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Preparation and characterization of silicon-substituted hydroxyapatite coating by a biomimetic process on titanium substrate
Surface & Coatings Technology 203 (2009) 1075–1080
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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 ev i e r. c o m / l o c a t e / s u r f c o a t
Preparation and characterization of silicon-substituted hydroxyapatite coating by a biomimetic process on titanium substrate Erlin Zhang a,⁎, Chunming Zou b, Songyan Zeng b a b
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 28 August 2008 Accepted in revised form 30 September 2008 Available online 17 October 2008 Keywords: Biomimetic Coating Silicon Hydroxyapatite Titanium
a b s t r a c t A biomimetic method has been used to prepare silicon-substituted hydroxyapatite coatings on titanium substrates. The surface structures of the coatings were characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared spectroscopy (FTIR). Si substituted hydroxyapatite (Si–HA) coatings with different Si contents were deposited successfully on the titanium substrate by immersing the pretreated titanium substrate into silicon containing supersaturated solutions (SSS) with different SiO2− 3 concentrations. The pretreatment of the Ti substrate in a mixed alkaline (NaOH + Ca(OH2)) followed by a heat treatment produced a 3D porous surface structure with rutile and CaTiO3 as main phases, which contributed mainly to the fast precipitation and deposition of Si–HA. FTIR results showed that Si in the Si–HA coating existed in the form of SiO44− groups. The cross-section microstructure was observed by scanning electronic microscopy and the shear strength was tested. The coating was about 5–10 μm in thickness and no interval was observed at the interface between the coating and the substrate. Shear strength testing showed that Si–HA/Ti exhibited higher shear strength than HA/Ti due to the existence of the SiO44− group in the coating. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Titanium and its alloys are used widely as orthopedic and dental implant materials. However, due to their poor surface bio-compatibility, the surface of titanium and its alloys has to be bio-modified to improve the surface osteoconductivity [1]. Many researches have focused on hydroxyapatite (HA) coating on titanium by several techniques [2–4]. HA has shown very good osteoconductivity, allowing direct bone formation across its surface by attachment, proliferation and differentiation of bone forming cells [5]. In addition, HA has a similar chemical composition and crystallographic structure to that of bone mineral [6]. However, the relatively slow rate of osseointegration limits its application [7,8]. Carlisle et al. have indicated the importance of silicon for bone formation and calcification through electron microprobe study. It has been shown that silicon is localized in active growth areas, such as the osteoid of the bone of young mice and rats, and silicon levels up to 0.5 wt.% were observed in these areas [9]. Further evidence of the importance of silicon on bone formation was found in bioactive silicate containing glass ceramics [10–12]. Schwarz and Milne [13] reported that deficiency of Si intake in rats retarded normal growth, and also caused disturbances in the development
⁎ Corresponding author. Tel./fax: +86 24 23971605. E-mail address: [email protected] (E. Zhang). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.09.038
of bone structures in terms of skull size and bone architecture. It is generally noted that silicon is essential to the growth and development of biological tissue such as bone, teeth and some invertebrate skeletons [9,14]. Recently, Si element was introduced into hydroxyapatite to improve further the osseointegration by different techniques [15,16]. Increasing evidence has proven that the presence of Si contributes to enhanced bioactivity of some bioactive ceramics in vitro [17,18]. Thian et al. have applied magnetron co-sputtering associated with appropriate heat-treatment to obtain a thin silicon-substituted hydroxyapatite (Si–HA) coating on titanium substrate [19,20].The quantitative histomorphometry results indicated that the percentage of bone ingrowth on Si–HA coating was significantly greater than that for pure HA phase [21]. Thin films of Si–HA have recently been deposited by other techniques such as electrostatic spraying [22], plasma spray [23], sol–gel [24] and pulse laser deposition [25]. Biomimetic process is one of the most promising techniques to produce a bonelike HA apatite layer on different substrates and hence has been utilized by many researchers [1,26]. The biomimetic process imitates the mode in which bone-like hydroxyapatite crystals are formed in a body simulated environment, and the coatings thereby are more readily degraded by osteoclasts [27], overcoming many of the drawbacks associated with other depositions techniques, such as high temperature. In the present study, a biomimetic technique was used to produce Si–HA coatings on titanium substrates. Primary results focused on the
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Table 1 Ion concentrations in SSS Concentration
HPO2− 4
Ca2+
Na+
HCO−3
Cl−
SO2− 4
Mg2+
K+
SiO2− 3
SSS-1/mM SSS-5/mM SSS-10/mM
1.0 1.0 1.0
2.5 2.5 2.5
142.0 142.0 142.0
4.2 4.2 4.2
142.8 134.8 124.8
0.5 0.5 0.5
– – –
5.0 5.0 5.0
1.0 5.0 10.0
possibility of producing Si–HA by a biomimetic process on titanium and the characterization of the coating. 2. Materials and methods 2.1. Pretreatment of titanium substrate Commercial pure titanium samples with a dimension of 12 × 12 × 2 mm were used. All samples were ground with SiC paper up to 800# and ultrasonically cleaned in deionzed water, acetone and alcohol, respectively. The samples were then immersed in a mixedalkali (NaOH and Ca(OH) 2,the volume fraction of Ca(OH)2 was 2 mL/L) solution at 90 °C for 8 h. After that, all samples were cleaned in distilled water at 60 °C, followed by a heat treatment at 630 °C for 1 h (with a heating rate of 3 °C/min), and then cooled in furnace down to room temperature. 2.2. Biomimetic coating process With reference to Hank's balanced salt solution (HBSS) [28], silicon containing supersaturated solution (SSS) was prepared by dissolving analysis grade chemical reagents in deionized water, as listed in Table 1. In the SSSs, Na2SiO3 was used as Si ion source. The pretreated samples were immersed in 500 ml of SSS at 37 ± 1 °C, then washed with deionized water and acetone, and then air dried for characterization. By changing the Si ion concentration in the SSS solutions, Si– HA coatings with different Si contents were prepared. Before the immersion, the pH value of the SSS was kept at 7.40. The solution was refreshed every 12 h in order to accelerate the formation of Si–HA coating. For comparison, hydroxyapatite coating was also prepared on titanium substrate by immersing the pretreated samples in Hank's solution. Table 2 listed the chemical composition of Hank's solution.
Fig. 1. Schematic of the shear strength testing according to ASTM F-1044.
with a crosshead moving rate of 1 mm/min, as shown in Fig. 1. This test relies on a bonding agent to remove the film with an applied force. The shear strength was calculated from the fractured force over the fracture area. At least, five samples were used for each condition. However, as a bonding agent (for example, epoxy resin) is required in this test, it is possible that the bonding agent may penetrate through macropores or cracks in the film and partially bond to the substrate, thereby compromising the validity of the test result. Therefore, careful attention must be paid to the issue of adhesive penetration. 3. Results 3.1. Synthesis process Fig. 2 shows the time dependence of pH values of SSSs during the biomimetic coating process. The pH value increased to the maximum after 6 h immersion for SSS1 and SSS5 and after 4 h for SSS10 solution, followed by rapid decreasing in the following immersion. After approximate 70 h for SSS1 and SSS5, and 50 h for SSS10, respectively, pH value decreased very slowly, indicating that a stable stage was reached. In the synthesis process, the SSS was refreshed every 12 h in order to keep the high Si ion concentration and pH value. 3.2. Surface characterization
2.3. Characterization The pH value of the SSSs was measured with a PH-4CT pH meter with an accuracy of 0.001 during coating process. To identify the phase constitutes, the surface was examined with small angle X-ray scattering which was conducted on a X-ray diffractometer (XRD, D/ MAX-RB, Rigaku) with an incident angle of 2° against the surface of the specimens, and the measurements were performed with a continuous scanning mode at a rate of 4°/min. The surface microstructure and morphology was observed on a Hitachi S-4700 scanning electronic microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). The coatings were peeled off and mixed with KBr, compressed into plate and subsequently analyzed by a Fourier transformed infrared (FTIR) spectroscopy on a Nicolet Avatar-360.
