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A new evaporation-based method for the preparation of biomimetic calcium phosphate coatings on metals
Materials Science and Engineering C 29 (2009) 1334–1337
Contents lists available at ScienceDirect
Materials Science and Engineering C 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 / m s e c
Short communication
A new evaporation-based method for the preparation of biomimetic calcium phosphate coatings on metals Ke Duan a,b,⁎, Allen Tang b, Rizhi Wang b a b
Key Lab of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, Sichuan, 610031, China Department of Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4
a r t i c l e
i n f o
Article history: Received 28 August 2008 Received in revised form 12 October 2008 Accepted 23 October 2008 Available online 5 November 2008 Keywords: Calcium phosphate Biomimetic Coating Metal Surface
a b s t r a c t This study reports a new method to prepare biomimetic calcium phosphate coatings on titanium, stainless steel, CoCrMo, and tantalum. The method does not require surface etching, high supersaturation, or tight control of solution conditions. Metallic samples were dipped into a supersaturated calcium phosphate solution, withdrawn, and left to dry at room temperature. Calcium phosphate crystallites formed on and completely covered the surfaces by repeating the dip-and-dry treatment. The crystallite-covered surfaces readily grew to calcium phosphate coatings when immersed in the supersaturated solution. The mechanism of the treatment was suggested to be an evaporation-induced surface crystallization process. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Calcium phosphates (CaPs) are extensively plasma-sprayed as surface coatings on metallic bone implants to improve clinical performances [1]. Studies have found higher implant stability and longer service-life with CaP-coated implants [2,3]. However, the prevalent plasma-spray is a line-of-sight technique and cannot uniformly coat internal surfaces, such as the porous bead coatings on joint implants [4]. In addition, high temperature plasma flame may cause CaP input powders to decompose into other phases such as tricalcium phosphate and calcium oxide [5,6]. To address the limitations of plasma-spray, biomimetic coating methods were developed [7]. In these methods, CaP crystals were induced to nucleate on the surfaces from supersaturated solutions. Because the biomimetic coatings were grown from solutions and usually at near body temperature, their compositions, morphologies, and structures were considered to be more similar to the bone mineral when compared with those from the plasma-spray technique. The widely used biomimetic coating methods involve pretreatment of the metal with strong base or acid to transform the surface into an alkali salt or hydrated oxide (e.g. sodium titanate, titania) [8–11]. The alkali salt and oxide show negative surface charges, and thus can attract Ca2+ and initiate CaP nucleation. A disadvantage of the methods is that they are based on etching and cause surface defects. ⁎ Corresponding author. Key Lab of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, Sichuan, 610031, China. Tel./fax: +86 28 87601371. E-mail address: [email protected] (K. Duan). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.10.028
The defects may adversely affect the mechanical integrity of implants, especially for fine structures such as porous metals. In addition, these methods can be used to coat titanium (Ti) and tantalum (Ta) but not stainless steel or CoCrMo alloy because their alkali salts and oxides do not show negative surface charges [8,10]. Several studies have reported the preparation of biomimetic coatings on Ti, Ti6Al4V, and CoCrMo without surface etching [12–14]. These studies used coating solutions with high degrees of supersaturation to overcome the poor nucleating ability of the untreated metal surfaces. The high supersaturation was obtained by controlled, gradual release of CO2 to increase the solution pH or by tight control of solution conditions (i.e. concentrations, temperature, pH, and agitation). However, under the high supersaturations, the solutions became prone to bulk precipitation; these studies reported the occurrence of “turbidity” [14] or a precipitate “throughout the whole solution” [12]. Although biomimetic coatings were achieved in these studies, CaP surface nucleation in competition with bulk precipitation or the requirement of tight solution condition controls may adversely affect the robustness and scaling-up of the coating technique. In this letter, we report a simple method to prepare biomimetic CaP coatings on a variety of orthopedic metals without using surface etching or tight solution controls. 2. Materials and methods 2.1. Materials Commercially pure sheets of Ti (2 mm thick, Johnson Matthey, Edmonton, AB, Canada), 316 stainless steel (1 mm thick, Alaskan
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Fig. 1. SEM micrographs of (a) a cleaned Ti sample, and Ti samples treated with (b) 4 times and (c) 8 times of dip-and-dry; (d) shows the corresponding EDS spectra.
