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Electrophoretic deposition of compositehydroxyapatite–silica–chitosan coatings

K. Grandfield, I. Zhitomirsky⁎

Department of Materials Science and Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4L7

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +1 905 525 9140E-mail address: zhitom@mcmaster.ca (I. Zh

1044-5803/$ – see front matter © 2006 Elsevidoi:10.1016/j.matchar.2006.10.016

A B S T R A C T

Article history:Received 17 August 2006Received in revised form21 October 2006Accepted 25 October 2006

Electrophoretic deposition (EPD) method has been developed for the fabrication ofnanocomposite silica–chitosan coatings. Cathodic deposits were obtained on variousconductive substrates using suspensions of silica nanoparticles in a mixed ethanol–watersolvent, containing dissolved chitosan. Co-deposition of silica and hydroxyapatite (HA)nanoparticles resulted in the fabrication of HA–silica–chitosan coatings. The depositionyield has been studied at a constant voltage mode at various deposition durations. Themethod enabled the formation of coatings of different thickness in the range of up to100 μm. Deposit composition, microstructure and porosity can be varied by variation of HAand silica concentration in the suspensions. It was demonstrated that EPD can be used forthe fabrication of HA–silica–chitosan coatings of graded composition and laminates. Themethod enabled the deposition of coatings containing layers of silica–chitosan and HA–chitosan nanocomposites using suspensions with different HA and silica content. Obtainedcoatings were studied by X-ray diffraction, thermogravimetric and differential thermalanalysis, scanning electron microscopy and energy dispersive spectroscopy. Themechanism of deposition is discussed.

© 2006 Elsevier Inc. All rights reserved.

Keywords:Electrophoretic depositionHydroxyapatiteSilicaChitosanComposite

1. Introduction

Composite materials are currently under investigation forapplications in biomedical implants. Significant interest hasbeen generated in the composites containing hydroxyapatiteCa10(PO4)6(OH)2 (HA). HA is an important material for biomed-ical applications, as its chemical composition is similar to thatof bone tissue. Synthetic HA is a biocompatible prostheticmaterial, bonding strongly to the bone and promoting theformation of bone tissue on its surface. Composites containingHA and other materials, such as silica, bioglasses andpolymers have been extensively studied [1–4].

Silica and bioglasses are of particular interest for biomed-ical applications [5,6]. There have been a number of interestingstudies that revealed the biological functions of silica [3–10]. Itwas found that silica has an important role in biomineraliza-tion. The formation of HA on the surface of silica is a subject of

; fax: +1 905 528 9295.itomirsky).

er Inc. All rights reserved

intensive investigations [7–9]. Silica coatings on HA [5] havebeen utilized as multifunctional surface modification agentsfor the fabrication of HA–polymer composites. The coatingsprevent low pH dissolution of HA, promote bond formationbetween the HA and polymer matrices and enhancebiocompatibility.

The use of polymers offers the advantage of low temper-ature processing of composite materials. Significant interesthas been generated in the formation of nanocompositescontaining HA and silica in a chitosan matrix. Chitosan is anatural cationic polysaccharide that can be produced by thealkalineN-deacetylation of chitin. Important properties of thismaterial, such as antimicrobial activity, chemical stability,biocompatibility, and advanced mechanical properties, havebeen utilized in biotechnology [11,12]. HA–chitosan [13–15]and silica–chitosan [16–18] composite scaffolds and films havebeen developed.

.

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The development of advanced composites includes notonly the selection of components but also materials design.The unique performance of natural biomaterials arises fromthe precise hierarchical organization, graded composition andporosity, orientation of HA nanocrystals and other factors [19–21]. Recent studies highlighted the importance of the fabrica-tion of artificial implant materials of graded composition,microstructure and porosity [21–25]. The investigation offunctional gradation of living tissue resulted in the develop-ment of advanced biomaterials. Several investigations havebeen focused on the development of laminated compositescontaining alternating layers of HA and other materials suchas silica [26], zirconia [27,28], bioglass [29], collagen [30], andpolymers [31,32]. Step-wise or continuously graded compo-sites have been designed to improve interfacial bondingbetween different materials. New materials design resultedin improved mechanical behavior, blood permeability andoptimized interactions with cells.

