A Polymer-Free Paclitaxel Eluting Coronary Stent: Effects of Solvents, Drug Concentrations and Coating Methods

A Polymer-Free Paclitaxel Eluting Coronary Stent: Effects of Solvents,

Drug Concentrations and Coating Methods

SUJAN LAMICHHANE, ANNEMARIE GALLO, and GOPINATH MANI

Biomedical Engineering Program, The University of South Dakota, 4800 N. Career Ave, Suite 221, Sioux Falls, SD 57107, USA

(Received 9 December 2013; accepted 28 March 2014; published online 5 April 2014)

Associate Editor K. A. Athanasiou oversaw the review of this article.

Abstract—Some polymer coatings used in drug-eluting stents(DES) cause adverse reactions. Hence, the use of self-assembled monolayers (SAMs) as a polymer-free platformto deliver an anti-proliferative drug (paclitaxel—PAT) from2D metal substrates was previously demonstrated. In thisstudy, we optimized the PAT coating on SAMs coated 3Dcoronary stents. For the optimization process, we investi-gated the effects of solvents (ethanol, DMSO, and theirmixtures), drug concentrations (2, 3, 4, 8, and 12 mg/mL) inthe coating solution, and coating methods (dip and spray) onPAT deposition. A solvent mixture of 75:25 v/vEt-OH:DMSO was determined to be the best for obtainingsmooth and homogenous PAT coating. PAT coated stentsprepared using 8 mg/mL and 3 mg/mL concentrations ofPAT by dip and spray coating methods, respectively, wereoptimal in terms of carrying adequate drug doses (0.35 lg/mm2 for dipping and 0.76 lg/mm2 for spraying) as well asnegligible defects observed in the coating. PAT was success-fully released from SAMs coated stents in a biphasic mannerwith an initial burst followed by a sustained release for up to10 weeks. Thus, this study sheds light on the effects ofsolvents, drug concentrations, and coating methods onpreparing a polymer-free DES.

Keywords—Drug-eluting stent, Coatings, Surface modifica-

tion, Drug delivery.

INTRODUCTION

Cardiovascular stents are currently implanted inmillions of patients every year to treat atherosclerosis,a disease which causes blockages in arteries that supplyblood to the heart. However, the arterial injury thatoccurs during a stent implantation causes a cascade of

biological events resulting in the growth of scar tissue(neointimal hyperplasia) to re-occlude the artery.12

Hence, drug-eluting stents (DES) are currently used todeliver an anti-proliferative drug such as paclitaxel(PAT) to inhibit neointimal hyperplasia and therebykeep the artery open for a longer period of time.23

Although DES are successful in reducing the rates ofre-occlusion of an artery, still a few concerns exist.Most DES use polymers to carry a drug onto the stent.However, there are a few limitations associated withthe use of some polymers (not all) as drug deliverycarriers in stents: (a) polymer coatings can undergomechanical damage during stent expansion18;(b) hypersensitivity and inflammatory reactions topolymer coatings on stents25,26; (c) a delay in thehealing of an artery after the implantation of polymercoated stents leading to late stent thrombosis.4 Hence,the research for developing novel drug delivery plat-forms for stents is currently aimed at developing eithera more biocompatible polymer platform or a com-pletely polymer-free platform. Drug delivery usingself-assembled monolayer (SAM) belongs to the poly-mer-free platform category.15 A SAM is a coating ofwell-ordered organic molecules that can be depositedon a wide variety of material substrates includingmetals, metal oxides, and ceramics.24 A SAM consistsof a head group that attaches to the material substrate,a long hydrocarbon chain that facilitates the orderingprocess, and a terminal functional group that deter-mines the overall functionality of the monolayer. Awide variety of biomolecules including proteins, DNA,and antibodies have been immobilized to the SAMscoated metal surfaces for potential biomedical appli-cations.14 We have previously demonstrated the use ofSAMs to deliver PAT from two-dimensional (2D)cobalt–chromium (CoCr) alloy metal plates.17 In thisstudy, we explored the use of SAMs platform to deliver

Address correspondence to Gopinath Mani, Biomedical Engi-

neering Program, The University of South Dakota, 4800 N. Career

Ave, Suite 221, Sioux Falls, SD 57107, USA. Electronic mail:

[email protected] Lamichhane and Annemarie Gallo have contributed

equally to this study.

