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Paclitaxel-eluting composite fibers: Drug release and tensile mechanical properties Meital Zilberman, Amir Kraitzer Department of Biomedical Engineering, Faculty of Engineering, Tel-Aviv University, Tel-Aviv 69978, Israel Received 6 August 2006; revised 19 September 2006; accepted 10 January 2007 Published online 2 July 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.a.31286 Abstract: New core/shell fiber structures loaded with paclitaxel were developed and studied. These composite fibers are ideal for forming thin, delicate, biomedically important structures for various applications. Possible appli- cations include fiber-based endovascular stents that mechan- ically support blood vessels while delivering drugs for pre- venting restenosis directly to the blood vesel wall, or drug delivery systems for treatment of cancer. The core/shell fiber structures were formed by ‘‘coating’’ dense core fibers with porous paclitaxel-containing poly(DL-lactic-co-glycolic acid) (PDLGA) structures. Shell preparation (‘‘coating’’) was per- formed by freeze-drying water in oil emulsions. The present study focused on the effects of the emulsion’s formulation (composition) and processing conditions on the paclitaxel release profile and on the fibers’ tensile mechanical proper- ties. In general, the porous PDLGA shell released *40% of the paclitaxel, with most of the release occurring during the first 30 days. The main release mechanism during the tested period is diffusion, rather than polymer degradation. The release rate and quantity increased with increased drug con- tent or decreased polymer content, whereas the organic:aqu- eous phase ratio had practically no effect on the release pro- file. These new composite fibers are strong and flexible enough to be used as basic elements for stents. We demon- strated that proper selection of processing conditions based on kinetic and thermodynamic considerations can yield poly- mer/drug systems with the desired drug release behavior and good mechanical properties. Ó 2007 Wiley Periodicals, Inc. J Biomed Mater Res 84A: 313–323, 2008 Key words: paclitaxel; composite fibers; controlled drug release; poly(DL-lactic-co-glycolic acid); porous structure INTRODUCTION Coronary stenting has become an established mode of treatment in percutaneous transluminal cor- onary interventions. It has been shown to be more effective than conventional balloon angioplasty in reducing late restenosis. 1–3 However, in-stent reste- nosis can occur as a result of in-stent neointimal hyperplasia caused by proliferation and migration of vascular smooth muscle cells induced by vessel wall injury. 4 The pathology of restenosis stems from a complex interaction between cellular and acellular components of the vessel wall and the blood. 5 Ani- mal models have shown that some antiproliferative and antiinflammatory agents elute slowly from poly- mer coatings and are associated with reduced neoin- timal formation. In humans, promising preliminary results have been reported for the two antiprolifera- tive agents paclitaxel 6,7 and sirolimus. 8 Paclitaxel is a potent cell proliferation inhibitor and is known to be very effective in the treatment of cancer as well as in preventing restenosis. 9,10 How- ever, it is potentially cardiotoxic, and the dose of paclitaxel that can be delivered is relatively small. 11,12 Paclitaxel has been shown to markedly attenuate stent-induced intimal thickening of the lumen. 13,14 Paclitaxel’s antiproliferative effect is re- versible. 15 Its short cellular residence time (1 h), along with the reversible antiproliferative activity, suggest that it should be formulated in sustained- release dosage form. 16 The TAXUS trials revealed significant inhibition of coronary stenosis by pacli- taxel. 17–20 Drug-eluting polymer-coated stents have thus moved into the limelight as vehicles for local drug administration. 21 Bioresorbable stents are designed to support the vascular wall during the vessel healing process, gradually transferring the mechanical load to the vessel wall as the stent mass and strength decrease over time. Restenosis commonly occurs within 3–6 months after coronary intervention, and rarely there- after. Interest in bioresorbable vascular stents, with Correspondence to: M. Zilberman; e-mail: meitalz@eng. tau.ac.il Contract grant sponsor: Israeli Ministry of Health; contract grant number: 5821 Contract grant sponsors: RAMOT (Horowitz) Founda- tion, Tel-Aviv University ' 2007 Wiley Periodicals, Inc.

