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Bioactive coating on titanium using calcium-containing methylsiloxane
Surface & Coatings Technology 203 (2008) 52–58
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Bioactive coating on titanium using calcium-containing methylsiloxane Yasuto Hoshikawa a,⁎, Eiichi Yasuda a,1, Takamasa Onoki a, Masaru Akao b, Yasuhiro Tanabe c a b c
Materials and Structures Laboratory, Tokyo Institute of Technology, R3-21, 4259, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10, Surugadai, Kanda, Chiyoda-ku, Tokyo 101-0062, Japan Department of Chemical Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
a r t i c l e
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Article history: Received 27 December 2007 Accepted in revised form 25 July 2008 Available online 31 July 2008 Keywords: Sol–gel method Methyltriethoxysilane Methylsiloxane Bone-like apatite Bioactive coating Adhesive strength
a b s t r a c t A bioactive coating of Ca-containing methylsiloxane on a titanium surface was investigated. The coatings have double-layered structure consisted of underlying methylsiloxane (MS) and with an incorporated Ca content methylsiloxane (MS-Ca) top layer. The top layer of MS-Ca was prepared from methyltriethoxysilane containing 2.5 mol% pentaethoxyniobium and 2.5 mol% pentaethoxytantalum followed by the addition of calcium nitrate tetrahydrate with the Ca/Si molar ratio of 0.05 to 0.5 to enhance the formation of bone-like apatite (BLA) in a simulated body fluid (SBF). The adhesive strength of the coating with a 0.05 Ca/Si ratio was 3.5 MPa, while the adhesive strength of the coating tended to decrease to 2 MPa with the increasing Ca content. The adhesive strength of the coating was related to the porosity, which depended on the Ca content. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Titanium (Ti) has desirable mechanical properties and biocompability to be utilized as an orthopaedic implant. However, the material surface needs to be bioactivated because of its bioinert property. Some effective coatings to make the surface bioactive have been reported as follows: hydroxyapatite (HA) [1–5], titania [6], glass–ceramic [7,8], organic silicate [9–12], etc. Commercially available coatings are manufactured by the plasma spraying of HA powders on metal surfaces. However, there are some significant problems such as low fatigue strength, weak adherence between HA and the substrate, etc, caused by the decomposition of HA with high temperature processing [1,2]. The sol–gel derived organic silicate is a favorable procedure for a new process such as the bioactive coating on a Ti implant instead of the plasma spraying. It can be prepared at low temperature, and able to coat a complicated surface by a dipping process. In recent reports, there are two types of the bioactive coatings derived from organic silicate. One includes a bioactive promoter such as bioglass, glass– ceramics and HA powder [9–11]. This type is an organic silicate composite using an organic silane to adhere to the particles to induce a bioactivity on the metallic surface. The other is an organic silicatecalcium hybrid composed of organic silane and calcium (Ca) as calcium nitrate tetrahydrate (Ca(NO3)2 4H2O) and calcium acetate [12,13]. Quick release of the Ca ions plays a significant bioactive role in a body fluid situation. Therefore, the coatings have a favorable ⁎ Corresponding author. Tel.: +81 3 5841 7368; fax: +81 3 5800 3806. E-mail addresses: [email protected] (Y. Hoshikawa), [email protected] (E. Yasuda). 1 Tel.: +81 3 5734 2042; fax: +81 3 5734 2062. 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.07.027
bioactivity compared to the former one. Organic silicate coatings on a metal substrate, however, were not reported from the viewpoint of adhesive strength of the coating and metal substrate. We prepared methyltriethoxysilane (MTES)-derived methylsiloxane (MS) coatings on a Ti substrate by the sol–gel method. Its material was one of the organic silicates that has an organosiloxane structure called poly methylsilsesquioxane [14]. The coating has a double-layered structure. Underlying coating is MS derived by MTES. Top coating is with an incorporated Ca context MS (MS-Ca) composed of MTES, calcium nitrate tetrahydrate (Ca(NO3)2 4H2O), pentaethoxyniobium (Nb(OC2H5)5) and pentaethoxytantalum (Ta(OC2H5)5). The coatings composed of doublelayered MS-Ca/MS were a homogeneous and stable film, which has a bioactivity with a good adherence on the Ti surface using a low temperature process [12]. The Ca content plays an important role in the bioactivity. In the present study, we investigated the relation between the bioactivity and adhesive strengths of the coatings with various Ca contents in order to determine the suitable composition of the implant coating. 2. Materials and methods 2.1. Coating solutions A MTES solution was prepared from 14 mL MTES (0.070 mol; ShinEtsu Chemical Co., Ltd., Tokyo), 12.2 mL ethanol, 2.5 mL distill water and 0.35 mL 1 N hydrochloric acid. After stirring for 1 h, the solution was aged at 60 °C for 3 h. The polymerized product formed a MS network called the MS solution, which was applied to the underlying coating on the Ti substrates. Metal alkoxides, Nb(OC2H5)5 and Ta(OC2H5)5 (High Purity Chemical Laboratory Co., Ltd., Inc., Tokyo) were mixed in an acetylacetone
Y. Hoshikawa et al. / Surface & Coatings Technology 203 (2008) 52–58 Table 1 Starting composition and aging time of coating solutions in the present study Coating solution
MS MS-Ca05 MS-Ca10 MS-Ca30 MS-Ca50
Reagents molar ratio
Aging time (h)
MTES Ta Nb AcAc HCI (OEt)5 (OEt)5
H2O Ca EtOH (NO3)2·4H2O
1 1 1 1 1
2 2 2 2 2
0 0.025 0.025 0.025 0.025
0 0.025 0.025 0.025 0.025
0 0.1 0.1 0.1 0.1
0.005 0.005 0.005 0.005 0.005
0 0.05 0.1 0.3 0.5
3 6 6 6 6
3 24 18 6 3
MTES: methyltriethoxysilane, SiCH3 (OC2H5)3 AcAc: acetylacetone, CH2 ((fO)CH3)2 Et: ethyl group, –C2H5.
(AcAc) solvent for 30 min under a dry argon gas to control the hydrolysis rate [15]. The molar ratio of each metal alkoxide and AcAc was 1:2. The solutions were called the AcAc modified metal alkoxide. Ca(NO3)2 4H2O was used as the Ca source. Amount of 0.83, 1.65, 4.95 and 8.25 g Ca(NO3)2 4H2O were dissolved in 12.2 mL of ethanol. Their Ca/Si molar ratios were calculated to be 0.05, 0.10, 0.30 and 0.50, respectively. They were called the Ca solutions. To the MTES solution prepared by a procedure similar to the MS solution before aging were added 0.91 g AcAc modified Nb(OC2H5)5 and 1.06 g AcAc modified Ta (OC2H5)5. After stirring the solutions for 30 min, the Ca solutions were added. After stirring for 30 min, the solutions with Ca/Si 0.05, 0.10, 0.30 and 0.50 were aged at 60 °C for 24, 18, 6 and 3 h, respectively, of which the solution opaqueness was estimated. They were called MSCa05, MS-Ca10, MS-Ca30 and MS-Ca50 solution with respect to the Ca/Si ratios of 0.05, 0.10, 0.30 and 0.50. These solutions were applied as top coating on the Ti substrate. The molar ratios and aging times of the coating solutions are given in Table 1.