The design of composite coatings for biomedical applica-tions is inevitably related to the development of depositiontechniques. Electrophoretic deposition (EPD) is an importanttool for the deposition of ceramics, glasses and polymers [33–35]. The possibility of deposition of chitosan and otherfunctional materials opens new opportunities in the fabrica-tion of advanced coatings for biomedical implants [36]. In theprevious investigations [37–39] we obtained composite HA–chitosan coatings with various HA content. These results pavethe way for the fabrication of novel functionally graded andlaminated composites containing HA and other bioceramics.

The goal of this investigation was the fabrication ofcomposite HA–silica–chitosan coatings by EPD. New compos-ite materials were obtained as monolayers, laminates orfunctionally graded coatings.

Fig. 1 –TG (a, b) and DTA (c, d) data for the deposits preparedfrom the suspensions containing 3.6 (a, c) and 0.72 g/l silica(b, d).

2. Experimental Procedures

Stoichiometric HA nanoparticles for EPD were prepared by awet chemical technique described in a previous work [40]. Ca(NO3)2·4H2O, (NH4)2HPO4, and NH4OH (Aldrich) were used.Precipitation was performed at 70 °C by the slow addition of a0.6 M ammonium phosphate solution to a 1.0 M calciumnitrate solution. The pH of the solutions was adjusted to 11 byNH4OH. Stirring was performed for 8 h at 70 °C and then for24 h at room temperature. The precipitate was washed withwater and finally with ethanol. It has been previously reported[39] that the average length of the needle-shape HA crystals,prepared by this method, is about 200 nm and the averageaspect ratio is 8. Silica powder with average particle size of7 nm and chitosan (Mw=200,000) with a degree of deacetyla-tion of about 85% was purchased from Aldrich. According tothemanufacturer, the silica powderwas amorphous. Chitosanwas dissolved in a 1% acetic acid solution and then used forthe preparation of suspensions for EPD.

EPD was performed from the suspensions of HA and silicananoparticles in a mixed ethanol−water solvent (17 vol.%water), containing 0.5 g/l chitosan. Before the deposition, thesuspensions were ultrasonicated for 1 h to achieve ahomogeneous dispersion of the nanoparticles. The EPD cellfor the deposition included a cathodic substrate centered

between two parallel platinum counter electrodes. Thedistance between the cathode and counter electrode was15 mm. The EPD was performed at a constant voltage of 10–30 V. Cathodic deposits were obtained on Ti foils(50×50×0.1 mm), graphite, 316L stainless steel (316L SS) foils(30×30×0.1 mm) and wires (0.125 mm diameter). For thermo-gravimetric analysis (TG) and differential thermal analysis(DTA), the deposits were scraped from the electrodes anddried at room temperature for 24 h before the analysis. Thethermoanalyzer (Netzsch STA-409) was operated in air be-tween room temperature and 1200 °C at a heating rate of 5 °C/min. The X-ray diffraction (XRD) studies were carried out witha diffractometer (Nicolet I2) using monochromatized CuKα

radiation at a scanning speed of 1°/min. The microstructuresof the deposited coatings were studied using a JEOL JSM-7000Fscanning electron microscope (SEM) equipped with energydispersive spectroscopy (EDS).

3. Experimental Results and Discussion

Silica–chitosan composite deposits were obtained on thecathodic substrates using the 0.5 g/l chitosan solutionscontaining 0–3.6 g/l silica. Fig. 1 compares TG/DTA data fortwo different deposits prepared at a deposition voltage of 30 V.The observed weight loss can be attributed to the liberation ofthe adsorbed water and burning out of chitosan. The TGcurves indicate that the weight loss below 600 °C occurs inseveral steps. The total weight loss in the temperature rangeof 20–1200 °C was found to be 41.4 and 22.5 wt.% for thedeposits prepared from the 0.72 and 3.6 g/l silica suspensions,respectively. The silica content in the deposits was found to be58.6 and 77.5 wt.%. The TG data indicates that the silicacontent in the deposits can be varied by the variation of silicaconcentration in the suspensions used for deposition. Theresults of TG are in a good agreement with the DTA data. TheDTA data showed exotherms in the range of 200–600 °C relatedto the burning out of chitosan. The exotherms are moredistinct for the deposit containing a lower amount of silica andtherefore larger amount of chitosan. Similar exotherms werereported for pure chitosan powders and deposits [37].