Paclitaxel-eluting composite fibers: Drug release and tensile mechanical properties

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Page 1: Paclitaxel-eluting composite fibers: Drug release and tensile mechanical properties

Paclitaxel-eluting composite fibers: Drug release andtensile mechanical properties

Meital Zilberman, Amir KraitzerDepartment of Biomedical Engineering, Faculty of Engineering, Tel-Aviv University, Tel-Aviv 69978, Israel

Received 6 August 2006; revised 19 September 2006; accepted 10 January 2007Published online 2 July 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.a.31286

Abstract: New core/shell fiber structures loaded withpaclitaxel were developed and studied. These compositefibers are ideal for forming thin, delicate, biomedicallyimportant structures for various applications. Possible appli-cations include fiber-based endovascular stents that mechan-ically support blood vessels while delivering drugs for pre-venting restenosis directly to the blood vesel wall, or drugdelivery systems for treatment of cancer. The core/shell fiberstructures were formed by ‘‘coating’’ dense core fibers withporous paclitaxel-containing poly(DL-lactic-co-glycolic acid)(PDLGA) structures. Shell preparation (‘‘coating’’) was per-formed by freeze-drying water in oil emulsions. The presentstudy focused on the effects of the emulsion’s formulation(composition) and processing conditions on the paclitaxelrelease profile and on the fibers’ tensile mechanical proper-ties. In general, the porous PDLGA shell released *40% of

the paclitaxel, with most of the release occurring during thefirst 30 days. The main release mechanism during the testedperiod is diffusion, rather than polymer degradation. Therelease rate and quantity increased with increased drug con-tent or decreased polymer content, whereas the organic:aqu-eous phase ratio had practically no effect on the release pro-file. These new composite fibers are strong and flexibleenough to be used as basic elements for stents. We demon-strated that proper selection of processing conditions basedon kinetic and thermodynamic considerations can yield poly-mer/drug systems with the desired drug release behaviorand good mechanical properties. � 2007 Wiley Periodicals,Inc. J BiomedMater Res 84A: 313–323, 2008

Key words: paclitaxel; composite fibers; controlled drugrelease; poly(DL-lactic-co-glycolic acid); porous structure

INTRODUCTION

Coronary stenting has become an establishedmode of treatment in percutaneous transluminal cor-onary interventions. It has been shown to be moreeffective than conventional balloon angioplasty inreducing late restenosis.1–3 However, in-stent reste-nosis can occur as a result of in-stent neointimalhyperplasia caused by proliferation and migration ofvascular smooth muscle cells induced by vessel wallinjury.4 The pathology of restenosis stems from acomplex interaction between cellular and acellularcomponents of the vessel wall and the blood.5 Ani-mal models have shown that some antiproliferativeand antiinflammatory agents elute slowly from poly-mer coatings and are associated with reduced neoin-timal formation. In humans, promising preliminary

results have been reported for the two antiprolifera-tive agents paclitaxel6,7 and sirolimus.8

Paclitaxel is a potent cell proliferation inhibitorand is known to be very effective in the treatment ofcancer as well as in preventing restenosis.9,10 How-ever, it is potentially cardiotoxic, and the dose ofpaclitaxel that can be delivered is relativelysmall.11,12 Paclitaxel has been shown to markedlyattenuate stent-induced intimal thickening of thelumen.13,14 Paclitaxel’s antiproliferative effect is re-versible.15 Its short cellular residence time (1 h),along with the reversible antiproliferative activity,suggest that it should be formulated in sustained-release dosage form.16 The TAXUS trials revealedsignificant inhibition of coronary stenosis by pacli-taxel.17–20 Drug-eluting polymer-coated stents havethus moved into the limelight as vehicles for localdrug administration.21

Bioresorbable stents are designed to support thevascular wall during the vessel healing process,gradually transferring the mechanical load to thevessel wall as the stent mass and strength decreaseover time. Restenosis commonly occurs within 3–6months after coronary intervention, and rarely there-after. Interest in bioresorbable vascular stents, with

Correspondence to: M. Zilberman; e-mail: [email protected] grant sponsor: Israeli Ministry of Health;

contract grant number: 5821Contract grant sponsors: RAMOT (Horowitz) Founda-

tion, Tel-Aviv University

' 2007 Wiley Periodicals, Inc.

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or without drug delivery, is therefore increasing.These stents are usually made of fibers. For example,Tamai et al.22,23 described the Igaki/Tamai stent, abioresorbable balloon expandable zigzag coil designbased on a PLLA monofilament. Tranilast is an anti-allergic drug that also inhibits the migration andproliferation of VSMCs induced by platelet-derivedgrowth factor and transforming growth factor b1.5,24

The tranilast eluting Igaki–Tamai stent was made ofa PLLA monofilament mixed with tranilast, with ashape similar to that of the regular Igaki–Tamai stentwithout the drug. The radial compression strengthof the drug-loaded stent was *10% lower than thatof the neat stent. We have also developed and stud-ied a fiber-based expandable stent design,25–28 whichis prepared using a linear, continuous coil arrayprinciple, by which four furled lobes convert into asingle large lobe upon balloon expansion (Fig. 1).This expandable stent design is based on melt-spunbioresorbable fibers that were made from a relativelyhigh molecular weight PLLA. It demonstrated excel-lent initial radial compression strength and goodin vitro degradation resistance for 20 weeks.26

