Biocompatible Two-Layer Tantalum/Titania−Polymer Hybrid Coating

Biocompatible Two-Layer Tantalum/Titania-Polymer HybridCoating

Elisa Cortecchia,† Annalisa Pacilli,‡ Gianandrea Pasquinelli,‡ and Mariastella Scandola*,†

Dipartimento di Chimica “G. Ciamician”, Universita di Bologna and INSTM UdR Bologna, Via Selmi 2,40126 Bologna, Italy, and Dipartimento Clinico di Scienze Radiologiche e Istocitopatologiche, Sezione di

Patologia Clinica, Universita di Bologna, Policlinico S. Orsola-Malpighi, Via Massarenti 9, 40138 Bologna, Italy

Received June 4, 2010; Revised Manuscript Received July 16, 2010

Using a two-step procedure, radiopaque and biocompatible coatings were obtained, consisting of a tantalum layerdeposited by sputtering technique and of an upper organic-inorganic hybrid layer synthesized via sol-gel. Asshown by radiographic images, tantalum confers to plastic substrates good X-ray visibility, adjustable via controlof deposition time, but its adhesion to the substrate is poor and manipulation easily damages the metal layer.Polymer-titania hybrid coatings, synthesized using poly-ε-caprolactone (PCL) or carboxy-terminated polydim-ethylsiloxane (PDMS) as organic precursors, were applied on the metal layer as biocompatible protective coatings.Biocompatibility is demonstrated by cytotoxicity tests conducted using vascular wall resident-mesenchymal stemcells (VW-MSCs). Both coatings show very good adhesion to the substrate, showing no sign of detachment uponlarge substrate deformations. Under such conditions, SEM observations show that the PCL-containing hybridforms cracks, whereas the PDMS-based hybrid does not crack, suggesting possible applications of the latter materialas a protective layer of sputtered tantalum radiopaque markers for flexible medical devices.

Introduction

X-ray identification and traceability nowadays are an essentialrequirement for medical devices in many surgical and medicalapplications. As a matter of fact, medical devices such asvascular stents or catheters with good radiopacity can be easilypositioned (and later on detected) inside the human body throughfluoroscopy and X-ray radiography, thus, avoiding invasiveprocedures on patients. The use of traceable devices hassignificantly improved patient treatment quality and has gener-ated in the literature growing interest toward the developmentof new radiopaque biomaterials. Medical devices with improvedradiopacity can be obtained following two main strategies.Radiopacity is achieved in one case by modifying the bulkmaterial that constitutes the device,1-3 and in the other byapplication at the device surface either of a radiopaque coating4,5

or of small bulk radiopaque markers attached to the devicestructure.6,7 As concerns the first route, many polymericmaterials with improved visibility have been obtained by theincorporation into the polymer of radiopaque compounds, suchas barium8,9 and strontium salts,10 or by linking to the polymerchain through covalent bonds radiopaque, typically halogen-containing, moieties.11-14 Highly radiopaque materials havebeen also successfully produced by blending polymers withradiopaque metals.15-17 Although it is possible to obtain devicescharacterized by an excellent level of radiopacity, the principaldrawback in choosing this strategy is that the addition of theradiopaque component in the bulk material often leads to changein its properties,15,18,19 negatively affecting device performance.

Impact on bulk material properties can be substantiallyreduced if a coating of an intrinsically radiopaque material isapplied at the device surface rather than incorporating it in the

bulk structure. In this context, deposition of a thin coating layerof a radiopaque compound at the device surface is a quite simpleprocess compared with the complex and costly fixing technolo-gies currently used to apply external small radiopaque bulkymarkers to biomedical devices. A strong improvement of deviceX-ray visibility has been obtained by the application of a surfacelayer of a radiopaque metal, such as gold20 or tantalum,21 usingphysical vapor deposition techniques or electroplating. Theincrease of visibility is significant even if a µm-thick layers areapplied. However, such metal coatings have some maindrawbacks, such as (i) low adhesion to the substrate leading,upon metal corrosion, to the release of ions inside the humanbody and (ii) inability of the metal coating to follow substratedeformation, thus, causing surface cracking.22

An alternative to metal coatings is represented by radiopaquehybrid organic-inorganic coatings. Organic-inorganic hybridsare materials that combine characteristics of organic polymerswith those of inorganic metal oxides. They show novelproperties with respect to those of the pure constituents, thanksto the existence of strong interactions (either covalent ornoncovalent) between organic and inorganic components.23

Hybrids can be synthesized through the sol-gel process, amethod developed for the synthesis of pure inorganic metaloxides starting from alkoxide precursors, which proceeds witha sequence of hydrolysis and condensation reactions leading tothe formation of an amorphous inorganic network.24 Sol-gelreactions involving titanium tetraisopropoxide (TIPT) as theinorganic phase precursor in the synthesis of titania (TiO2) arereported in Scheme 1 as an example.

A simple strategy to synthesize organic-inorganic hybridsconsists in conducting the sol-gel reaction in the presence ofa suitable polymeric component that may either enter thereaction condensation step forming covalent bonds with thesynthesized inorganic oxide or strongly interact with the latterin an alternative manner.25 Sol-gel is a process that needs tobe carried out in a solvent; this feature allows hybrid materials

* To whom correspondence should be addressed. Tel.: +39 051 2099577.Fax: +39 051 2099456. E-mail: [email protected].

† Dipartimento di Chimica “G. Ciamician”.‡ Dipartimento Clinico di Scienze Radiologiche e Istocitopatologiche,

Sezione di Patologia Clinica.