Annals of Biomedical Engineering, Vol. 42, No. 6, June 2014 (� 2014) pp. 1170–1184

DOI: 10.1007/s10439-014-1003-y

0090-6964/14/0600-1170/0 � 2014 Biomedical Engineering Society

1170

PAT from three-dimensional (3D) CoCr alloy cardio-vascular stents.

Extensive literature is currently available for poly-mer-based drug delivery platforms for stents specifi-cally addressing the uses of different coating methodsto deposit drug-containing polymers on stents,8

methods to obtain homogeneous drug-containingpolymer coatings on stents,2 and the techniques toadjust drug dose in the polymer coatings.19 However,very few reports are available for polymer-free drugdelivery platforms for stents especially addressing thedetailed preparation, characterization, and optimiza-tion of such platforms. To the best of our knowledge,no prior studies have been carried out on the follow-ing: (a) effects of different parameters such as solvents,drug concentrations in the coating solution, andcoating methods on the deposition (morphology,homogeneity, and drug doses) of PAT on a polymer-free stent; (b) determining the maximum PAT dosethat can be coated on a polymer-free stent withoutacquiring defects in the coating; (c) effect of balloonexpansion on the PAT coated polymer-free stent; (d)use of SAMs platform to deliver PAT from 3D car-diovascular stents. These are the specific objectives ofthis study.

MATERIALS AND METHODS

Materials

Cobalt-chromium (CoCr) alloy stents of 11.2 mm inlength were purchased from Fortimedix B.V. (Neth-erlands). The dimensions of the stent struts were0.083 mm 9 0.092 mm (width 9 thickness) and thetotal surface area of the stent was 65 mm2. Absoluteethanol, methanol, acetone, 16-phosphonohexadeca-noic acid (16-PHDA), dimethyl sulfoxide (DMSO),HPLC-grade water and acetonitrile, and phosphate-buffered saline with 0.05% Tween-20 (PBS/T-20) wereall purchased from Sigma-Aldrich (USA). Tetrahy-drofuran (THF, anhydrous) was obtained from AlfaAesar (USA). Paclitaxel was purchased from Chemie-Tek (Indianapolis, IN). All chemicals were used asreceived.

Preparation of Chemically Cleaned and SAMsCoated CoCr Stents

CoCr alloy stents were chemically cleaned asdescribed previously.6 Briefly, the stents were sonicatedin organic solvents such as ethanol (Et-OH), acetone,and methanol twice for 10 min each followed bynitrogen (N2) gas drying. A carboxylic acid terminatedphosphonic acid SAM was coated on the chemicallycleaned stents using a previously described procedure.17

Briefly, the cleaned stents were immersed in 3 mL of a1 mM solution of 16-PHDA in THF for 24 h. Afterthat, the stents were immediately transferred to an ovenwithout rinsing, and heat treated in air at 120 �C for18 h. The samples were then removed from the ovenand sonicated in THF and deionized water (di-H2O) for1 min each followed by N2 gas drying.

PAT Deposition on the SAMs Coated Co–CrAlloy Stents

Paclitaxel was deposited on the SAMs coated CoCralloy stents by two different coating methods: (a) dipcoating; (b) spray coating.

Solvents Used for Preparing PAT Solutions

Initially, five different solutions of PAT were pre-pared in Et-OH (100%), DMSO (100%), and themixture of Et-OH and DMSO at different ratios(75:25, 50:50, and 25:75 v/v, Et-OH:DMSO). Theconcentration of PAT in these solutions was 4 mg/mL.The five different solutions of PAT prepared were thenused for dip and spray coating methods to investigatethe effect of solvents (Et-OH, DMSO, and their mix-ture at different ratios) on the morphology andhomogeneity of PAT coating on the stents.

Dip Coating

Dip coating was carried out by immersing the stentsin 1.2 mL of different PAT solutions (prepared asdescribed in ‘‘Solvents Used for Preparing PAT Solu-tions’’ section) at room temperature for 1 h. The stentswere then taken out of the solutions and dried at roomtemperature in air for 18 h.

Spray Coating

A MediCoat stent coating system (Sono-Tek Cor-poration, Milton, NY) was used to spray coat thestents with different PAT solutions (‘‘Solvents Used forPreparing PAT Solutions’’ section). The parametersused to spray coat the stents are listed in Table 1.