In one of our recent projects we presented a newconcept of core/shell composite fiber structureswhich combines a dense polymer core fiber and adrug/protein-loaded porous shell structure.29,30 Inthis structure the drug or protein is located in a sep-arate compartment (a ‘‘shell’’) around a melt spun‘‘core’’ fiber. The shell is prepared using mild pro-cessing conditions and can therefore include any bio-active agent, while preserving its activity. A sche-matic representation of the composite fiber structure

is presented in Figure 2(a). Our new fibers are idealfor forming thin, delicate, biomedically importantstructures for various applications, such as fiber-based endovascular stents (Fig. 1). Use such compos-ite fibers, with bioresorbable core and shell, ratherthan only the basic PLLA core fiber will result inbioresorbable stents that mechanically support bloodvessels while delivering paclitaxel directly to theblood vessel wall. Our novel coating technique canalso be used for stable stents, where only the drug-loaded coat degrades with time. Such paclitaxel-elut-ing fibers can also be used for cancer treatment.Studies on paclitaxel-eluting bioresorbable micro-spheres for cancer treatment10,31,32 have shown thatits release in an aqueous medium is relatively slowdue to paclitaxel’s high hydrophobicity. Porousstructures that encapsulate the drug, such as ourfiber’s ‘‘shell,’’ may therefore be advantageous fordrug-eluting stents as well as for cancer treatmentapplications.

The current study focuses on composite core/shellfiber structures loaded with paclitaxel. The porousshell (drug-containing section) was prepared usingthe technique of freeze drying an inverted emulsion.The effects of the emulsion’s formulation (compo-nents) and processing conditions on the emulsion’sstability and on the resulting shell’s microstructurehave already been reported.30 The current articledescribes the effects of the emulsion’s formulation(components) and processing conditions on thefibers’ properties, that is paclitaxel’s release profileand tensile mechanical properties in light of theshell’s morphology. It should be noted that these

Figure 1. The design concept of the vascular fiber-based stent: (a) predilated, (b) dilated, (c) predilated, side view, (d)dilated, side view.28 [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

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paclitaxel-eluting composite fiber structures can alsoserve as a good model for composite fibers loadedwith other hydrophobic bioactive agents for a widerange of biomedical applications.

MATERIALS AND METHODS

Materials

EthilonTM monofilament nylon sutures (model W597Ethicon, USA) were used as core fibers.

Bioresorbable porous structures (the shell coating) weremade of 75/25 poly(DL-lactic-co-glycolic acid) (PDLGA), in-herent viscosity (i.v.) ¼ 0.65 dL/g (in CHCl3 at 308C,*97,100 g/mole), Absorbable Polymer Technologies, USA.

Paclitaxel (GenexolTM) was purchased from Sam Yang,Seoul, Korea.

Surface active agents

1. Pluronic L121TM, a triblock copolymer of ethyleneoxide (EO) and propylene oxide, (PEO-PPE-PEO),with a mean molecular weight of *4400 Da wasreceived as a gift from BASF, USA.

2. Poly(vinyl alcohol) (PVA), 87–89% hydrolyzed, mo-lecular weight ¼ 13,000–23,000 Da was purchasedfrom Sigma.

Preparation of core/shell fiber structures

Fiber surface treatment

The sutures were surface-treated in order to dispose theoriginal fiber’s coating and to enhance adhesion betweenthe core fiber and the coating. The nylon fibers wereslightly stretched on special holders and dipped in a 75/25v/v formic acid/ethanol solution for 15 s. The fibers werethen washed and dried in a vacuum oven at 658C for80 min.

Emulsion formation

A known amount of PDLGA was dissolved in chloro-form to form an organic solution and paclitaxel was addedto the solution. Double-distilled water was then pouredinto the organic phase (in a test tube) and homogenizationof the emulsion was performed using a hand-held homog-enizer (OMNI TH, 7 mm rotor) operating at 16,500 rpm(medium rate) for 3 min, for most investigated samples.Certain samples were prepared using homogenizationrates of 5500 rpm (low rate) or 25,000 rpm (high rate) andhomogenization durations of 1 and 4 min in order toinvestigate the effect of processing conditions on theporous shell structure. An emulsion formulation containing

Figure 2. The structure of the core/shell composite fibers: (a) A schematic representation showing the concept, (b) and(c) SEM fractographs of the standard specimen. [Color figure can be viewed in the online issue, which is available atwww.interscience.wiley.com.]