Biomacromolecules 2010, 11, 2446–24532446

10.1021/bm100619t 2010 American Chemical SocietyPublished on Web 07/27/2010

to be easily applied as thin coatings on both flat and irregularlyshaped substrates by either doctor blade or dip coatingprocedures. The possibility to easily design hybrid materialswith tailored properties by choosing convenient organic andinorganic phase constituents, combined with the very simplesynthetic production procedure, are the key factors that havepromoted the investigation of this class of materials for a widerange of applications. When the polymer phase is properlychosen, hybrid coatings are significantly more flexible than metalcoatings.26 Moreover, unlike the latter, hybrids show very goodadhesion to a variety of different types of substrates, includingmetal and plastics.27

Good radiopacity of organic-inorganic hybrids has beenrecently reported, suggesting that these materials might findapplications in medical fields where X-ray visibility isrequired.28,29 Because radiopacity is associated with the presenceof the inorganic component, the latter content must be quitehigh for thin hybrid coatings to reach a good level of radiopacity.As a consequence, the coating layer tends to be rigid and toform cracks upon deformation.

The present work presents an alternative solution for enhanc-ing medical device radiopacity that combines advantages of bothmetallic and hybrid coatings. High metal radiopacity is associ-ated with good mechanical and protective properties oforganic-inorganic hybrids in the form of a double-layer coating.Tantalum, a highly radiopaque metal already employed in themedical field,30-34 is chosen as the constituent of the first layerdeposited on sample substrates by means of a sputteringtechnique. The second layer is composed of a hybrid whoseconstituents are well-known biocompatible organic and inor-ganic components, such as the polymers poly-ε-caprolactone(PCL), polydimethylsiloxane (PDMS), and titanium oxide,respectively.

Experimental Section

Materials. Poly-ε-caprolactone (PCL, Mn ) 10 kDa, Aldrich),polydimethylsiloxane dicarboxypropyl-terminated (PDMS, Mn ) 28kDa, ABCR Gmbh), titanium tetraisopropoxide (TIPT, 97%, Aldrich),tetrahydrofurane (THF, absolute, Fluka), concentrated hydrochloric acid(HCl, 37%, Aldrich), hydrogen peroxide (Carlo Erba Reagents), smoothmuscle growth medium-2 (SmGM-2, Lonza), and Trypan blue solution(Sigma) were used as received without further purification.

Instrumental Techniques. Tantalum was deposited on a polypro-pylene (PP) substrate (PP sheet, 150 µm thick, without any pretreatmentprior to Ta deposition) employing a Bal-Tec MED 010 sputter coater

and a Ta sputtering target (99.9% purity, PI-KEM Ltd.). Sputteringwas carried out in an argon atmosphere (pressure: 8 × 10-3mbar,current: 120 mA, target-to-substrate distance: 5 cm), after evacuationof the sputtering chamber (Ar pressure <10-4mbar). The superficialoxide layer on Ta target was preliminarily removed by applying acurrent of 100 mA for 5 min (Ar pressure 8 × 10-3 mbar). Scanningelectron microscopy (SEM) observations were carried out using aPhilips 515 scanning electron microscope at 15.0 kV. SEM pictureswere analyzed by means of Zeiss AxioVision 3.0 software. Sputtercoating with gold was applied to hybrid-coated samples prior to SEManalysis. AFM characterization was carried out on a Veeco NanoscopeIIIa in tapping-mode. Images were collected using a Si probe operatedat a resonance frequency of 330 kHz and scanned at a rate of 1 Hz.Radiographic images were recorded on a mammographic apparatus(Diamond MGX Instrumentarium Co., Imaging Division) equipped witha radiogenic tube Varian M113SP (experimental parameters: 22 kV, 4mA/s, 20 cm sample-to-detector distance) and on an angiographicapparatus (OEC 9800 Plus, GE Medical Systems, experimentalparameters: 55 kV, 20 cm sample-to-detector distance). X-ray filmswere digitized with an EPSON V10 scanner at 300dpi resolution andobtained images were analyzed by means of ImageJ 1.42q software.Thermogravimetric (TGA) measurements were carried out using a TA-TGA 2950. The analyses were performed at 10 °C/min from roomtemperature to 600 °C under air flow. Infrared spectra were recordedusing a Nicolet 380 FT-IR spectrometer (32 scans from 4000 to 400cm-1, resolution: 4 cm-1). Insoluble solid samples were ground withKBr (1 mg sample/100 mg KBr) and were pelletized under pressure.Soluble samples were dissolved in chloroform and were cast on KBrdiscs, allowing total solvent evaporation prior to measurement. Allsamples were dried overnight under vacuum in a desiccator over P2O5

before FT-IR measurements. Differential scanning calorimetry (DSC)measurements were performed using a TA DSC 2010 apparatus.Samples were placed in open Al pans and subjected to two consecutiveruns from -100 to 100 °C at 20 °C/min. Quench cooling was performedbetween scans.

Synthesis of Hybrids and Coating on Substrates. PCL hybrid(HPCL) was synthesized starting from a ratio of organic to inorganicprecursor of 50/50 w/w; the synthesis procedure was as follows: 2 mLof THF were added to 1.0 g of TIPT under vigorous stirring (solution1); a solution containing 1.0 g of PCL in 4 mL of THF (solution 2)was then slowly added to solution 1; the mixture was stirred for a fewminutes to obtain a clear homogeneous solution (solution 3). Solution3 was then cast on Teflon sheet, on PP sheet, and on Ta-coated PPsheet and the excess solution was removed with the aid of a slidingglass rod maintained at 200 µm distance from the substrates (doctorblade technique). After 24 h at room temperature (gelation), all sampleswere thermally treated for 2 h in an oven at 100 °C under vacuum(curing). PDMS hybrid (HPDMS) was synthesized starting from anorganic to inorganic precursor ratio of 70/30 w/w; the syntheticprocedure was the same as that described above for HPCL with the twofollowing changes: solution 1 contained 0.6 g of TIPT, 2.4 mL of THF,and 5.2 µL of HCl (molar ratio HCl/TIPT ) 0.06:1) and solution 2contained 1.4 g of PDMS in 6 mL of THF.