Copper and Brass, Vancouver, BC, Canada), and Ta (0.5 mm thick, Goodfellow, Oakdale, PA, USA) were cut into 20 × 20 mm samples. Ti and stainless steel samples were ground to 800 grit surface finish; Ta samples were used without grinding. Two clinical devices, porous Ta cylinders (Ø3.15 × 8 mm, Zimmer, Warsaw, IN, USA) and a CoCrMo (ASTM F-75) artificial hip stem, were also used. The samples were sonicated in acetone, ethanol, and distilled water, each for 5 min. The
samples were then rinsed in concentrated sulfuric acid (96 wt.%, Fisher) for 1 min to clean the surfaces. 2.2. Coating method A supersaturated calcium phosphate solution was prepared: 2.32 mM NH4H2PO4, 3.87 mM CaCl2, 150 mM NaCl, 40 mM HCl, and
Fig. 2. Photographs showing the metal samples coated by the dip-and-dry treatment. (a) The upper-left one is a control sample showing the gloss of uncoated Ti, which was cleaned and immersed in ACS without the dip-and-dry treatment. The other seven samples in the upper two rows are CaP-coated Ti prepared by the treatment. The third row shows four CaPcoated Ta. (b) Eleven CaP-coated stainless steel samples. The upper-left one is a control sample, which was cleaned and immersed in ACS without the treatment. (c) A coated CrCoMo alloy hip stem. Inset: The clear border of the CaP coating reflects the depth of ACS solution for the dip-and-dry treatment. (d) Three CaP-coated porous Ta cylinders in the right, showing a whitish color. The control sample in the left shows the gloss of bare metal. Coating condition for all samples: 8 times dip-and-dry and 2 h immersion in ACS.
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Fig. 3. SEM micrographs of a CaP coating prepared on Ti by 8 times of dip-and-dry and immersed in ACS for 2 h.
50 mM tri(hydroxymethyl)aminomethane (Tris) (all Fluka); it was termed as the accelerated calcification solution (ACS) [15]. The ACS was prepared by the following procedure. Appropriate amount/ volume of NaCl, CaCl2 and HCl were dissolved in 450 ml of distilled water, and NH4H2PO4 and Tris were dissolved in 500 ml of water. The latter solution was slowly added into the former one under stirring. The pH was adjusted to 7.30 ± 0.05 at room temperature (20-24 °C) by adding NaOH, and the volume was adjusted to 1 liter. The pH was retested and, if necessary, fine-adjusted again to the value. In this lab, the ACS was stable without precipitation for more than 2 days at room temperature. The cleaned samples were held by a pair of stainless steel tweezers, vertically dipped into ACS for approximately 5 s, and withdrawn from the solution, with a wetting film remaining on the surfaces. The samples were placed on a watch glass and left to completely dry in open air at room temperature (20–24 °C). Every sample was treated 8 times with this dip-and-dry, and then rinsed with water and air-dried. Then the samples were immersed in 50 ml of ACS, kept in a 37 °C water bath for 2 h. Metals which received the cleaning but did not undergo the dip-and-dry treatment were immersed in ACS as controls. After the immersion, all samples were rinsed with distilled water and air-dried. 2.3. Characterizations Surface morphologies were studied with a scanning electron microscope (SEM, Hitachi 3000 N, 5 kV). Elemental compositions were analyzed by an energy dispersive X-ray spectrometer (EDS,10 kV) attached to the SEM. Coatings were also scraped off and analyzed by X-ray diffraction (XRD, Rigaku Rotaflex, CuKα, 20 kV, 20 mA, 2°/min.).