Fig. 3 shows the XRD pattern of the pretreated Ti substrate. The diffraction patterns of CaTiO3 and rutile as well as Ti substrate were detected by XRD. With the consideration of the pretreatment process, it can be deduced that rutile was produced in the heating process at 630 °C in the pretreatment process. Ca2+ was incorporated in the
2.4. Shear strength According to ASTM standard F1044, the shear strength measurement was conducted on an INSTRON Series IX materials testing system Table 2 Chemical composition of Hank's solution (g/L) NaCl
KCl
CaCl2
NaHCO3
MgSO4 ·7H2O
Na2HPO4 ·12H2O
KH2PO4
8.00
0.40
0.14
0.35
0.20
0.12
0.06
Fig. 2. pH values of different SSSs in the biomimetic coating process.
E. Zhang et al. / Surface & Coatings Technology 203 (2009) 1075–1080
Fig. 3. XRD pattern of the pretreated titanium substrate.
mixed-alkaline treatment and transformed into CaTiO3 in the following process. Fig. 4 shows the microstructure of the pretreated Ti substrate. In the low magnification microstructure, Fig. 4a, fine porous structure
Fig. 4. Surface microstructure of the pretreated Ti substrate. a) low magnification, b) high magnification.
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Fig. 5. XRD pattern of the coatings on titanium after immersion in different SSS.
with some holes or cracks was observed. Fine needle like phase were clearly shown. Combined with the XRD results in Fig. 3, these needlelike phases could be the mixture of rutile and CaTiO3. Due to the fine size, it is not impossible to distinguish them by EDS. High magnification microstructure in Fig. 4b clearly shows the very fine pore structure. EDS analysis result shows that the Ca content in the surface layer is as high as 0.87 at.%. Fig. 5 shows the XRD patterns of the coatings on titanium substrates obtained from different SSSs. Diffraction peaks of HA, rutile and titanium were detected. The present of HA peaks indicates that HA phase was produced as a new phase in the coatings obtained from all SSSs. All peaks belong to HA are broadened, especially in the coating obtained from high Si containing solutions, indicating that the HA phase is not well crystallized or is nanometer in size in the coating. However, no phase corresponding to Si was detected by XRD. FTIR was used to quantify the phosphate groups of Si–HA coatings obtained from different SSSs, as shown in Fig. 6. A weak OH band at 3570 cm− 1 is obscured by a broad band between 3200 and 3600 cm− 1, which is assigned to moisture in the sample. The bands at 1636 and 1449 cm− 1 correspond to carbonate for all the samples [29]. Intense bands at 1093, 1035 cm− 1, and week band at 960 cm− 1 can be observed, which correspond to P–O stretching vibration modes, as well as the bands at 605 and 563 cm− 1 correspond to the O–P–O bending mode [15,30]. The band appearing at 870 cm− 1 for Si–HA, is assigned to Si–O vibration modes of SiO44− groups [30,31]. With the increasing of Si content, the band corresponding to P–O stretching vibration modes shifts from 1036 cm− 1 to 1100 cm− 1, and the band appearing at 870 cm− 1 becomes weak. A band around 2360 cm− 1 was detected in all coatings. This band was also detected in the FTIR spectra of Si–HA [29,30] and HA [30], but disappeared after the Si–HA
Fig. 6. FTIR spectra of the Si–HA coatings obtained from different SSSs.
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powder was heat treated at 1200 °C for 2 h [29]. With the consideration of the precipitation process, the band might be due to the carbonate. The carbonate presence in the specimens is due to absorbance of carbon dioxide from the air during synthesis of the samples. Another broad band at 803 cm− 1 might be assigned to Si–O–Si vibration modes of SiO4 group [32]. It means that the existing form of SiO44−groups in Si– HA coating from SSS5 and SSS10 has been changed due to some reasons, i.e. SiO44−groups have been polymerized [33]. From above analysis, it was deduced that Si exists in a form of SiO44− group. 3.3. Microstructure observation Fig. 7 shows the typical surface morphology of the coatings obtained form different SSSs. Large amounts of cracks were found in the surface due to the dehydration shrinkage, as shown in Fig. 7a and b. A relatively dense coating was observed on the surface of the coating obtained from SSS10, shown in Fig. 7c. Si element besides O, Ti, Ca, and P elements was detected by EDS in all coatings, indicating that a Si-containing compound was formed in the coatings. Si content as high as 1.14–1.20 at.% was detected by EDS at the coating obtained from SSS10 while only 0.42–0.50 at.% and 0.14–0.18 at.% were detected in the coatings from SSS5 and SSS1, respectively. Meanwhile, the Ca/ (P + Si) ratio of the coating obtained from SSS10 was 1.73–1.79 while only 1.68–1.71 and 1.61–1.62 were detected in the coatings from SSS5 and SSS1, respectively. Fig. 8 shows a typical interface microstructure between the coating and titanium substrate. An even coating was observed on all samples. High contents of O, Ca, P as well as Si were detected in all coatings. The element line scanning results also reveal that Si-content in the coating gradually increases from the interface to the outer surface. No interval was observed at the interface between the coating and the substrate, which shows that there is a highly dense coating and a good interface bonding. In addition, with the increasing of the Si content in SSS, the coating became more thin and dense. For example, the thickness changes from 10 μm for SSS1 to 5 μm for SSS10.
Fig. 8. Cross section of the Si–HA coatings obtained from SSS1 (a) and elements line scanning (b).
3.4. Shear strength From the appearances of the tested samples, almost all specimens fractured at the interface between the Si–HA coating and the titanium substrate. Only several samples fractured at the interface between the Si– HA and the epoxy resin or within the Si–HA coating, which were not
Fig. 7. SEM surface micrographs of coatings obtained from different SSSs, (a ) SSS1; (b) SSS5; (c) SSS10.
Fig. 9. Interfacial shear strength between Si–HA coating and Ti substrate.