Cytotoxicity Tests. Cytotoxicity tests were conducted using humanvascular wall resident-mesenchymal stem cells (VW-MSCs) derivedfrom human femoral artery.35 After sterilization under UV light for2 h (1 h each side), samples were exposed to cell culture mediumSmGM-2 for 24 h at 37 °C (RH g 95%; CO2: 5%). The medium wasthen recovered and used to incubate for 24 h VW-MSCs, previouslyseeded (50.000 cells/well) in a 12-well culture plate (Corning Incor-porated, Euroclone). After that, cells were detached with 0.25%trypsin-EDTA, diluted in trypan blue solution, and both vital and deadcells were counted with a Neubauer hemocytometer. A negative controlwas run by exposing cells to plain fresh culture medium for 24 h, whilefor positive control cells were exposed to 1 mM H2O2 for 2 h beforecounting. Five replicates were run for each experiment. Cells wereobserved using a NIKON Eclipse TS100 inverted microscope.

Scheme 1. TIPT Sol-Gel Reactions Leading to TiO2 Synthesis

Tantalum/Titania-Polymer Hybrid Coating Biomacromolecules, Vol. 11, No. 9, 2010 2447

Results and Discussion

Sputtering is a physical vapor deposition technique that allowsto deposit a thin coating of a chosen material on a substrate36

yielding high purity layers whose thickness can be controlledby varying deposition time. Sputtering has been previously usedfor the production of radiopaque metal coatings on biomedicaldevices.22 In this work tantalum coatings with different thick-nesses were deposited on plastic flexible substrates made ofpolypropylene (PP). Deposition times of 30, 60, and 90 minwere applied, and the obtained samples were labeled, respec-tively, Ta1, Ta2, and Ta3. To evaluate the thickness of thedeposited layer, Ta was also contemporarily sputtered onaluminum stubs half-covered with adhesive tape. Removal oftape yielded a neat step whose height corresponds to thethickness of the deposited Ta layer. Metal layer thickness,evaluated by SEM, gradually increases from 1 to 5 µm withchanging sputtering time from 30 to 90 min (Table 1). Thesample sputtered for 30 min was also analyzed by AFM,obtaining a value of 0.91 µm for the metal layer thickness, ingood agreement with the value from SEM.

All PP-coated samples were then analyzed using X-rayradiographic equipment (mammograph) to verify their radio-pacity and the obtained X-ray images are reported in Figure 1.It is clearly seen that the uncoated PP is almost undistinguishablefrom the background, whereas application of the tantalumcoating provides good X-ray visibility to the plastic substrates.In addition, contrast of the X-ray images, that is, radiopacity,clearly increases with tantalum coating thickness.

The intensity of the transmitted X-ray radiation (I) through asample, which decreases as the radiopacity of the analyzedmaterial increases, can be correlated with the radiopaque layerthickness by the following equation:37

where I0 is the intensity of the incident radiation, m is the massattenuation coefficient of the material, F is the density, and x isthe sample thickness. Application of eq 1 requires the knowledgeof radiation intensity, an absolute value not provided by X-rayradiographs that show differences of radiopacity as changes ofimage contrast.

An ASTM standard method38 is available for the quantitativedetermination of radiopacity of materials for medical use throughradiographic image analysis. In the test method, radiopacity is

quantitatively evaluated from the radiographic image as thedifference between the sample tested and the background interms of either optical density (logarithmic ratio of transmittedto incident light through the radiographic image) or pixelintensity (gray scale value ranging from 0 to 255). Radiopacityvalues for the three tantalum-coated PP samples and for plainPP were calculated according to the ASTM standard fromdigitalized radiographic images as the difference in pixelintensity and results are shown in Table 1. As already suggestedby simple observation of the radiographs in Figure 1, the valueof relative pixel intensity for the Ta-coated substrates increaseswith thickness of the tantalum layer.

X-ray images of aluminum sheets with different thickness(not shown) were also collected and their radiopacity valueswere calculated following the same procedure. The relative pixelintensity versus Al thickness is shown in Figure 2A and, uponreplotting the data in a logarithmic thickness scale (Figure 2B),a linear correlation is obtained (eq 2, correlation coefficient R2

) 0.993):

where RadAl is the radiopacity of the aluminum sampleexpressed in relative pixel intensity and t is the thickness ofthe sample. An analogous linear dependence of optical densityon thickness was earlier observed by Watts et al.39 in aradiographic evaluation of aluminum standards for density. InFigure 2C, the relative pixel intensity of the Ta-coated PP films

Table 1. Tantalum Coatings Characterization

samplesputteringtime (min)

Ta layerthickness (µm) radiopacitya

PP 2Ta1 30 1.0 19Ta2 60 2.5 40Ta3 90 5.0 53

a Calculated as the difference between the pixel intensity of the samplein the radiographic image and the pixel intensity of the background (seebackground in Figure 1A).

Figure 1. X-ray images of (A) background, (B) PP film substrate, (C)Ta1, (D) Ta2, and (E) Ta3. Brighter pictures correspond to higherradiopacity.