3. Results The cleaned Ti showed a flat surface with some grinding marks (Fig. 1a), and EDS detected only Ti (Fig. 1d). After 4 times of dip-anddry, the Ti surfaces were partially covered by CaP crystallites with a tripod morphology. Each arm of the tripods was about 400 nm long (Fig. 1b). EDS analysis detected 0.26 at.% of Ca and 0.12 at.% of P, in addition to the Ti substrate (Fig. 1d). After 8 times of dip-and-dry, the CaP crystallites completely covered the Ti surfaces, and the Ca and P content increased to 0.91 at.% and 0.63 at.%, respectively (Fig. 1c and d). After 8 times of dip-and-dry, the Ti surfaces appeared uniformly golden indicating the presence of a thin surface film. The thickness of the crystallite layer was difficult to measure but was estimated to be below 400 nm. After 8 times of dip-and-dry and 2 h of immersion in ACS, all Ti samples were completely covered with white CaP coatings (Fig. 2a). In contrast, all Ti control samples showed either no or negligible coating coverage. SEM micrographs showed that the coating (Fig. 3a and b) consisted of aggregation of CaP flakes 2 to 3 μm long and about 100 nm thick; the flaky morphology is typical of octacalcium phosphate (OCP, Ca8(PO4)4(HPO4)2.5H2O) [9]. The XRD pattern of the coating showed peaks at 4.7°, 9.5°, and 16.1°, which were characteristic of OCP (Fig. 4). The groups of peaks at 26° and ~ 32° could be indexed to OCP or apatite phase, because their XRD patterns strongly overlap in the regions. Therefore, the coating was mainly OCP, but may also contain apatite as the minor phase. The coating method (i.e. 8 times of dip-and-dry followed by immersion in ACS) has been applied on more than 60 flat Ti, more than 20 flat stainless steel, more than 30 flat Ta, 10 porous Ta samples, and a CoCrMo hip stem (Fig. 2b to d); uniform coatings were obtained for all samples. 4. Discussion
Fig. 4. XRD pattern of a CaP coating prepared on Ti by 8 times of dip-and-dry and immersion in ACS for 2 h. Black bars show the standard pattern of hydroxyapatite (Ca10(OH)2(PO4)6, JCPDS 9-432); grey bars show the pattern of OCP (JCPDS 26-1056).
In this study, a new dip-and-dry method was used to deposit a CaP crystallite layer on the surfaces, which then acted as crystal growth seeds to facilitate the subsequent coating growth. The mechanism of the treatment may be suggested to be an evaporation-induced surface crystallization process (Fig. 5). The dip step left a continuous wetting film on the sample surface. During the dry step, the wetting film gradually evaporated, and the supersaturation level increased. The slow increase in supersaturation may have driven CaP crystallites to nucleate on the surfaces to form adherent crystallites. CaP crystallites may also have formed within the wetting film, and they may either adhere to, loosely attach to, or be separated from the surface. Complete drying of the wetting film formed a “crust” on the surface consisting of CaP crystallites and salt crystals (NaCl, Tris etc.). By the next dip step, the salt crystals dissolved and the non-adherent CaP crystallites detached; a new wetting film formed. In the next drying step, new CaP crystallites formed again to increase coverage. The repeated dip-and-dry treatment thus formed a complete CaP
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phase. However, further studies are needed to better understand this issue. The novelty of this method is that the surface nucleation occurred in a thin liquid film instead of a bulk solution. Therefore, it could obtain a supersaturation high enough for surface nucleation but without increasing the risk of bulk precipitation or the need of tight solution condition control. This should be beneficial to the robustness of the coating technique. Moreover, although simple, the method provided a driving force strong enough to coat non-negatively charged surfaces (i.e. stainless steel, CoCrMo). The method is versatile; it can also coat polymers such as surfacehydrolyzed poly-(lactic-co-glycolic acid) (not shown here). In addition, we recently found that the H2SO4 cleaning procedure for metals could be replaced by treatment in boiling water for 10 min, thus avoiding the concerns associated with the use of concentrated acid. The bonding strength and biological performances of the coatings will be studied in a future study. 5. Conclusion This study reports a new method to prepare biomimetic calcium phosphate coatings on titanium, stainless steel, CoCrMo alloy, and porous tantalum. The method does not require surface etching, high supersaturation, or tight control of the solution conditions. The method is simple, reproducible, and can be applied on a variety of metals. An evaporation-induced surface crystallization process was suggested to be the mechanism. The method may be promising for coating orthopedic implants. Acknowledgements
Fig. 5. A schematic showing the suggested mechanism of the dip-and-dry treatment. (a) A metal sample was dipped into ACS and withdrawn. (b) A wetting film (dotted border) remained on the surface. (c) The film dried on the surface to form a “crust” consisting of adherent CaP crystallites (grey hemisphere), non-adherent CaP crystallites (grey sphere), and NaCl and Tris salt crystals (cubes). (d) The sample was dipped into ACS again; the non-adherent CaP crystallites detached; the NaCl and Tris crystals dissolved; the adherent CaP crystallites remained on the surface, with a new wetting film. (e) By repeated cycles between C and D, the adherent CaP crystallites stepwise increased their surface coverage to complete.
