Acta Biomaterialia 2 (2006) 567–574

www.actamat-journals.com

Dip coated silicon-substituted hydroxyapatite films

Natalia Hijon, M. Victoria Cabanas, Juan Pena, Marıa Vallet-Regı *

Departamento de Quımica Inorganica y Bioinorganica, Facultad de Farmacia, Universidad Complutense, 28040 Madrid, Spain

Received 18 January 2006; received in revised form 28 April 2006; accepted 11 May 2006

Abstract

Silicon-substituted hydroxyapatites have been deposited onto Ti6Al4V substrates by sol–gel technology. The Ca10(PO4)6�x�y-(SiO4)x(CO3)y(OH)2�x+y coatings obtained, with silicon contents up to x = 1 (2.8 wt.%), show a homogeneous and crack-free surfacecomposed of particles smaller than 20 nm. The silicon enters into the apatite structure in the form of SiO4�

4 groups that partially sub-stitute the PO3�

4 groups. The Si content and the Ca/P molar ratio of the coatings agree with those originally introduced in the sols. Layerswith thicknesses around 600 nm show adhesion strengths superior to 20 MPa as determined by a pull-out test. The formation of an apa-tite layer onto these coatings after immersion in a simulated body fluid is enhanced by the presence of silicon.� 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Silicon-substituted hydroxyapatite; Coatings; Sol–gel

1. Introduction

Silicon incorporation into the apatite structure consti-tutes a method to upgrade calcium phosphate implants[1]. In addition to the well known features of hydroxyapa-tite (HA), silicon is known to be essential in the early stagesof bone mineralisation and soft tissue development [2,3].This improvement has been evidenced in vitro andin vivo [4–8], leading to an adequate osteointegration andensuring the implant success.

Silicon-substituted calcium phosphates have been pre-pared by different methods in powdered form, as reviewedby Vallet-Regı et al. [1], including sol–gel, hydrothermal,solid state reaction, controlled crystallization. However,fewer works, using deposition techniques such as magne-tron sputtering [7,9] and electrospraying [10], have reportedthe preparation of silicon substituted apatites in the formof coatings. With regard to the deposition of related com-pounds, films of Ca2SiO4 prepared by plasma spray, induceearly apatite deposition and osteoblast growth [11], whilePietak et al. [12] studied the crystallization kinetics of

1742-7061/$ - see front matter � 2006 Acta Materialia Inc. Published by Else

doi:10.1016/j.actbio.2006.05.004

* Corresponding author. Tel.: +34 913941861; fax: +34 913941786.E-mail address: vallet@farm.ucm.es (M. Vallet-Regı).

Si–tricalcium phosphate coatings obtained by dip coat-ing.

Nowadays, the technique most frequently employed toprepare commercial coated implants is plasma spray [13].However, it has some disadvantages that cannot be easilyavoided: inability to coat implants with complex shapes,differences in the chemical composition, delamination,etc. Other line-of-sight deposition methods such as sputter-ing or laser ablation cannot achieve, for example, the coat-ing of porous substrates [14,15]. Solution based methodsare emerging options for the preparation of these coatingsdue to some of their features: better control of coatingmorphology, chemistry and structure, covering of intricatepieces, simple technology, and so forth.

In this sense, sol–gel technology has been employed byseveral authors to prepare hydroxyapatite layers onto dif-ferent types of substrates [16–31], where the influence ofparameters such as the nature of the precursors, sol tem-perature, aging time and temperature, extraction rate, etc.have been studied in order to optimize the deposition con-ditions of pure HA films.

Taking into consideration all this background, the aimof this work is to prepare and characterize silicon-substi-tuted hydroxyapatite (Si-HA) coatings deposited ontoTi6Al4V substrates by the sol–gel method.

vier Ltd. All rights reserved.

568 N. Hijon et al. / Acta Biomaterialia 2 (2006) 567–574

2. Materials and methods

Silicon-substituted hydroxyapatite coatings, accordingto the formula: Ca10(PO4)6�x(SiO4)x(OH)2�xhx (Si-HA),where x varies from 0.25 to 1 and h expresses the anionicvacancies generated, have been prepared. Fig. 1 schemat-izes the sol preparation and the coating procedure.