I ) I0e-mFx (1)

Figure 2. Radiopacity values, reported as relative pixel intensitycalculated from a digitalized radiographic image, as a function ofsample thickness: for aluminum sheets (A, linear scale; B, semiloga-rithmic scale) and for tantalum-coated PP sheets (C, semilogarithmicscale). Linear regression lines are reported in B and C.

RadAl ) 17.5603 + 30.4901 log t (2)

2448 Biomacromolecules, Vol. 11, No. 9, 2010 Cortecchia et al.

(from Figure 1) is plotted as a function of coating thickness ina logarithmic scale, showing a good linear correlation betweenradiopacity and Ta layer thickness (eq 3, R2 ) 0.999):

where RadTa is the radiopacity of the tantalum sample expressedin relative pixel intensity and t is again sample thickness. Thisresult demonstrates that it is possible to obtain Ta layers withcontrolled radiopacity on medical devices by simply adjustingsputtering deposition time.

Radiopacity data have been obtained in the present work usinga mammographic apparatus characterized by an energy radiationof 22 kV. However, for some surgical procedures such asvascular interventions, radiographic equipment that uses higherenergy radiation is employed. Because higher energy radiationimplies lower visibility of materials, thickness of the radiopaquemetallic coating should be adjusted considering the specificapplication of the medical device to be coated, and thecorrelation between thickness and radiopacity obtained in thiswork can be applied to perform this calculation. As an example,X-ray images of an aluminum sheet with a thickness of 650µm were collected both with the mentioned mammographicapparatus and with angiographic equipment using much higherenergy radiation (55 kV, radiograph in Supporting Information).The obtained sample radiopacity (pixel intensity values) de-creases from 102 to 16 when analyzed with the above instru-ments, respectively. By application of the correlation reportedabove for Ta coatings (eq 3) it is found that, to obtain the samevisibility under the angiographic equipment as that of thementioned Al sheet, the thickness of the Ta coating should be49 µm.

As already mentioned, metal coatings have some significantdrawbacks (mainly fragility and poor resistance to corrosion)that strongly suggest application of a protective layer. In thispaper, protection of the metal layer is achieved by coating itwith an organic-inorganic hybrid. Hybrid components areselected considering that biocompatibility is the first requirementthat a coating for biomedical devices needs to fulfill. Titania isan inorganic oxide that has already found commercial applica-tion in different fields such as food, cosmetics, and medicine.40-42

In particular, titania coatings are currently investigated as abiocompatibility enhancer in medical devices.43-45 Hybridscontaining titania can be obtained through a sol-gel process46

starting from the cheap alkoxide precursor titanium isopropoxideTIPT, which is highly reactive toward sol-gel hydrolysisreaction.47 It is known that TIPT reacts with polyesters andpolycarbonates through transesterification reactions leading tothe production of hybrid materials where organic and inorganiccomponents are strongly linked together (Class II hybrids23)through organotitanium ester bonds.29,48 Indeed, earlier studieson poly(ε-caprolactone)-titania hybrids showed that PCLundergoes transesterification reactions in the presence of TIPT.27

Due to its biocompatibility and slow degradation rate, PCL hasfound a number of biomedical applications, including ascomponent for drug delivery devices and degradable staples (forwound closure).49 PCL is therefore a good candidate for thedevelopment of a hybrid protective coating for biomedicaldevices.

The PCL-titania hybrid (HPCL) synthesized as described inthe Experimental Section was investigated by TGA and FT-IRanalyses, in the form of a self-standing film, obtained by castingthe sol-gel colloidal solution on Teflon sheet by the doctorblade method. As previously reported for polyester-titania and

polycarbonate-titania hybrids,29,48 evaluation of the high tem-perature solid residue in TGA experiments, run in the presenceof air, allows to ascertain whether the synthetic procedureapplied for hybrid production leads to complete hydrolysis ofthe titania precursor (TIPT). The method compares the TGAsolid residue value at T ) 600 °C, which consists of solidtitanium oxide, with the calculated amount of titania residueexpected from the starting organic to inorganic precursor ratio(1:1 in the present case), on the assumption that the sol-gelreaction has run to completion and that the hybrid sample isfree from reaction byproduct. The experimental TGA solidresidue at 600 °C for HPCL is 22.5%, a value that very wellcompares with that theoretically expected (21.9%), thus, con-firming reliability of the synthetic procedure applied that allowsto finely tune hybrid composition through the precursors ratio.

FT-IR spectrum of the self-standing HPCL film was recordedand it is compared in Figure 3A with the spectrum of plainPCL in the range 2000-500 cm-1. Both samples show thecharacteristic carbonyl vibration band at 1733 cm-1, while thehybrid also shows in addition a new band at 1535 cm-1 and abroad multiple absorption region in the range 1000-500 cm-1,where vibrations of Ti-O-Ti bonds are located.50 As alreadyreported in earlier work,27 the band at 1535 cm-1 is attributedto the presence of organotitanium ester carboxylates, whichinteract with titanium atoms through the carbonyl oxygen, givingrise to a bidentate bridging structure. Ester moieties connectorganic and inorganic phases in this hybrid.