crystallite layer on the surface. The crystallite layer supported the subsequent growth of CaP coatings due to their similarities in structures and compositions. In this study, the coatings grown from ACS were mainly OCP, whereas the coatings obtained from the simulated body fluid (SBF) were apatite [8,10]. The difference may be attributed to the different ion concentrations of the solutions. SBF and ACS are supersaturated with both OCP and apatite, and a theoretical analysis suggested that when compared with apatite, OCP has lower thermodynamic stability but higher nucleation rate [16]. In addition, OCP is widely considered as a precursor phase that can convert to apatite. The competition between OCP nucleation/growth and conversion may partly account for the different phases obtained [17]. For SBF, because it has lower Ca and P concentrations, the OCP nucleation/growth may be slower than the conversion to apatite, so only apatite is obtained. In comparison, since ACS has higher Ca and P concentrations, the OCP nucleation/ growth rate may exceed the conversion, so OCP becomes the major
The authors are grateful to the Natural Sciences and Engineering Research Council of Canada and the Canadian Institute of Health Research for the financial supports. R.W. is incumbent of the Canada Research Chair in Biomaterials. The porous tantalum samples were kindly provided by Zimmer Inc. References [1] K. De Groot, R. Geesink, C.P. Klein, P. Serekian, J. Biomed. Mater. Res. 21 (1987) 1375. [2] R.G.T. Geesink, in: R.G.T. Geesink, M.T. Manley (Eds.), Hydroxyapatite Coatings in Orthopaedic Surgery, Raven, New York, 1993, p. 151. [3] L.I. Havelin, B. Engesaeter, B. Espehaug, O. Furnes, S.A. Lie, S.E. Vollset, Acta Orthop. Scand. 71 (2000) 337. [4] R.M. Pilliar, Orthop. Clin. North Am. 36 (2005) 113. [5] S.R. Radin, P. Ducheyne, J. Mater. Sci., Mater. Med. 3 (1992) 33. [6] J. Weng, X. Liu, X. Zhang, K. de Groot, J. Biomed. Mater. Res. A 30 (1996) 5. [7] K. Duan, R. Wang, J. Mater. Chem. 16 (2006) 2309. [8] H.M. Kim, F. Miyaji, T. Kokubo, T. Nakamura, J. Biomed. Mater. Res. 32 (1996) 409. [9] H. Wen, Q. Liu, J.R. de Wijn, K. de Groot, F. Cui, J. Mater. Sci., Mater. Med. 9 (1998) 121. [10] T. Miyazaki, H.M. Kim, F. Miyaji, T. Kokubo, H. Kato, T. Nakamura, J. Biomed. Mater. Res. 50 (2000) 35. [11] C. Ohtsuki, H. Iida, S. Hayakawa, A. Osaka, J. Biomed. Mater. Res. 35 (1997) 39. [12] F. Barrere, M.M.E. Snel, C.A. Van Blitterswijk, K. de Groot, B. Layrolle, Biomaterials 25 (2004) 2901. [13] P. Li, J. Biomed. Mater. Res. 66 (2003) 79. [14] J.P. Zitelli, P. Higham, Materials Research Symposium 599 (Mineralization in Natural and Synthetic Biomaterials), Materials Research Society, Warrendale, 1999, p. 117. [15] S.H. Li, Q. Liu, J. de Wijn, J. Woke, B. Zhou, K. de Groot, J. Mater. Sci., Mater. Med. 8 (1997) 543. [16] X. Lu, Y. Leng, Biomaterials 26 (2005) 1097. [17] Soten, G.A. Ozin, J. Mater. Chem. 9 (1999) 703.
