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Synthesis and characterization of copolymers of methylmethacrylate and 2-hydroxyethyl methacrylate for the aqueous solubilization of PaclitaxelDavide Silvestri ab; Mariacristina Gagliardi a; Niccoletta Barbani a; Caterina Cristallini c; Paolo Giusti ab

a Department of Chemical Engineering, Industrial Chemistry and Materials Science, University of Pisa, Pisa,Italy b Interdepartmental Centre for the Study and Evaluation of Biomaterials, Endo-prosthesis 'NicolinoMarchetti' (C.I.B.E.), Pisa, Italy c CNR Institute for Composite and Biomedical Materials IMCB, Pisa, Italy

Online Publication Date: 01 February 2009

To cite this Article Silvestri, Davide, Gagliardi, Mariacristina, Barbani, Niccoletta, Cristallini, Caterina and Giusti, Paolo(2009)'Synthesisand characterization of copolymers of methylmethacrylate and 2-hydroxyethyl methacrylate for the aqueous solubilization ofPaclitaxel',Drug Delivery,16:2,116 — 124

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Drug Delivery, 2009; 16(2): 116–124

R e s e a R c h a R t i c l e

Synthesis and characterization of copolymers of methylmethacrylate and 2-hydroxyethyl methacrylate for the aqueous solubilization of Paclitaxel

Davide Silvestri1,2, Mariacristina Gagliardi1, Niccoletta Barbani1, Caterina Cristallini3, and Paolo Giusti1,2

1Department of Chemical Engineering, Industrial Chemistry and Materials Science, University of Pisa, Pisa, Italy, 2Interdepartmental Centre for the Study and Evaluation of Biomaterials and Endo-prosthesis ‘Nicolino Marchetti’ (C.I.B.E.), Pisa, Italy, and 3CNR Institute for Composite and Biomedical Materials IMCB, Pisa, Italy

Address for Correspondence: Dr Mariacristina Gagliardi, Department of Chemical Engineering, Industrial Chemistry and Materials Science, University of Pisa, via Diotisalvi 2, 56126 Pisa, Italy. Tel: +39 050 2217877. Fax: +39 050 2217866. Email: [email protected]

(Received 7 October 2008; accepted 4 December 2008)

ISSN 1071-7544 print/ISSN 1521-0464 online © 2009 Informa UK LtdDOI: 10.1080/10717540802666980

abstractThe aim of the present work is the modification of a hydrophobic polymeric macromolecule, polymethylmethacrylate, by introducing hydrophilic moieties of 2-hydroxyethyl methacrylate within the polymer chain. Synthesis, characterization, and drug delivery control capabilities exerted on a highly hydrophobic drug (Paclitaxel) are illustrated. In particular, the dependency of the drug delivery kinetic on the fraction of hydrophilic units inserted in the copolymer chain was studied. Results showed that it is pos-sible to have an increase of the kinetic delivery introducing hydrophilic units. In addition, a double control, diffusive and due to the relaxation of the molecules, on drug delivery was obtained.

Keywords: Acrylic copolymer; poly[(methyl methacrylate)-co-(2-hydroxyethyl methacrylate)]; drug delivery polymers; controlled drug release.

http://www.informapharmascience.com/drd

Introduction

Commonly, in biomedical applications, polymeric systems are used with the aim of controlling the rate of drug release from solid dosage forms (Strathman, 1996). Controlled drug release systems are widely employed to treat localized disease conditions in specific anatomical sites, or to control adverse local biological response to an implantable medical device (Anderson, 2003; Richey et al., 2000; Wang et al., 2007; Pires et al., 2005; Kohane, 2006), improving the performance of the treatment and reducing side-effects (Ho Wah et al., 1987). Furthermore, this kind of device allows prediction of the delivery rate, maintaining the drug concentration within the thera-peutic range (Swarbrick and Boylan , 1990).

The drug delivery control can be obtained using formulations containing natural, synthetic, or semi-

synthetic polymers (Sivakumar and Panduranga Rao, 2000; Pillai and Panchagnula, 2001; Bures et al., 2001; Manu et al., 2004; Miyata et al., 2004) used as a reser-voir and loaded with one or more drugs. The controlling effect of a polymeric material on drug release depends on its physicochemical properties and system manufac-turing (Tarcha, 2000).