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17.5% w/v polymer in the organic solution, 1.43% w/wpaclitaxel (relative to the polymer load), and an organic toaqueous (O:A) phase ratio of 2:1 v/v was used as the ref-erence formulation. Other formulations included 15 and22.5% w/v polymer, 0.71, 2.86, and 7.14% w/w paclitaxel,and O:A phase ratios of 4:1 and 1.3:1. All formulations arepresented in Table I. Surface active agents were added tothe emulsion in some of the samples: Pluronic (1% w/wrelative to the polymer quantity) was added to the poly-mer solution and PVA (1% w/v relative to the water quan-tity) was added to the water.

Core/shell fiber structure formation

The treated core nylon fibers were dip-coated (whileplaced on holders) in fresh emulsions and then frozen im-mediately in a liquid nitrogen bath. The holders þ sampleswere then placed in a precooled (�1058C) freeze dryer(Virtis 101 equipped with a nitrogen trap) capable of work-ing with organic solvents (freezing temperature of the con-denser was approximately �1058C) and freeze dried inorder to preserve the microstructure of the emulsion-basedcore/shell fiber structures. Drying was performed in twostages:

1. The freeze dryer chamber pressure was reduced to100 mTorr, while the temperature remained at�1058C.

2. The condenser was turned off and its plate tempera-ture slowly increased to room temperature, while thepressure was monitored between 100 and 700 mTorr.During this step the liquid nitrogen trap condensedthe excess water and solvent vapors.

The samples were stored in desiccators until use.

In vitro drug release studies

The composite core/shell fiber structures wereimmersed in phosphate buffered saline (PBS) at 378C for112 days in order to determine paclitaxel’s release kineticsfrom these structures. The release studies were conductedin closed glass vessels containing 3 mL PBS medium,using a horizontal bath shaker operated at a constant rateof 130 rpm. The medium was removed (completely) peri-odically, at each sampling time (1, 3, 7, 14, 21, 28, 42, 56,70, 84, and 112 days), and fresh medium was introduced.This experiment enabled simulating conditions close tothese that exist in a blood vessel.

The paclitaxel content of the medium samples wasdetermined using Agilent 1100 high performance liquidchromatography (HPLC). A reverse phase column (Lith-rosther 100 RP 18) was used (inner diameter D ¼ 4 mm, L¼ 250 mm, pore size 5 lm), and was kept at 258C. The mo-bile phase consisted of a mixture of acetonitrile and doubledistilled water (50/50, v/v) at a flow rate of 0.8 mL/minwith a pump (Agilent 1100), gradient t ¼ 0, 50/50, t ¼ 15min, 50/50, t ¼ 21, 0/100, with a 10-min post process. 20lL samples were injected with an autosampler. The col-umn effluent was detected at 232 nm with an adjustablewavelength detector (UV DAD). The area of each elutedpeak was integrated using Chemstation software.

The cumulative release profiles were determined relativeto the initial amount of paclitaxel in the composite fibers(quantity released during the incubation period þ the resi-due remaining in the fibers). All experiments were per-formed in triplicates. Results are presented as means 6standard errors. The effects of the emulsion’s compositionon the release profile were studied by examining the fol-lowing parameters: polymer content in the organic phase(%w/v), drug content (relative to polymer content, %w/w), and organic:aqueous (O:A) phase ratio. The effects ofthe process parameters on the release profile were studiedby examining the stirring rate and time.

Residual drug recovery from composite fibers

Residual paclitaxel recovery from the composite fiberswas measured as follows: the fibers were placed in 1 mLmethylene chloride and 3 mL of 50/50 acetonitrile/watersolution was added. Methylene chloride evaporation wasperformed under a nitrogen stream and the paclitaxel con-centration was then estimated using HPLC. An extractionfactor was used for correction. To determine this recoveryefficiency of the extraction procedure, known weights ofpaclitaxel were dissolved in 1 mL of methylene chlorideand subjected to the same extraction procedure in tripli-cate. Recoveries were always greater than 75% and the val-ues of the residual drug was corrected appropriately.