In a first step, the triethyl phosphite, P(OCH2CH3)3, washydrolyzed in water for 24 h. This precursor has been cho-sen in consideration of its faster hydrolysis when comparedto other phosphorus precursors such as trialkyl phosphates[32]. After that, tetraethyl orthosilane, Si(OCH2CH3)4, wasadded. The reaction was carried out maintaining a molarratio of H2O/(P + Si) = 4. A 4 M aqueous calcium nitratesolution, Ca(NO3)2 Æ 4H2O, was poured into the sol-containing phosphorus and silicon. The amounts ofreagents were calculated considering a Ca/(P + Si) molarconstant ratio, equal to HA stoichiometry (1.67), assumingthat the silicate ions would substitute the phosphate groups(Table 1).

The mixed sol was stirred for 15 min and aged at 60 �Cin an oven for 6 h. Before the sols were used to make coat-ings, they were diluted with ethanol in a ratio of 1:1. Theseconditions were chosen according to previous resultsobtained for HA coatings deposited by sol–gel using theseprecursors [30,31].

Fig. 1. Scheme for the preparation of coatings, inclu

Table 1Quantities of reactants, expected molar ratios and theoretical composition of

Ca source[M]

P source[M]

Ca/PM ratio

Si source[M]

x = 0.00 10 · 10�2 6 · 10�2 1.67 –x = 0.25 10 · 10�2 5.75 · 10�2 1.74 0.25 · 10�2

x = 0.50 10 · 10�2 5.50 · 10�2 1.82 0.50 · 10�2

x = 0.75 10 · 10�2 5.25 · 10�2 1.91 0.75 · 10�2

x = 1.00 10 · 10�2 5.0 · 10�2 2.00 1.00 · 10�2

Ca source: Ca(NO3)2 Æ 4H2O; P source: P(OCH2CH3)3; Si source: Si(OCH2CH

Ti6Al4V discs with dimensions 12 mm · 1 mm wereabraded with a 320 silicon grit range carbide paper, pol-ished with 9, 3 and 1 lm diamond paste and washed withdistilled water, alcohol and acetone. For the preparationof the films, these substrates were dip coated into the pre-cursor sol and extracted with a withdrawal speed of2500 lm/s. After the deposition, the coatings were driedat 100 �C (1 h), annealed in air at 550 �C (10 min) and thencooled to room temperature. Finally, the coatings werewashed with ethanol in an ultrasonic bath for 2 min.

A static bioactivity test was performed by soaking theannealed coatings in 6 mL of simulated body fluid (SBF)[33] at 37 �C up to 7 days. The SBF was previously filteredand all operations were carried out in a laminar flux cabin,in order to avoid bacterial contamination. After soaking,the coatings were removed from the fluid and washed withwater and ethanol.

The viscosity of the sols was measured by using aHaaker ReoStress RS75 rheometer, at a shear rate rangefrom 1 to 200 s�1 at 20 �C.

The coatings were characterized by X-ray diffraction(XRD) in a Philips X-Pert MPD diffractometer equippedwith a thin-film (grazing incidence) attachment and usingCu Ka radiation; the X-ray generator operated at 45 kVand 40 mA. Data were collected over the 2h range of20–50� with a step size of 0.02� and a count time of 10 s

ding the sol synthesis and the dip-coating device.

deposited coatings

Ca/(P + Si)M ratio

Si, wt.% Ca10(PO4)6�x(SiO4)x(OH)2�xhx

1.67 – Ca10(PO4)6(OH)2

1.67 0.7 Ca10(PO4)5.75(SiO4)0.25(OH)1.75h0.25

1.67 1.4 Ca10(PO4)5.5(SiO4)0.5(OH)1.5h0.5

1.67 2.1 Ca10(PO4)5.25(SiO4)0.75(OH)1.25h0.75

1.67 2.8 Ca10(PO4)5(SiO4)1(OH)1h1

3)4.