Contents lists available at ScienceDirect
Materials Science and Engineering C 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 / m s e c
Short communication
A new evaporation-based method for the preparation of biomimetic calcium phosphate coatings on metals Ke Duan a,b,⁎, Allen Tang b, Rizhi Wang b a b
Key Lab of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, Sichuan, 610031, China Department of Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4
a r t i c l e
i n f o
Article history: Received 28 August 2008 Received in revised form 12 October 2008 Accepted 23 October 2008 Available online 5 November 2008 Keywords: Calcium phosphate Biomimetic Coating Metal Surface
a b s t r a c t This study reports a new method to prepare biomimetic calcium phosphate coatings on titanium, stainless steel, CoCrMo, and tantalum. The method does not require surface etching, high supersaturation, or tight control of solution conditions. Metallic samples were dipped into a supersaturated calcium phosphate solution, withdrawn, and left to dry at room temperature. Calcium phosphate crystallites formed on and completely covered the surfaces by repeating the dip-and-dry treatment. The crystallite-covered surfaces readily grew to calcium phosphate coatings when immersed in the supersaturated solution. The mechanism of the treatment was suggested to be an evaporation-induced surface crystallization process. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Calcium phosphates (CaPs) are extensively plasma-sprayed as surface coatings on metallic bone implants to improve clinical performances [1]. Studies have found higher implant stability and longer service-life with CaP-coated implants [2,3]. However, the prevalent plasma-spray is a line-of-sight technique and cannot uniformly coat internal surfaces, such as the porous bead coatings on joint implants [4]. In addition, high temperature plasma flame may cause CaP input powders to decompose into other phases such as tricalcium phosphate and calcium oxide [5,6]. To address the limitations of plasma-spray, biomimetic coating methods were developed [7]. In these methods, CaP crystals were induced to nucleate on the surfaces from supersaturated solutions. Because the biomimetic coatings were grown from solutions and usually at near body temperature, their compositions, morphologies, and structures were considered to be more similar to the bone mineral when compared with those from the plasma-spray technique. The widely used biomimetic coating methods involve pretreatment of the metal with strong base or acid to transform the surface into an alkali salt or hydrated oxide (e.g. sodium titanate, titania) [8–11]. The alkali salt and oxide show negative surface charges, and thus can attract Ca2+ and initiate CaP nucleation. A disadvantage of the methods is that they are based on etching and cause surface defects. ⁎ Corresponding author. Key Lab of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu, Sichuan, 610031, China. Tel./fax: +86 28 87601371. E-mail address: [email protected] (K. Duan). 0928-4931/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msec.2008.10.028
The defects may adversely affect the mechanical integrity of implants, especially for fine structures such as porous metals. In addition, these methods can be used to coat titanium (Ti) and tantalum (Ta) but not stainless steel or CoCrMo alloy because their alkali salts and oxides do not show negative surface charges [8,10]. Several studies have reported the preparation of biomimetic coatings on Ti, Ti6Al4V, and CoCrMo without surface etching [12–14]. These studies used coating solutions with high degrees of supersaturation to overcome the poor nucleating ability of the untreated metal surfaces. The high supersaturation was obtained by controlled, gradual release of CO2 to increase the solution pH or by tight control of solution conditions (i.e. concentrations, temperature, pH, and agitation). However, under the high supersaturations, the solutions became prone to bulk precipitation; these studies reported the occurrence of “turbidity” [14] or a precipitate “throughout the whole solution” [12]. Although biomimetic coatings were achieved in these studies, CaP surface nucleation in competition with bulk precipitation or the requirement of tight solution condition controls may adversely affect the robustness and scaling-up of the coating technique. In this letter, we report a simple method to prepare biomimetic CaP coatings on a variety of orthopedic metals without using surface etching or tight solution controls. 2. Materials and methods 2.1. Materials Commercially pure sheets of Ti (2 mm thick, Johnson Matthey, Edmonton, AB, Canada), 316 stainless steel (1 mm thick, Alaskan
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Fig. 1. SEM micrographs of (a) a cleaned Ti sample, and Ti samples treated with (b) 4 times and (c) 8 times of dip-and-dry; (d) shows the corresponding EDS spectra.