Different Concentrations of PAT Used forCoating the Stents

Once the optimal solvent mixture (75:25 v/vEt-OH:DMSO) for obtaining a smooth and homoge-neous PAT coating on stents was determined, thesolvent mixture was then used to prepare PAT solu-tions at different concentrations (2, 3, 4, 8, and 12 mg/mL). The different concentrations of PAT solutionsprepared were used to coat the stents by dipping andspraying. SEM characterization was performed to

A Polymer-Free Paclitaxel Eluting Coronary Stent 1171

analyze the PAT coated stents in an unexpanded stateas well as in an expanded state.

Stent Expansion Using Balloon Catheter

The PAT coated stents were mounted on an angi-oplasty balloon catheter and were expanded to 3 mmin diameter for 1 min using a balloon pressure of9.5 atm. SEM was used to determine the integrity ofPAT coating on the expanded stents.

Scanning Electron Microscopy (SEM)

SEM (Quanta 450, FEI, USA) was the primarysurface characterization technique used in this study toobserve the morphology and homogeneity of PATcoating on stents. SEM images of different magnifica-tions were acquired using an accelerating voltage of30 kV at several different parts of the stents, includingstraight struts, curved struts, and strut intersections(labeling of different parts of the stents is shown inFig. 1a). SEM images were captured from at least 15distinct spots on each stent used in this study, and therepresentative images are provided here. Prior to SEMimaging, the stents were sputter coated with a 15 nmthickness of gold–palladium to prevent surface charg-ing by the electron beam.

Drug Quantification

The amount of PAT deposited on the stents wasdetermined as follows. The PAT coated stents weresonicated in 2 mL of Et-OH for 10 min thrice, withfresh solvent used each time. The three ethanol samplescollected were characterized using high performanceliquid chromatography (HPLC) to determine theamount of PAT extracted from the stents.

Criteria Used for Selecting the Optimal PAT CoatedStent

The SEM images and drug quantification studies ofPAT coated stents prepared using different concentrations

of PATwere analyzed and the optimal PAT coated stentin each coating method category (dip and spray) wasselected based on the following criteria: (a) smoothmorphology of PAT coating; (b) homogenous coating;(c) maintaining coating integrity with negligible defectsafter stent expansion; (d) adequate amount of drugloading. The optimal stent selected in each coatingmethod category was then used for additional surfacecharacterization using optical surface profilometry andcontact angle goniometry, followed by drug-elutionstudies.

Optical Surface Profilometry (OSP)

The 3D surface topography and average roughnessvalues (Sa) of the chemically cleaned, SAMs coated,and PAT deposited stents were obtained using aWYKO NT8000 optical surface profilometer (BrukerCorporation, operated at Michigan Metrology, LLC).The average surface roughness was calculated fromnine distinct spots on a stent sample and reported herealong with its corresponding standard deviation.

Contact Angle Goniometry

A VCA optima system (AST products) was used tomeasure contact angles on chemically cleaned, SAMscoated, and PAT deposited stents. A series of testswere conducted to determine the volume (0.5 lL) ofdroplet required to be accurately placed on stent struts.Then, the determined volume of 0.5 lL of di-H2O andmethylene iodide was carefully placed on top of thestent strut, and static contact angles were measured onboth sides of the drop after 15 s. The contact anglesreported here represent the mean ± standard devia-tions of four distinct spots on a stent for each category.Using the water and methylene iodide contact angles,the surface energies of the samples were determined bythe harmonic mean method.

In Vitro Drug Release Studies

The PAT coated stents (n = 3 for each dip and spraycoated stents) were immersed in 20 mL of PBS/T-20and incubated in a circulating water bath at 37 �C. As astandard procedure, a non-ionic surfactant, Tween-20,was added to increase the solubility of PAT in PBS andto maintain sink conditions.11 The stents were taken outof the solution at several pre-determined time points(1, 2, 3, 5, 7, 10, 14, 18, 22, 26, 33, 40, 48, 57, 63, and70 days) and transferred to a fresh PBS/T-20 solutionuntil the next time point. The PBS/T-20 solutions col-lected at each time point were then analyzed for theamount of PAT released using HPLC. As a standardprocedure, prior to the HPLC analysis, 1 mL of Et-OH

TABLE 1. Parameters used in spray coating.