In the last years the use of polymeric matrices for different applications in the drug release systems was significantly investigated (Taylor, 1996; Giavaresi et al., 2004; Chu et al., 2004; Al-Lamee and Cook, 2003) and different controlled-release systems were developed for drug delivery applications to maintain a drug level in the body within a specific therapeutic range and controlled dosage (Schmaljohann, 2006; Nishiyama and Kataoka, 2006).

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Synthesis and characterization of copolymers of methylmethacrylate 117

Drug release kinetics in controlled drug delivery sys-tems are affected by various parameters, like the initial drug loading, the thickness of the reservoir, and the hydrophilicity of the polymeric materials (Langer, 2000). In systems where polymeric matrix is highly hydropho-bic and the active principle is poorly soluble in aqueous environment, drug delivery kinetic can be very slow and the amount of drug released can be under the minimum therapeutic level. For this reason, polymeric systems with greater hydrophilicity are widely studied, with the specific aim to increase the drug elution rate.

The goal of the present work was the synthesis of a copolymer constituted of methylmethacrylate (MMA) and 2-hydrossiethylmethacrylate (HEMA) monomeric units, obtained while varying the molar composition of the mac-romolecule, with the aim of improving the hydrophilicity of the material so that it is able to control the drug deliv-ery performance for the elution a hydrophobic active principle, as Paclitaxel. The choice of the hydrophilic unit depended on chemical properties: HEMA can uptake a great amount of water and its degree of swelling can reach high values. It may also avoid an irreversible entrapment of the drug inside the polymeric matrix that was in some cases observed (Acharya and Park, 2006).

Materials and methods

In the present work the homopolymer poly(methyl- methacrylate) (PMMA) and its copolymers with 2-hydroxyethylmethacrylate (HEMA), poly[(methyl- methacrylate)-co-(2-hydroxyethylmethacrylate)] (P(MMA-co-HEMA)) in different initial MMA and HEMA molar ratios, respectively, 82:18 and 73:27, were synthesized, characterized, and tested in order to evaluate chemical–physical properties and control-led drug delivery capability. Only two formulations were chosen with a low HEMA percentage because

the main goal was to increase the hydrophilicity of PMMA without losing its biostability. For this reason, synthesized copolymers could be merely interpreted as a modification of the PMMA macromolecule.

Monomers used were MMA (Sigma® Aldrich, St. Louis, MO, molecular weight: 100.12 g/mol, specific gravity: 0.94 g/cm3) and HEMA (Sigma® Aldrich, St. Louis, MO, molecular weight: 130.14 g/mol, specific gravity: 1.07 g/cm3). A radical reaction was performed using a water/ethyl alcohol blend as dispersing medium, poly vinyl pyrrolidone (PVP, Sigma®

Aldrich, St. Louis, MO, average molecular weight: 55,000 g/mol) as stabilizer, and 2,2’-azo-bis- isobutyronitrile (AIBN, Fluka, Sigma® Aldrich, St. Louis, MO, molecular weight: 164.21 g/mol, purity ≥ 98%) as initiator.

Copolymers synthesized were used to obtain film-shaped samples after the dissolution of the polymeric pow-der in an appropriate solvent. Polymeric solutions were prepared using as solvents a chloroform (CH

3Cl, Carlo

Erba Reagenti®, Milan, Italy) and N,N-dimethylformamide (DMF, Carlo Erba Reagenti®, Milan, Italy) mixture. Drug delivery tests were carried out on these samples using as active molecule Paclitaxel (PTX, Sigma® Aldrich, St. Louis, MO, molecular weight: 853.9 g/mol, purity ≥ 99%).

Films were obtained by non-solvent induction phase separation (NIPS) technique, using water as coagulant medium.