Copper and Brass, Vancouver, BC, Canada), and Ta (0.5 mm thick, Goodfellow, Oakdale, PA, USA) were cut into 20 × 20 mm samples. Ti and stainless steel samples were ground to 800 grit surface finish; Ta samples were used without grinding. Two clinical devices, porous Ta cylinders (Ø3.15 × 8 mm, Zimmer, Warsaw, IN, USA) and a CoCrMo (ASTM F-75) artificial hip stem, were also used. The samples were sonicated in acetone, ethanol, and distilled water, each for 5 min. The
samples were then rinsed in concentrated sulfuric acid (96 wt.%, Fisher) for 1 min to clean the surfaces. 2.2. Coating method A supersaturated calcium phosphate solution was prepared: 2.32 mM NH4H2PO4, 3.87 mM CaCl2, 150 mM NaCl, 40 mM HCl, and
Fig. 2. Photographs showing the metal samples coated by the dip-and-dry treatment. (a) The upper-left one is a control sample showing the gloss of uncoated Ti, which was cleaned and immersed in ACS without the dip-and-dry treatment. The other seven samples in the upper two rows are CaP-coated Ti prepared by the treatment. The third row shows four CaPcoated Ta. (b) Eleven CaP-coated stainless steel samples. The upper-left one is a control sample, which was cleaned and immersed in ACS without the treatment. (c) A coated CrCoMo alloy hip stem. Inset: The clear border of the CaP coating reflects the depth of ACS solution for the dip-and-dry treatment. (d) Three CaP-coated porous Ta cylinders in the right, showing a whitish color. The control sample in the left shows the gloss of bare metal. Coating condition for all samples: 8 times dip-and-dry and 2 h immersion in ACS.
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Fig. 3. SEM micrographs of a CaP coating prepared on Ti by 8 times of dip-and-dry and immersed in ACS for 2 h.
50 mM tri(hydroxymethyl)aminomethane (Tris) (all Fluka); it was termed as the accelerated calcification solution (ACS) [15]. The ACS was prepared by the following procedure. Appropriate amount/ volume of NaCl, CaCl2 and HCl were dissolved in 450 ml of distilled water, and NH4H2PO4 and Tris were dissolved in 500 ml of water. The latter solution was slowly added into the former one under stirring. The pH was adjusted to 7.30 ± 0.05 at room temperature (20-24 °C) by adding NaOH, and the volume was adjusted to 1 liter. The pH was retested and, if necessary, fine-adjusted again to the value. In this lab, the ACS was stable without precipitation for more than 2 days at room temperature. The cleaned samples were held by a pair of stainless steel tweezers, vertically dipped into ACS for approximately 5 s, and withdrawn from the solution, with a wetting film remaining on the surfaces. The samples were placed on a watch glass and left to completely dry in open air at room temperature (20–24 °C). Every sample was treated 8 times with this dip-and-dry, and then rinsed with water and air-dried. Then the samples were immersed in 50 ml of ACS, kept in a 37 °C water bath for 2 h. Metals which received the cleaning but did not undergo the dip-and-dry treatment were immersed in ACS as controls. After the immersion, all samples were rinsed with distilled water and air-dried. 2.3. Characterizations Surface morphologies were studied with a scanning electron microscope (SEM, Hitachi 3000 N, 5 kV). Elemental compositions were analyzed by an energy dispersive X-ray spectrometer (EDS,10 kV) attached to the SEM. Coatings were also scraped off and analyzed by X-ray diffraction (XRD, Rigaku Rotaflex, CuKα, 20 kV, 20 mA, 2°/min.).