Parameters Values

Distance from nozzle tip to stent 8.89 mm

Ultrasonic power 0.7 W

Syringe pump dispense rate 0.33 lL/s

Focusing gas pressure 3.45 MPa

Drying gas pressure 13.8 MPa

Rotation rate 7.33 rad/s

Horizontal translation speed 2.5 mm/s

Number of loops 120

Drying time in nitrogen 1.8 9 103 s

Drying time in air 9 9 102 s

LAMICHHANE et al.1172

was added to the PBS/T-20 solution to remove anyphysically adsorbed PAT on the polypropylene con-tainer surfaces used in drug-elution studies.16

High-Performance Liquid Chromatography (HPLC)

The HPLC analysis was performed using a Waterse2695 system equipped with a Waters 2489 UV/Visdetector. A Nova-Pak C18 column (3.9 mm 9 150 mm;particle size: 4 lm) was used. The HPLC protocol fordetermining PAT was followed as reported previously.16

Briefly, a mobile phase of 45% water and 55% aceto-nitrile was used at a flow rate of 1 mL/min. A 10 lLvolume of the sample was injected for the analysis. Thecolumn was maintained at a temperature of 35 �C, andthe UV detector was set to detect the wavelength at227 nm. For the calibration, a linear plot was obtainedwith correlation coefficients (R2) of 0.9998, 0.9997, and

0.9996 for the concentration ranges of 0.003–0.1, 0.1–1,and 1–100 lg/mL, respectively.

Statistical Analysis

The experimental data collected are provided asmean ± standard deviation. A one-way analysis ofvariance (ANOVA) was carried out to determine thestatistical significance at p< 0.05.

RESULTS

SEM Characterization of the Chemically Cleanedand SAMs Coated Stents

Figure 1 shows the SEM images of chemicallycleaned and SAMs coated stents. Low magnification

FIGURE 1. SEM images of labeling of different parts of a stent (a), chemically cleaned stent (b–d), and SAMs coated stent (e–g).


(Figs. 1b and 1e) and high magnification images(curved struts in Figs. 1c and 1f and stent intersectionsin Figs. 1d and 1g) are provided. The chemicallycleaned stent surfaces appear smooth with no surfacedefects or irregularities (Figs. 1b–1d). Similar mor-phology was observed for the SAMs coated stents(Figs. 1e–1g) as well.

Effect of Solvents on the Deposition of PATon the SAMs Coated Stents

The SEM images of the PAT coated stents preparedby dip coating using a 4 mg/mL concentration of PATwith solvents Et-OH, DMSO, and their mixture atdifferent ratios are shown in Fig. 2. Three images (onelow and two high magnifications) are provided foreach of the PAT coated stents. PAT coating preparedusing 100% Et-OH showed a non-homogenous dis-tribution of the drug (Figs. 2a–2c). The drug coatingwas too thick at the strut intersections and producedcracks (Fig. 2b). Also, large needle shaped PAT crys-tals with rough morphology were evident in some partsof the stent (Fig. 2c). However, after the addition of25% volume of DMSO to Et-OH (75:25 v/vEt-OH:DMSO), the PAT coating appeared smoothand uniform with no defects observed throughout thestent (Figs. 2d–2f). For 50:50 v/v Et-OH:DMSO,although the coating was smooth and uniform in mostparts of the stent (Fig. 2g), shrinkage was observedespecially near the curved struts (Figs. 2h and 2i). Withthe further addition of DMSO (25:75 v/vEt-OH:DMSO), the inner loop of curved struts showeda large accumulation of needle shaped PAT crystals(Fig. 2k) with few cracks (Fig, 2l). The PAT coatingprepared using 100% DMSO showed a non-uniformprecipitation of the drug (Figs. 2m–2o).

Figure 3 shows the low and high magnificationSEM images of the PAT coated stents prepared byspray coating using a 4 mg/mL concentration of PATwith different solvents. For 100% Et-OH, a non-homogenous drug coating was clearly evident from theSEM images (Figs. 3a–3c). The agglomerates andclumps of PAT were unevenly present on the stentstruts (Figs. 3b and 3c). For 75:25 v/v Et-OH:DMSO),the coating looked smooth with the drug homoge-neously distributed throughout the stent (Figs. 3d–3f).For 50:50 v/v Et-OH:DMSO, some defects such aswebbing were observed in the curved stent struts(Figs. 3g and 3h). Also, shrinkage of the drug coatingwas observed in some of the inner loops of the curvedstruts (Fig. 3i). For 25:75 v/v Et-OH:DMSO, a non-homogenous distribution of the drug was observedwith an unevenly distributed small PAT crystals andcracks at the strut intersections (Figs. 3j–3l). For 100%DMSO as well, the coating was not uniform with

loosely present PAT crystals, and bridging of drugcoating between curved struts was observed (Figs. 3mand 3n).