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inhibits measuring of the adhesive strength between coating and Ti substrate. Therefore, we prepared physiological salt solution (PS) adjusted to pH 7.4 with TRIS instead of SBF. The double-layered MSCa/MS coatings used for the adhesion test were soaked in 120 mL PS adjusted to pH 7.4 at 37 °C for 1 day. They rinsed with pure water, and dried at room temperature for 1 day. The total coating thickness of the single-layered MS and the double-layered MS-Ca/MS were measured using a coating thickness tester (LZ-330J, Kett Co., Ltd.), which is capable of measuring the coating thickness of non-ferrous substrates by the eddy-current method. Fig. 1 shows a tear-off attachment for the adhesion test. Steel rods (8 mm diameter) were glued to the coated plates using an epoxy resin and fixed for 24 h at room temperature. The tensile tests were performed at a crosshead speed of 0.1 mm/min using a universal testing machine (Auto Graph AK-100kG, SHIMADZU Co., Ltd.). The number of tested specimens of between 8 and 10 were used for the tensile test. After testing, the microstructure of the fractured surface of the coatings was observed by optical microscopy (Metaphot, Nikon Co., Ltd.) and field emission scanning electron microscopy (FE-SEM; S-4500, HITACHI Co., Ltd.). Cross-section of MS-Ca05 and MS-Ca50 were observed by E-SEM. The plates coated each double-layered MS-Ca/MS as MS-Ca05 and MSCa50 after PS soaking were embedded into polyester resin, and cut at vertically against Ti plate. After polishing with alumina (Ra 0.1 µm), the microstructures of the coating were observed. The bulk samples were prepared from the MS-Ca05, MS-Ca10, MSCa30 and MS-Ca50 solutions after drying at 60 °C for 168 h and at 80 °C for 96 h for the density measurement. The bulk samples were soaked in 120 mL of PS at 37 °C for 1 day. They were rinsed with pure water, and dried at room temperature for 1 day and at 60 °C for 1 day. The bulk density was measured using the cubic bulk volume and mass. The true density was measured by a densimeter (Accupyc 1330, SHIMADZU Co., Ltd.) followed by Archimedes method substituted with He gas.
2.2. Coating techniques on Ti plates 3. Results The pure Ti plate size used for the bioactivity test was 10 × 10 × 1 mm3, and for adhesion test, it was 10 × 40 × 3 mm3. The surfaces of the plates were polished with 2000 grit SiC paper and washed in acetone using an ultrasonic bath. The plates were dipped into the MS solution for 10 s and then pulled up. The MS coated plates were placed on a watch glass and dried at 80 °C for 24 h. The coating was called single-layered MS. The MS-Ca solutions were applied as drops to the MS coated plates at a rate of 0.3 µL/mm2 and then dried at 80 °C for 24 h to produce a doublelayered structure consisting of a MS-Ca top layer and MS underlying layer. The coating was called double-layered MS-Ca/MS, or various solutions of MS-Ca with Ca/Si 0.05, 0.10, 0.30 and 0.50 were corresponded to MS-Ca05, MS-Ca10, MS-Ca30 and MS-Ca50, respectively.
3.1. Bioactivity test Fig. 2 shows the XRD patterns of the double-layered MS-Ca/MS coatings with different ratios of Ca/Si on the Ti plates after soaking in SBF for 3 and 6 days. The diffraction patterns of the coatings showed that MS-Ca10, MS-Ca30 and MS-Ca50 deposited low-crystalline HA, so-called BLA on their surfaces at 3 days, while the MS-Ca05 sample deposited HA at 6 days. Except for the HA peaks, the samples had intense peaks from the α-titanium used as a substrate. The MS-Ca10 had low HA peaks compared to the MS-Ca30 and MS-Ca50 as shown in Fig. 2(a). Fig. 3 shows a comparison of the SEM images for the SBF
2.3. Bioactivity test using SBF The bioactivity on the coating samples soaked in simulated body fluid (SBF) was examined for bone-like apatite (BLA) formation [16]. The SBF was prepared by dissolving reagent-grade NaCl, NaHCO3, KCl, K2HPO4 3H2O, MgCl2 6H2O, CaCl2 and Na2SO4 into distilled water, and buffered at pH 7.40 with trishydroxymethylaminomethane (TRIS) and hydrochloric acid at 37.0 °C. The coated specimen was soaked into 80 mL of SBF at 37 °C for 3 and 6 days. The specimens were rinsed with distilled water and dried at room temperature. The surfaces were examined by X-ray diffractometry (XRD; RINT-2000, RIGAKU Co., Ltd.) and environmental scanning electron microscopy (E-SEM; ESEM2700, Nikon Co., Ltd.) to examine the bioactivity. 2.4. Adhesive strength test The adhesive strengths of the coatings on Ti substrate were evaluated using the Japanese Industrial Standards (JIS) A 6909 tear-off test. BLA layer formed on the coating surface after soaked in SBF
Fig. 1. Setup measuring adhesive strength between coating layer and Ti plate, tear-off testing based on JIS A 6909.
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soaked surfaces, between the coating MS-Ca05 (a), MS-Ca10 (b), MSCa30 (c) and MS-Ca50 (d) for 6 days. Spherical particles estimated to be due to the deposition of BLA were formed on the surfaces during the early stages. The MS-Ca05 was not completely covered by BLA as shown in Fig. 3(a). The MS-Ca30 and MS-Ca50 showed many more spherical particles and intense HA peaks compared to the MS-Ca05 and MS-Ca10. 3.2. Adhesive strength test
Fig. 2. XRD patterns of the double-layered MS-Ca/MS coatings with varied Ca/Si ratios after soaking in SBF (a) at 3 days and (b) at 6 days. HA: hydroxyapatite, Ti: α-titanium.
The total thickness of the single-layered MS and the doublelayered MS-Ca/MS coatings ranged from 3 to 5 and 40 to 50 µm, respectively. Fig. 4 shows SEM observations of cross-section for each MS-Ca05 (a) and MS-Ca50 (b) coatings. There were two layers and adhered between MS-Ca and Ti with intermediacy layer of underlying MS. The top layer of MS-Ca05 was dense; on the other hand, MS-Ca50 had many pores in the layer. The average tensile strength of the single-layered MS coating was 6.2 ± 2.2 MPa. All the samples peeled off at the MS-epoxy resin interface but did not to peel at the MS-Ti surface interface. Therefore, the adhesive strength between the single-layered MS and Ti surface was higher than 6.2 MPa. Fig. 5 shows optical microscopy of the fractured surface for the double-layered MS-Ca/MS coatings after tear-off test. There were two types of fracture areas as noted point (X) and (Y) in Fig. 5. Microstructure of the fracture area (X) and (Y) were shown in Figs. 6 and 7, respectively. The area (X) was fractured during interior portion of MS-Ca layer. In Fig. 6(a), the microstructures of MS-Ca05 layer had some of nano-pores under 100 nm. MS-Ca10 layer increased nanopores more than MS-Ca05 as shown in Fig. 6(b). MS-Ca30 and MSCa50 layers had larger pores in the order, MS-Ca50: N1 µm, MS-Ca30: N500 nm, and these pores were percolated as shown in Fig. 6(c) and (d). Fig. 8 shows the variations in the adhesive strengths of the doublelayered MS-Ca/MS coatings on Ti. The tensile strength for each coatings of MS-Ca05, MS-Ca10, MS-Ca30 and MS-Ca50 were 3.5, 2.8,
Fig. 3. SEM photographs of the surface of double-layered MS-Ca/MS coatings after soaking in SBF. (a), (b), (c) and (d) indicate MS-Ca05, MS-Ca10, MS-Ca30 and MS-Ca50 coating at 6 days, respectively; (a) arrowhead indicates non-BLA covered surface.
