), the MH of IPN decreases also due to the IPN's morphology change where within the “softer” PMAA matrix “float” small domains from the “harder” PAAm. For similar systems it is known that the overall MH is mostly determined by the MH of the “softer” component (PMAA).11 Thus, the morphology of the IPN defines and corresponds to the observed MH dependence on IPN's composition.
The IPN morphology, presented in Fig. 4, in fact resembles a nanocomposite-like structure, where nanodomains from PAAm are dispersed into PMAA matrix (φPMAA > 0.52). The phase separation in IPN is controlled by the PAAm to PMAA (2nd to 1st network) ratio.12 This specific IPN's morphology is expected also to change the drug delivery profile, i.e. it appears to be an additional tool to control the drug diffusion in and out of the IPN. This is another parameter that would control the characteristics of the newly developed IPN as delivery vehicle for sustained drug release.
The morphology of PMAA/PAAM IPN (Fig. 4) differs from the morphology, observed for PAA/PAAm IPN.3 There, the 2nd to the 1st network ratio increase resulted only into the growth of the 2nd network domains without the formation of the interconnected cylinders morphology, i.e. without an inversion of the phases to take place. The reason for this difference is the enhanced hydrophobicity of PMAA as compared to PAA.
The VPM loading in IPN PMAA/PAAm resulted into a change in their morphology. The morphology of a broken surface of IPN samples with different composition after VPM loading is shown in Fig. S2.† There, the inclusion of the drug into the polymer matrix is clearly seen. The VPM inclusions are uniformly dispersed within the IPN matrix for all IPN compositions. The IPN's morphology after VPM loading is much coarser and the fine phase separation observed for the neat IPN samples (Fig. 4) is hardly seen because the specific phase separation of IPN PMAA/PAAm is utilized for the in situ VPM deposition within the IPN samples.
Tgs of IPN samples depend on the IPN's composition (Fig. S4†) and as φPMAA increases, the IPN's Tg decreases because PMAA is the component with lower Tg. The lower Tg of SN PMAA corresponds to its lower MH values (Fig. 3) as compared to the Tg and MH values of SN PAAm. The IPN's Tg dependence on composition is well described by the straight line drawn according to the additivity law (Fig. S4†).
VPM is a crystalline solid which melts at 144 °C (Fig. 5). It was interesting to check if the VPM loading resulted into change in the thermal properties of both the polymer vehicle as well as of the loaded drug. Along with the reversing heat flow for neat VPM, in Fig. 5 are presented the same for IPMA4 (φPMAA = 0.39) unloaded and VPM loaded. When VPM is loaded in the IPN, the drug's melting peak disappears due to the VPM amorphization. The drugs' amorphization is a way to improve their solubility and dissolution rate as well as their bioavailability.15 The drug becomes amorphous in the current case due to its interaction with the polymer matrix (IPN) and most probably with the PMAA component. This interaction is proved by the slight increase (∼1 °C) in Tg of IPMA4 after VPM loading (Fig. 5).
The changes in the thermal properties of both PMAA/PAAm IPN and VPM are similar to the results obtained for IPN PAA/PAAm loaded with VPM.3 There, the drug and the polymer vehicle mutually influenced their thermal properties due to the strong interaction between them.
As a result of the interaction, the IPN's Tg increased and the drug melting peak disappeared. The Tg's increase observed for the PMAA/PAAm IPN, however, is not as strong as it was reported for the PAA/PAAm IPN which could be related to the fact that PAA is a stronger acid as compared to PMAA.
One needs to mention here that the wavenumber decrease in the IPN PMAA/PAAm is less strong as compared to the IPN PAA/PAAm.16 The reason could be the more pronounced ionic interaction between COOH and CONH2 groups in the latter as compared to the former due to PAA's stronger acidity (lower pKa) as compared to PMAA.
Thus, the IR study of the neat IPN PMAA/PAAm demonstrates an interaction between the PMAA and PAAm through hydrogen bonds formation between their COOH and CONH2 groups.
In Fig. 6, the IR spectra of the neat VPM and IPMA1 (φPMAA = 0.86), loaded and unloaded with VPM, are presented. In the neat IPMA1 spectrum, the typical for its components PMAA (in blue) and PAAm (in red) IR bands appear.