Inte

nsity

(a.

u.)

25 30 35 40 45

2θ (o)(2

00)

(002

)

(102

)

(210

)

(211

)/(1

12)

(300

)(2

02)

(301

)

20

Ti6Al4V

X=0

X=1

X=0.5

Fig. 2. X-ray diffraction patterns of the x = 0, 0.5 and 1 Si-HA coatings.

N. Hijon et al. / Acta Biomaterialia 2 (2006) 567–574 569

per step, with a fixed omega angle of 0.5�. Fourier trans-form infrared (FTIR) spectroscopy was carried out in aNicolet Nexus spectrometer using an ATR Golden Gate.Spectra were obtained at 4 cm�1 resolution averaging 64scans. The surface of the films was examined by scanningelectron microscopy (SEM) in a JEOL 6400 instrumentworking at 20 kV. Ca, P and Si distribution in the coatingswas determined by energy dispersive X-ray spectrometry(EDS) with a Oxford Pentafet Super A/W analyzer micro-scope system. The samples were also characterized by elec-tron diffraction (ED) and transmission electron microscopy(TEM) in a JEOL 2000 FX electron microscope, workingat 200 kV on material scraped from the surface coatingwith a metallic blade. The local composition of severalcrystals was established by EDS. For this purpose, a JEOL2000 FX electron microscope equipped with an EDAX-analyser system, Link ISIS 300, was employed.

The adhesive strength was measured by a pull-out test,in accordance with American Society for Testing Materials(ASTM) specifications [34]: a pair of metallic jigs werebonded to the coated and the uncoated titanium substrates,these ensembles were joined by means of Loctite 480 adhe-sive. After leaving the set for 1 day, for complete solidifica-tion of the glue, the adhesive strength of the film wasmeasured by applying a tensile stress using a universalmechanical tester MicrotestSCM 3000 at a crosshead speedof 0.5 mm/s until fracture occurred. Five measurementswere done for each data point.

3. Results and discussion

From previous experience of the sol–gel deposition ofHA coatings [30,31], a compromise can be establishedbetween temperatures and times of sol aging. Briefly, thehigher the temperature, the shorter the aging period toobtain pure HA coatings and, unfortunately, also the sta-bility of the sol decreases.

Taking into account these previous results, the deposi-tion of silicon-substituted hydroxyapatite coatings wasperformed working with aqueous sols aged at 60 �C for6 h. These conditions allow pure apatite films to be depos-ited with a maximum withdrawal speed of 800 lm/s, whichyields coatings with thicknesses of around 200 nm, with astability period for the sol of around 24 h before powderwas formed onto the substrate. In order to increase the sta-bility of the sol as well as the withdrawal rate for preparingSi-HA coatings, these aqueous sols were diluted with etha-nol in a 1:1 ratio. Under such conditions the stability of thesols increases, i.e. the interval available to deposit Si-HAincreases. Besides, the withdrawal rate can be increasedup to 2500 lm/s, which allows layers of around 600 nmto be deposited. All the precursor sols used for the coatingdeposition showed a pale yellow color with viscosity valuesaround 5 mPa, regardless of silicon content or ethanoldilution.

Fig. 2 shows the XRD patterns of the layers with x = 0,0.5 and 1 after thermal treatment at 550 �C; only maxima

attributable to an apatite-like phase (powder diffraction file9-432 JCPDS 2003) or to the Ti6Al4V substrate (powderdiffraction file 44-1294 JCPDS 2003) can be seen. In nocases can the presence of additional phases be detected, suchas calcium oxide (Powder Diffraction File 37–1497 JCPDS2003) or carbonate (Powder Diffraction File 5–586 and41–1475 JCPDS 2003), which may appear as a consequenceof the Ca/P ratio increase with the silicon substitution.Moreover, these results indicate that the introduction ofsilicon into the precursor sol does not alter the depositionconditions in such a way that compounds like calcite mayappear together with or instead of apatite, as has beenobserved after minimal variations in these parameters inthe deposition of pure HA films [30,31]. However, somedifferences on the XRD patterns can be appreciated withincreasing silicon content, namely a progressive broadnessof the apatite maxima; in fact, from x = 0.5 the maximaattributable to the (002), (300) and (202) reflections canbe hardly distinguished from the background. A similar factwas also observed for Si-HA coatings obtained via sputter-ing deposition [9]. This broadening effect can be explainednot only by considering the great degree of distortiongenerated in the structure due to the substitution of PO3�

4

by SiO4�4 groups and the creation of vacancies in the OH�

site, but also by a decrease in the crystal size, as previouslyobserved in Si-HA in powder form [35,36], or by the factthat the silicon causes texture of the films [9]. This decreasehas been established to affect, particularly, (001) planes,causing the growth perpendicular to the [001] direction to

570 N. Hijon et al. / Acta Biomaterialia 2 (2006) 567–574

be hindered in the presence of tetraethyl orthosilane, result-ing in more needle-shaped crystallites [1].

The FTIR spectra of prepared coatings (Fig. 3) show thecharacteristic absorption bands of carbonate-hydroxyapa-tite. The intense bands in the ranges 570–605 and 960–1088 cm�1 can be attributed to the major absorptionmodes of the phosphate groups, the O–P–O bending modeand P–O stretching vibration modes, respectively [37].These vibrations modes, which fundamentally correspondto the stretching modes, become wider and less defined incoatings containing silicon. The absorption band at634 cm�1, attributable to the OH� vibrational mode, canbe clearly appreciated in the spectra corresponding to thex = 0 coating but not in films where silicon is introduced,that is, the hydroxylation level decreases in coatings con-taining silicate groups. On the other hand, an additionalband at 520 cm�1 is observed in all coatings containing sil-icon, which is assigned to Si–O–Si bonding vibrations [38].Finally, carbonate groups substituting the PO3�

4 ions in theapatite structure, can be detected in all coatings by the

Wavenumbers (cm-1)

x=0

x=0.25

x=0.50

x=0.75

x=1.00

Tra

nsm

itta

nce

%

CO32-

CO32-

PO43-

PO43-

OH-

6008001000120014001600

Fig. 3. FTIR spectra of the coatings obtained.

appearance of bands at 875, 1410 and 1456 cm�1 [39].Usually, these CO2�

3 groups are observed in HA coatingsdeposited by the sol–gel technique [29,30] and can beattributed to the decomposition of the precursors,P(OCH2CH3)3 and Si(OCH2CH3)4 [35].

The modification of the PO3�4 bands in the presence of

silicon could be associated with the incorporation of sili-cate ions into some phosphate sites of the lattice, whichprovoke changes in the bonding and symmetry of thePO3�

4 groups. In addition, the contribution of the intenseSi–O–Si asymmetric stretching mode around 1080 cm�1

[40] could also justify the broadening of these bands. More-over, Gibson et al. [35] indicate the appearance of threeadditional peaks at 945, 890 and 840 cm�1 for Si-HApowders with a 0.4 wt.% of Si. According to those findings,the presence of a peak at 945 cm�1 could justify the broad-ening of the 960 cm�1 band observed in the x > 0.5coatings; however, the observation of the other bands iscomplicated due to the overlapping with the carbonatecharacteristic bands (Fig. 3).

The analysis by XRD and FTIR spectroscopy of Si-HAcoatings indicates the introduction of silicate ions into theapatite lattice. The results obtained by other authors forpowdered material using X-ray and neutron diffraction[35,41,42], show that these silicate ions are substitutingphosphate groups. Consequently, to compensate the extranegative charge of the SiO4�

4 groups, some of the OH� ionswould be lost to retain the charge balance; hence, initiallythe composition of the coatings could be described asCa10(PO4)6�x(SiO4)x(OH)2�x. These results agree with thedecrease/disappearance of the band attributable to theOH� vibration mode observed in the FTIR spectra ofSi-HA coatings (Fig. 3).