The HPCL hybrid was then layered on top of Ta-sputtered PPsubstrates (Ta1, Ta2, and Ta3) by the doctor blade procedure,and the obtained samples were, respectively, labeled HPCL-Ta1,HPCL-Ta2, and HPCL-Ta3. Figure 4 shows a picture and thecorresponding radiographic image of a HPCL-Ta1 sample inwhich the Ta layer underlying the hybrid is star-shaped. Becausethe hybrid is coated over the whole rectangular sample shownin Figure 4, the picture (A) clearly shows that the hybrid coatingis transparent, implying the presence of nanosized inorganicdomains. Comparison of the radiographic image of HPCL-Ta1in Figure 4B with that of the plain PP substrate in Figure 1Bshows that, in the area outside the Ta star, the hybrid contributessome additional radiopacity to HPCL-Ta1. Indeed, a differencein relative pixel intensity corresponding to 5 units in the grayscale is observed, the increase in radiopacity being attributedto the presence of titanium atoms in the inorganic phase of thehybrid material.

RadTa ) 19.4687 + 48.8589 log t (3)

Figure 3. FT-IR spectra in the range 2000-500 cm-1 of (A) plainPCL (broken line) and HPCL hybrid (solid line); (B) plain PDMS (brokenline) and HPDMS hybrid (solid line).


As highlighted in Figure 4, employment of the sputteringtechnique for radiopaque metallic layer deposition offers theadditional interesting feature of easily obtaining specific shapes,by simply applying a suitable hollow-mask over the substrateon which the metal must be deposited. The ability of easy-shaping radiopaque coatings for medical devices is extremelyimportant because it is the presence of one or more radiopaquemarkers with given shape that permits surgeons to unambigu-ously localize position and orientation of implanted devicesunder X-ray radiation, without confusing them with internalbody parts. Hence, implantable devices, catheters, and so on,carrying markers that associate a desired-shape radiopaque Talayer with an upper protective hybrid layer, as proposed inthis work, represent a new option with interesting potentialapplications.

The double-layered HPCL-Ta sample discussed above (HPCL-Ta1) was subjected to cytotoxicity tests. VW-MSCs were chosenas a cellular biocompatibility model, considering that medicaldevices for vascular surgery could be a potential field ofapplication for the double-layered X-ray traceable coatingdeveloped in this work. The double coating procedure describedabove consists of various steps and cytotoxicity tests were runon the sample after each single step, starting from the plainsubstrate (PP film) and going through the two coating steps,that is, after Ta sputtering (Ta1) and after subsequent hybridcoating (HPCL-Ta1).

Figure 5 compares microscope images of cells exposed topositive control (A), to negative control (B), and to mediumconditioned with HPCL-Ta1 (C). It can be noted that exposureto medium conditioned with HPCL-Ta1 does not alter the spindle-shaped morphology typical of VW-MSCs and that no significantdifferences are observed with respect to cells exposed to plainfresh medium (negative control). On the other hand, challengingthe cells with 1 mM H2O2 as positive control, rapidly determineschanges of morphology such as membrane wrinkling and altersadhesion properties of VW-MSCs that shift from spindle toround shaped morphology; during the 2 h of exposure tohydrogen peroxide solution, a typical suffering response of cellsto adverse growing conditions leading to diffuse cell death isobserved. The percentage of cell mortality upon exposure todifferently conditioned mediums, including the double layeredHPCL-Ta1 sample (Table 2), is comparable with that obtainedfrom negative control.

Statistical analysis ANOVA (p . 0.05) confirms that nosignificant difference between negative controls and testedsamples is present. In addition, cell counts reveal not only theabsence of sample-induced mortality, but also the absence ofany obstacle to cell proliferation, suggesting that no toxicelements are released by the samples in culture medium after24 h of incubation. These preliminary results show that HPCL-

Ta1 is biocompatible and that it can be proposed as a suitablematerial for applications in the medical field.

In addition to the primary biocompatibility demand, specificbiomedical devices may require that the radiopaque coatingexhibits a degree of deformability that allows it to properlyfollow device deformation without cracking or detaching.Flexibility of the PCL-titania hybrid coating was evaluated

Figure 4. Picture of HPCL-Ta1 sample (A) and corresponding radio-graphic image (B).

Figure 5. Optical microscope images of VW-MS cells grown in vitrowith (A) 1 mM H2O2 for 2 h (positive control), (B) fresh medium(negative control), and (C) HPCL-Ta1 conditioned medium for 24 h.

Table 2. Cytotoxicity Test Results

sample cell mortalitya (%) No. living cellsb

PP 4.4 ((2.1) 65600Ta1 4.1 ((3.3) 56600HPCL-Ta1 4.1 ((2.4) 75800HPDMS-Ta1 5.9 ((2.6) 76800positive controlc 100.0 ((0.0) 0negative controld 4.9 ((2.4) 77500a Values are given as means out of five replicates. For each value

correspondent standard deviation is reported within brackets. b Numberof living cells present in each well after 24 h incubation in medium startingfrom 50000 seeded cells. c Cells exposed to 1 mM H2O2 solution for 2 h.d Cells exposed to fresh culture medium for 24 h.


under bending deformation. As shown in Figure 6A for sampleHPCL-Ta1, the double coating stands deformation of the PP filmsubstrate toward which it shows very good adhesion (noevidence of partial detachment).

Simple visual inspection of the coating surface after substratedeformation does not reveal the presence of cracks, whoseexistence was however evidenced by SEM. Before SEMobservation, the sample (a stripe with given dimensions, Scheme2), is manually bent until its ends come into contact; then someof the deformation is released and sample ends are fixed ontothe SEM stub as shown in the picture. The SEM picture of HPCL

-Ta1 in Figure 6B shows that the hybrid coating is not able tofully comply with substrate deformation and it breaks formingparallel cracks perpendicular to the bending direction.