Materials synthesis

Reactions were conducted in a three-neck round-bottom flask immersed in a thermostatic oil-bath. An Allihn reflux condenser, refrigerated with water at room temperature, was employed to avoid the loss of mono-mers. A Teflon® impeller was used, moved by a Heidolf RZR 2020 engine at 400 rpm. Reaction was carried out under N

2 flux. Reaction temperature was uniformly

maintained at 65°C and controlled by a thermocouple. The time of reaction was 3 hr. In Table 1 the reaction

Table 1. Reaction recipes for polymerizations, reactivity ratios, percentage molecular compositions, and monomer conversions.

Polymerization mixtures

Reaction set-up (co)polymer Nomenclature MMA (mol) HEMA (mol)

Reaction solvent: water 1 ethyl alcohol (60:40 v/v) (ml) PVP (g) AIBN (g)

PMMA 0.046 — 100 4 0.06

P(MMA-co-HEMA) 82:18 0.037 0.008 100 4 0.06

P(MMA-co-HEMA) 73:27 0.033 0.012 100 4 0.06

Reaction results Reactivity ratios and theoretical macromolecular compositions

Copolymer rMMA

rHEMA

rMMA

·rHEMA

FMMA

(%) FHEMA

(%)

P(MMA-co-HEMA) 82:18 0.898 0.635 0.57 81.7 18.3

P(MMA-co-HEMA) 73:27 0.874 0.662 0.58 72.9 27.1

Conversion of monomers

Material xMMA

xHEMA

PMMA 0.990 —

P(MMA-co-HEMA) 82:18 0.996 0.997

P(MMA-co-HEMA) 73:27 0.989 0.998

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118 Davide Silvestri et al.

composition of mixtures for every single polymerization is reported. Synthesis of PHEMA was not performed in the present work.

Kinetic study and monomers conversions

The consumption of monomers during the reaction was evaluated through high performance liquid chromatog-raphy (HPLC) method analyzing samples withdrawn at different times from the reacting mass. HPLC apparatus consisted in a 410 LC pump, an ALLTECH C18 3U column with a non-polar phase (dimensions 100 mm × 4.6 mm), and a UV detector (Perkin-Elmer®, Waltham, MA). Analyses were carried out at room temperature.

Reagents used were specific HPLC grade acetonitrile (ACN, Carlo Erba Reagenti®, Milan, Italy) and double deionized water. The mobile phase was constituted of 30% ACN and 70% water, test time was 10 min, mobile phase internal flux rate was 1 ml/min, injected sample volume was 100 l, and UV wavelength was 210 nm.

Because it was not possible to gather reactivity ratios from literature for the present reaction under used spe-cific conditions, the Kelen-Tüdòs method (Cowie, 1973) was used to evaluate these parameters and also the actual composition and the structure of the macromol-ecules at the end of the reaction.

Polymeric powder obtained was dissolved in chlo-roform (solvent of PMMA) and after in DMF (solvent of PHEMA), and the solution was filtered in order to recover the undissolved copolymer: this operation was carried out to extract from the synthesized material PMMA or PHEMA oligomers or unreacted monomers. After purification, polymeric powders were dried in a vented stove at 40°C for 10 hr.

Preparation of film samples

Polymeric solutions (10% in weight of polymer on sol-vent volume) were obtained dissolving the powder in a 64.5/35.5 (v/v) chloroform/DMF blend (Domíguez, 1993); 50 l of polymeric solutions were shed onto bare AISI 316L and carbofilm-coated supports, and deposi-tion samples were immersed in a water coagulation bath obtaining a solid layer of polymer deposited on the solid substrate and to obtain a total extraction of DMF solvent. Samples used for drug delivery tests were obtained by introducing into the polymeric solution a fraction of Paclitaxel (2% in weight of drug with respect of the sol-vent volume). Active principle was added and dissolved because the CH

3Cl/DMF mixture is a good solvent also

for Paclitaxel.After the immersion in the coagulation bath, drug-

loaded samples were weighed, evaluating the effective amount of drug present in the sample at the beginning of the drug delivery tests.