Based on these results, the mixture of Et-OH andDMSO at a ratio of 75:25 v/v was determined to be theoptimal solvent mixture for producing a smooth andhomogenous PAT coating on stents with no defectsirrespective of the coating method (dip or spray)employed.

Effect of Different Concentrations of PAT on theDeposition of Drug on the SAMs Coated Stents

The optimized solvent mixture (75:25 v/vEt-OH:DMSO) was used to prepare different concen-trations of PAT at 2, 3, 4, 8, and 12 mg/mL. The dif-ferent concentrations of PAT prepared were used todip and spray coat the stents. SEM images were takenbefore and after expanding the PAT coated stentsusing a balloon catheter. The SEM images of PATcoated stents prepared by the dip coating method usingdifferent concentrations of PAT are shown in Fig. 4.For the drug concentrations of 2, 3, and 4 mg/mL ofPAT, the coating was smooth and uniform with nodefects (Figs. 4a–4f) observed before stent expansion.The SEM images of expanded stents for these drugconcentrations are shown in Figs. 4k–4p. No mor-phological changes in the drug coating were observedafter stent expansion. Similar results were observed for8 mg/mL of PAT (Figs. 4g and 4h) in which thecoating was smooth and uniform before stent expan-sion. After the stent expansion, although minor drugflakes were observed in very few curved struts, theintegrity of the coating was well maintained through-out the stent (Figs. 4q and 4r). For 12 mg/mL con-centration of PAT, a greater amount of the drugaccumulated in the curved struts leading to the for-mation of cracks (Figs. 4i and 4j). Upon stent expan-sion, the cracks appeared more prominent with thedrug flaking off from the stent struts (Figs. 4s and 4t).

Figure 5 shows the SEM images of PAT coated stentsprepared by the spray coating method using differentconcentrations of PAT. A homogeneous drug coatingwas observed for the stents prepared using 2 and 3 mg/mL concentrations of PAT (Figs. 5a–5d). These stentsalso maintained their coating integrity upon expansion(Figs. 5k–5n). For 4 mg/mL concentration of PAT, nodefects were observed in the unexpanded stents(Figs. 5e–5f). However, upon expansion, a few cracksanddrugflakeswere observed on someof the inner loopsof strut intersections and curved struts (Figs. 5o and5p).For 8 and 12 mg/mL concentrations of PAT, the drugaccumulated at the curved struts (Figs. 5g and 5i) andstrut intersections (Figs. 5h and 5j) leading to theformation of cracks (Fig. 5h) even before the stent


expansion. More irregularities were observed after thestent expansionwith drug agglomerates flaking off fromthe stent surfaces (Figs. 5q–5t).

The total amount of the drug loaded on the stentsusing different concentrations of PAT in coatingsolution is listed in Table 2. Depending on the con-centration of PAT used in the coating solution, thetotal amount of the drug loaded on stents ranged from3.2 to 71.1 lg and from 27.2 to 249.6 lg for dip andspray coating methods, respectively. The total surface

area of the stents used in this study is 64.9352 mm2.Hence, the drug dosage (in lg/mm2) per stent was alsocalculated and provided in Table 2. The drug dosageranged from 0.05 to 1.1 lg/mm2 and from 0.42 to3.84 lg/mm2 for dip and spray coating methods,respectively (Table 2). It was determined that the PATcoated stents prepared using 8 mg/mL concentrationof PAT for dip coating and 3 mg/mL concentration ofPAT for spray coating showed an adequate amount ofthe drug doses (0.35 lg/mm2 for dip coating and

FIGURE 2. PAT coated stents prepared by dip coating method using 100% Et-OH (a–c), 75:25 v/v (Et-OH:DMSO) (d–f), 50:50 v/v(Et-OH:DMSO) (g–i), 25:75 v/v (Et-OH:DMSO) (j–l), and 100% DMSO (m–o).


0.76 lg/mm2 for spray coating) as well as negligibledefects observed in the coating before or after stentexpansion. Thus determined, these two groups ofoptimal stents were further characterized using opticalsurface profilometry and contact angle goniometry,and then used for the drug-elution studies.