Morphological characterization

The morphology of the composite core/shell fiber struc-tures (cryogenically fractured surfaces) was observed usinga Jeol JSM-6300 scanning electron microscope (SEM) at an

TABLE IThe Examined Specimens and their Shells’ Structural

Characteristics

Process Parameters AmountMean Poresize [lm] Porosity

Polymer content[% w/v]

15 5.8 6 2.3 8517.5 6.5 6 2.3 85.222.5 5.4 6 2.1 82

Paclitaxel content[% w/w]

0.71 5.4 6 2.6 891.43 6.5 6 2.3 85.22.86 21.2 6 6 857.14 79.1 6 17 N/A

Organic to aqueousphase ratio [v/v]

4:1 6.1 6 3.1 87.62:1 6.5 6 2.3 85.2

1.3:1 7.8 6 3.8 94.2Homogenization

rate [rpm]5500 7.7 6 3.5 92.7

16,500 6.5 6 2.3 85.225,000 5.8 6 1.9 86

Homogenizationduration [s]

60 7 6 3.7 86.8180 6.5 6 2.3 85.2240 5.9 6 2.6 81.6

Surfactant content[1% w/w][1% w/v]

None 6.5 6 2.3 85.2Pluronic 8.2 6 3.0 88PVA 6.2 6 2.8 87.5

The measurement error of the porosity is 10%.

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accelerating voltage of 5 kV. The SEM samples were Ausputtered prior to observation. The mean pore diameterand porosity of the observed morphologies was analyzedusing Sigma Scan Pro software and statistics were drawnusing SPSS 10 software. Statistical significance was deter-mined using the ANOVA (Tukey-Kramer) method. Toevaluate the porosity of the samples, for each SEM fracto-graph the area occupied by the pores was calculated, usingthe Sigma Scan Pro software. The porosity was determinedas the area occupied by the pores divided by the total area.

Measurements of tensile mechanical properties

The fibers’ tensile mechanical properties were measuredat room temperature, under unidirectional tension at a rateof 50 mm/min (ASTM D 3379), using a 5500 Instronmachine. The tensile strength was defined as the maxi-mum strength in the stress-strain curve. The maximalstrain was defined as the breaking strain. Young’s modu-lus was defined as the slope of the stress-strain curve inthe elastic (linear) region. Six samples were tested for eachpoint. The means and standard deviations were calculatedusing the SPSS 10 software. ANOVA (Tukey-Kramer) wasused for group comparison.

RESULTS AND DISCUSSION

The freeze drying technique is unique in being ca-pable of preserving the liquid structure after becom-ing solid. We used this technique in order to pro-duce inverted emulsions in which the continuousphase contained polymer and drug dissolved in asolvent, with water being the dispersed phase. SEMfractographs showing the bulk morphology of thestandard specimens are presented in Figure 2(b,c).The diameter of the treated core fiber was in therange of 170–190 lm and shell thickness of 30–60lm was obtained for most emulsion formulations.Relatively high contents of hydrophobic components(such as PDLGA and paclitaxel) resulted in anincrease in shell thickness, due to higher emulsion’sviscosity.30 There are no gaps between core and shell[Fig. 2(c)], indicating that the quality of the interfacebetween the fiber and the porous coating is high, thatis the surface treatment enabled good adhesionbetween core and shell. The shell’s porous structure inall studied specimens contained round-shaped pores,usually within the 5–10 lm range, with a porosityexceeding 80% (Table I). The shell’s microstructurewas uniform in each sample, probably due to rapidquenching of the emulsion, which enabled preserva-tion of its microstructure.30 The pores were partiallyinterconnected by smaller inner pores. The emulsionswere stable only within a certain formulation range andsurfactants were not required for achieving stability.30

The effects of emulsion formulation and processingcondition on the shell morphology are described indetails in one of our recent publication.30

Cumulative release of paclitaxel from the referencespecimen for 4 months is presented in Figure 3. Bothrelease scales, that is quantity and %, are presented.Although a relatively small percentage of the loadeddrug was released, this is the relevant quantity forthe stent application.9 Paclitaxel was released in anexponential manner, that is the rate decreased withtime. Such a release profile is typical of diffusion-controlled systems. A minor burst effect of less than3% was obtained during the first days of release.The paclitaxel release from the porous shell was rela-tively slow, mainly due to paclitaxel’s extremelyhydrophobic nature. Moreover, the release ratedecreased with time, since the drug had a progres-sively longer distance to pass and a lower drivingforce for diffusion. A mathematical–physical modelbased on Fick’s 2nd law of diffusion was developedlately by us.33 This model uses a time-dependentbioactive agent diffusion coefficient, which increaseswith time due to the degradation of the host poly-mer. It indeed showed that the drug controlledrelease from these systems is controlled mainly bydiffusion and that the host polymer’s degradationrate has a minor effect on drug and protein release.