3. Results The cleaned Ti showed a flat surface with some grinding marks (Fig. 1a), and EDS detected only Ti (Fig. 1d). After 4 times of dip-anddry, the Ti surfaces were partially covered by CaP crystallites with a tripod morphology. Each arm of the tripods was about 400 nm long (Fig. 1b). EDS analysis detected 0.26 at.% of Ca and 0.12 at.% of P, in addition to the Ti substrate (Fig. 1d). After 8 times of dip-and-dry, the CaP crystallites completely covered the Ti surfaces, and the Ca and P content increased to 0.91 at.% and 0.63 at.%, respectively (Fig. 1c and d). After 8 times of dip-and-dry, the Ti surfaces appeared uniformly golden indicating the presence of a thin surface film. The thickness of the crystallite layer was difficult to measure but was estimated to be below 400 nm. After 8 times of dip-and-dry and 2 h of immersion in ACS, all Ti samples were completely covered with white CaP coatings (Fig. 2a). In contrast, all Ti control samples showed either no or negligible coating coverage. SEM micrographs showed that the coating (Fig. 3a and b) consisted of aggregation of CaP flakes 2 to 3 μm long and about 100 nm thick; the flaky morphology is typical of octacalcium phosphate (OCP, Ca8(PO4)4(HPO4)2.5H2O) [9]. The XRD pattern of the coating showed peaks at 4.7°, 9.5°, and 16.1°, which were characteristic of OCP (Fig. 4). The groups of peaks at 26° and ~ 32° could be indexed to OCP or apatite phase, because their XRD patterns strongly overlap in the regions. Therefore, the coating was mainly OCP, but may also contain apatite as the minor phase. The coating method (i.e. 8 times of dip-and-dry followed by immersion in ACS) has been applied on more than 60 flat Ti, more than 20 flat stainless steel, more than 30 flat Ta, 10 porous Ta samples, and a CoCrMo hip stem (Fig. 2b to d); uniform coatings were obtained for all samples. 4. Discussion
Fig. 4. XRD pattern of a CaP coating prepared on Ti by 8 times of dip-and-dry and immersion in ACS for 2 h. Black bars show the standard pattern of hydroxyapatite (Ca10(OH)2(PO4)6, JCPDS 9-432); grey bars show the pattern of OCP (JCPDS 26-1056).
In this study, a new dip-and-dry method was used to deposit a CaP crystallite layer on the surfaces, which then acted as crystal growth seeds to facilitate the subsequent coating growth. The mechanism of the treatment may be suggested to be an evaporation-induced surface crystallization process (Fig. 5). The dip step left a continuous wetting film on the sample surface. During the dry step, the wetting film gradually evaporated, and the supersaturation level increased. The slow increase in supersaturation may have driven CaP crystallites to nucleate on the surfaces to form adherent crystallites. CaP crystallites may also have formed within the wetting film, and they may either adhere to, loosely attach to, or be separated from the surface. Complete drying of the wetting film formed a “crust” on the surface consisting of CaP crystallites and salt crystals (NaCl, Tris etc.). By the next dip step, the salt crystals dissolved and the non-adherent CaP crystallites detached; a new wetting film formed. In the next drying step, new CaP crystallites formed again to increase coverage. The repeated dip-and-dry treatment thus formed a complete CaP
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phase. However, further studies are needed to better understand this issue. The novelty of this method is that the surface nucleation occurred in a thin liquid film instead of a bulk solution. Therefore, it could obtain a supersaturation high enough for surface nucleation but without increasing the risk of bulk precipitation or the need of tight solution condition control. This should be beneficial to the robustness of the coating technique. Moreover, although simple, the method provided a driving force strong enough to coat non-negatively charged surfaces (i.e. stainless steel, CoCrMo). The method is versatile; it can also coat polymers such as surfacehydrolyzed poly-(lactic-co-glycolic acid) (not shown here). In addition, we recently found that the H2SO4 cleaning procedure for metals could be replaced by treatment in boiling water for 10 min, thus avoiding the concerns associated with the use of concentrated acid. The bonding strength and biological performances of the coatings will be studied in a future study. 5. Conclusion This study reports a new method to prepare biomimetic calcium phosphate coatings on titanium, stainless steel, CoCrMo alloy, and porous tantalum. The method does not require surface etching, high supersaturation, or tight control of the solution conditions. The method is simple, reproducible, and can be applied on a variety of metals. An evaporation-induced surface crystallization process was suggested to be the mechanism. The method may be promising for coating orthopedic implants. Acknowledgements
Fig. 5. A schematic showing the suggested mechanism of the dip-and-dry treatment. (a) A metal sample was dipped into ACS and withdrawn. (b) A wetting film (dotted border) remained on the surface. (c) The film dried on the surface to form a “crust” consisting of adherent CaP crystallites (grey hemisphere), non-adherent CaP crystallites (grey sphere), and NaCl and Tris salt crystals (cubes). (d) The sample was dipped into ACS again; the non-adherent CaP crystallites detached; the NaCl and Tris crystals dissolved; the adherent CaP crystallites remained on the surface, with a new wetting film. (e) By repeated cycles between C and D, the adherent CaP crystallites stepwise increased their surface coverage to complete.