Y. Hoshikawa et al. / Surface & Coatings Technology 203 (2008) 52–58
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2.3 and 2.0 MPa, respectively, and the standard deviations for all the samples were less than 1.0 MPa. The double-layered MS-Ca/MS coatings peeled off with fracturing as shown in Fig. 5. Therefore, values for tensile strength of the double-layered MS-Ca/MS coatings equal adhesive strength between coating and Ti. The porosity (P0) is calculated using the bulk density (dB) and true density (dT) as follows. P0 ¼
1−
dB dT
100:
Table 2 shows the bulk density, true density and porosity of each bulk MS-Ca sample. The porosity of the bulk MS-Ca tended to increase with the increasing calcium content. 4. Discussion 4.1. Bioactivity of MS-Ca materials
Fig. 4. SEM photographs of the cross-section of samples after soaking in physiological salt solution adjusted to pH 7.4 at 37 °C for 1 day. (a) and (b) indicate MS-Ca05 and MSCa50 coating, respectively.
The formation of a BLA on the bulk of Ca-containing organic silicate, which was derived from tetraethoxysilane (TEOS), polydimethylsiloxane (PDMS) and Ca(NO3)2·4H2O (TEOS-PDMS-Ca), was reported by Tsuru et al. [17,18]. The silanol (Si–OH) groups of the silicate are essential chemical species that induce the heterogeneous nucleation of BLA [19]. The effect of the dissolved calcium ions from the surface helps the BLA deposition since it increases the degree of supersaturation in the SBF with regard to the BLA. The coating of the double-layered MS-Ca/MS formed BLA as shown in Fig. 3. It seems that a material of the MS-Ca has a similar mechanism to form BLA as that of TEOS-PDMS-Ca reported by Tsuru et al. On the other hand, the ability of BLA formation for the MS-Ca05 and MS-Ca10 coatings was lower
Fig. 5. Photographs of the fractured surface of samples after tear-off test. (a), (b), (c) and (d) indicate MS-Ca05, MS-Ca10, MS-Ca30 and MS-Ca50 coating, respectively; (X) and (Y) indicate fracture area as shown in Figs. 6 and 7, respectively.
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Fig. 6. SEM photographs of microstructure on area (X) as shown in Fig. 5 after tear-off testing. (a), (b), (c) and (d) indicate MS-Ca05, MS-Ca10, MS-Ca30 and MS-Ca50 coating, respectively.
Fig. 7. SEM photographs of microstructure on area (Y) as shown in Fig. 5 after tear-off testing. (a), (b), (c) and (d) indicate MS-Ca05, MS-Ca10, MS-Ca30 and MS-Ca50 coating, respectively.
Y. Hoshikawa et al. / Surface & Coatings Technology 203 (2008) 52–58
Fig. 8. Adhesive strength of the double-layered MS-Ca/MS coatings with varied Ca/Si ratios.
than MS-Ca30 and MS-Ca50. The coating layer ranging from 40 to 50 µm thick was very thin and low Ca/Si ratio MS-Ca didn't have enough Ca contents compared to bulk sample with 1 mm thick. However, the high Ca/Si ratio MS-Ca can release a greater amount of Ca ions than the low Ca/Si ratio coatings for the supersaturation to form BLA. Tsuru et al. also reported a similar result for the BLA formation of the organic silicate coating prepared by vinyltrimethoxysilane (VTMS) and calcium acetate (VTMS-Ca) on polyamide substrate [13]. The VTMS-Ca coating had difficulty to form the BLA for 14 days of soaking in SBF whereas the bulk clearly formed BLA before 7 days of soaking. The ability of the BLA formation for the MSCa coatings increased with an increase in the amount of Ca as shown in Figs. 2 and 3. It can maintain a favorable bioactivity of the coating to enhance the Ca release in SBF. The porosity of the MS-Ca also increased with the increasing Ca as shown in Table 2. Tsuru et al. reported that the Ca-containing organic silicate of TEOS-PDMS-Ca with the appropriate hydrochloric acid affects the micrometer-range structure such as the porosity and pore size, whereas the chemical bonding structure like Si–O–Si is unchanged by the Si29–NMR observation. For the sample with a ratio Ca/Si 0.05, BLA formation was enhanced by the increase in the hydrochloric acid [18]. It suggested that with a large content of hydrochloric acid, the gel network was well established at the gelation by consuming the free Si–OH groups due to condensation resulting in greater pores for organic silicate of TEOS-PDMS-Ca. It is also important for bioactivity to increase Si–OH groups because of inducing more heterogeneous nucleation of apatite. As shown in Fig. 6 and Table 2, MS-Ca material that included high Ca content tends to enhance poresize and porosity. In consequence of it, surface area of the MS-Ca with Si–OH groups is expanded and it has influence on the BLA forming ability of the coating. That is, it seems the effect of Ca incorporated in MS-Ca material elevates bioactivity not only to accelerate supersaturation for apatite but also to increase Si–OH group in the MS-Ca surface. This result indicates that it is important for the bioactive coating of the organic silicate to include a high Ca content. The MS-Ca in the present study can include a large amount of Ca at the Ca/Si of 0.5, whereas the
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previously reported organic silicates prepared by TEOS-PDMS-Ca or VTMS-Ca [13,17,18] are less than 0.05. In a previous study, the Ca-containing organic silicate derived from only MTES and Ca(NO3)2·4H2O could not be made in the absence of a metal alkoxide such as Nb(OC2H5)5 and Ta(OC2H5)5. The metal alkoxides, such as Nb(OC2H5)5 and Ta(OC2H5)5, were incorporated into the siloxane network for copolymerization with the MTES polymerization. Therefore, the terminal of the methylsiloxane structure has Nb–OH and Ta–OH groups as well as Si–OH [20]. The electronegativity of O, Si, Nb and Ta by Pauling are 3.4, 1.9, 1.6 and 1.5, respectively. In general, if the difference in the electronegativity for two atoms is large, their polarization becomes stronger. The difference in the electronegativity during Si–O is 1.5, whereas Nb–O and Ta–O are 1.8 and 1.9, respectively. The degree of polarization δ− for the oxygen atom as M–Oδ− bonding, Ta–Oδ− and Nb–Oδ− are stronger than Si–Oδ− because the significant difference in the electronegativity indicates the ability of ionic bonding during M–O. Therefore the terminal groups of Nb–Oδ−H and Ta–Oδ−H, which has a strong negative polarization, are coordinate Ca ions having a positive charge during the growing methylsiloxane network of polymerization. It seems that the siloxane polymers with Nb–OH and Ta–OH are able to trap more positive Ca ions compared to only the siloxane one. 4.2. Adhesive strength of the coatings All of the double-layered MS-Ca/MS coatings were fractured on the tear-off testing, whereas the single-layered MS coating was not peeling off. Therefore, adhesive strength of the single-layered MS coating just only composed by MTES is enough high compared to double-layered MS-Ca/MS. Cross sectional observation of the double-layered coatings in Fig. 4, microstructure of the top layer composed MS-Ca has many pores, which is tended to increase with increasing Ca composition, whereas underlying MS layer is dense. Microstructure of the fracture surface (Y) as shown in Fig. 7 is described partial micro-pore structure remains on dense layer assumed underlying MS layer. It indicated fractured surface (Y) as shown in Fig. 7 is nearby interface between MS-Ca and MS. Therefore, the double-layered MS-Ca/MS coatings are peeled off during the MS-Ca layer without underlying MS layer. In fact, all of the coatings for double-layered MS-Ca/MS, in which the value of the adhesive strength is less than 3.5 MPa, is lower enough than the single-layered MS coating which is more than 6.2 MPa. It implies that underlying MS of the double-layered coatings do not have peeling between MS and Ti interface. Therefore, peeling of the double-layered coatings is decided adhesive strength between MS-Ca and MS. With increasingly Ca/Si ratios, pore size of the MS-Ca was expanded as shown in Fig. 6 with increasing porosity from Table 2,
Table 2 Density and porosity of MS-Ca bulks after soaking in physiological salt solution adjusted to pH 7.4 at 37 °C for 1 day
Bulk density (g/cm3) True density (g/cm3) Porosity (%)
MS-Ca05
MS-Ca10
MS-Ca30
MS-Ca50
1.25 1.40 11
1.03 1.45 29
0.50 1.46 52
0.36 1.46 66
Fig. 9. Natural logarithmic function for adhesive strength and porosity of the doublelayered MS-Ca/MS coatings.