Fragility and low extensibility exhibited by the HPCL hybridmight originate, in principle, from crystallinity of the organicpolymeric phase (PCL is a partially crystalline polyester thatmelts around 60 °C) and by the stiffening effect of the titaniaparticles. DSC results (not shown) demonstrate that, though plainPCL easily crystallizes, crystallization of the polymer in thehybrid is totally inhibited. This result agrees with earlier findingsconcerning various hybrids,27 where hindrance to the polymer

component crystallization was attributed to strong organic-inorganic interactions, especially in polyester-titania hybridswhere transesterification leads to polymer chain shortening andorganotitanium ester bond formation. The present HPCL hybridcontains a considerable amount (22%w/w) of nanosized titaniaparticles that bridge rather short PCL chains in a highly cross-linked network and this might explain low deformability of thiscoating. HPCL is a material that, according to its biocompatibilityand above-mentioned properties, may be suitable for productionof protective coatings for a range of biomedical devices, but itdoes not seem the best choice for coatings that need to standconsiderable bending deformation.

Therefore, in this work, another hybrid was developed usingpolydimethylsiloxane (PDMS) as the organic component. PDMSis well-known in the medical field as a material with goodbiocompatibility and biodurability.49 PDMS has found wide-spread applications in health care, in the fluid form (for medicaldevice lubrication, siliconization of needles and syringes,excipient for topical formulation, etc.), in the cross-linked gelform (as soft skin adhesive), and in the cross-linked solidelastomeric form (for the production of catheters, heart valves,mammary prosthesis, etc.).49,51 Moreover, being a totallyamorphous polymer with a very low glass transition temperature,PDMS is suitable for the production of highly flexible hybridmaterials.52,53 With the aim of obtaining a coating that is ableto stand high deformations, PDMS was investigated in this workas an organic component for hybrid protective coating produc-tion. In particular, a dicarboxypropyl-terminated PDMS wasemployed, on the assumption that carboxyl terminals can reactwith the inorganic phase precursor (TIPT) during the sol-gelprocess, forming organotitanium ester bonds. Because suchbonds can form only at chain terminals, a low degree of cross-linking is expected and, as a consequence, PDMS high chainflexibility should be preserved.

To additionally decrease rigidity of the final material, thehybrid containing PDMS has a higher organic to inorganicprecursor ratio (70/30 w/w) than the PCL-titania hybriddiscussed above (50/50 w/w). TGA analysis of the self-standingPDMS-titania hybrid (HPDMS), obtained by casting the sol-gelcolloidal solution on Teflon sheet by the doctor blade method,showed a 45% residue at 600 °C, a value well above that(10.7%) corresponding to the expected titania content, owingto the additional presence of silicon oxide deriving from thermaldegradation of PDMS. No precise calculation of SiO2 producedupon thermal degradation of PDMS in air can be carried outbecause, as reported in the literature,54 part of the SiO2 isproduced in the gas phase from oxidation of prevolatilizedoligomers and, thus, can be washed out by the TGA carriergas. FT-IR analysis of HPDMS was carried out and Figure 3Bcompares the spectrum of the hybrid with that of plain PDMSin the range 2000-500 cm-1. The plain polymer spectrumshows characteristic bands of PDMS (2964 cm-1 CH3

stretching; 1260 cm-1 CH3 bending; 1094 cm-1 and 1021cm-1 Si-O-Si stretching; 800 cm-1 Si-C stretching)55 and,in addition, a band (see magnification in insert) at 1711 cm-1

associated with the stretching of carbonyl groups of carboxylchain terminals. In the HPDMS spectrum, the signal related tocarboxyl moieties disappears while a new band located at1537 cm-1 is observed. The latter is attributed, in agreementwith previous assignments,29,56 to the stretching vibration ofthe carbonyl group interacting with titanium atom and itsappearance demonstrates formation of organotitanium esterbonds between titanium oxide and PDMS chain ends.

Figure 6. Picture of HPCL-Ta1 sample deformed by hand (A) and SEMimage (B) of HPCL-Ta1 surface after bending deformation accordingto Scheme 2. White scale bar ) 100 µm.

Scheme 2. Bending of Coated Samples and Fixing on SEM Stub


Once the PDMS hybrid is coated on Ta-sputtered samples,the obtained double-layered coating is transparent but, unlikeHPCL hybrid (Figure 4B), no radiopacity enhancement isobserved when HPDMS is used as the protective coating ofTa-sputtered PP, due to the low percentage of titanium oxidein HPDMS (ca. 10 vs 22% in HPCL). An interesting result onthe PDMS-containing hybrid coating concerns its behaviorunder bending deformation. HPDMS-Ta1, HPDMS-Ta2, andHPDMS-Ta3 were deformed according to Scheme 2 and SEMobservation of the deformed coating surface showed noappreciable new features, indicating that the HPDMS hybridcoating is able to uniformly follow substrate deformation,without cracking. The hybrid coating very firmly adheres tothe plastic substrate showing no tendency to detach underdeformation. These results show that selection of a low-Tg

polymeric component able to form a rather loose networkvia bond formation between chain end functional groups andtitania particles is a good strategy to obtain a hybrid protectivelayer able to follow large bending deformations of aradiopaque Ta-sputtered substrate.