Optical Surface Profilometry Characterization

The parameters that were used in the OSP analysisare provided in Table 3. The 3D surface topography

images of the chemically cleaned, SAMs coated, andPAT coated stents are shown in Fig. 6. The averagesurface roughness (Sa) values of the samples are alsoincluded in the images. The chemically cleaned stentsshowed an Sa of 3.4 ± 0.6 nm. No significant differ-ence in the Sa was observed between the SAMscoated (4.6 ± 1.4 nm) and the chemically cleaned.This suggests that the SAMs coating was uniform andfollowed the contour of the underlying stent surface.The PAT coated stents prepared by dip coatingshowed an Sa of 4.4 ± 1.7 nm. This suggests the

FIGURE 3. PAT coated stents prepared by spray coating method using 100% Et-OH (a–c), 75:25 v/v (Et-OH:DMSO) (d–f), 50:50 v/v(Et-OH:DMSO) (g–i), 25:75 v/v (Et-OH:DMSO) (j–l), and 100% DMSO (m–o).


deposition of smooth and homogeneous PAT coatingon the SAMs coated stents. The PAT coated stentsprepared by spray coating showed an Sa of7.8 ± 2.1 nm, which suggests that the PAT coatingprepared by the spray coating was slightly rougherthan that of the dip coating.

Contact Angle Goniometry Characterization

The water contact angles of the chemically cleaned,SAMs coated, PAT coated (by dipping), and PATcoated (by spraying) are 112.9 ± 4.6�, 84.9 ± 2.9�,105.7 ± 12.3�, and 100.1 ± 4.1�, respectively. The

FIGURE 4. SEM images of unexpanded (a–j), and expanded (k–t) PAT coated stents prepared by dip coating method usingdifferent concentrations of PAT: 2 mg/mL (a, b, k, l), 3 mg/mL (c, d, m, n), 4 mg/mL (e, f, o, p), 8 mg/mL (g, h, q, r), and 12 mg/mL (i, j,s, t).


methylene iodide contact angles of the chemicallycleaned, SAMs coated, PAT coated (by dipping), andPAT coated (by spraying) are 77.5 ± 6.9�, 81.9 ± 5�,85.4 ± 3.5�, and 84.4 ± 4.8�, respectively. Based onthe harmonic mean method, the chemically cleanedstent surface showed a lower surface energy of

22.9 dynes/cm. After SAMs coating, the surface energysignificantly increased to 29.7 dynes/cm. This suggeststhe successful deposition of –COOH terminated mon-olayers on the stent surfaces. An increase in the surfaceenergy is expected since the terminal groups (–COOH)of SAMs are hydrophilic in nature. After PAT coating,

FIGURE 5. SEM images of unexpanded (a–j), and expanded (k–t) PAT coated stents prepared by spray coating method usingdifferent concentrations of PAT: 2 mg/mL (a, b, k, l), 3 mg/mL (c, d, m, n), 4 mg/mL (e, f, o, p), 8 mg/mL (g, h, q, r), and 12 mg/mL (i, j,s, t).


the surface energy significantly reduced to 22.6 dynes/cm. The decrease in the surface energy is expected sincePAT is a hydrophobic drug with plenty of –CH3 and –CH2 groups in its chemical structure.

In Vitro Drug Release Studies

A biphasic drug release profile with an initial burstfollowed by a slow and sustained release of PAT wasobserved for up to 10 weeks for both dip (Fig. 7) andspray (Fig. 8) coated stents. Figure 7b shows theamount of the drug released between every two con-secutive time points for dip coated stents. On the firstday, 4.14 ± 0.95 lg of PAT was released, which isshown in the primary Y-axis of Fig. 7b. This initialburst was followed by a sustained release ranging from0.3 to 1 lg of PAT for the later time points till day-70which is shown in the secondary Y-axis of Fig. 7b.Similarly, for the spray coated stents, on the first day,6.24 ± 2.36 lg of PAT was released, which is shown inthe primary Y-axis of Fig. 8b. The initial burst wasthen followed by a sustained release ranging from 0.2to 0.5 lg of PAT for later time points till day-70, whichis shown in the secondary Y-axis of Fig. 8b.

DISCUSSION

A DES should have a smooth and homogeneouscoating. The coating should remain intact during thestent expansion process. Also, an adequate amount ofdrug dose should be present in the coating to inhibitneointimal hyperplasia. Hence, the optimization ofcoating process is crucial for achieving the abovementioned criteria, especially for a polymer-free DES,since the number of reports available on this topic isvery limited. In this study, the effects of solvents,concentrations of drug in the coating solution, andcoating methods were investigated for optimizing thePAT coating on the SAMs coated CoCr alloy cardio-vascular stents.