Drug delivery tests

Drug delivery tests were carried out in vitro by immers-ing the samples in 2 ml of a pH = 7.4 phosphate buffer solution (PBS) containing 0.05% w/v of sodium dodecyl sulfate (SDS, Sigma®, molecular weight: 288 g/mol, purity ≥ 99%). A PBS+SDS aqueous solution was used for drug delivery tests to simulate a biological envi-ronment; because the Paclitaxel solubility in aqueous solution is low (0.3 g/mL) (Goldspiel, 1997), the SDS surfactant was added in the delivery medium to increase the solubility of Paclitaxel in aqueous solution repro-ducing similar in vivo conditions. Samples were main-tained in a thermostatic and stirred bath at 37°C for all the testing period. The delivery medium was withdrawn after established periods and then replaced with fresh solution, in order to ensure that drug concentration in the release medium was small enough to maintain the assumption of perfect sink conditions. A total of 12 withdraws per sample were carried out in 24 days.

Three samples were prepared and tested for each mate-rial, and a mean value of released drug was evaluated.

Materials characterization

A morphological characterization was performed on polymeric powders and film-shaped samples obtained by phase inversion. Morphological characterization was carried out through Scanning Electron Microscopy (SEM) JEOL JSM 5600. FT-IR spectra and chemical imaging maps were obtained using a Perkin Elmer Mod. Spectrum GX. Calorimetric analyses were obtained using a Perkin Elmer DSC 7. Thermo-gravimetric analyses were carried out using a Thermogravimetric Analyzer TGA 6.

Drug delivery kinetic was monitored by HPLC analyses using the same apparatus employed for the analysis of the polymerization kinetic, and the mobile phase was consti-tuted by 58% ACN and 42% H

2O, test time was 10 min,

internal flux rate was 1 ml/min, injected volume was 100 l, and wavelength was 210 nm at room temperature.

Results and discussion

Copolymerization kinetic, evaluation of reactivity ratios, and monomers conversions

Using the chromatographic method reported in the pre-vious section, the residual amounts of monomers in the reacting mass were calculated with time. These values were utilized for the evaluation of reactivity ratios and the theoretical macromolecular composition using the Kelen-Tüdòs mathematical model.

Reactivity ratios evaluated are reported in Table 1. In systems where both the reactivity ratios are less than

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Synthesis and characterization of copolymers of methylmethacrylate 119

the unit, copolymerization is favored, and only short sequences of single homopolymer tend to form (Cowie, 1973). Then, we can suppose a random structure for P(MMA-co-HEMA) 73:27 and 82:18 copolymers, inter-mediate between perfect random (r

MMA·r

HEMA = 1) and

alternate (rMMA

·rHEMA

= 0) structures.Also, HPLC analyses allowed evaluating the mono-

mers consumption during the polymerization reactions. Monomers conversions are reported in Table 1.

Morphological analysis

Morphological analyses were performed using Scanning Electronic Microscopy. Analyses performed on powder samples showed a micro-particulate structure. For syn-thesized materials, dimension of the particles decreased, increasing the amount of HEMA units in the polymer chain while the dispersion decreased.

Analyses carried out on film-shaped samples showed a dense and uniform structure for both copolymers, while an asymmetric structure for PMMA that is micro-porous in the lower section and dense in the upper. In Figure 1 results of SEM analyses are reported.

FT-IR analysis

FT-IR analysis allowed evaluating the effective mac-romolecular composition. Starting from the spectra of the homopolymers, two characteristic bands for both homopolymers were identified and taken as reference.

PMMA spectrum shows intense bands in correspond-ence to 1720 cm−1, due to the stretching of the ester (C=O)–O bond and within the region between 1200 and 1100 cm−1, due to the asymmetric vibrations associated to (C=O)–O and C–O. The PHEMA spectrum is character-ized by two intense bands due to the stretching of C=O

and C–O. The stretching of the C=O group is evident at 1729 cm−1, although it is usually verified at 1715 cm−1. This shift at a greater frequency could be justified with an increment of the spring constant of the bond associated to the presence of a O atom bonded to the C atom. Further, characteristics peaks associated to alcoholic function were identified (stretching of O–H at 3400 cm−1, stretching of C–O at 1024 cm−1, and bending of O–H at 1389 cm−1).