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and adhesive strength of the double-layered MS-Ca/MS coatings were decreased as shown in Fig. 8. Therefore, it seems that adhesive strength of the coatings is affected microstructure of the MS-Ca material. Fig. 9 shows the natural logarithmic function of the adhesive strength of the double-layered MS-Ca/MS coatings with different porosities from Table 2. The variation in lnσ with porosity is a decreasing linear relation. Therefore, relationship of the adhesive strength σ and the porosity P0 is equal to ln σ ¼ −BP0 þ ln σ 0
ð1Þ
where σ0 is the strength of the nonporous ceramics and B is the experimental constant. Formula (1) is transformed as follows σ ¼ σ 0 e−BP0 :
ð2Þ
5. Conclusions The double-layered MS-Ca/MS coatings can include a large amount of Ca with Ca/Si molar ratios of 0.5 compared to previous reports so that the coating could have a favorable bioactivity similar to the bulk one. The coatings with Ca/Si ratios of 0.05, 0.1, 0.3 and 0.5 formed BLA at 6, 3, 3, 3 days, respectively. The adhesive strengths of the coating on Ti became low with an increase in the Ca content due to the increase in the micropores. With the addition of Ca into the organic silicate, the adhesive strength made sacrifices to enhance the bioactivity. The coatings have an adhesive strength ranging from 2.0 to 3.5 MPa. The coatings, which kept the bioactivity and the desirable mechanical properties, can be made by low temperature process; therefore, the coating method is useful to apply to a novel bioactive coating on titanium for orthopedic implants. Acknowledgments
This means that adhesive strength of the coating σ exponentially becomes weak with the increasing porosity P0. The formula (2) meets relationship of the compressive strength σ of porous ceramics by Duckworth [21]. It means that mechanical properties of the coating layer is affected its porosity as similarly ceramics. Therefore, adhesive strength of the coatings is also decided porosity from relationship of formula (2). Effect of the Ca incorporation in MS-Ca material is summarized as follows; the increasing Ca content of double-layered MS-Ca/MS coatings degraded the mechanical properties of the MS-Ca material due to the increase in its micropores. But it played an enhanced role in bioactivity. It is important for load-bearing implant coated bioactive ceramics that coating–substrate interface obtains a sufficient mechanical properties, whereas its difficulty. Piveteau et al. reported adhesive strength of sol–gel derived titanium dioxide mixed calcium phosphate (TiO2-CaP) on Ti substrate by dipping process [22]. The coating thickness was 5 µm, and it was heated for 10 min at 850 °C in air to be crystallized. Adhesive strength of the TiO2-CaP coating was 16 to 18 MPa. Adhesive strength of the double-layered MS-Ca/MS coatings is lower than the TiO2-CaP coating. In preparation of the doublelayered MS-Ca/MS coatings, heat temperature treatments were very low (b80 °C), and the layers was thick (40 to 50 µm). Crystallinity of the double-layered MS-Ca/MS materials seems very low because of its low heat temperature treatment. The crystallinity and thickness of coating material affects mechanical adhesion [22]. Therefore, it seems the double-layered MS-Ca/MS coatings have lower adhesive strength than the TiO2-CaP coating. It is difficult to define sufficient adhesive strength for the orthopedic implants, even more whether adhesive strength of the double-layered MS-Ca/MS coatings is suitable for one or not. As an example, adhesive strength of the soldering used heating coil for windshield defroster of a car rear window is 2.9 MPa [23]. Adhesive strength of the MS-Ca10 and MS-Ca05 had 2.8 MPa and 3.5 MPa, respectively. So the double-layered MS-Ca/MS coatings have even or more adhesion compared with the practical soldering. It is not the same for the orthopedic implant as the adhesive strength. However, it shows the coating method with low temperature process has good advantage against other processes with high temperature; nonetheless because of its low heat temperature treatment, the coatings on Ti substrates kept the same adhesion as industrial soldering. Therefore, coating technique of the double-layered MSCa/MS has a potential to apply to a bioactive coating for bone implants.
This work was supported by a Grant-in-Aid for Cooperative Research Project of Nationwide Joint-Use Research Institutes on Development Base of Joining Technology for New Metallic Glasses and Inorganic Materials and was partially supported by a Grant-in-Aid for Young Scientists (B), 18760516, 2006, from The Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors appreciated Mr. Hajime Ishii from the technical staffs of MSL for his help with the setup utilized by the Collaborative Research Project of Materials and Structures Laboratory, Tokyo Institute of Technology. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
K. de Groot, R. Geesink, C.P.T. Klein, P. Serekian, J. Biomed. Mater. Res. 21 (1987) 1375. S.R. Radin, P. Ducheyne, J. Mater. Sci. Med. 3 (1992) 33. T. Onoki, T. Hashida, Surf. Coat. Technol. 200 (2006) 6801. P. Zhou, M. Akao, Bio-Med. Mater. Eng. 7 (1997) 67. A. Montenero, G. Gnappi, F. Ferrari, M. Cesari, E. Salvioli, S. Kaciulis, M. Fini, J. Mater. Sci. 35 (2000) 2791. P. Li, I. Kangasniemi, K. de Groot, T. Kokubo, J. Am. Ceram. Soc. 77 (1994) 1307. L.L. Hench, P.J. Buscemi, United States Patent, No.4159358 (1979). D.R. Bloyer, J.M. Gomez-Vega, E. Saiz, J.M. Mcnaney, R.M. Cannon, A.P. Tosima, Acta Mater. 47 (1999) 4221. C. Garcia, S. Cere, A. Duran, J. Non-Cryst. Solids 348 (2004) 218. C. Damia, P. Sharrok, Mater. Lett. 60 (2006) 3192. J. Ballarre, D.A. Lopez, W.H. Schreiner, A. Duran, S.M. Cere, Appl. Surf. Sci. 253 (2007) 7260. Y. Hoshikawa, E. Yasuda, T. Onoki, M. Akao, Y. Tanabe, Ceram. Transact.198 (2007) 129. K. Tsuru, S. Hayakawa, C. Ohtsuki, A. Osaka, J. Sol-Gel Sci. Technol. 13 (1998) 237. D.F. Merill, Canadian Patent, 868996 (1971. 4.20). D.C. Bradly, R.C. Mehrotra, D.P. Gaur, Metal Alkoxides, Academic Press, London, 1978, p. 209. T. Kokubo, H. Kushitani, S. Sakka, T. Kitsugi, T. Yamamuro, J. Biomed. Mater. Res. 24 (1990) 721. K. Tsuru, C. Ohtsuki, A. Osaka, T. Iwamoto, J.D. Mackenzie, J. Mater. Sci. Med. 8 (1997) 157. K. Tsuru, Y. Aburatani, T. Yabuta, S. Hayakawa, C. Ohtsuki, A. Osaka, J. Sol-Gel Sci. Technol. 21 (2001) 89. P. Li, C. Otuki, T. Kokubo, K. Nakanishi, N. Soga, T. Nakamura, T. Yamamuro, J. Am. Ceram. Soc. 75 (1992) 2094. M. Fukushima, Doctoral dissertation of Tokyo Institute of Technology (2004) 68. W. Duckworth, J. Am. Ceram. Soc. 36 (1953) 68. L.D. Piveteau, B. Gasser, L. Schlapbach, Biomaterials 21 (2000) 2193. Y. Yamamoto, K. Kamiya, S. Sakka, Yoguo-Kyokai-Shi 90 (1982) 328 [In Japanese].