E. Zhang et al. / Surface & Coatings Technology 203 (2009) 1075–1080
adopted in the calculation of the shear strength. No penetration of the epoxy resin to the fracture interface was observed under optical microscopy. This demonstrates that the measured shear strength represents the coating-substrate interfacial shear strength. Fig. 9 shows the interfacial shear strength between the Si–HA coating and the titanium substrate. The shear strength of the HA coating/Ti substrate prepared by a similar biomimetic process was also plotted. Without the addition of Si ion, the shear strength of the HA-coating/Ti substrate was about 9.2± 2.0 MPa. With the addition of 0.14–0.18 at.% Si, the shear strength significantly increased to a very high value, 17.5± 3.7 MPa, increased by 90%. Although the shear strength slightly decreased with the further increasing of the Si content, no significant difference in the shear strength was observed from Fig. 9. The average shear strength was 15.7 MPa. 4. Discussion 4.1. Synthesis process Fast deposition or precipitation of HA or Si–HA is desired. Surface microstructure and chemical properties of the titanium substrate play a very important role in the deposition of Si–HA coating in the biomimetic coating process. Fig. 3 and Fig. 4 clearly show that a very fine 3D porous structure was produced after the pretreatment. The high surface area of the 3D porous structure will absorb ions in the solution and accelerate the precipitation of HA or Si–HA. Also, Ca2+ in the coating, rutile and CaTiO3 contribute to the precipitation of Si–HA on the Ti substrate. Previous study showed that rutile can accelerate the deposition rate of HA on titanium substrate [34]. CaTiO3 in the coating has the potential to form Ti–OH on the surface when the sample is in contact with SSS or SBF. The formation of Ti–OH induces the precipitation and deposition of HA on titanium substrate [35]. It has been reported that Si–HA has a higher dissolution with the preferential release of silicon into the medium than HA and this fact may account for the high cell proliferation on Si–HA in comparison with HA [36]. XRD results in Fig. 5 and the FTIR in Fig. 6 indicate that Si– HA coatings with different Si contents have been produced successfully on the titanium substrates by the proposed biomimetic method although no phase corresponding to Si was detected by XRD. The Si content of the coating ranges from 0.14 to 1.2 at.%, and the Ca/(P + Si) atomic ratio from 1.61 to 1.79. The coating shows a uniform thickness of 5–10 μm which is significantly thicker as compared with the Si–HA coating produced by magnetron co-sputtering process, which generally ranged between 400 nm and 750 nm [20]. The EDS results indicate that this method is a controllable method for Si–HA coating preparation, by which the composition of the coatings, especially Si content, can be controlled on demand. By using Na2SiO3, a metastable solution can be prepared with different silicon ion concentrations (1 mM, 5 mM and 10 mM). The chemical reactions in SSS can be envisaged as following: 2− H2 O + SiO2− 3 Y H2 SiO4
ð1Þ
3− + H2 SiO2− 4 Y H + HSiO4
ð2Þ
4− + HSiO3− 4 Y H + SiO4
ð3Þ
3− + HPO2− 4 Y H + PO4
ð4Þ
H
+
+ HCO−3
Y H2 O + CO2 z
ð5Þ
1 4− − 10Ca2+ + ð20−4x− yÞPO3− 4 + xSiO4 + yOH Y Ca10 ðPO4 Þ13ð20 − 4x − yÞ ðSiO4 Þx ðOHÞy A 3
ð6Þ 4− The first three reactions make SiO2− 3 exist in the form of SiO4 in SSSs and be incorporated into HA in the form of SiO44−. Reactions
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(2)–(4) can support H+ ions. Reaction (5) increases the pH value by consuming H+ ions that released from the reactions (2)–(4), which leads the solution to a supersaturated stage. Then Reaction (6) consumes OH−, SiO44− and PO3− 4 , and Si–HA forms on titanium substrates. In Porter's experiment, the pH value must be maintained at 10.5 with the addition of ammonia, which results in the releasing of irritant gas [37], while in the present experiment it is not necessary to maintain pH value, so there is no releasing of irritant gas. It has been reported that the degree of crystallization is a key factor to osteoconductivity of hydroxyapatite ceramics in the subcutaneous of dogs [37]. The hydroxyapatite with lower degree of crystallization performed an obvious osteoconduction , while the hydroxyapatite with higher degree of crystallization did not. The biomimetic process imitates the mode in which bone-like hydroxyapatite crystals are formed in the body [38], so substituted Si element can be co-precipitated into the HA crystal lattice. The Si–HA coating prepared by the biomimetic method had also a lower degree of crystallization, as shown in Fig. 5. Hence, the Si–HA coating would lead to better bone repair and regeneration, and such coatings with different silicon contents could have a great potential for the design of future implant materials. 4.2. Shear strength The HA coating provides a bioactive surface on a metal implant for bone ongrowth. Therefore, it is important that the bonding strength of the coating-implant interface should be sufficiently high to withstand the interfacial stresses encountered in the in vivo environment. In other processes, such as plasma deposition or electrophorectical deposition (EPD), high temperature is involved in order to obtain high bonding/or shear strength. One of the big problems is the deleterious effects of high temperature on the properties of the coating and the metal substrate. High sintering temperature can result in phase transformation and grain growth of the metal substrate, which may cause the mechanical properties of the metal substrate to decrease significantly, and also the metal substrate catalyzed decomposition of the HA to anhydrous calcium phosphates, which leads to enhanced in vitro dissolution rates of HA coating [39,40]. The high temperature sintering in EPD process also resulted in cracks in the HA coating due to the shrinkage [40], which significantly reduced the interfacial bonding strength. In order to improve the bonding strength of the HA coating/substrate, a dual coating technique was developed to “fill” the cracks in the EPD HA coating [40–42]. Also heat treatment was applied to the plasma sprayed HA coating to improve the bonding strength [43,44]. In addition, bioglass [45], titanium powder [46] or TiO2 [47] were adopted to produce HA-glass or HA-titanium composites coating to get a high bonding strength. It was reported the shear strength of dual HA coatings prepared by EPD on Ti was ~ 11 MPa and Ti6Al4 V was ~14 MPa [40]. It was also reported that the shear strength of the bovine cortical bone was 34 MPa with a standard deviation of 17 MPa [40]. Table 3 lists the interfacial bond strength of HA/Ti substrate reported elsewhere. Significantly, the interfacial bond strength of thermally sprayed HA was in a range of 13–30 MPa.
Table 3 Shear strength of HA coating and titanium substrate Substrate/HA coating
Coating process
Shear strength, MPa
Ref.