Fig. 2 –Deposit weight versus deposition time for (a) the 3.6 g/l silica suspension and (b) the 2.5 (a) and 5 g/l HA (b)suspensions.

Fig. 3 –SEM image of the silica–chitosan composite deposit ona 316L SS wire prepared from the 1.8 g/l silica suspension:uncoated (a) and coated area (b).

Fig. 4 –SEM images of the silica–chitosan composite depositson 316L SS wires prepared from the 1.8 g/l silica suspensionfor deposition times of (a) 1 (b) 3 and (c) 10 min.

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The formation of the cathodic deposits indicates that silicaparticles are positively charged in the chitosan solutions. It issuggested that the positive charge of the silica nanoparticles isattributed to the adsorbed chitosan. The mechanism of theformation of composite silica–chitosan deposits can beconsidered as the electrophoretic co-deposition of silica andchitosan. The deposition mechanism is similar to thatproposed for the HA–chitosan composite films [37–39]. Thewater soluble and positively charged chitosanwas prepared byprotonation in the acidic solutions:

CHIT–NH2 þ H3Oþ→CHIT–NHþ3 þ H2O ð1Þ

It is suggested that the chitosan macromolecules adsorbedon the silica surface provide the electrosteric stabilization andcharging of the silica nanoparticles.

The cathodic reduction of water results in increasing pH atthe cathode surface:

2H2O þ 2e−→H2 þ 2OH− ð2Þ

Electrophoresis provides accumulation of the chitosanmacromolecules and silica nanoparticles at the electrodesurface and the formation of cathodic deposits. The neutral-ization of the positively charged chitosan macromoleculesresults in the deposition of insoluble chitosan:

CHIT–NHþ3 þ OH−→CHIT–NH2 þ H2O ð3Þ

In this work, EPD of silica–chitosan and HA–chitosancoatings has been studied at a constant voltage mode inorder to find similar experimental conditions for co-deposi-tion of the materials. The results of the investigation ofdeposition yield (Fig. 2) have been further utilized for the

Fig. 6 –X-ray diffraction patterns of the composite silica–chitosan (a) and HA–chitosan (b) deposits prepared from thesuspensions, containing 1.8 g/l silica and 2.5 g/l HA,respectively.

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fabrication of HA–silica–chitosan composites, includingmonolayers, laminates or functionally graded coatings. Fig. 2shows deposit weight versus deposition time for the silica–chitosan and HA–chitosan deposits at a constant voltage of30 V. Deposit weight increased with increasing depositiontime indicating the possibility of the formation of deposits ofdifferent thickness. The increase in the HA content in thesuspension resulted in increasing deposition rate. However,for the suspensions containing 5 g/l HA, the reduction indeposit weight was observed at deposition durations above6min, which can be attributed to deposit spalling. The depositspalling is related to lower adhesion of the deposits withhigher HA content. It is in this regard that the TG/DTA data forHA–chitosan composites [39] showed that the HA content inthe deposits increases with increasing HA concentration inthe suspensions. The adhesion of the composite films withhigh HA content can be improved by the deposition of aninterfacial chitosan-rich layer. The method developed in thiswork and described below provides such a possibility.

Fig. 3 shows a SEM image of a silica–chitosan coating on a316L SS wire. The deposit thickness is about 100 μm. Depositsof different thickness on 316L SS wires obtained at a cellvoltage of 30 V are shown in Fig. 4. The deposit thickness canbe changed by the variation of deposition time at a constantvoltage. This result is in good agreement with the experimen-tal data shown in Fig. 2a.