For IPMA1 loaded with VPM, the bands characteristic for neat VPM could be also seen, but these bands are wider as compared to the neat drug (VPM, Fig. 6) which is an indication for the VPM interaction with the polymer vehicle.10 The characteristic VPM's bands are presented in Table S2.†17,18 The results from the IR study on the IPMA1 loaded with VPM suggest the formation of R3NH+:COO− ion pair19 between the VPM NH2 and COO− from PMAA. This is proved by the disappearing of the VPM's N–H stretching vibration band at ∼2800–2300 cm−1 due to the protonation of the VPM's amine group when loaded into the IPN. The protonation is caused by its interaction with COO− from PMAA. The IR spectra confirm the proposed at the beginning of this work ionic interaction between the polymer vehicle and the cationic drug, which was also indirectly proved by the DSC results (VPM amorphization).
From the same spectra, however, it could be concluded that the polymer–drug interaction takes place also through the formation of hydrogen bonds (Fig. 6). The C–H stretching vibrations of the methoxy groups in VPM, which appear at ∼2841 cm−1 in the neat VPM, are shifted to 2838 cm−1 in IPMA1, loaded with VPM, which is also an indication for drug–polymer interaction. The same interaction is the reason for the slight decrease in the wavenumber of the CN stretching vibrations of the saturated alkyl nitrile in the neat VPM (∼2237 cm−1) which appears at ∼2235 cm−1 in the IPMA1. The changes in the peak shape (Fig. 6) and the bands positions (Table S2†) for the stretching vibrations of the –OCH3 group and –CN group of VPM arise from the hydrogen bonding between VPM and the polymer matrix.
Thus the IR study shows that the IPN PMAA/PAAm interact with the loaded VPM via: (i) ionic interactions between COO− from PMAA and R3NH+ from VPM as well as (ii) via hydrogen bonds between the polymer matrix and the drug. These results confirm the role of the IPN functionality for the polymer vehicle–drug interaction and hence for the sustained drug release profiles that these IPN could ensure.
The VPM release profiles depend on the IPN composition. The IPMA5 shows a good potential for VPM sustained release. This IPN has EE ∼ 50%, almost linear VPM release profile between 2nd and the 8th hours (regression coefficient = 0.984) but releases only ∼50% of the loaded VPM most probably due to the strong ionic interaction with the cationic drug as it was the case with the SN PMAA.
The IPMA1 is the IPN with the less satisfactory performance in terms of VPM's sustained delivery. For this IPN composition, it was expected the strongest interaction polymer vehicle-cationic drug (VPM) as it has the highest PMAA content, however this sample has the lowest EE (∼10%) and shows burst effect although it releases ∼90% of the loaded VPM for 24 h.
The best performing IPN is IPMA7, i.e. the one with the lowest PMAA content. This IPN shows the highest EE (∼55%), no burst effect and nearly zero order release profile (regression coefficient = 0.990) between the 2nd and the 8th hours, i.e. under conditions similar to the ones in the intestines.
In summary, the best sustained drug delivery performance is ensured by the right combination of functionality and swelling ability of PMAA/PAAm IPN. The best illustration is IPMA7 where these two factors are playing together thus creating the best performing sustained drug delivery system for VPM. Thus, the concept for IPN versatile application for sustained drug delivery was successfully proved.
The VPM's release profiles strongly depend on the IPNs' composition, the IPNs with prevailing PAAm showing a potential for VPM's sustained release. The IPMA7 could be considered as the most appropriate for sustained drug delivery because the balance between PMAA and PAAm enables the most precise control over the VPM release. Besides the pure functionality, the swelling ability of the polymer vehicles appears to have a strong impact on both the drug entrapment efficiency as well as on the drug release profiles.
In summary, new IPN were synthesized and characterized and their potential for sustained drug delivery of a cationic drug (VPM) was demonstrated. It was proved that the IPN's composition is a strong factor that controls the properties of the system and hence its behavior as a drug delivery system. These advantageous properties of the IPN PMAA/PAAm could be further utilized for controlled release of other drugs with different physicochemical characteristics and therapeutic requirements.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14067j |
This journal is © The Royal Society of Chemistry 2016 |