Several organic solvents such as ethanol, methanol,acetone, DMSO, chloroform, and methylene chloridehave been used for making PAT solutions.13,22 How-ever, ethanol and DMSO are the most commonly usedsolvents for PAT because of the better solubility andlonger stability of the drug in these solvents.22 Also,the implant coatings that have been prepared usingEt-OH and DMSO are commonly used without havingany biocompatibility issues.1,7 DMSO has also beenshown to inhibit the growth of smooth muscle cells,which is the main cell type involved in the occurrenceof neointimal hyperplasia.5 Hence, Et-OH, DMSO,and their mixture at different ratios were evaluated forthe PAT coating on stents. The PAT coating preparedusing either 100% Et-OH or 100% DMSO did notproduce a smooth and homogeneous coating. Ethanolhas a lower boiling point (78 �C). Hence, the solventevaporated quickly, which resulted in the formation oflarge and non-uniform PAT crystals on the stent.DMSO has a higher boiling point (189 �C). Hence, thesolvent evaporated very slowly, which resulted in theformation of non-uniform precipitation of PAT onstents. However, the mixture of Et-OH and DMSO,especially at a ratio of 75:25 v/v Et-OH:DMSO, pro-duced an optimal coating. This suggested that whenEt-OH and DMSO were mixed together, the evapo-ration time of the mixed solvent was adjusted forfavoring the formation of a smooth, homogeneous,

TABLE 2. Total amount of drug loaded and drug doses on dip and spray coated stents prepared using different concentrations ofPAT in the coating solution.

PAT concentration in

coating solution (mg/mL)

Dip coated stents Spray coated stents

Total drug loaded (lg) Drug dose (lg/mm2) Total drug loaded (lg) Drug dose (lg/mm2)

2 3.15 0.05 27.18 0.42

3 4.56 0.07 49.51 0.76

4 4.98 0.08 52.43 0.81

8 22.52 0.35 178.62 2.75

12 71.09 1.1 249.55 3.84

TABLE 3. Parameters used in optical surface profilometrycharacterization.

Measurement Attribute Values

Magnification 99.59

Measurement array size 640 9 480

Lateral sampling 0.1 lm

Field of view 64 lm 9 48 lm

3D filter Gaussian – Short Wave Pass

Long Wavelength

Cutoff = 0.01 mm

Short Wavelength

Cutoff = 0.001 mm

Height resolution < 0.0003 lm

Bearing ratio offsets peak/valley 1%/1%

Stylus X kc/ks 60 lm/0.6 lm

Stylus Y kc/ks 35 lm/0.4 lm

Stylus filter type Gaussian


and thin film-like morphology of PAT coating onstents.

The commercially available polymer-based DEStypically carry 1 lg/mm2 of the drug.10 Our objectivewas to determine the maximum drug dose that can becoated on a polymer-free stent without acquiringdefects in the coating before or after the expansion ofstents. An increase in the concentration of PAT in thecoating solution resulted in an increased drug dose onthe stents for both the coating methods employed.Although higher amount of PAT concentration(12 mg/mL) in the coating solution resulted in greateramount of drug doses (1.1 lg/mm2 for dipping and3.84 lg/mm2 for spraying) on stents, the coatings ofsuch high drug doses had some defects. It was deter-mined in this study that 0.35 and 0.76 lg/mm2 of PATcan be coated on the SAMs coated stents using dip andspray coating methods, respectively, with negligibledefects observed before or after the expansion ofstents. Although these drug doses were lesser than thatof the commercially available polymer-based DES,

several reports have previously shown that the doses inthe range of 0.1–0.7 lg/mm2 are clinically relevant forinhibiting neointimal hyperplasia.3,9,20 Bhargava et al.3

showed that 0.2 and 0.4 lg/mm2 doses of PAT onstents were able to inhibit neointimal hyperplasia in aporcine model. In another study, Jabara et al.9 haveshown that a dose of 0.15 lg/mm2 of PAT not onlyinhibited neointimal hyperplasia but also reduced thedrug toxicity and promoted vessel healing. Shinkeet al.21 compared the effects of a stent carrying a doseof 0.025 lg/mm2 of PAT with a commercially availablestent carrying a dose of 1 lg/mm2. Both the dosesinhibited neointimal hyperplasia with no significantdifferences observed between them. Based on theseliterature, it is believed that the drug doses coated onthe SAMs coated stents are adequate for inhibitingneointimal hyperplasia.