Ti–6Al–4 V/HA
Plasma spray Plasma spray + heat treatment Plasma spray Plasma spray Plasma spray EPD EPD Biomimetic process
24.5 ± 2.4 26.7 ± 1.4
[43]
23.5–30.2 12.9 17.3 11 ± 7.8 13.9 ± 4.3 15.7 ± 3.4
[48] [46] [46] [40] [40] This work
Ti–6Al–4 V/HA Ti/HA Ti/Ti–HA composite coating Ti/HA Ti–6Al–4 V/HA Ti/Si–HA
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E. Zhang et al. / Surface & Coatings Technology 203 (2009) 1075–1080
In the biomimetic process, HA coating was formed in a simulated body fluid. No high temperature was involved. Therefore, the HA coating has more close structure to the natural bone tissue and has good osteoconductivity, especially at the early stage of the bone healing process [49]. However, the low interface bonding or shear strength of the HA coating/Ti substrate limits their clinical application. For example, the shear strength between the HA coating and the titanium substrate in this paper is about 9.2 MPa, close to the value of HA/Ti produced by EPD reported in Ref. [40]. The substitution Si ion into the HA coating significantly improves the shear strength to an average 16 MPa, which is close to the value of the HA coating/Ti prepared by plasma spray. The significant improvement in the shear strength provides the potential clinical application of Si–HA coating from the mechanical properties point of view and in light of the fact that biomimetic process has a low processing cost and is a non-line-of-sight process. 5. Conclusions Uniform Si–HA coatings with 5–10 μm in thickness were successfully prepared by a biomimetic process on titanium substrates. The Si content in the coatings ranges from 0.14 to 1.2 at.% and the Ca/(P + Si) atomic ratio from 1.61 to 1.79. The biomimetic mineral phase was not well crystallized and amorphous phase existed in the coating. Silicon was confirmed to exist in the form of SiO44− groups in Si–HA coating. Si– HA coating showed high adhesion strength to the titanium substrate. Acknowledgments
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
One of authors (Erlin Zhang) would like to acknowledge the financial support from the Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), and Shenyang Science and Technology Institute (Program No. 1062109-1-00). Many thanks to Prof. Ma M.Z. at Yanshan University, China, for his help in small angle XRD analysis. References [1] [2] [3] [4] [5] [6] [7] [8]
F. Li, Q.L. Feng, F.Z. Cui, H.D. Li, H. Schubert, Surf. Coat. Technol. 154 (2002) 88. L.M. Sun, C.C. Berndt, K.A. Gross, A. Kucuk, J. Biomed. Mater. Res. 58 (2001) 570. J.C. Knowles, K. Gross, C.C. Berndt, W. Bonfield, Biomaterials 17 (1996) 639. M. Hamdi, S. Hakamata, A.M. Ektessabi, Thin Solid Films 377 (2000) 484. P. Ducheyne, Q. Qiu, Biomaterials 20 (1999) 2287. W. Suchanek, M. Yoshimoto, J. Mater. Res. 13 (1998) 94. L.L. Hench, J. Am. Ceram. Soc. 74 (1991) 1487. H. Oonishi, L.L. Hench, J. Wilson, F. Sugihara, E. Tsuji, S. Kushitani, H. Iwaki, J. Biomed. Mater. Res. 44 (1999) 31.
[38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]
E.M. Carlisle, Science 167 (1970) 279. S. Hayakawa, K. Tsuru, C. Ohtsuki, A. Osaka, J. Am. Ceram. Soc. 82 (1999) 2155. T. Kasuga, Y. Hosoi, M. Nogami, M. Niinomi, J. Am. Ceram. Soc. 84 (2001) 450. L.L. Hench, J. Am. Ceram. Soc. 81 (1998) 1705. K. Schantz, D.B. Milne, Nature 239 (1972) 333. E.M. Carlisle, Science 178 (1972) 619. T. Tian, D. Jiang, J. Zhang, Q. Lin, Mater. Sci. Eng. C 28 (2008) 57. S.R. Kim, J.H. Lee, Y.T. Kim, D.H. Riu, S.J. Jung, Y.J. Lee, S.C. Chung, Y.H. Kim, Biomaterials 24 (2003) 1389. N. Patel, S.M. Best, W. Bonfield, I.R. Gibson, K.A. Hing, E. Damien, P.A. Revell, J Mater. Sci-Mater. M. 13 (2002) 1199. K.A. Hing, P.A. Revell, N. Smith, T. Buckland, Biomaterials 27 (2006) 5014. E.S. Thian, J. Huang, M.E. Vickers, S.M. Best, Z.H. Barber, W. Bonfield, J. Mater. Sci. 41 (2006) 709. E.S. Thian, J. Huang, S.M. Best, Z.H. Barber, W. Bonfield, J. Mater. Sci-Mater. M. 16 (2005) 411. E.S. Thian, J. Huang, S.M. Best, Z.H. Barber, R.A. Brooks, N. Rushton, W. Bonfield, Biomaterials 27 (2006) 2692. J. Huang, S. Jayasinghe, S.M. Best, M.J. Edirisinghe, R.A. Brooks, N. Rushton, W. Bonfield, J. Mater. Sci-Mater. M. 16 (2005) 1137. S.J. Ding, T.H. Huang, C.T. Kao, Surf. Coat. Tech. 165 (2003) 248. N. Hijon, M. Victoria Cabanas, J. Pena, M. Vallet-Regi, Acta Biomater. 2 (2006) 567. E.L. Solla, P. Gonzalez, J. Serra, S. Chiussi, B. Leon, J.G. Lopez, Appl. Surf. Sci. 254 (2007) 1189. W.-H. Song, Y.-K. Jun, Y. Han, S.-H. Hong, Biomaterials 25 (2004) 3341. E.L. Zhang, K. Yang, T. Nonferr. Metal. Soc. 15 (2005) 1199. S.R. Sousa, M.A. Barbosa, Biomaterials 17 (1996) 397. I.R. Gibson, S.M. Best, W. Bonfield, J. Biomed. Mater. Res. 44 (1999) 422. T. Leventouri, C.E. Bunaciu, V. Perdikatsis, Biomaterials 24 (2003) 4205. A. Fidalgo, L.M. Ilharco, J. Non-Cryst. Solids 283 (2001) 144. I. Rehman, W. Bonfield, J. Mater. Sci. Mater. Med. 8 (1997) 1. X.L. Tang, X.F. Xiao, R.F. Liu, Mater. Lett. 59 (2005) 3841. B.C. Yang, M. Uchida, H.M. Kim, X.D. Zhang, T. Kokubo, Biomaterials 25 (2004) 1003. H.M. Kim, F. Miyaji, T. Kokubo, T. Nakamura, J. Biomed. Mater. Res. A 32 (1996) 409. C.M. Botelho, R.A. Brookes, S.M. Best, M.A. Lopes, J.D. Santos, N. Rushton, W. Bonfield, (Trans Tech Publications LTD, Zurich, Switzerland, 2004). A.E. Porter, N. Patel, J.N. Skepper, S.M. Best, W. Bonfield, Biomaterials 24 (2003) 4609. Y. Liu, E.B. Hunziker, N.X. Randall, K. de Groot, P. Layrolle, Biomaterials 24 (2003) 65. L. Brannon-Peppas, Biomaterials 11 (1990) 224. M. Wei, A.J. Ruys, M.V. Swain, S.H. Kim, B.K. Milthorpe, C.C. Sorrell, J. Mater. SciMater. M. 10 (1999) 401. A.J.D. Farmer, N. Gane, R. Sekel, Int. Ceram. Mogogr. 1 (1994). F. Witte, F. Feyerabend, P. Maier, J. Fischer, M. Stormer, C. Blawert, W. Dietzel, N. Hort, Biomaterials 28 (2007) 2163. S.W.K. Kweh, K.A. Khor, P. Cheang, Biomaterials 23 (2002) 775. C.W. Yang, T.M. Lee, T.S. Lui, E. Chang, Mater. Sci. Eng. C 26 (2006) 1395. M.F. Morks, J. Mech. Behav. Biomed. Mater. 1 (2008) 105. X.B. Zheng, M.H. Huang, C.X. Ding, Biomaterials 21 (2000) 841. X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 125 (2000) 407. C.Y. Yang, R.M. Lin, B.C. Wang, T.M. Lee, E. Chang, Y.S. Hang, P.Q. Chen, J. Biomed. Mater. Res. 37 (1997) 335. S. Nishiguchi, H. Kato, H. Fujita, M. Oka, H.-M. Kim, T. Kokubo, T. Nakamura, Biomaterials 22 (2001) 2525.