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Bioactive coating on titanium using calcium-containing methylsiloxane Yasuto Hoshikawa a,⁎, Eiichi Yasuda a,1, Takamasa Onoki a, Masaru Akao b, Yasuhiro Tanabe c a b c
Materials and Structures Laboratory, Tokyo Institute of Technology, R3-21, 4259, Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, 2-3-10, Surugadai, Kanda, Chiyoda-ku, Tokyo 101-0062, Japan Department of Chemical Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
a r t i c l e
i n f o
Article history: Received 27 December 2007 Accepted in revised form 25 July 2008 Available online 31 July 2008 Keywords: Sol–gel method Methyltriethoxysilane Methylsiloxane Bone-like apatite Bioactive coating Adhesive strength
a b s t r a c t A bioactive coating of Ca-containing methylsiloxane on a titanium surface was investigated. The coatings have double-layered structure consisted of underlying methylsiloxane (MS) and with an incorporated Ca content methylsiloxane (MS-Ca) top layer. The top layer of MS-Ca was prepared from methyltriethoxysilane containing 2.5 mol% pentaethoxyniobium and 2.5 mol% pentaethoxytantalum followed by the addition of calcium nitrate tetrahydrate with the Ca/Si molar ratio of 0.05 to 0.5 to enhance the formation of bone-like apatite (BLA) in a simulated body fluid (SBF). The adhesive strength of the coating with a 0.05 Ca/Si ratio was 3.5 MPa, while the adhesive strength of the coating tended to decrease to 2 MPa with the increasing Ca content. The adhesive strength of the coating was related to the porosity, which depended on the Ca content. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Titanium (Ti) has desirable mechanical properties and biocompability to be utilized as an orthopaedic implant. However, the material surface needs to be bioactivated because of its bioinert property. Some effective coatings to make the surface bioactive have been reported as follows: hydroxyapatite (HA) [1–5], titania [6], glass–ceramic [7,8], organic silicate [9–12], etc. Commercially available coatings are manufactured by the plasma spraying of HA powders on metal surfaces. However, there are some significant problems such as low fatigue strength, weak adherence between HA and the substrate, etc, caused by the decomposition of HA with high temperature processing [1,2]. The sol–gel derived organic silicate is a favorable procedure for a new process such as the bioactive coating on a Ti implant instead of the plasma spraying. It can be prepared at low temperature, and able to coat a complicated surface by a dipping process. In recent reports, there are two types of the bioactive coatings derived from organic silicate. One includes a bioactive promoter such as bioglass, glass– ceramics and HA powder [9–11]. This type is an organic silicate composite using an organic silane to adhere to the particles to induce a bioactivity on the metallic surface. The other is an organic silicatecalcium hybrid composed of organic silane and calcium (Ca) as calcium nitrate tetrahydrate (Ca(NO3)2 4H2O) and calcium acetate [12,13]. Quick release of the Ca ions plays a significant bioactive role in a body fluid situation. Therefore, the coatings have a favorable ⁎ Corresponding author. Tel.: +81 3 5841 7368; fax: +81 3 5800 3806. E-mail addresses: [email protected] (Y. Hoshikawa), [email protected] (E. Yasuda). 1 Tel.: +81 3 5734 2042; fax: +81 3 5734 2062. 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.07.027
bioactivity compared to the former one. Organic silicate coatings on a metal substrate, however, were not reported from the viewpoint of adhesive strength of the coating and metal substrate. We prepared methyltriethoxysilane (MTES)-derived methylsiloxane (MS) coatings on a Ti substrate by the sol–gel method. Its material was one of the organic silicates that has an organosiloxane structure called poly methylsilsesquioxane [14]. The coating has a double-layered structure. Underlying coating is MS derived by MTES. Top coating is with an incorporated Ca context MS (MS-Ca) composed of MTES, calcium nitrate tetrahydrate (Ca(NO3)2 4H2O), pentaethoxyniobium (Nb(OC2H5)5) and pentaethoxytantalum (Ta(OC2H5)5). The coatings composed of doublelayered MS-Ca/MS were a homogeneous and stable film, which has a bioactivity with a good adherence on the Ti surface using a low temperature process [12]. The Ca content plays an important role in the bioactivity. In the present study, we investigated the relation between the bioactivity and adhesive strengths of the coatings with various Ca contents in order to determine the suitable composition of the implant coating. 2. Materials and methods 2.1. Coating solutions A MTES solution was prepared from 14 mL MTES (0.070 mol; ShinEtsu Chemical Co., Ltd., Tokyo), 12.2 mL ethanol, 2.5 mL distill water and 0.35 mL 1 N hydrochloric acid. After stirring for 1 h, the solution was aged at 60 °C for 3 h. The polymerized product formed a MS network called the MS solution, which was applied to the underlying coating on the Ti substrates. Metal alkoxides, Nb(OC2H5)5 and Ta(OC2H5)5 (High Purity Chemical Laboratory Co., Ltd., Inc., Tokyo) were mixed in an acetylacetone
Y. Hoshikawa et al. / Surface & Coatings Technology 203 (2008) 52–58 Table 1 Starting composition and aging time of coating solutions in the present study Coating solution
MS MS-Ca05 MS-Ca10 MS-Ca30 MS-Ca50
Reagents molar ratio
Aging time (h)
MTES Ta Nb AcAc HCI (OEt)5 (OEt)5
H2O Ca EtOH (NO3)2·4H2O
1 1 1 1 1
2 2 2 2 2
0 0.025 0.025 0.025 0.025
0 0.025 0.025 0.025 0.025
0 0.1 0.1 0.1 0.1
0.005 0.005 0.005 0.005 0.005
0 0.05 0.1 0.3 0.5
3 6 6 6 6
3 24 18 6 3
MTES: methyltriethoxysilane, SiCH3 (OC2H5)3 AcAc: acetylacetone, CH2 ((fO)CH3)2 Et: ethyl group, –C2H5.