Copolymers showed all homopolymers character-istic bands, and in particular peaks between 1009 and 929 cm−1 (characteristic band of the PMMA homopoly-mer) and between 1104 and 1044 cm−1 (characteristic band of the commercial PHEMA homopolymer).

Using the spectral calculator, three theoretical spec-tra were numerically evaluated and the ratios of peak areas were calculated. From the ratios value, a work line was constructed, and, from its expression and the evaluation of the ratios for real synthesized copolymers, macromolecular compositions were obtained. Results are summarized in Table 2.

FT-IR analysis indicated a final composition of copol-ymer chain very similar to the starting reacting mixture, with the confirmation of the introduction of hydrophilic units of HEMA in the hydrophobic PMMA with the desired amount. In Figure 2, IR spectra of tested materi-als are reported.

Chemical imaging

Drug-loaded polymeric films were analyzed by Spectrum Spotlight obtaining maps where the drug distribution is represented. Correlation maps between the spectra map of each sample and the spectrum of the drug were cre-ated. In Figure 3, correlation maps are reported, respec-tively, (A) PMMA, (B) P(MMA-co-HEMA) 82:18, and (C) P(MMA-co-HEMA) 73:27. Obtained results showed that drug was homogeneously dispersed within the poly-meric matrices (correlation assumed values close to one

Table 2. Macromolecular compositions evaluated by FT-IR analyses.

Material Ratio % MMA % HEMA

P(MMA-co-HEMA) 82:18 0.7 83 17

P(MMA-co-HEMA) 73:27 1.3 71 29 Figure 2. FT-IR spectra of tested materials.

PHEMA

P(MMA-co-HEMA) 73:27

Tran

smitt

ance

[%]

P(MMA-co-HEMA) 82:18

PMMA

3850 3650 3450 3250 3050 2850 2650 2450 2250

Wave number [cmˆ-1]2050 1850 1650 1450 1250 1050 850 650

Figure 1. SEM analyses onto powders and films. PMMA (a.1) powder, (a.2) film section; P(MMA-co-HEMA) 82:18 (b.1) powder, (b.2) film section; P(MMA-co-HEMA) 73:27 (c.1) powder, (c.2) film section.

a. 1)

14kU x5,000 5µm 22 29 SEI 14kU x5,000 5µm 22 29 SEI 14kU x5,000 5µm 21 22 SEI

14kU x500 50µm 23 22 SEI14kU x850 20µm 24 22 SEI14kU x550 20µm 22 29 SEI

b. 1) c. 1)

a. 2) b. 2) c. 2)

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120 Davide Silvestri et al.

for all materials and there are no considerable segregated domains of drug).

Calorimetric analysis

Glass transition temperatures (tg) for synthesized

copolymers were evaluated by Differential Scanning Calorimetry (DSC). Results were compared to homopol-ymers t

g. Samples underwent two scans, and t

g were

evaluated using the second scan thermograms.For PMMA, the t

g was 130°C, while for commercial

PHEMA it was 115°C. Introduction of HEMA units within the copolymer macromolecule increased the chain flexibility (t

g showed a decrease). Results obtained

are reported in Table 3.

Experimental behavior can be compared to theo-retical approaches proposed by Fox (equation 1) (Brostow et al., 2008) and Gordon-Taylor (equation 2a) (Gordon, 1952):

where wMMA

and wHEMA

are the weight fractions of functional units present in the copolymer chain, t

g,MMA

and tg,HEMA

are the glass transition temperatures of

1

t

w

t

w

tg

MMA

g MMA

HEMA

g HEMA

= +, ,

(1)

(2a)tw t k w t

w k wgMMA g MMA MMA g HEMA

MMA MMA

=⋅ + ⋅ − ⋅

+ ⋅ −, .( )

( )

1

1

Figure 3. Correlation maps (A) PMMA, (B) P(MMA-co-HEMA) 82:18, and (C) P(MMA-co-HEMA) 73:27