However, the FTIR spectra indicate that all the coatings(with and without Si) contain carbonate groups that enterinto the phosphate sites. Considering this substitution, thecomposition of the deposited coatings could be betterdescribed as:

Ca10ðPO4Þ6�x�yðSiO4ÞxðCO3ÞyðOHÞ2�xþy

when x = y, i.e. the number of carbonates are equal to thatof silicates, no hydroxyl vacancies will be created. As theFTIR spectra indicate an OH� signal reduction, x mustbe greater than y.

As mentioned above, the carbonate incorporation is dueto the thermal decomposition of the precursors. Someauthors [1,35] observed that the amount of carbonatesincreases with the Si(OCH2CH3)4 addition, resulting, dueto the competition between CO2�

3 and SiO4�4 groups for

the phosphate sites, in not all of the silicon being incorpo-rated into the apatite structure. In this sense, in sampleswith considerable silicon content, a new band correspond-ing to Si–O–Si stretching mode appears in the FTIR spectraat 800 cm�1, indicating that this silicon remains in the HAsurface in the form of monomeric or polymeric SiO4�

4 [5].In the present work no significant differences can be

appreciated in the carbonate content between coatings with

Fig. 5. (a) TEM micrographs (b) ED patterns and (c) EDS spectra of Si-HA coatings with x = 0.5 and x = 1.

N. Hijon et al. / Acta Biomaterialia 2 (2006) 567–574 571

different silicon content, probably because the increase ofSi(OCH2CH3)4 is compensated by the decrease in theamount of P(OCH2CH3)3. Consequently, in the Si-HAcoatings described in this work the quantity of carbonateis similar for different silicon contents, and its presencedoes not seem to prevent the SiO4�

4 incorporation intothe HA lattice.

The SEM study showed an excellent coverage of thesubstrate surface. All the films obtained show a smoothsurface where the component particles can be hardly distin-guished and no cracks or other defects are visible (Fig. 4).This figure also includes the distribution of elements in themicrograph. An homogeneous distribution can be appreci-ated for the different components: Ca, P, Si. A minor con-centration of this last element can be appreciated whencompared to the other two.

On the other hand, TEM and ED analyses of materialscraped from the surface of silicon-substituted hydroxyap-atite coatings were performed. Examples of representativecrystals with x = 0.5 and x = 1 are illustrated in Fig. 5.The electron micrographs show that the coatings are con-stituted by crystals smaller than 20 nm. These crystals weresimilar in size and morphology, independent of silicon con-tent. Highly ordered zones can be appreciated in some ofthe crystals shown in Fig. 5. The corresponding ED patternis characteristic of a polycrystalline material with diffrac-tion rings that can be indexed to the interplanar distancesof an apatite-like structure. The EDS analysis performedon these crystals show that the composition of all studiedcrystals was based on Ca, P and Si with a molar amountratio similar to that introduced in the precursor sol (Table1). Fig. 5 shows an evident increase of the Si signal whichconfirms the different silicon content between x = 0.5(1.38 ± 0.02 wt.%) and x = 1 (2.82 ± 0.02 wt.%) coatings.

The adhesion test showed values higher than 20 MPa,the strength at which breakage occurs in the glue, in theinterface glue/coating or between the substrate and themetallic holder; in no case was fracture of the coating

Fig. 4. SEM micrograph of Si-HA (x = 0.5) coating. The Ca, P and Sidistribution maps are overlapped.

observed. Thus, the value of 20 MPa must be consideredas a lower limit. No significant differences were observedin coatings deposited as a function of the silicon content.These minimum tensile strength adhesion values are com-parable to those obtained for calcium phosphate coatingsprepared by the sol–gel technique [43], plasma spray [44]electrodeposition [45], or pulsed laser deposition [46].