The shell’s microstructure may affect the drugrelease profile, which is determined by both thermo-dynamic and kinetic parameters. The thermody-namic parameters are actually the emulsion’s formu-lation, that is the polymer and drug contents and theO:A ratio, whereas the kinetic considerations areactually the processing conditions, which include therate and duration of homogenization. Surfactantswere also added in certain cases. The effects of thesethermodynamic and kinetic parameters on the fiber’sproperties, that is drug release profile and tensile

Figure 3. Paclitaxel release profile from the standardcore/shell fiber specimen. [Color figure can be viewed inthe online issue, which is available at www.interscience.wiley.com.]

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mechanical properties are described below, withrespect to the shell’s microstructure.

In vitro paclitaxel release from compositecore/shell fiber structures

Effect of emulsion formulation

The effects of the emulsion formulation (composi-tion) on the drug release from the porous shell andon the shell’s microstructure are presented in Figures4 and 5, respectively. The release rate and theamount of drug released increased with the decreasein polymer content [Fig. 4(a)]. The quantity releasedfrom the formulation containing 15% w/v polymerwas significantly higher than that obtained for 17.5and 22.5% w/v formulations. Since the pore sizewas almost unchanged with the emulsion’s polymercontent [Fig. 5(a,b) and Table I], but less dense ‘‘poly-meric walls’’ were probably created between adjacentpores, it is suggested that a relatively low polymercontent reduces the binding region between the ma-trix and paclitaxel, resulting in a higher diffusioncoefficient which enables more effective drugrelease.

The drug content significantly affected the releaseprofile, as presented in Figure 4(b). The release rateand the amount of drug released increased with theincrease in paclitaxel content, mainly due to a higherdrug concentration gradient between the matrix andthe release medium. Furthermore, a relatively largeburst effect was evident when a high drug quantitywas loaded, that is specimens containing 7.14% w/wpaclitaxel released 7% during the first 24 h com-pared with 3% from specimens containing 2.86% w/wpaclitaxel. The SEM observations indicate that higherdrug contents result in a larger pore size, due toemulsion instability [Fig. 5(c)]. Paclitaxel is a hydro-phobic drug and a higher paclitaxel content in theorganic phase of the emulsion therefore results inhigher interfacial tension (difference between thesurface tension of the organic and aqueous phases),leading to a less stable emulsion with a larger poresize (Table I). Larger pores are expected to reducethe release rate for a given porosity and interconnec-tivity. It can be concluded that the driving force fordiffusion has a greater effect than the morphologicalchanges, since the release rate in this systemincreased with the drug content, in spite of the mor-phological changes which favor the opposite drugrelease behavior.

The effect of the O:A phase ratio on paclitaxel’srelease profile from the composite fibers is presentedin Figure 4(c). The release profile as well as the poresize and porosity [Fig. 5(d) and Table I] exhibitedalmost no change in the O:A range of 2:1 to 4:1. It

should be mentioned that the relatively narrow O:Arange chosen for this study resulted from stabilityconsiderations. The porosity of samples derivedfrom emulsions with O:A ratios higher than 4:1 maynot be high enough to enable effective release ofwater-insoluble agents such as paclitaxel. On theother hand, samples derived from emulsions withO:A ratios less than 2:1 are not stable. For example,the 1.3:1 O:A structures are not stable and exhibiteda relatively large pore distribution,30 with a porosityof 94.2% (Table I).

Figure 4. The effect of emulsion formulation on therelease profile of paclitaxel from core/shell fiber structures:(a) effect of polymer content: deep blue square—15% w/v,pink circle—17.5% w/v, dark green triangle—22.5% w/v,(b) effect of paclitaxel content: red square—0.7% w/w,pink circle—1.4% w/w, deep blue triangle—2.9% w/w,light blue square—7.1% w/w, (c) effect of O:A phase ratio:pink square—4:1 v/v, light green diamond—2:1 v/v.[Color figure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]

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Effect of processing conditions

The kinetic parameters of the fiber processing stepinclude the rate and the duration of homogenization.The effect of the emulsion’s homogenization rate onthe drug release from the porous shell is presentedin Figure 6. The homogenization rate exhibited someeffect on the release profile and the shell’s porousstructure (Table I) in the chosen relatively broadrange. An increase in homogenization rate resultedin a slight decrease in pore size, which enabledsome increase in drug release rate and quantity. Thehomogenization duration did not exhibit a signifi-cant effect on paclitaxel’s release profile for samplesprepared using homogenization durations whichexceeded 180 s. This is in agreement with the simi-larity in pore size and shape (Table I). However, rel-atively short homogenization times (such as 60 s),may lead to some instability which will result inlocal continuous paths for drug diffusion and willtherefore exhibit higher release rates.