A

186.7

110

100

50

0

0 100 200 242.7

0.9199

0.9278

0.9362

0.9438

0.9518

0.9601

0.9677

0.9757

0.9837

0.9916

1.0000Corr

−50

−100

−150−182.0

−238.5

Micrometers

Mic

rom

eter

s

−100

186.7

110

100

50

0

0 100 200 242.70.9008

0.9106

0.9205

0.9304

0.9403

0.9501

0.9600

0.9699

0.9901

0.9802

1.0000Corr

−50

−100

−150

−182.0−238.5

Micrometers

Mic

rom

eter

s

−100

186.7

110

100

50

0

0 100 200 242.70.8674

0.8787

0.8905

0.9013

0.9126

0.9243

0.9352

0.9464

0.9690

0.9577

0.9808Corr

−50

−100

−150

−182.0−238.5

Micrometers

Mic

rom

eter

s

−100

B

C

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Synthesis and characterization of copolymers of methylmethacrylate 121

homopolymers, and k is the thermal expansion coef-ficient of the material. Assuming k = 1, equation (2a) can be rewritten as:

In Figure 4, differential scanning calorimetric curves are reported, while Figure 5 shows experimental and theoretical behavior for all tested polymers.

Also in this case an intermediate behavior of copoly-mers with respect of the control homopolymers was observed, with a single t

g similar to theoretical glass

transition temperature, indicating the real formation of a copolymer structure.

Drug-loaded samples were tested by DSC, evaluating the effect of the presence of the active principle on glass transition temperatures with respect of the pure materi-als. The presence of the drug caused a plastificant effect: t

g of drug-loaded materials were smaller than t

g values of

pure materials.

Thermo-gravimetric analysis

From thermo-gravimetric analyses, degradation events for all polymers were detected. Two kinds of degradation events for PMMA were observed during the thermo-gravimetric analysis: (1) the formation of cyclic intermolecular anhydrides (280–320°C), and (Anderson, 2003) the depolymerization of the chain (350–420°C).

Degradation of PHEMA was detected in a range of temperatures included between 220°C and 350°C. This event was typified of the degradation of the chain and the formation of 2-hydroxyethyl methacrylate and ethyl-ene dimethacrylate.

Copolymers 82:18 and 73:27 showed two thermal events, the former was detected at 200–280°C and it was correlated to the thermal degradation of HEMA, the sec-ond was picked out at 330–420°C and was associated to the depolymerization of the PMMA fraction present in the copolymer chain. The first event occurred at a lower tem-perature than the homopolymer. In order to explain this lowering of the temperature range, a FT-IR analysis was performed on the samples after thermogravimetric analy-ses. The spectra showed a decrease of the ratio between the band area of characteristic peaks for PMMA and PHEMA, only due to partial depolymerization of the HEMA, prob-ably in the terminal blocks of the copolymer chain.

Degradation events did not occur in the typical range of temperature of the cyclic intermolecular anhydride for-mation; the presence of the HEMA prevented their forma-tion for its hydrophilicity (Radhesh Kumar et al., 2002). In Figure 6, the thermograms of tested samples are reported.

Figure 5. Experimental and theoretical glass transition temperature for P(MMA-co-HEMA) copolymers.

150145140135130

Tg [˚

C]

125120115110105100

0 10 20 30 40

Experimental Fox eq. Gordon-Taylor eq. Theroretical

50PMMA weight [%]

60 70 80 90 100

Figure 6. Thermogravimetric curves for P(MMA-co-HEMA) copolymers and PMMA and PHEMA homopolymers.

100908070

Wei

ght [

%]

605040302010

00 80 130 180 230 280

Temperature [°C]330 380 430 480

PMMA

P HEMA

P(MMA-co-HEMA) 82:18P(MMA-co-HEMA) 73:27

Table 3. tg obtained from DSC analyses.

Material

tg [°C]

(non-containing drug)

tg [°C]

(drug-loaded samples)

PMMA 130 121

P(MMA-co-HEMA) 82:18 123 111

P(MMA-co-HEMA) 73:27 121 112

PHEMA (commercial) 115 104

(2b)t w t w tg MMA g MMA HEMA g HEMA= × + ×, .

Figure 4. Differential scanning calorimetric curves of tested materials.

Hea

t flo

w

40 60

PHEMA

P(MMA-co-HEMA)

P(MMA-co-HEMA) 82:18

PMMA

80 100 120Temperature [°C]

140 160 180 200

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122 Davide Silvestri et al.