Dipping and spraying are commonly used for coat-ing stents. For every concentration of PAT used, thespray coating loaded significantly more amount of drugthan that of the dip coating. This could be due to the

FIGURE 6. 3D optical surface profilometry topography images of chemically cleaned stents (a), SAMs coated stents (b), PATcoated stents prepared by dip coating method (c), and PAT coated stents prepared by spray coating method (d).


inherent differences between spray coating and dipcoating methods. In spray coating, the drug solutionwas sprayed on the stent at a gas pressure of 3.45 MPaalong with a slow and constant horizontal back andforth movement of the stent at a speed of 2.5 mm/s for120 cycles (50 min), followed by drying under N2 gas ata pressure of 13.8 MPa for 30 min. Hence, this methodwas able to load more amount of drug dose (up to0.76 lg/mm2) on the stents without acquiring anydefects in the coating. However, in dip coating, the drugis loaded on the stent during the immersion process(1 h), followed by air drying at room temperature for18 h. Hence, only 0.35 lg/mm2 (~half the amount ofdrug dose that was coated using spraying) of PAT wascoated on the stents by dip coating without defects.

The different drug release profiles of TAXUS, acommercially available PAT eluting stent, have beencompared previously.10 The drug release was differentdepending on the solvent ratio used to prepare thePAT containing polymer coating and the drug topolymer ratio. Based on these factors, three differentformulations including slow, medium, and fast releas-ing polymer platforms were prepared with the samedrug dose density and the total drug dose. Althoughthe amount of drug released from these formulationswas different, all the release profiles showed an initialburst release (typically within 1 or 2 days) followed bya slow and sustained release for up to 10 or 15 days.10

Based on this literature, the type of drug release profile(biphasic—an initial burst followed by a slow and

FIGURE 7. Cumulative PAT released from dip coated stents (a), amount of PAT released between every two consecutive timepoints of dip coated stents (b).


sustained release) obtained for the SAMs platform issimilar to that of the currently used DES.

We have previously reported the nature of PATcoating on –COOH terminated SAMs coated 2D CoCralloy surfaces.17 The two bonding interactions thatassist in the coating of PAT on –COOH terminatedsurfaces are (a) the hydrogen bonding between thePAT and –COOH terminated surfaces; (b) an extensiveintermolecular hydrogen bonding between the PATmolecules. PAT is released by the breakage of hydro-gen bonds between the drug and the –COOH termi-nated surfaces, and between the PAT molecules by theions in the physiological solution such as PBS/T-20.17

In agreement with the 2D studies, the PAT wasreleased from the SAMs coated 3D cardiovascularstents in a biphasic manner with an initial burst

followed by a slow and sustained release for up to10 weeks. Our future studies are focused on investi-gating the in vivo drug uptake and biocompatibility ofthese optimized PAT coated stents in an animal model.

CONCLUSIONS

The effects of solvents, drug concentrations in thecoating solution, and coating methods were investi-gated for optimizing the PAT coating on SAMs coated3D cardiovascular stents. A ratio of 75:25 v/vEt-OH:DMSO was determined as an optimal solventmixture to produce a smooth and homogeneous PATcoating on stents irrespective of the coating methods(dipping or spraying) employed. A 8 mg/mL (for dip

FIGURE 8. Cumulative PAT released from spray coated stents (a); amount of PAT released between every two consecutive timepoints of spray coated stents (b).


coating) and 3 mg/mL (for spray coating) concentra-tions of PAT in the coating solution was determined tobe the optimal concentrations for producing adequatedrug doses (0.35 lg/mm2 by dip coating and 0.76 lg/mm2 by spray coating) on the stents with negligibledefects observed in the coating before or after the stentexpansion. PAT was successfully released from SAMscoated cardiovascular stents in a biphasic manner withan initial burst followed by a slow and sustainedrelease for up to 10 weeks.

ACKNOWLEDGMENTS

This study was supported by a National ScientistDevelopment Grant Award (10SDG2630103) from theAmerican Heart Association (AHA). The authors aregrateful to the AHA for providing the financial sup-port.

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Documents

A Polymer-Free Paclitaxel Eluting Coronary Stent: Effects of Solvents, Drug Concentrations and Coating Methods