Effect of surfactants

The effect of surfactants on the shell’s microstruc-ture and on the resulting release profile was also

investigated. Pluronic incorporation into the emul-sion resulted in an increase in the release rate andquantity, whereas incorporation of PVA resulted in adecrease in both parameters, as presented in Figure7. Both surfactants were incorporated in concentra-tions of 1% w/w and pluronic was also studiedusing a concentration of 10% w/w, but did not fur-ther increase the release rate. Incorporation of thepluronic surfactant in the emulsion changed the

Figure 6. The effect of the stirring rate on the release pro-file of paclitaxel from core/shell fiber structures: deep bluediamond—low rate, pink square—medium rate, dark greentriangle—high rate. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]

Figure 5. SEM fractographs of composite fibers showing the effect of the emulsion’s formulation on the shell’s micro-structure: (a) The standard specimen containing 17.5% w/v polymer, 1.43% w/w pacitaxel, and phase ratio of 2:1, (b) for-mulation with 15% w/v polymer, (c) formulation with 2.9% w/w paclitaxel, (d) formulation with O:A ratio of 4:1.

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shell’s microstructure [Fig. 8(b)]. Relatively largevoids appeared between domains of the regular po-rous structure, instead of the regular homogenousstructure. The large voids between the regular po-rous regions introduced local continuous paths fordrug diffusion and therefore probably enabled someincrease in release rate and quantity. The pore sizeand porosity indeed increased due to pluronic incor-poration (Table I). The PVA surfactant had almostno effect on the shell’s morphology [Fig. 8(c)], butresulted in a decrease in the release rate [Fig. (7)].

Effects of process-structure on the drug-releaseprofile

The paclitaxel release profile obtained for moststudied structures during the test period demon-strated a very low initial burst effect, accompanied bya decrease in release rate with time. It is clear that asecond release phase should occur after more than 4months. This means that during the first releasephase, most of the drug is released within the firstmonth and a second release phase should occur lateras the polymer undergoes degradation into very smallfragments. Such a release profile can be advantageousfor our application, especially since it is known thatrestenosis may occur within 6 months after the proce-dure. It should be mentioned that the specimensmaintained their mechanical integrity throughout theentire test period, without visible cracking or dis-charge of degradation products to the medium.

The emulsion formulation and processing condi-tions could be expected to affect the release profile.However, this was observed only in certain cases.The emulsion’s stability in the studied systemsactually determined the porous structure, and a morehydrophobic organic phase was expected to exhibit aporous structure with larger pores, due to higherinterfacial tension leading to coalescence of aqueousdomains. Such an increase in pore size should resultin a decreased surface area and a lower diffusion

Figure 8. SEM fractographs of composite fibers showing the effect of surfactants on the shell’s microstructure: (a) Thestandard specimen (no surfactants), (b) formulation with 1% w/w Pluronic, (c) formulation with 1% w/v PVA.

Figure 7. The effect of the addition of surfactant on therelease profile of paclitaxel from core/shell fiber structurespink square—no surfactant, deep blue triangle—1% w/wPluronic, black diamond—1% w/w PVA. [Color figure canbe viewed in the online issue, which is available atwww.interscience.wiley.com.]

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rate. More hydrophobic organic phases are thereforeexpected to enable lower drug release rates and quan-tities. Our results indicate that an increase in pacli-taxel content results in an increase in the drug releaserate and quantity, in spite of the increase in pore size.The driving force for diffusion therefore has a greatereffect than the microstructure. When the polymercontent of the organic phase was decreased, therelease rate and quantities increased, although themicrostructure hardly changed. The emulsion’s for-mulation affected the release profile through micro-structure mainly for Pluronic-loaded samples, wherea change in the microstructure enabled more effectivediffusion of the hydrophobic paclitaxel from the po-rous shell. Furthermore, a small decrease in pore sizedue to an increase in homogenization rate resulted ina higher paclitaxel release rate. It can therefore alsobe conclude that attempts should be made to obtain amore delicate porous shell structure with a higher in-ternal interface in order to exhibit a higher releaserate of pacitaxel from the composite fiber and torelease a larger quantity during the first 30 days. Thismay be achieved by using better surfactants that arecompatible with the inverted emulsion.

Tensile mechanical properties

The nylon sutures were surface-treated, asdescribed in the experimental section, in order todispose of the fiber’s original coating and to enhancethe adhesion between the core fiber and the coating.