Swelling tests

Materials were treated to swelling tests with the specific aim to evaluate the water uptake by each material and the inclination to the swelling in aqueous environment.

Film-shaped samples not containing drug were prepared and immersed in 2 ml of bidistilled water, then withdrawn at specific times, blotted, and weighted. From experimental results the swelling percentage parameter (SW%) was evaluated using equation (3):

Obtained results are reported in Figure 7, where SW% versus time is represented.

Results showed the effect of the increased hydro-phylicity of the PMMA macromolecule after insert-ing HEMA units. In particular, is possible to highlight that the SW% of materials was similar at early times, then differences between PMMA homopolymer and

copolymers occurred after 24 hr, finally copolymer 73:27 showed only after 58 hr a slow increase that copolymer 82:18 did not show. It is possible to conclude that the increased hydrophylicity of the PMMA due to insertions of HEMA moieties did not affect the first period of test, when the water uptake was similar. After 1 day HEMA units showed their hydrophilic effect, causing a great increment of the SW% that became marked, especially for the 73:27 copolymer.

Drug release kinetic

Figure 8 reports the released drug amount normalized with respect to the exposed area to the delivery solu-tion. At an early time, introducing HEMA units in the PMMA chain, copolymers showed an increased kinetic in the first period of the test with respect to pure PMMA, but no significant differences between copolymers and PMMA homopolymer occurred. At a later time the amount of drug released was higher and the increase of the percentage of HEMA in the macromolecule increased this amount. In particular, obtained results indicated and confirmed the significant role played by the presence of hydroxyethyl methacrylate moieties on the polymer chain in affecting the drug elution mecha-nism. Introducing these units in fact was possible to improve the hydrophilic properties of the copolymer chain, increasing the drug elution amount and kinetic in the aqueous environment.

Drug release from copolymeric matrices showed the same behavior in the first part of the test (about 10 days), after this period the increased hydrophilicity of the copolymer P(MMA-co-HEMA) 73:27 with respect to the P(MMA-co-HEMA) 82:18 predominated and a faster drug delivery occurred.

Results obtained indicated that for Paclitaxel it was possible to control the drug delivery rate by varying the HEMA moieties in the proposed range of compositions.

Experimental results were elaborated using a math-ematical model (Ritger and Peppas, 1987). Drug delivery kinetic for biostable matrices can be described, in gen-eral, using the kinetic equation:

where Mt is the amount of drug released at a generic

time t, M∞

is the amount of drug eluted when a pla-teau occurs, k is the kinetic constant of the delivery process, and n the kinetic order. From the value of the kinetic order, polymers/drug film-shaped systems can be classified as characterized by a Fickian diffu-sion only (n ≤ 0.50), a release due to fickian diffusion and relaxation of macromolecules (0.50 ≤ n ≤ 0.89),

SWw t w

w%

( ) ( )

( )= − 0

0(3)

(4)M

Mk tt n

∞= ⋅

60

50

40

30

% S

W

PMMAP(MMA-co-HEMA) 82:18

P(MMA-co-HEMA) 73:27

20

10

00 6030 90 120

Time [hours]150

Figure 7. Effect of the copolymer composition on water uptake, swelling coefficient versus time.

Figure 8. Amount of Paclitaxel released with time normalized with respect of the polymeric matrix amount.

0.7

0.5

0.6

0.4

Dru

g re

leas

ed/p

olym

etric

mat

rix[µ

g/m

g]

PMMA P(MMA-co-HEMA) 82:18 P(MMA-co-HEMA) 73:27

0.3

0.2

0.1

00 105 15 20

Time [d]25

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Synthesis and characterization of copolymers of methylmethacrylate 123

and only the relaxation controlled drug delivery (n ≥ 0.89). Experimental results suggested that for PMMA and P(MMA-co-HEMA) 82:18 a Fickian transport dur-ing the entire delivery period occurs, while for more hydrophilic P(MMA-co-HEMA) 73:27 a Fickian control in the first part of the test occurs, and relaxation con-trol appears in the second period, combined to Fickian transport.