The composition and microstructure of the depositsdepends on the HA and silica concentration in the suspen-sions. The deposits prepared from the suspensions with HA orsilica concentration above ∼ 0.7 g/l showed porous micro-structures (Fig. 5a). It is suggested that the porosity is

Fig. 5 –SEM images of (a) the HA–chitosan composite depositprepared from a 2.5 g/l HA suspension and (b) the silica–chitosan composite deposit prepared from a 1.8 g/l silicasuspension.

attributed to the packing of the needle-shape HA particles.The deposits, prepared from the silica suspensions showedlower porosity, which can be attributed to lower particle sizeand better packing (Fig. 5b). The porosity decreased withdecreasing concentration of silica and HA nanoparticles in thedeposits. The deposits prepared from the suspensions con-taining 0–0.5 g/l silica or HA were smooth and dense. Thedeposits contained silica or HA nanoparticles distributed in apolymer matrix. Fig. 6 shows X-ray diffraction patterns of thecomposite coatings. The XRD pattern of the HA–chitosancoatings shows diffraction peaks attributed to HA (JCPDS file09-0432). The crystallinity of HA is important for implantapplications. The higher HA crystallinity usually results inimprovedmechanical properties and better chemical stability.

The XRD studies of silica–chitosan coatings revealed abroad peak at 2θ∼13°. However, phase identification presentsdifficulties. The X-ray diffraction studies of as-received silicapowders showed similar peak.

The obtained results indicate a possibility of co-depositionof silica and HA to form composite HA–silica–chitosan films.Fig. 7 shows the microstructures of the composite filmsprepared from the suspensions containing different amountsof silica. The HA–silica–chitosan deposits showed lowerporosity compared to the HA–chitosan films (Fig. 5a). TheSEM images showed larger number of silica particles in thedeposits prepared from the suspensions with higher silicaconcentration. It is suggested that an increase in silicaconcentration in the suspensions results in an increasedconcentration of silica in the deposits, better packing andlower porosity of the deposits. The EDS spectra shown in Fig. 8indicate that the intensity of Si peaks increases withincreasing silica concentration in the suspensions used forthe deposition. Numerous EDS analyses showed the silica/HAweight ratios of 0.43, 0.81 and 2.44±0.03 for the depositsprepared from the 2.5 g/l HA suspensions containing 0.9, 1.8and 3.6 g/l silica, respectively.

The obtained results coupled with the results of previousstudies pave the way for the fabrication of the deposits ofgraded composition and laminates. Recent studies highlightedthe importance of the development of advanced solvent–

Fig. 8 –EDS data for the HA–silica–chitosan compositedeposits obtained from 2.5 g/l HA suspensions containing 0.9(a), 1.8 (b) and 3.6 g/l silica (c).

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binder–dispersant systems suitable for the deposition ofdifferent materials [41]. It was shown that the use of similarbath compositions and similarly charged particles allows forthe deposition of consecutive layers of different ceramics.Well-dispersed and stable suspensions of fine particles may beobtained by selection of an effective dispersant. However, fineparticles with high surface area promote deposit crackingduring drying, which could be prevented by the use of binders.Various binder materials have been investigated for electro-phoretic deposition in order to increase the adherence andstrength of the deposited material and prevent cracking. Manyadvanced polymer binder materials are electrically neutralpolymers. It is suggested that charged ceramic particlestransport adsorbed polymer to the electrode surface, thusallowing the polymer binder to be included in the deposit. Someproblems are related to the competitive adsorption of disper-sant, binder and charging additives on the particle surface andcontrol of the amount of binder transported to the electrodesurface. From this point of view, the use of chitosan forelectrophoretic deposition offers important advantages.

Fig. 7 –SEM images of the HA–silica–chitosan compositedeposits prepared from the 2.5 g/l HA suspensions contain-ing (a) 0.9 (b) 1.8 (c) 3.6 g/l silica.