(AcAc) solvent for 30 min under a dry argon gas to control the hydrolysis rate [15]. The molar ratio of each metal alkoxide and AcAc was 1:2. The solutions were called the AcAc modified metal alkoxide. Ca(NO3)2 4H2O was used as the Ca source. Amount of 0.83, 1.65, 4.95 and 8.25 g Ca(NO3)2 4H2O were dissolved in 12.2 mL of ethanol. Their Ca/Si molar ratios were calculated to be 0.05, 0.10, 0.30 and 0.50, respectively. They were called the Ca solutions. To the MTES solution prepared by a procedure similar to the MS solution before aging were added 0.91 g AcAc modified Nb(OC2H5)5 and 1.06 g AcAc modified Ta (OC2H5)5. After stirring the solutions for 30 min, the Ca solutions were added. After stirring for 30 min, the solutions with Ca/Si 0.05, 0.10, 0.30 and 0.50 were aged at 60 °C for 24, 18, 6 and 3 h, respectively, of which the solution opaqueness was estimated. They were called MSCa05, MS-Ca10, MS-Ca30 and MS-Ca50 solution with respect to the Ca/Si ratios of 0.05, 0.10, 0.30 and 0.50. These solutions were applied as top coating on the Ti substrate. The molar ratios and aging times of the coating solutions are given in Table 1.
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inhibits measuring of the adhesive strength between coating and Ti substrate. Therefore, we prepared physiological salt solution (PS) adjusted to pH 7.4 with TRIS instead of SBF. The double-layered MSCa/MS coatings used for the adhesion test were soaked in 120 mL PS adjusted to pH 7.4 at 37 °C for 1 day. They rinsed with pure water, and dried at room temperature for 1 day. The total coating thickness of the single-layered MS and the double-layered MS-Ca/MS were measured using a coating thickness tester (LZ-330J, Kett Co., Ltd.), which is capable of measuring the coating thickness of non-ferrous substrates by the eddy-current method. Fig. 1 shows a tear-off attachment for the adhesion test. Steel rods (8 mm diameter) were glued to the coated plates using an epoxy resin and fixed for 24 h at room temperature. The tensile tests were performed at a crosshead speed of 0.1 mm/min using a universal testing machine (Auto Graph AK-100kG, SHIMADZU Co., Ltd.). The number of tested specimens of between 8 and 10 were used for the tensile test. After testing, the microstructure of the fractured surface of the coatings was observed by optical microscopy (Metaphot, Nikon Co., Ltd.) and field emission scanning electron microscopy (FE-SEM; S-4500, HITACHI Co., Ltd.). Cross-section of MS-Ca05 and MS-Ca50 were observed by E-SEM. The plates coated each double-layered MS-Ca/MS as MS-Ca05 and MSCa50 after PS soaking were embedded into polyester resin, and cut at vertically against Ti plate. After polishing with alumina (Ra 0.1 µm), the microstructures of the coating were observed. The bulk samples were prepared from the MS-Ca05, MS-Ca10, MSCa30 and MS-Ca50 solutions after drying at 60 °C for 168 h and at 80 °C for 96 h for the density measurement. The bulk samples were soaked in 120 mL of PS at 37 °C for 1 day. They were rinsed with pure water, and dried at room temperature for 1 day and at 60 °C for 1 day. The bulk density was measured using the cubic bulk volume and mass. The true density was measured by a densimeter (Accupyc 1330, SHIMADZU Co., Ltd.) followed by Archimedes method substituted with He gas.
2.2. Coating techniques on Ti plates 3. Results The pure Ti plate size used for the bioactivity test was 10 × 10 × 1 mm3, and for adhesion test, it was 10 × 40 × 3 mm3. The surfaces of the plates were polished with 2000 grit SiC paper and washed in acetone using an ultrasonic bath. The plates were dipped into the MS solution for 10 s and then pulled up. The MS coated plates were placed on a watch glass and dried at 80 °C for 24 h. The coating was called single-layered MS. The MS-Ca solutions were applied as drops to the MS coated plates at a rate of 0.3 µL/mm2 and then dried at 80 °C for 24 h to produce a doublelayered structure consisting of a MS-Ca top layer and MS underlying layer. The coating was called double-layered MS-Ca/MS, or various solutions of MS-Ca with Ca/Si 0.05, 0.10, 0.30 and 0.50 were corresponded to MS-Ca05, MS-Ca10, MS-Ca30 and MS-Ca50, respectively.
3.1. Bioactivity test Fig. 2 shows the XRD patterns of the double-layered MS-Ca/MS coatings with different ratios of Ca/Si on the Ti plates after soaking in SBF for 3 and 6 days. The diffraction patterns of the coatings showed that MS-Ca10, MS-Ca30 and MS-Ca50 deposited low-crystalline HA, so-called BLA on their surfaces at 3 days, while the MS-Ca05 sample deposited HA at 6 days. Except for the HA peaks, the samples had intense peaks from the α-titanium used as a substrate. The MS-Ca10 had low HA peaks compared to the MS-Ca30 and MS-Ca50 as shown in Fig. 2(a). Fig. 3 shows a comparison of the SEM images for the SBF
2.3. Bioactivity test using SBF The bioactivity on the coating samples soaked in simulated body fluid (SBF) was examined for bone-like apatite (BLA) formation [16]. The SBF was prepared by dissolving reagent-grade NaCl, NaHCO3, KCl, K2HPO4 3H2O, MgCl2 6H2O, CaCl2 and Na2SO4 into distilled water, and buffered at pH 7.40 with trishydroxymethylaminomethane (TRIS) and hydrochloric acid at 37.0 °C. The coated specimen was soaked into 80 mL of SBF at 37 °C for 3 and 6 days. The specimens were rinsed with distilled water and dried at room temperature. The surfaces were examined by X-ray diffractometry (XRD; RINT-2000, RIGAKU Co., Ltd.) and environmental scanning electron microscopy (E-SEM; ESEM2700, Nikon Co., Ltd.) to examine the bioactivity. 2.4. Adhesive strength test The adhesive strengths of the coatings on Ti substrate were evaluated using the Japanese Industrial Standards (JIS) A 6909 tear-off test. BLA layer formed on the coating surface after soaked in SBF
Fig. 1. Setup measuring adhesive strength between coating layer and Ti plate, tear-off testing based on JIS A 6909.
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soaked surfaces, between the coating MS-Ca05 (a), MS-Ca10 (b), MSCa30 (c) and MS-Ca50 (d) for 6 days. Spherical particles estimated to be due to the deposition of BLA were formed on the surfaces during the early stages. The MS-Ca05 was not completely covered by BLA as shown in Fig. 3(a). The MS-Ca30 and MS-Ca50 showed many more spherical particles and intense HA peaks compared to the MS-Ca05 and MS-Ca10. 3.2. Adhesive strength test
Fig. 2. XRD patterns of the double-layered MS-Ca/MS coatings with varied Ca/Si ratios after soaking in SBF (a) at 3 days and (b) at 6 days. HA: hydroxyapatite, Ti: α-titanium.