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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 ev i e r. c o m / l o c a t e / s u r f c o a t
Preparation and characterization of silicon-substituted hydroxyapatite coating by a biomimetic process on titanium substrate Erlin Zhang a,⁎, Chunming Zou b, Songyan Zeng b a b
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, PR China School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China
a r t i c l e
i n f o
Article history: Received 28 August 2008 Accepted in revised form 30 September 2008 Available online 17 October 2008 Keywords: Biomimetic Coating Silicon Hydroxyapatite Titanium
a b s t r a c t A biomimetic method has been used to prepare silicon-substituted hydroxyapatite coatings on titanium substrates. The surface structures of the coatings were characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and Fourier transformed infrared spectroscopy (FTIR). Si substituted hydroxyapatite (Si–HA) coatings with different Si contents were deposited successfully on the titanium substrate by immersing the pretreated titanium substrate into silicon containing supersaturated solutions (SSS) with different SiO2− 3 concentrations. The pretreatment of the Ti substrate in a mixed alkaline (NaOH + Ca(OH2)) followed by a heat treatment produced a 3D porous surface structure with rutile and CaTiO3 as main phases, which contributed mainly to the fast precipitation and deposition of Si–HA. FTIR results showed that Si in the Si–HA coating existed in the form of SiO44− groups. The cross-section microstructure was observed by scanning electronic microscopy and the shear strength was tested. The coating was about 5–10 μm in thickness and no interval was observed at the interface between the coating and the substrate. Shear strength testing showed that Si–HA/Ti exhibited higher shear strength than HA/Ti due to the existence of the SiO44− group in the coating. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Titanium and its alloys are used widely as orthopedic and dental implant materials. However, due to their poor surface bio-compatibility, the surface of titanium and its alloys has to be bio-modified to improve the surface osteoconductivity [1]. Many researches have focused on hydroxyapatite (HA) coating on titanium by several techniques [2–4]. HA has shown very good osteoconductivity, allowing direct bone formation across its surface by attachment, proliferation and differentiation of bone forming cells [5]. In addition, HA has a similar chemical composition and crystallographic structure to that of bone mineral [6]. However, the relatively slow rate of osseointegration limits its application [7,8]. Carlisle et al. have indicated the importance of silicon for bone formation and calcification through electron microprobe study. It has been shown that silicon is localized in active growth areas, such as the osteoid of the bone of young mice and rats, and silicon levels up to 0.5 wt.% were observed in these areas [9]. Further evidence of the importance of silicon on bone formation was found in bioactive silicate containing glass ceramics [10–12]. Schwarz and Milne [13] reported that deficiency of Si intake in rats retarded normal growth, and also caused disturbances in the development
⁎ Corresponding author. Tel./fax: +86 24 23971605. E-mail address: [email protected] (E. Zhang). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.09.038
of bone structures in terms of skull size and bone architecture. It is generally noted that silicon is essential to the growth and development of biological tissue such as bone, teeth and some invertebrate skeletons [9,14]. Recently, Si element was introduced into hydroxyapatite to improve further the osseointegration by different techniques [15,16]. Increasing evidence has proven that the presence of Si contributes to enhanced bioactivity of some bioactive ceramics in vitro [17,18]. Thian et al. have applied magnetron co-sputtering associated with appropriate heat-treatment to obtain a thin silicon-substituted hydroxyapatite (Si–HA) coating on titanium substrate [19,20].The quantitative histomorphometry results indicated that the percentage of bone ingrowth on Si–HA coating was significantly greater than that for pure HA phase [21]. Thin films of Si–HA have recently been deposited by other techniques such as electrostatic spraying [22], plasma spray [23], sol–gel [24] and pulse laser deposition [25]. Biomimetic process is one of the most promising techniques to produce a bonelike HA apatite layer on different substrates and hence has been utilized by many researchers [1,26]. The biomimetic process imitates the mode in which bone-like hydroxyapatite crystals are formed in a body simulated environment, and the coatings thereby are more readily degraded by osteoclasts [27], overcoming many of the drawbacks associated with other depositions techniques, such as high temperature. In the present study, a biomimetic technique was used to produce Si–HA coatings on titanium substrates. Primary results focused on the
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Table 1 Ion concentrations in SSS Concentration
HPO2− 4
Ca2+
Na+
HCO−3
Cl−
SO2− 4
Mg2+
K+
SiO2− 3
SSS-1/mM SSS-5/mM SSS-10/mM
1.0 1.0 1.0
2.5 2.5 2.5
142.0 142.0 142.0
4.2 4.2 4.2
142.8 134.8 124.8
0.5 0.5 0.5
– – –
5.0 5.0 5.0
1.0 5.0 10.0
possibility of producing Si–HA by a biomimetic process on titanium and the characterization of the coating. 2. Materials and methods 2.1. Pretreatment of titanium substrate Commercial pure titanium samples with a dimension of 12 × 12 × 2 mm were used. All samples were ground with SiC paper up to 800# and ultrasonically cleaned in deionzed water, acetone and alcohol, respectively. The samples were then immersed in a mixedalkali (NaOH and Ca(OH) 2,the volume fraction of Ca(OH)2 was 2 mL/L) solution at 90 °C for 8 h. After that, all samples were cleaned in distilled water at 60 °C, followed by a heat treatment at 630 °C for 1 h (with a heating rate of 3 °C/min), and then cooled in furnace down to room temperature. 2.2. Biomimetic coating process With reference to Hank's balanced salt solution (HBSS) [28], silicon containing supersaturated solution (SSS) was prepared by dissolving analysis grade chemical reagents in deionized water, as listed in Table 1. In the SSSs, Na2SiO3 was used as Si ion source. The pretreated samples were immersed in 500 ml of SSS at 37 ± 1 °C, then washed with deionized water and acetone, and then air dried for characterization. By changing the Si ion concentration in the SSS solutions, Si– HA coatings with different Si contents were prepared. Before the immersion, the pH value of the SSS was kept at 7.40. The solution was refreshed every 12 h in order to accelerate the formation of Si–HA coating. For comparison, hydroxyapatite coating was also prepared on titanium substrate by immersing the pretreated samples in Hank's solution. Table 2 listed the chemical composition of Hank's solution.
Fig. 1. Schematic of the shear strength testing according to ASTM F-1044.
with a crosshead moving rate of 1 mm/min, as shown in Fig. 1. This test relies on a bonding agent to remove the film with an applied force. The shear strength was calculated from the fractured force over the fracture area. At least, five samples were used for each condition. However, as a bonding agent (for example, epoxy resin) is required in this test, it is possible that the bonding agent may penetrate through macropores or cracks in the film and partially bond to the substrate, thereby compromising the validity of the test result. Therefore, careful attention must be paid to the issue of adhesive penetration. 3. Results 3.1. Synthesis process Fig. 2 shows the time dependence of pH values of SSSs during the biomimetic coating process. The pH value increased to the maximum after 6 h immersion for SSS1 and SSS5 and after 4 h for SSS10 solution, followed by rapid decreasing in the following immersion. After approximate 70 h for SSS1 and SSS5, and 50 h for SSS10, respectively, pH value decreased very slowly, indicating that a stable stage was reached. In the synthesis process, the SSS was refreshed every 12 h in order to keep the high Si ion concentration and pH value. 3.2. Surface characterization
2.3. Characterization The pH value of the SSSs was measured with a PH-4CT pH meter with an accuracy of 0.001 during coating process. To identify the phase constitutes, the surface was examined with small angle X-ray scattering which was conducted on a X-ray diffractometer (XRD, D/ MAX-RB, Rigaku) with an incident angle of 2° against the surface of the specimens, and the measurements were performed with a continuous scanning mode at a rate of 4°/min. The surface microstructure and morphology was observed on a Hitachi S-4700 scanning electronic microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). The coatings were peeled off and mixed with KBr, compressed into plate and subsequently analyzed by a Fourier transformed infrared (FTIR) spectroscopy on a Nicolet Avatar-360.