Table 2 reports the results of cytotoxicity tests performedon HPDMS-Ta1. Cell mortality upon exposure to mediumconditioned with the double layered HPDMS-Ta1 sample iscomparable with that obtained from negative control, takinginto account standard deviation. Besides the absence of anyevidence of sample-induced mortality, cell counts show thatcells are free to proliferate, implying that no toxic elementsare released from the HPDMS-Ta1 material in the culturemedium. Worth pointing out is the fact that after a 24 hexposure to medium and subsequent room temperature drying,no weight change was gravimetrically detected for HPDMS-Ta1. In addition, neither visual observation nor surfaceanalysis by SEM (see Supporting Information, Figure S2)revealed appreciable differences before/after medium expo-sure. These results indicate good water resistance of thecoating. The data collected in this work provide evidence ofbiocompatibility of the double-layered HPDMS-Ta1 coatingsinvestigated and suggest that such coatings are very promisingmaterials for applications in the area of radiopaque flexiblemedical devices.

Conclusions

In the present work, a two-step procedure for the produc-tion of coatings able to enhance radiopacity of medicaldevices has been successfully developed. In the first step, athin layer of tantalum is deposited by sputtering on a plasticsubstrate. The metallic layer confers to the polymer substrategood X-ray visibility and the correlation found betweenradiopacity and coating thickness allows to tailor the formersimply by controlling sputtering deposition time. The appliedmetal deposition technique also permits easy shaping of theradiopaque layer, allowing production of radiopaque markersfor medical devices that can be unambiguously identified bysurgeons during implantation and in subsequent radiologicalinvestigations. The second step consists in the applicationof a protection layer made of an organic-inorganic hybridmaterial that can be applied on top of the metallic film eitherby dip coating or by doctor blade method. SynthesizedPCL-titania and PDMS-titania hybrids strongly adhere tosubstrates and show good biocompatibility as highlighted bycytotoxicity tests. The PDMS-titania hybrid coating ischaracterized by high flexibility that allows it to stand largesubstrate deformations without detaching nor cracking, thus,being suitable for application on flexible devices.

Acknowledgment. The authors gratefully thank Prof. StefanoMignani and Mrs. Rita Luciani (Centro Mammografico, Poli-clinico Universitario Sant’Orsola, Bologna University) for theradiographic images and Prof. Andrea Alessandrini (Diparti-mento di Fisica, Modena and Reggio Emilia University) forAFM characterization.

Supporting Information Available. (1) Radiographic imageof an aluminum sheet using angiographic equipment; and (2)SEM pictures of HPDMS-Ta1 sample before and after exposureto culture medium. This material is available free of charge viathe Internet at http://pubs.acs.org.

References and Notes

(1) Wang, Y. PCT Patent Application WO2009126479, 2009.(2) Kruft, M. A. B.; Benzina, A.; Blezer, R.; Koole, L. H. Biomaterials

1996, 17, 1803–1812.(3) Mirji, S. P.; Listro, A.; O’Neil, C. J.; Areva, S.; Acquarulo, L.; Johnson,

L. ANTEC 2008 Plast. 2008, 2213–2217.(4) Li, J. U.S. Patent Application 20090281558, 2009.(5) Glocker, D. A.; Romach, M. M. U.S. Patent Application 20070106374,

2007.(6) Simpson, J. A.; Boylan, J. F., Cornish, W. U.S. Patent Appl.

20090099645, 2009.(7) Allen, J.; Birdsall, M. Patent WO2009099958, 2009.(8) Chan, W. A.; Bini, T. B.; Venkatraman, S. S.; Boey, F. Y. C.

J. Biomed. Mater. Res., Part A 2006, 79A, 47–52.(9) Constant, M. J.; Keeley, E. M.; Cruise, G. M. J. Biomed. Mater. Res.,

Part B 2009, 89B, 306–313.(10) Wang, X.; Ye, J.; Wang, Y. Acta Biomater. 2007, 3, 757–763.(11) Rusu, M. C.; Rusu, M.; Popa, M.; Delaite, C.; Riess, G. Polym. Bull.

2007, 59, 25–30.(12) Kiran, S.; James, N. R.; Joseph, R.; Jayakrishnan, A. Biomaterials

2009, 30, 5552–5559.(13) Mawad, D.; Poole-Warren, L. A.; Martens, P.; Koole, L. H.; Slots,

T. L. B.; Hooy-Corstjens, C. S. J. Biomacromolecules 2007, 9, 263–268.

(14) Carbone, A. L.; Song, M.; Uhrich, K. E. Biomacromolecules 2008, 9,1604–1612.

(15) O’Brien, B.; Stinson, J.; Carroll, W. Acta Biomater. 2008, 4, 1553–1559.

(16) Pacetti, S. D.; Kramer-Brown, P. A.; Santos, R. J.; Morris, J. W.;Malik, S. M. U.S. Patent Application 20080195194, 2008.

(17) Carlson, J. M.; Carr, S.; Devereaux, P.; Haverty, D.; Lavelle, S.;McGloughlin, T.; Tofail, S. PCT Patent Application WO2008030517,2008.

(18) Makita, M.; Yamakado, K.; Nakatsuka, A.; Takaki, H.; Inaba, T.;Oshima, F.; Katayama, H.; Takeda, K. Radiat. Med. 2008, 26, 533–538.

(19) Manero, J. M.; Ginebra, M. P.; Gil, F. J.; Planell, J. A.; Delgado,J. A.; Morejon, L.; Artola, A.; Gurruchaga, M.; Goni, I. Proc. Inst.Mech. Eng., Part H 2004, 218, 167–172.

(20) Edelman, E. R.; Seifert, P.; Groothuis, A.; Morss, A.; Bornstein, D.;Rogers, C. Circulation 2001, 103, 429–434.

(21) Cheng, Y.; Cai, W.; Li, H. T.; Zheng, Y. F. J. Mater. Sci. 2006, 41,4961–4964.