The total thickness of the single-layered MS and the doublelayered MS-Ca/MS coatings ranged from 3 to 5 and 40 to 50 µm, respectively. Fig. 4 shows SEM observations of cross-section for each MS-Ca05 (a) and MS-Ca50 (b) coatings. There were two layers and adhered between MS-Ca and Ti with intermediacy layer of underlying MS. The top layer of MS-Ca05 was dense; on the other hand, MS-Ca50 had many pores in the layer. The average tensile strength of the single-layered MS coating was 6.2 ± 2.2 MPa. All the samples peeled off at the MS-epoxy resin interface but did not to peel at the MS-Ti surface interface. Therefore, the adhesive strength between the single-layered MS and Ti surface was higher than 6.2 MPa. Fig. 5 shows optical microscopy of the fractured surface for the double-layered MS-Ca/MS coatings after tear-off test. There were two types of fracture areas as noted point (X) and (Y) in Fig. 5. Microstructure of the fracture area (X) and (Y) were shown in Figs. 6 and 7, respectively. The area (X) was fractured during interior portion of MS-Ca layer. In Fig. 6(a), the microstructures of MS-Ca05 layer had some of nano-pores under 100 nm. MS-Ca10 layer increased nanopores more than MS-Ca05 as shown in Fig. 6(b). MS-Ca30 and MSCa50 layers had larger pores in the order, MS-Ca50: N1 µm, MS-Ca30: N500 nm, and these pores were percolated as shown in Fig. 6(c) and (d). Fig. 8 shows the variations in the adhesive strengths of the doublelayered MS-Ca/MS coatings on Ti. The tensile strength for each coatings of MS-Ca05, MS-Ca10, MS-Ca30 and MS-Ca50 were 3.5, 2.8,
Fig. 3. SEM photographs of the surface of double-layered MS-Ca/MS coatings after soaking in SBF. (a), (b), (c) and (d) indicate MS-Ca05, MS-Ca10, MS-Ca30 and MS-Ca50 coating at 6 days, respectively; (a) arrowhead indicates non-BLA covered surface.
Y. Hoshikawa et al. / Surface & Coatings Technology 203 (2008) 52–58
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2.3 and 2.0 MPa, respectively, and the standard deviations for all the samples were less than 1.0 MPa. The double-layered MS-Ca/MS coatings peeled off with fracturing as shown in Fig. 5. Therefore, values for tensile strength of the double-layered MS-Ca/MS coatings equal adhesive strength between coating and Ti. The porosity (P0) is calculated using the bulk density (dB) and true density (dT) as follows. P0 ¼
1−
dB dT
100:
Table 2 shows the bulk density, true density and porosity of each bulk MS-Ca sample. The porosity of the bulk MS-Ca tended to increase with the increasing calcium content. 4. Discussion 4.1. Bioactivity of MS-Ca materials
Fig. 4. SEM photographs of the cross-section of samples after soaking in physiological salt solution adjusted to pH 7.4 at 37 °C for 1 day. (a) and (b) indicate MS-Ca05 and MSCa50 coating, respectively.
The formation of a BLA on the bulk of Ca-containing organic silicate, which was derived from tetraethoxysilane (TEOS), polydimethylsiloxane (PDMS) and Ca(NO3)2·4H2O (TEOS-PDMS-Ca), was reported by Tsuru et al. [17,18]. The silanol (Si–OH) groups of the silicate are essential chemical species that induce the heterogeneous nucleation of BLA [19]. The effect of the dissolved calcium ions from the surface helps the BLA deposition since it increases the degree of supersaturation in the SBF with regard to the BLA. The coating of the double-layered MS-Ca/MS formed BLA as shown in Fig. 3. It seems that a material of the MS-Ca has a similar mechanism to form BLA as that of TEOS-PDMS-Ca reported by Tsuru et al. On the other hand, the ability of BLA formation for the MS-Ca05 and MS-Ca10 coatings was lower
Fig. 5. Photographs of the fractured surface of samples after tear-off test. (a), (b), (c) and (d) indicate MS-Ca05, MS-Ca10, MS-Ca30 and MS-Ca50 coating, respectively; (X) and (Y) indicate fracture area as shown in Figs. 6 and 7, respectively.
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Fig. 6. SEM photographs of microstructure on area (X) as shown in Fig. 5 after tear-off testing. (a), (b), (c) and (d) indicate MS-Ca05, MS-Ca10, MS-Ca30 and MS-Ca50 coating, respectively.
Fig. 7. SEM photographs of microstructure on area (Y) as shown in Fig. 5 after tear-off testing. (a), (b), (c) and (d) indicate MS-Ca05, MS-Ca10, MS-Ca30 and MS-Ca50 coating, respectively.
Y. Hoshikawa et al. / Surface & Coatings Technology 203 (2008) 52–58
Fig. 8. Adhesive strength of the double-layered MS-Ca/MS coatings with varied Ca/Si ratios.
than MS-Ca30 and MS-Ca50. The coating layer ranging from 40 to 50 µm thick was very thin and low Ca/Si ratio MS-Ca didn't have enough Ca contents compared to bulk sample with 1 mm thick. However, the high Ca/Si ratio MS-Ca can release a greater amount of Ca ions than the low Ca/Si ratio coatings for the supersaturation to form BLA. Tsuru et al. also reported a similar result for the BLA formation of the organic silicate coating prepared by vinyltrimethoxysilane (VTMS) and calcium acetate (VTMS-Ca) on polyamide substrate [13]. The VTMS-Ca coating had difficulty to form the BLA for 14 days of soaking in SBF whereas the bulk clearly formed BLA before 7 days of soaking. The ability of the BLA formation for the MSCa coatings increased with an increase in the amount of Ca as shown in Figs. 2 and 3. It can maintain a favorable bioactivity of the coating to enhance the Ca release in SBF. The porosity of the MS-Ca also increased with the increasing Ca as shown in Table 2. Tsuru et al. reported that the Ca-containing organic silicate of TEOS-PDMS-Ca with the appropriate hydrochloric acid affects the micrometer-range structure such as the porosity and pore size, whereas the chemical bonding structure like Si–O–Si is unchanged by the Si29–NMR observation. For the sample with a ratio Ca/Si 0.05, BLA formation was enhanced by the increase in the hydrochloric acid [18]. It suggested that with a large content of hydrochloric acid, the gel network was well established at the gelation by consuming the free Si–OH groups due to condensation resulting in greater pores for organic silicate of TEOS-PDMS-Ca. It is also important for bioactivity to increase Si–OH groups because of inducing more heterogeneous nucleation of apatite. As shown in Fig. 6 and Table 2, MS-Ca material that included high Ca content tends to enhance poresize and porosity. In consequence of it, surface area of the MS-Ca with Si–OH groups is expanded and it has influence on the BLA forming ability of the coating. That is, it seems the effect of Ca incorporated in MS-Ca material elevates bioactivity not only to accelerate supersaturation for apatite but also to increase Si–OH group in the MS-Ca surface. This result indicates that it is important for the bioactive coating of the organic silicate to include a high Ca content. The MS-Ca in the present study can include a large amount of Ca at the Ca/Si of 0.5, whereas the