Fig. 3 shows the XRD pattern of the pretreated Ti substrate. The diffraction patterns of CaTiO3 and rutile as well as Ti substrate were detected by XRD. With the consideration of the pretreatment process, it can be deduced that rutile was produced in the heating process at 630 °C in the pretreatment process. Ca2+ was incorporated in the
2.4. Shear strength According to ASTM standard F1044, the shear strength measurement was conducted on an INSTRON Series IX materials testing system Table 2 Chemical composition of Hank's solution (g/L) NaCl
KCl
CaCl2
NaHCO3
MgSO4 ·7H2O
Na2HPO4 ·12H2O
KH2PO4
8.00
0.40
0.14
0.35
0.20
0.12
0.06
Fig. 2. pH values of different SSSs in the biomimetic coating process.
E. Zhang et al. / Surface & Coatings Technology 203 (2009) 1075–1080
Fig. 3. XRD pattern of the pretreated titanium substrate.
mixed-alkaline treatment and transformed into CaTiO3 in the following process. Fig. 4 shows the microstructure of the pretreated Ti substrate. In the low magnification microstructure, Fig. 4a, fine porous structure
Fig. 4. Surface microstructure of the pretreated Ti substrate. a) low magnification, b) high magnification.
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Fig. 5. XRD pattern of the coatings on titanium after immersion in different SSS.
with some holes or cracks was observed. Fine needle like phase were clearly shown. Combined with the XRD results in Fig. 3, these needlelike phases could be the mixture of rutile and CaTiO3. Due to the fine size, it is not impossible to distinguish them by EDS. High magnification microstructure in Fig. 4b clearly shows the very fine pore structure. EDS analysis result shows that the Ca content in the surface layer is as high as 0.87 at.%. Fig. 5 shows the XRD patterns of the coatings on titanium substrates obtained from different SSSs. Diffraction peaks of HA, rutile and titanium were detected. The present of HA peaks indicates that HA phase was produced as a new phase in the coatings obtained from all SSSs. All peaks belong to HA are broadened, especially in the coating obtained from high Si containing solutions, indicating that the HA phase is not well crystallized or is nanometer in size in the coating. However, no phase corresponding to Si was detected by XRD. FTIR was used to quantify the phosphate groups of Si–HA coatings obtained from different SSSs, as shown in Fig. 6. A weak OH band at 3570 cm− 1 is obscured by a broad band between 3200 and 3600 cm− 1, which is assigned to moisture in the sample. The bands at 1636 and 1449 cm− 1 correspond to carbonate for all the samples [29]. Intense bands at 1093, 1035 cm− 1, and week band at 960 cm− 1 can be observed, which correspond to P–O stretching vibration modes, as well as the bands at 605 and 563 cm− 1 correspond to the O–P–O bending mode [15,30]. The band appearing at 870 cm− 1 for Si–HA, is assigned to Si–O vibration modes of SiO44− groups [30,31]. With the increasing of Si content, the band corresponding to P–O stretching vibration modes shifts from 1036 cm− 1 to 1100 cm− 1, and the band appearing at 870 cm− 1 becomes weak. A band around 2360 cm− 1 was detected in all coatings. This band was also detected in the FTIR spectra of Si–HA [29,30] and HA [30], but disappeared after the Si–HA
Fig. 6. FTIR spectra of the Si–HA coatings obtained from different SSSs.
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powder was heat treated at 1200 °C for 2 h [29]. With the consideration of the precipitation process, the band might be due to the carbonate. The carbonate presence in the specimens is due to absorbance of carbon dioxide from the air during synthesis of the samples. Another broad band at 803 cm− 1 might be assigned to Si–O–Si vibration modes of SiO4 group [32]. It means that the existing form of SiO44−groups in Si– HA coating from SSS5 and SSS10 has been changed due to some reasons, i.e. SiO44−groups have been polymerized [33]. From above analysis, it was deduced that Si exists in a form of SiO44− group. 3.3. Microstructure observation Fig. 7 shows the typical surface morphology of the coatings obtained form different SSSs. Large amounts of cracks were found in the surface due to the dehydration shrinkage, as shown in Fig. 7a and b. A relatively dense coating was observed on the surface of the coating obtained from SSS10, shown in Fig. 7c. Si element besides O, Ti, Ca, and P elements was detected by EDS in all coatings, indicating that a Si-containing compound was formed in the coatings. Si content as high as 1.14–1.20 at.% was detected by EDS at the coating obtained from SSS10 while only 0.42–0.50 at.% and 0.14–0.18 at.% were detected in the coatings from SSS5 and SSS1, respectively. Meanwhile, the Ca/ (P + Si) ratio of the coating obtained from SSS10 was 1.73–1.79 while only 1.68–1.71 and 1.61–1.62 were detected in the coatings from SSS5 and SSS1, respectively. Fig. 8 shows a typical interface microstructure between the coating and titanium substrate. An even coating was observed on all samples. High contents of O, Ca, P as well as Si were detected in all coatings. The element line scanning results also reveal that Si-content in the coating gradually increases from the interface to the outer surface. No interval was observed at the interface between the coating and the substrate, which shows that there is a highly dense coating and a good interface bonding. In addition, with the increasing of the Si content in SSS, the coating became more thin and dense. For example, the thickness changes from 10 μm for SSS1 to 5 μm for SSS10.
Fig. 8. Cross section of the Si–HA coatings obtained from SSS1 (a) and elements line scanning (b).
3.4. Shear strength From the appearances of the tested samples, almost all specimens fractured at the interface between the Si–HA coating and the titanium substrate. Only several samples fractured at the interface between the Si– HA and the epoxy resin or within the Si–HA coating, which were not
Fig. 7. SEM surface micrographs of coatings obtained from different SSSs, (a ) SSS1; (b) SSS5; (c) SSS10.
Fig. 9. Interfacial shear strength between Si–HA coating and Ti substrate.