The tensile stress–strain curves of the treated nylonfibers and of fibers coated with the standard emul-sion are presented in Figure 9. These fibers werenamed composite fiber A and their evaluated me-chanical tensile properties are presented in Table II.Two methods were used for evaluating the mechani-cal properties of the core/shell fibers: (a) using thetotal diameter of the fiber, (b) using an effective di-ameter which is actually the treated core fiber. Thetreated core fiber lost 33% of its strength and 33% ofits Young’s modulus due to its coating. However, itshould be noted that the highly porous shell actuallycannot carry the load. The second method of evalua-tion therefore affords the real effect of coating, whichis an 18% decrease in tensile strength and a 20%decrease in Young’s modulus (Table II). Theseresults demonstrate that the process of fiber coating,which includes exposure to the emulsion, quenchingin liquid nitrogen and freeze drying, results in someactual decrease in tensile strength and modulus.However, the fibers remained strong and flexible.Two additional composite fibers were tested in orderto investigate the effect of the emulsion’s viscosityon the fibers’ mechanical properties: fibers coatedwith a more viscous emulsion (22.5% w/v ( polymerinstead of 17.5% w/v of the standard emulsion,named composite fiber B) and fibers coated with aless viscous emulsion (higher solvent volume of 5mL (equivalent to 14% w/v polymer) instead of 4mL (17.5 % w/v polymer), named composite fiberC). Both types of fibers exhibited mechanical proper-ties similar to those obtained for the fibers that werecoated with the standard emulsion (Table II).

The mechanical properties of these fibers are veryimportant, since they are intended for construction ofendovascular stents. Our results indicate that the newcomposite fibers are strong and flexible enough to beused as basic elements for stents. The decrease in

Figure 9. Tensile stress-strain curves of nylon fibers. Fibertype: 1—deep blue line surface treated core fiber, 2—lightgreen line standard core/shell fiber structure (total diame-ter is considered), 3—red line standard core/shell fiberstructure (effective diameter is considered). [Color figurecan be viewed in the online issue, which is available atwww.interscience.wiley.com.]

TABLE IIThe Fibers’ Tensile Mechanical Properties

Fiber TypeStrength(MPa)

Modulus(MPa)

Strain(%)

Treated core fiber 396 6 50 880 6 15 48.0 6 5.5Composite fiber A(*) 267 6 32 590 6 7 47.4 6 4.8Composite fiber A(**) 325 6 40 700 6 12 47.9 6 5.0Shell derived from the

standard emulsionComposite fiber

B(**)—shell preparedfrom emulsion withhigh polymer content

331 6 35 713 6 17 37.8 6 5.3

Composite fiberC(**)—shell preparedfrom emulsion withhigh solvent volume

337 6 41 695 6 21 39.0 6 4.9

*Evaluation based on the total fiber diameter.**Evaluation based on the effective fiber diameter.

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mechanical properties due to the coating processremains within acceptable limits compared with mon-olithic or reservoir fibers, that is our experience withstents with good radial compression strength26,28,34

indicates that such stents can be constructed from ourcomposite fibers.

SUMMARY AND CONCLUSIONS

New bioresorbable core/shell fiber structures forbiomedical applications were developed and stud-ied. These structures were composed of a nylon coreand a porous PDLGA shell loaded with the antipro-liferative agent paclitaxel, prepared using freeze dry-ing of inverted emulsions. Investigation of thesecomposite fibers focused on the effects of the emul-sion’s composition (formulation) and processing con-ditions on the drug release profile from the fibersand on the fibers’ tensile mechanical properties.

In general, extremely porous ‘‘shell’’ structures(mean porosity of *85% and mean pore size 6 lm)were obtained with good adhesion to the core fiber.These new fibers demonstrated good mechanicalproperties with a versatile drug release profile. Theporous PDLGA shell released *40% of the paclitaxeland most of the release occurred during the first 30days. Paclitaxel release from the porous shell wasrelatively slow due to its extremely hydrophobic na-ture, and the main release mechanism during thetested period was diffusion rather than polymer deg-radation.

The release rate and quantity increased with theincrease in drug content or the decrease in polymercontent, while the O:A phase ratio in the studiedrange had practically no effect on the release profile.Incorporation of the pluronic surfactant in the or-ganic phase increased the rate of drug releasethrough a microstructure with a larger surface areafor diffusion. The effect of emulsion formulation onthe release profile is more significant than the effectof the microstructure, owing to paclitaxel’s hydro-phobic nature. Our results indicate that the newcomposite fibers are strong and flexible enough to beused as basic elements for stents.

We have demonstrated that proper selection ofprocessing conditions, based on kinetic and thermo-dynamic considerations, can yield polymer/drugsystems with desired drug release behaviors andgood mechanical properties. These core/shell fiberstructures can be used in medical implants, such asvascular stents.

The authors thank Mr. Guy Alon and Mr. JonathanElsner, Deptartment of Biomedical Engineering, Tel-AvivUniversity, for their assistance with tensile mechanicalproperty measurements.

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