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previously reported organic silicates prepared by TEOS-PDMS-Ca or VTMS-Ca [13,17,18] are less than 0.05. In a previous study, the Ca-containing organic silicate derived from only MTES and Ca(NO3)2·4H2O could not be made in the absence of a metal alkoxide such as Nb(OC2H5)5 and Ta(OC2H5)5. The metal alkoxides, such as Nb(OC2H5)5 and Ta(OC2H5)5, were incorporated into the siloxane network for copolymerization with the MTES polymerization. Therefore, the terminal of the methylsiloxane structure has Nb–OH and Ta–OH groups as well as Si–OH [20]. The electronegativity of O, Si, Nb and Ta by Pauling are 3.4, 1.9, 1.6 and 1.5, respectively. In general, if the difference in the electronegativity for two atoms is large, their polarization becomes stronger. The difference in the electronegativity during Si–O is 1.5, whereas Nb–O and Ta–O are 1.8 and 1.9, respectively. The degree of polarization δ− for the oxygen atom as M–Oδ− bonding, Ta–Oδ− and Nb–Oδ− are stronger than Si–Oδ− because the significant difference in the electronegativity indicates the ability of ionic bonding during M–O. Therefore the terminal groups of Nb–Oδ−H and Ta–Oδ−H, which has a strong negative polarization, are coordinate Ca ions having a positive charge during the growing methylsiloxane network of polymerization. It seems that the siloxane polymers with Nb–OH and Ta–OH are able to trap more positive Ca ions compared to only the siloxane one. 4.2. Adhesive strength of the coatings All of the double-layered MS-Ca/MS coatings were fractured on the tear-off testing, whereas the single-layered MS coating was not peeling off. Therefore, adhesive strength of the single-layered MS coating just only composed by MTES is enough high compared to double-layered MS-Ca/MS. Cross sectional observation of the double-layered coatings in Fig. 4, microstructure of the top layer composed MS-Ca has many pores, which is tended to increase with increasing Ca composition, whereas underlying MS layer is dense. Microstructure of the fracture surface (Y) as shown in Fig. 7 is described partial micro-pore structure remains on dense layer assumed underlying MS layer. It indicated fractured surface (Y) as shown in Fig. 7 is nearby interface between MS-Ca and MS. Therefore, the double-layered MS-Ca/MS coatings are peeled off during the MS-Ca layer without underlying MS layer. In fact, all of the coatings for double-layered MS-Ca/MS, in which the value of the adhesive strength is less than 3.5 MPa, is lower enough than the single-layered MS coating which is more than 6.2 MPa. It implies that underlying MS of the double-layered coatings do not have peeling between MS and Ti interface. Therefore, peeling of the double-layered coatings is decided adhesive strength between MS-Ca and MS. With increasingly Ca/Si ratios, pore size of the MS-Ca was expanded as shown in Fig. 6 with increasing porosity from Table 2,
Table 2 Density and porosity of MS-Ca bulks after soaking in physiological salt solution adjusted to pH 7.4 at 37 °C for 1 day
Bulk density (g/cm3) True density (g/cm3) Porosity (%)
MS-Ca05
MS-Ca10
MS-Ca30
MS-Ca50
1.25 1.40 11
1.03 1.45 29
0.50 1.46 52
0.36 1.46 66
Fig. 9. Natural logarithmic function for adhesive strength and porosity of the doublelayered MS-Ca/MS coatings.
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and adhesive strength of the double-layered MS-Ca/MS coatings were decreased as shown in Fig. 8. Therefore, it seems that adhesive strength of the coatings is affected microstructure of the MS-Ca material. Fig. 9 shows the natural logarithmic function of the adhesive strength of the double-layered MS-Ca/MS coatings with different porosities from Table 2. The variation in lnσ with porosity is a decreasing linear relation. Therefore, relationship of the adhesive strength σ and the porosity P0 is equal to ln σ ¼ −BP0 þ ln σ 0
ð1Þ
where σ0 is the strength of the nonporous ceramics and B is the experimental constant. Formula (1) is transformed as follows σ ¼ σ 0 e−BP0 :
ð2Þ
5. Conclusions The double-layered MS-Ca/MS coatings can include a large amount of Ca with Ca/Si molar ratios of 0.5 compared to previous reports so that the coating could have a favorable bioactivity similar to the bulk one. The coatings with Ca/Si ratios of 0.05, 0.1, 0.3 and 0.5 formed BLA at 6, 3, 3, 3 days, respectively. The adhesive strengths of the coating on Ti became low with an increase in the Ca content due to the increase in the micropores. With the addition of Ca into the organic silicate, the adhesive strength made sacrifices to enhance the bioactivity. The coatings have an adhesive strength ranging from 2.0 to 3.5 MPa. The coatings, which kept the bioactivity and the desirable mechanical properties, can be made by low temperature process; therefore, the coating method is useful to apply to a novel bioactive coating on titanium for orthopedic implants. Acknowledgments
This means that adhesive strength of the coating σ exponentially becomes weak with the increasing porosity P0. The formula (2) meets relationship of the compressive strength σ of porous ceramics by Duckworth [21]. It means that mechanical properties of the coating layer is affected its porosity as similarly ceramics. Therefore, adhesive strength of the coatings is also decided porosity from relationship of formula (2). Effect of the Ca incorporation in MS-Ca material is summarized as follows; the increasing Ca content of double-layered MS-Ca/MS coatings degraded the mechanical properties of the MS-Ca material due to the increase in its micropores. But it played an enhanced role in bioactivity. It is important for load-bearing implant coated bioactive ceramics that coating–substrate interface obtains a sufficient mechanical properties, whereas its difficulty. Piveteau et al. reported adhesive strength of sol–gel derived titanium dioxide mixed calcium phosphate (TiO2-CaP) on Ti substrate by dipping process [22]. The coating thickness was 5 µm, and it was heated for 10 min at 850 °C in air to be crystallized. Adhesive strength of the TiO2-CaP coating was 16 to 18 MPa. Adhesive strength of the double-layered MS-Ca/MS coatings is lower than the TiO2-CaP coating. In preparation of the doublelayered MS-Ca/MS coatings, heat temperature treatments were very low (b80 °C), and the layers was thick (40 to 50 µm). Crystallinity of the double-layered MS-Ca/MS materials seems very low because of its low heat temperature treatment. The crystallinity and thickness of coating material affects mechanical adhesion [22]. Therefore, it seems the double-layered MS-Ca/MS coatings have lower adhesive strength than the TiO2-CaP coating. It is difficult to define sufficient adhesive strength for the orthopedic implants, even more whether adhesive strength of the double-layered MS-Ca/MS coatings is suitable for one or not. As an example, adhesive strength of the soldering used heating coil for windshield defroster of a car rear window is 2.9 MPa [23]. Adhesive strength of the MS-Ca10 and MS-Ca05 had 2.8 MPa and 3.5 MPa, respectively. So the double-layered MS-Ca/MS coatings have even or more adhesion compared with the practical soldering. It is not the same for the orthopedic implant as the adhesive strength. However, it shows the coating method with low temperature process has good advantage against other processes with high temperature; nonetheless because of its low heat temperature treatment, the coatings on Ti substrates kept the same adhesion as industrial soldering. Therefore, coating technique of the double-layered MSCa/MS has a potential to apply to a bioactive coating for bone implants.
This work was supported by a Grant-in-Aid for Cooperative Research Project of Nationwide Joint-Use Research Institutes on Development Base of Joining Technology for New Metallic Glasses and Inorganic Materials and was partially supported by a Grant-in-Aid for Young Scientists (B), 18760516, 2006, from The Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors appreciated Mr. Hajime Ishii from the technical staffs of MSL for his help with the setup utilized by the Collaborative Research Project of Materials and Structures Laboratory, Tokyo Institute of Technology. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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