(22) Macionczyk, F.; Gerold, B.; Thull, R. Surf. Coat. Technol. 2001, 142,1084–1087.

(23) Sanchez, C.; Ribot, F. New J. Chem. 1994, 18, 1007–1047.(24) Brinker, J. C.; Sherer, G. Sol-Gel Science: The Physics and Chemistry

of Sol-Gel Processing, Academic Press: San Diego, 1990.(25) Judeinstein, P.; Sanchez, C. J. Mater. Chem. 1996, 6, 511–525.(26) Katayama, S.; Kubo, Y.; Yamada, N. J. Am. Ceram. Soc. 2002, 85,

1157–1163.(27) Mazzocchetti, L.; Cortecchia, E.; Scandola, M. ACS Appl. Mater. Inter.

2009, 1, 726–734.(28) Mazzocchetti, L.; Sandri, S.; Scandola, M.; Bergia, A.; Zuccheri, G.

Biomacromolecules 2007, 8, 672–678.(29) Mazzocchetti, L.; Scandola, M.; Pollicino, A. Polymer 2008, 49, 5215–

5224.(30) Malkani, A. L.; Price, M. R.; Crawford, C. H.; Baker, D. L. J.

Arthroplasty 2009, 24, 1068–1073.(31) Hanawa, T. J. Artif. Organs 2009, 12, 73–79.(32) Cheng, Y.; Cai, W.; Li, H. T.; Zheng, Y. F.; Zhao, L. C. Surf. Coat.

Technol. 2004, 186, 346–352.


(33) Casanova, R. M., Pathak, C. P. PCT Patent Appl. WO2007056762,2007.

(34) Castillo, M. V., Joquin Walker, J. U.S. Patent 6077880, 2000.(35) Pasquinelli, G.; Pacilli, A.; Alviano, F.; Foroni, L.; Ricci, F.; Valente,

S.; Orrico, C.; Lanzoni, G.; Buzzi, M.; Tazzari, P. L.; Pagliaro, P.;Stella, A.; Bagnara, G. P. Cytotherapy 2010, 12, 275–287.

(36) Mattox, D. M. Handbook of Physical Vapor Deposition (PVD)Processing; William Andrew Publishing/Noyes: Park Ridge, NJ, 1998.

(37) Barrett, C. S. Structure of metals, McGraw Hill Book Company Inc.:New York, 1943.

(38) ASTM Int. ASTM F640, Test Methods for Determining Radiopacityfor Medical Use; ASTM Int.: West Conshohocken, PA, 2007.

(39) Watts, D. C.; Mccabe, J. F. J. Dentist. 1999, 27, 73–78.(40) Burdock, G. A. Encyclopedia of Food and Color AdditiVes; CRC Press:

Boca Raton, FL, 1996.(41) Barel, A. O.; Paye, M.; Maibach, H. I. In Handbook of Cosmetic

Science and Technology, 3rd ed.; Informa Healthcare: London, 2009.(42) Brunette, D. M.; Tengvall, P.; Textor, M. Titanium in Medicine;

Springer: New York, 2001.(43) Mulhonen, V.; Kujala, S.; Vuotikka, A.; Aaritalo, V.; Peltola, T.;

Areva, S.; Narhi, T.; Tuukkanen, J. Acta Biomater. 2009, 5, 785–793.

(44) Chu, C. L.; Wang, R. M.; Hu, T.; Yin, L. H.; Pu, Y. P.; Lin, P. H.;Dong, Y. S.; Guo, C.; Chung, C. Y.; Yeung, K. W. K.; Chu, P. K. J.Mater. Sci.: Mater. Med. 2009, 20, 223–228.

(45) Yang, Y. Z.; Park, S. W.; Liu, Y. X.; Lee, K. M.; Kim, H. S.; Koh,J. T.; Menge, X. W.; Kim, K. H.; Ji, H. B.; Wang, X. D.; Ong, J. L.Vacuum 2008, 83, 569–574.

(46) Hench, L. L.; West, J. K. Chem. ReV. 1990, 90, 33–72.(47) Livage, J.; Henry, M.; Sanchez, C. Prog. Solid State Chem. 1988, 18,

259–341.(48) Cortecchia, E.; Mazzocchetti, L.; Scandola, M. Macromol. Chem. Phys.

2009, 210, 1834–1843.(49) Ratner, B. D., Hoffman, A. S., Schoen, F. J., Lemons, J. E.

Biomaterials Science: An Introduction to Materials in Medicine;Elsevier Academic Press: New York, 2004.

(50) McDevitt, N. T.; Baun, W. L. Spectrochim. Acta 1964, 20, 799–808.(51) De Jaeger, R.; Gleira, M. Inorganic Polymers; Nova Science Publish-

ers: U.K., 2007.(52) Yamada, N.; Yoshinaga, I.; Katayama, S. J. Mater. Chem. 1997, 7,

1491–1495.(53) Nakade, M.; Ichihashi, K.; Ogawa, M. J. Sol-Gel Sci. Technol. 2005,

36, 257–264.(54) Camino, G.; Lomakin, S. M.; Lazzari, M. Polymer 2001, 42, 2395–

2402.(55) Smith, A. L. Spectrochim. Acta 1960, 16, 87–105.(56) Takahashi, R.; Takenaka, S.; Sato, S.; Sodesawa, T.; Ogura, K.;

Nakanishi, K. J. Chem. Soc., Faraday Trans. 1998, 94, 3161–3168.

BM100619T


Documents

Biocompatible Two-Layer Tantalum/Titania−Polymer Hybrid Coating