Using equation (4), kinetic parameters were evalu-ated for all tested materials and results are summarized in Table 4.

Copolymer P(MMA-co-HEMA) 73:27 showed a kinetic order ≥ 0.50, then a more exhaustive coverage is necessary. The kinetic delivery can be split into two parts, the first where low values of released drug occurs, and the second characterized by a higher delivery.

This behavior can be explained using the theory proposed by Peppas and Sahlin (1989). In this model, drug release from swellable matrices is described as a result of two transport mechanisms, Fickian

diffusion and relaxation of the polymer chains. Using this model, drug release kinetic can be calculated by equation (5):

where kF is the kinetic constant of the delivery due to

diffusion phenomena, and kr the kinetic constant of

release due to relaxation of macromolecules. Using this theory, kinetic orders and kinetic constants obtained are reported in Table 5.

Experimental results are compared with the math-ematical model in Figure 9.

Conclusions

The aim of this study was the realization and characteriza-tion of copolymers with increased hydrophilicity with con-trolled transport properties for application as stable con-trolled drug release materials. Two copolymers based on MMA and HEMA monomers were synthesized by varying initial molar ratios of components, with the goal to insert hydrophilic fractions inside hydrophobic PMMA chain.

Final conversion of monomers in carried out reac-tions, supported by FT-IR investigation, showed that the actual macromolecular composition was very similar to that expected. Furthermore, kinetic studies allowed investigating about effective macromolecular composi-tions by the theoretical Kelen-Tüdos approach.

Calorimetric analysis showed that glass transition temperatures were unique for copolymers, the thermal behavior could be well represented by the Gordon-Taylor model, and a plastificant effect was detected due to the drug presence. TGA analyses showed a variation of the range temperature of degradative events with the change of the molar composition of the macromolecule, proportionally to polymeric material composition.

Swelling tests showed that the effects of the presence of HEMA moieties gained an importance in the second period of tests, when a difference between the PMMA homopoly-mer and its copolymers occurred. After, difference between copolymers 82:18 and 73:27 could be highlighted.

Finally, drug delivery tests from homogeneous dis-perses drug-loaded films (chemical imaging maps) showed that the introduction of 2-hydroxyethyl meth-acrylate moieties in the polymer material played an effect on elution properties of the material in terms of cumulative amount and kinetic. In particular, increasing the hydrophilic properties of the macromolecular chain the delivery kinetic accelerates. Drug delivery kinetic was directly related to the swelling behavior, in particular for the 73:27 copolymer. The kinetic model of Ritger-Peppas was used to evaluate kinetic parameters (drug delivery

Table 4. Kinetic parameters for the delivery process using equation (3).

Material Kinetic order Kinetic constant [t−n]

PMMA 0.010 0.030

P(MMA-co-HEMA) 82:18 0.251 0.056

P(MMA-co-HEMA) 73:27 0.535 0.049

Table 5. Kinetic orders and kinetic constants for P(MMA-co-HEMA) 73:27 using equation (4).

Kinetic order (n) Kinetic constant (k)

Early 0.2418 0.055 (Fickian diffusion)

Late 0.9607 0.049 (relaxation of macromolecules)

Figure 9. Experimental results compared to mathematical model for P(MMA-co-HEMA) 73:27 copolymer. Fraction of drug release is reported.

0.7

0.5

0.6

0.4

Frac

tion

of p

aclit

axel

rele

ased

Experiment Theoretical

0.3

0.2

0.1

00 105 15 20

Time [d]25

(5)M

Mk t k tt

F r0

= ⋅ + ⋅

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124 Davide Silvestri et al.

constant and kinetic order) and the Peppas-Sahlin model was otherwise used for the accurate description of the 73:27 copolymer, that showed a different behavior with respect of the other studied materials.

Tested materials were shown to be promising plat-forms to entrap and after release in aqueous environ-ment hydrophobic and poor soluble drugs such as Paclitaxel, with a perspective application for example in cardiovascular field (coating for drug eluting stents is a potential perspective for these materials).

Acknowledgements

The authors acknowledge Federico Lensi and Giovanni Cassar Scalia for part of the experimental work.

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

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