E. Zhang et al. / Surface & Coatings Technology 203 (2009) 1075–1080
adopted in the calculation of the shear strength. No penetration of the epoxy resin to the fracture interface was observed under optical microscopy. This demonstrates that the measured shear strength represents the coating-substrate interfacial shear strength. Fig. 9 shows the interfacial shear strength between the Si–HA coating and the titanium substrate. The shear strength of the HA coating/Ti substrate prepared by a similar biomimetic process was also plotted. Without the addition of Si ion, the shear strength of the HA-coating/Ti substrate was about 9.2± 2.0 MPa. With the addition of 0.14–0.18 at.% Si, the shear strength significantly increased to a very high value, 17.5± 3.7 MPa, increased by 90%. Although the shear strength slightly decreased with the further increasing of the Si content, no significant difference in the shear strength was observed from Fig. 9. The average shear strength was 15.7 MPa. 4. Discussion 4.1. Synthesis process Fast deposition or precipitation of HA or Si–HA is desired. Surface microstructure and chemical properties of the titanium substrate play a very important role in the deposition of Si–HA coating in the biomimetic coating process. Fig. 3 and Fig. 4 clearly show that a very fine 3D porous structure was produced after the pretreatment. The high surface area of the 3D porous structure will absorb ions in the solution and accelerate the precipitation of HA or Si–HA. Also, Ca2+ in the coating, rutile and CaTiO3 contribute to the precipitation of Si–HA on the Ti substrate. Previous study showed that rutile can accelerate the deposition rate of HA on titanium substrate [34]. CaTiO3 in the coating has the potential to form Ti–OH on the surface when the sample is in contact with SSS or SBF. The formation of Ti–OH induces the precipitation and deposition of HA on titanium substrate [35]. It has been reported that Si–HA has a higher dissolution with the preferential release of silicon into the medium than HA and this fact may account for the high cell proliferation on Si–HA in comparison with HA [36]. XRD results in Fig. 5 and the FTIR in Fig. 6 indicate that Si– HA coatings with different Si contents have been produced successfully on the titanium substrates by the proposed biomimetic method although no phase corresponding to Si was detected by XRD. The Si content of the coating ranges from 0.14 to 1.2 at.%, and the Ca/(P + Si) atomic ratio from 1.61 to 1.79. The coating shows a uniform thickness of 5–10 μm which is significantly thicker as compared with the Si–HA coating produced by magnetron co-sputtering process, which generally ranged between 400 nm and 750 nm [20]. The EDS results indicate that this method is a controllable method for Si–HA coating preparation, by which the composition of the coatings, especially Si content, can be controlled on demand. By using Na2SiO3, a metastable solution can be prepared with different silicon ion concentrations (1 mM, 5 mM and 10 mM). The chemical reactions in SSS can be envisaged as following: 2− H2 O + SiO2− 3 Y H2 SiO4
ð1Þ
3− + H2 SiO2− 4 Y H + HSiO4
ð2Þ
4− + HSiO3− 4 Y H + SiO4
ð3Þ
3− + HPO2− 4 Y H + PO4
ð4Þ
H
+
+ HCO−3
Y H2 O + CO2 z
ð5Þ
1 4− − 10Ca2+ + ð20−4x− yÞPO3− 4 + xSiO4 + yOH Y Ca10 ðPO4 Þ13ð20 − 4x − yÞ ðSiO4 Þx ðOHÞy A 3
ð6Þ 4− The first three reactions make SiO2− 3 exist in the form of SiO4 in SSSs and be incorporated into HA in the form of SiO44−. Reactions
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(2)–(4) can support H+ ions. Reaction (5) increases the pH value by consuming H+ ions that released from the reactions (2)–(4), which leads the solution to a supersaturated stage. Then Reaction (6) consumes OH−, SiO44− and PO3− 4 , and Si–HA forms on titanium substrates. In Porter's experiment, the pH value must be maintained at 10.5 with the addition of ammonia, which results in the releasing of irritant gas [37], while in the present experiment it is not necessary to maintain pH value, so there is no releasing of irritant gas. It has been reported that the degree of crystallization is a key factor to osteoconductivity of hydroxyapatite ceramics in the subcutaneous of dogs [37]. The hydroxyapatite with lower degree of crystallization performed an obvious osteoconduction , while the hydroxyapatite with higher degree of crystallization did not. The biomimetic process imitates the mode in which bone-like hydroxyapatite crystals are formed in the body [38], so substituted Si element can be co-precipitated into the HA crystal lattice. The Si–HA coating prepared by the biomimetic method had also a lower degree of crystallization, as shown in Fig. 5. Hence, the Si–HA coating would lead to better bone repair and regeneration, and such coatings with different silicon contents could have a great potential for the design of future implant materials. 4.2. Shear strength The HA coating provides a bioactive surface on a metal implant for bone ongrowth. Therefore, it is important that the bonding strength of the coating-implant interface should be sufficiently high to withstand the interfacial stresses encountered in the in vivo environment. In other processes, such as plasma deposition or electrophorectical deposition (EPD), high temperature is involved in order to obtain high bonding/or shear strength. One of the big problems is the deleterious effects of high temperature on the properties of the coating and the metal substrate. High sintering temperature can result in phase transformation and grain growth of the metal substrate, which may cause the mechanical properties of the metal substrate to decrease significantly, and also the metal substrate catalyzed decomposition of the HA to anhydrous calcium phosphates, which leads to enhanced in vitro dissolution rates of HA coating [39,40]. The high temperature sintering in EPD process also resulted in cracks in the HA coating due to the shrinkage [40], which significantly reduced the interfacial bonding strength. In order to improve the bonding strength of the HA coating/substrate, a dual coating technique was developed to “fill” the cracks in the EPD HA coating [40–42]. Also heat treatment was applied to the plasma sprayed HA coating to improve the bonding strength [43,44]. In addition, bioglass [45], titanium powder [46] or TiO2 [47] were adopted to produce HA-glass or HA-titanium composites coating to get a high bonding strength. It was reported the shear strength of dual HA coatings prepared by EPD on Ti was ~ 11 MPa and Ti6Al4 V was ~14 MPa [40]. It was also reported that the shear strength of the bovine cortical bone was 34 MPa with a standard deviation of 17 MPa [40]. Table 3 lists the interfacial bond strength of HA/Ti substrate reported elsewhere. Significantly, the interfacial bond strength of thermally sprayed HA was in a range of 13–30 MPa.
Table 3 Shear strength of HA coating and titanium substrate Substrate/HA coating
Coating process
Shear strength, MPa
Ref.