Concerning the in vitro bioactivity test, the SEMmicrograph of the films after their immersion in the acel-lular fluid (Fig. 6a) shows that the coating surface presentsa different morphology to that observed before immersion.Now, the surface of the film is covered by a new layer ofmaterial which appears to be constituted of numerousneedle-like crystallites, similar to those observed in otherbioactive systems [47,48]. The morphology of this newlayer was similar in all the coatings immersed, indepen-dent of silicon content. However, silicon introductionshortens the induction of bioactivity period. Hence, inSi-HA coatings this layer was observed after 24 h, whereasfor HA coatings, without silicon, it was necessary to waitfor 3 days.

Due to the nature of this induced layer (an apatite-likestructure), it is not easy to distinguish it from the depositedcoating by techniques such as XRD or FTIR. For this

Fig. 6. Si-HA (x = 0.75) coating immersed in SBF for 1 day: (a) scanning electron micrograph, (b) transmission electron micrograph, and ED pattern ofthe particles which constitute the new layer formed in SBF.

572 N. Hijon et al. / Acta Biomaterialia 2 (2006) 567–574

reason, an ED and TEM characterization was performedon material scraped from the surface of an SBF soakedcoating. The TEM study shows the presence of two typesof particles in coatings immersed 1 day in SBF. One typeof crystals was similar to that showed in Fig. 5, which cor-respond to the Si-HA sol–gel coating, while the othersshowed a needle-like shape (Fig. 6b), with a lower Ca/Pmolar ratio (1.4 ± 0.1), similar to the ratio observed in bio-logical apatites [49] and other layers formed in SBF [47,48].The corresponding ED pattern (Fig. 6c) is characteristic ofa polycrystalline material, with interplanar distances attrib-utable to an apatite-like phase, showing a lower degree ofcrystallinity when compared to those crystals present inthe deposited coating (Fig. 5).

The in vitro bioactivity test in SBF shows that HA andSi-HA coatings induce the apatite precipitation onto itssurface, and this layer can be detected earlier in silicon-con-taining films. A similar increase of the in vitro bioactivityrate has been also observed by other authors in powderedsamples [50,51]. In this sense, it is proposed that the disso-lution of HA is enhanced by the presence of silicon whichcauses an increase in the local concentrations of calciumand phosphate ions, thereby increasing the degree of satu-ration in the medium and, thus, facilitating the precipita-tion of calcium phosphate phase [7]. Two mechanismshave been proposed to explain the enhanced dissolutionof Si-HA materials: an elevated number of defects causean extensive dissolution at the grain boundaries and tri-ple-junctions [52], and the incorporation of silicate ionsinto the HA lattice distort and destabilize the structure,therefore making it energetically favorable for silicate ions

to dissolve preferentially [51]. These could contribute toexplaining the higher rate of formation of the apatite layerin the SBF medium.

4. Conclusions

The sol–gel technique, a low cost and simple to usemethod, has been successfully applied to preparing sili-con-substitute hydroxyapatite coatings according to the fol-lowing formula: Ca10(PO4)6�x�y(SiO4)x(CO3)y(OH)2�x+y

(0 6 x 6 1), where carbonates are included in the phosphatesites competing with the introduced silicates. These Si-HAcoatings, with thicknesses around 600 nm, show a crack-free and smooth surface, constituted by nanocrystals smal-ler than 20 nm. The Si-HA deposited coatings show tensilestrength values around 20 MPa, comparable to those pre-pared without silicon or to those deposited by other prepa-ration techniques. The apatite forming ability on the surfaceof these coatings was enhanced by the introduction of sili-cate into the hydroxyapatite lattice. Therefore, the Si-HAfilms deposited by dip coating represent a good alternativefor coating metallic substrates, with potential applicationsas dental and orthopedic implants.

Acknowledgements

Financial support of CICYT (Spain) through ResearchProject MAT2005-01486 is acknowledged. The XRD,and SEM and TEM measurements were performed atC.A.I. Difraccion de Rayos X and C.A.I. MicroscopiaElectronica, Universidad Complutense, respectively. The

N. Hijon et al. / Acta Biomaterialia 2 (2006) 567–574 573

authors thank Biomet Spain Orthopaedics for providingthe titanium alloy.

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