Chitosan provides effective electrosteric stabilization ofceramic particles. The adsorption of chitosan on the surface ofsilica and HA particles provides a positive charge and allowscathodic electrodeposition of the particles. Moreover, chitosanis an effective binder, which provides adhesion of the particlesto the substrate surface and prevents cracking. Due to thecationic nature and good film forming properties, chitosan canbe deposited independently to form insoluble deposits of purepolymer. Previous investigations indicate that the HA contentin the chitosan–HA deposits can be varied in the range of 0–100 wt.% HA by the variation of chitosan or HA content in thesuspensions.

Fig. 9 – (a) SEM image of the HA–chitosan composite depositon a graphite substrate (s), containing two different layers (a)and (b), prepared from 0.25 and 2.5 g/l HA suspensions,respectively. (b) SEM image of alternating layers preparedfrom the same suspensions.

Fig. 10 –SEM images (a, b) at different magnifications forcomposite deposits on 316L SS wires containing a layerdeposited from a 1.8 g/l silica suspension (a) and anadditional layer deposited from a 2.5 g/l HA suspension (b).

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As an extension of the previous investigations, we utilizedthe suspensions with different HA content for the fabricationof HA–chitosan films of graded composition. Fig. 9a shows adeposit, which contains a thin layer prepared from the 0.25 g/l HA suspension and a thicker layer prepared from the 2.5 g/l HA suspension. The fabrication of thin layers with largerchitosan content offers a possibility to improve the adhesionof composite films. The second layer has a HA content of about70 wt.%, which is similar to that of natural bone. Compositecoatingwere also obtained as laminates, containing individuallayers with different HA content, as shown in Fig. 9b.

It is important to note that natural bone is a composite ofcollagen fibrils reinforced with HA nanoparticles. Thesemineralized fibrils are assembled into laminates, which arethe basis for a large variety of structures at various hierarchi-cal levels [19]. The lamellar structure of bone and otherbiomaterials results in unique mechanical and other proper-ties. It is known that natural bone is a material of gradedcomposition and porosity [21]. The results of our work indicatethat electrophoretic deposition enables the formation ofcomposite HA–silica–chitosan laminates and materials ofgraded composition. As an example, Fig. 10 shows compositecoatings containing layers of silica–chitosan and HA–chitosannanocomposites. The use of chitosan enables cathodic EPD oflayers of different materials and interfacial bonding of thelayers. The microstructure and composition of the coatingscan be varied by deposition of silica–chitosan and HA–chitosan layers of different composition and microporosity.

An important task for the future research is the investiga-tion of biocompatibility, adhesion, mechanical and otherproperties of the novel coatings prepared by EPD and

optimization of the microstructure and properties. It isimportant to note that the use of chitosan with antimicrobialproperties offers a possibility to obtain antimicrobial coating.Themethod has a potential of co-deposition of othermaterialsand further functionalization of the coatings for a large varietyof applications.

4. Conclusions

Nanocomposite silica–chitosan coatings have been preparedby cathodic EPD. In the proposed method chitosan has beenutilized for the electrosteric stabilization and charging ofceramic nanoparticles. Good binding and film forming prop-erties of chitosan enabled the formation of relatively thickcoatings in the range of up to 100 μm. The silica content in thecoatings can be varied by the variation of concentration ofsilica nanoparticles in the suspensions. The deposition yieldcan be varied by the variation of the deposition time in aconstant voltagemode. Themethod enables the co-depositionof HA and silica to form HA–silica–chitosan coatings. Theincrease in silica concentration in the suspensions at aconstant HA concentration resulted in an increasing silica/HA ratio in the deposits. The proposed approach enabled theformation of HA–silica–chitosan coatings of graded composi-tion and laminates. Novel nanocomposites were obtained byvarying the concentration of HA nanoparticles in suspensionsused for the deposition of individual layers. The methodallowed the deposition of coatings containing layers of silica–chitosan and HA–chitosan nanocomposites using suspen-sions with different HA and silica content. The proposedmethod offers the advantage of room temperature processing.The problems related to the high temperature sintering ofceramic coatings on metallic substrates can be avoided.Moreover room temperature processing offers a possibility ofco-deposition of various materials.

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