Ti–6Al–4 V/HA
Plasma spray Plasma spray + heat treatment Plasma spray Plasma spray Plasma spray EPD EPD Biomimetic process
24.5 ± 2.4 26.7 ± 1.4
[43]
23.5–30.2 12.9 17.3 11 ± 7.8 13.9 ± 4.3 15.7 ± 3.4
[48] [46] [46] [40] [40] This work
Ti–6Al–4 V/HA Ti/HA Ti/Ti–HA composite coating Ti/HA Ti–6Al–4 V/HA Ti/Si–HA
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In the biomimetic process, HA coating was formed in a simulated body fluid. No high temperature was involved. Therefore, the HA coating has more close structure to the natural bone tissue and has good osteoconductivity, especially at the early stage of the bone healing process [49]. However, the low interface bonding or shear strength of the HA coating/Ti substrate limits their clinical application. For example, the shear strength between the HA coating and the titanium substrate in this paper is about 9.2 MPa, close to the value of HA/Ti produced by EPD reported in Ref. [40]. The substitution Si ion into the HA coating significantly improves the shear strength to an average 16 MPa, which is close to the value of the HA coating/Ti prepared by plasma spray. The significant improvement in the shear strength provides the potential clinical application of Si–HA coating from the mechanical properties point of view and in light of the fact that biomimetic process has a low processing cost and is a non-line-of-sight process. 5. Conclusions Uniform Si–HA coatings with 5–10 μm in thickness were successfully prepared by a biomimetic process on titanium substrates. The Si content in the coatings ranges from 0.14 to 1.2 at.% and the Ca/(P + Si) atomic ratio from 1.61 to 1.79. The biomimetic mineral phase was not well crystallized and amorphous phase existed in the coating. Silicon was confirmed to exist in the form of SiO44− groups in Si–HA coating. Si– HA coating showed high adhesion strength to the titanium substrate. Acknowledgments
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
One of authors (Erlin Zhang) would like to acknowledge the financial support from the Institute of Metal Research (IMR), Chinese Academy of Sciences (CAS), and Shenyang Science and Technology Institute (Program No. 1062109-1-00). Many thanks to Prof. Ma M.Z. at Yanshan University, China, for his help in small angle XRD analysis. References [1] [2] [3] [4] [5] [6] [7] [8]
F. Li, Q.L. Feng, F.Z. Cui, H.D. Li, H. Schubert, Surf. Coat. Technol. 154 (2002) 88. L.M. Sun, C.C. Berndt, K.A. Gross, A. Kucuk, J. Biomed. Mater. Res. 58 (2001) 570. J.C. Knowles, K. Gross, C.C. Berndt, W. Bonfield, Biomaterials 17 (1996) 639. M. Hamdi, S. Hakamata, A.M. Ektessabi, Thin Solid Films 377 (2000) 484. P. Ducheyne, Q. Qiu, Biomaterials 20 (1999) 2287. W. Suchanek, M. Yoshimoto, J. Mater. Res. 13 (1998) 94. L.L. Hench, J. Am. Ceram. Soc. 74 (1991) 1487. H. Oonishi, L.L. Hench, J. Wilson, F. Sugihara, E. Tsuji, S. Kushitani, H. Iwaki, J. Biomed. Mater. Res. 44 (1999) 31.
[38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]
E.M. Carlisle, Science 167 (1970) 279. S. Hayakawa, K. Tsuru, C. Ohtsuki, A. Osaka, J. Am. Ceram. Soc. 82 (1999) 2155. T. Kasuga, Y. Hosoi, M. Nogami, M. Niinomi, J. Am. Ceram. Soc. 84 (2001) 450. L.L. Hench, J. Am. Ceram. Soc. 81 (1998) 1705. K. Schantz, D.B. Milne, Nature 239 (1972) 333. E.M. Carlisle, Science 178 (1972) 619. T. Tian, D. Jiang, J. Zhang, Q. Lin, Mater. Sci. Eng. C 28 (2008) 57. S.R. Kim, J.H. Lee, Y.T. Kim, D.H. Riu, S.J. Jung, Y.J. Lee, S.C. Chung, Y.H. Kim, Biomaterials 24 (2003) 1389. N. Patel, S.M. Best, W. Bonfield, I.R. Gibson, K.A. Hing, E. Damien, P.A. Revell, J Mater. Sci-Mater. M. 13 (2002) 1199. K.A. Hing, P.A. Revell, N. Smith, T. Buckland, Biomaterials 27 (2006) 5014. E.S. Thian, J. Huang, M.E. Vickers, S.M. Best, Z.H. Barber, W. Bonfield, J. Mater. Sci. 41 (2006) 709. E.S. Thian, J. Huang, S.M. Best, Z.H. Barber, W. Bonfield, J. Mater. Sci-Mater. M. 16 (2005) 411. E.S. Thian, J. Huang, S.M. Best, Z.H. Barber, R.A. Brooks, N. Rushton, W. Bonfield, Biomaterials 27 (2006) 2692. J. Huang, S. Jayasinghe, S.M. Best, M.J. Edirisinghe, R.A. Brooks, N. Rushton, W. Bonfield, J. Mater. Sci-Mater. M. 16 (2005) 1137. S.J. Ding, T.H. Huang, C.T. Kao, Surf. Coat. Tech. 165 (2003) 248. N. Hijon, M. Victoria Cabanas, J. Pena, M. Vallet-Regi, Acta Biomater. 2 (2006) 567. E.L. Solla, P. Gonzalez, J. Serra, S. Chiussi, B. Leon, J.G. Lopez, Appl. Surf. Sci. 254 (2007) 1189. W.-H. Song, Y.-K. Jun, Y. Han, S.-H. Hong, Biomaterials 25 (2004) 3341. E.L. Zhang, K. Yang, T. Nonferr. Metal. Soc. 15 (2005) 1199. S.R. Sousa, M.A. Barbosa, Biomaterials 17 (1996) 397. I.R. Gibson, S.M. Best, W. Bonfield, J. Biomed. Mater. Res. 44 (1999) 422. T. Leventouri, C.E. Bunaciu, V. Perdikatsis, Biomaterials 24 (2003) 4205. A. Fidalgo, L.M. Ilharco, J. Non-Cryst. Solids 283 (2001) 144. I. Rehman, W. Bonfield, J. Mater. Sci. Mater. Med. 8 (1997) 1. X.L. Tang, X.F. Xiao, R.F. Liu, Mater. Lett. 59 (2005) 3841. B.C. Yang, M. Uchida, H.M. Kim, X.D. Zhang, T. Kokubo, Biomaterials 25 (2004) 1003. H.M. Kim, F. Miyaji, T. Kokubo, T. Nakamura, J. Biomed. Mater. Res. A 32 (1996) 409. C.M. Botelho, R.A. Brookes, S.M. Best, M.A. Lopes, J.D. Santos, N. Rushton, W. Bonfield, (Trans Tech Publications LTD, Zurich, Switzerland, 2004). A.E. Porter, N. Patel, J.N. Skepper, S.M. Best, W. Bonfield, Biomaterials 24 (2003) 4609. Y. Liu, E.B. Hunziker, N.X. Randall, K. de Groot, P. Layrolle, Biomaterials 24 (2003) 65. L. Brannon-Peppas, Biomaterials 11 (1990) 224. M. Wei, A.J. Ruys, M.V. Swain, S.H. Kim, B.K. Milthorpe, C.C. Sorrell, J. Mater. SciMater. M. 10 (1999) 401. A.J.D. Farmer, N. Gane, R. Sekel, Int. Ceram. Mogogr. 1 (1994). F. Witte, F. Feyerabend, P. Maier, J. Fischer, M. Stormer, C. Blawert, W. Dietzel, N. Hort, Biomaterials 28 (2007) 2163. S.W.K. Kweh, K.A. Khor, P. Cheang, Biomaterials 23 (2002) 775. C.W. Yang, T.M. Lee, T.S. Lui, E. Chang, Mater. Sci. Eng. C 26 (2006) 1395. M.F. Morks, J. Mech. Behav. Biomed. Mater. 1 (2008) 105. X.B. Zheng, M.H. Huang, C.X. Ding, Biomaterials 21 (2000) 841. X. Nie, A. Leyland, A. Matthews, Surf. Coat. Technol. 125 (2000) 407. C.Y. Yang, R.M. Lin, B.C. Wang, T.M. Lee, E. Chang, Y.S. Hang, P.Q. Chen, J. Biomed. Mater. Res. 37 (1997) 335. S. Nishiguchi, H. Kato, H. Fujita, M. Oka, H.-M. Kim, T. Kokubo, T. Nakamura, Biomaterials 22 (2001) 2525.