crystallite layer on the surface. The crystallite layer supported the subsequent growth of CaP coatings due to their similarities in structures and compositions. In this study, the coatings grown from ACS were mainly OCP, whereas the coatings obtained from the simulated body fluid (SBF) were apatite [8,10]. The difference may be attributed to the different ion concentrations of the solutions. SBF and ACS are supersaturated with both OCP and apatite, and a theoretical analysis suggested that when compared with apatite, OCP has lower thermodynamic stability but higher nucleation rate [16]. In addition, OCP is widely considered as a precursor phase that can convert to apatite. The competition between OCP nucleation/growth and conversion may partly account for the different phases obtained [17]. For SBF, because it has lower Ca and P concentrations, the OCP nucleation/growth may be slower than the conversion to apatite, so only apatite is obtained. In comparison, since ACS has higher Ca and P concentrations, the OCP nucleation/ growth rate may exceed the conversion, so OCP becomes the major
The authors are grateful to the Natural Sciences and Engineering Research Council of Canada and the Canadian Institute of Health Research for the financial supports. R.W. is incumbent of the Canada Research Chair in Biomaterials. The porous tantalum samples were kindly provided by Zimmer Inc. References [1] K. De Groot, R. Geesink, C.P. Klein, P. Serekian, J. Biomed. Mater. Res. 21 (1987) 1375. [2] R.G.T. Geesink, in: R.G.T. Geesink, M.T. Manley (Eds.), Hydroxyapatite Coatings in Orthopaedic Surgery, Raven, New York, 1993, p. 151. [3] L.I. Havelin, B. Engesaeter, B. Espehaug, O. Furnes, S.A. Lie, S.E. Vollset, Acta Orthop. Scand. 71 (2000) 337. [4] R.M. Pilliar, Orthop. Clin. North Am. 36 (2005) 113. [5] S.R. Radin, P. Ducheyne, J. Mater. Sci., Mater. Med. 3 (1992) 33. [6] J. Weng, X. Liu, X. Zhang, K. de Groot, J. Biomed. Mater. Res. A 30 (1996) 5. [7] K. Duan, R. Wang, J. Mater. Chem. 16 (2006) 2309. [8] H.M. Kim, F. Miyaji, T. Kokubo, T. Nakamura, J. Biomed. Mater. Res. 32 (1996) 409. [9] H. Wen, Q. Liu, J.R. de Wijn, K. de Groot, F. Cui, J. Mater. Sci., Mater. Med. 9 (1998) 121. [10] T. Miyazaki, H.M. Kim, F. Miyaji, T. Kokubo, H. Kato, T. Nakamura, J. Biomed. Mater. Res. 50 (2000) 35. [11] C. Ohtsuki, H. Iida, S. Hayakawa, A. Osaka, J. Biomed. Mater. Res. 35 (1997) 39. [12] F. Barrere, M.M.E. Snel, C.A. Van Blitterswijk, K. de Groot, B. Layrolle, Biomaterials 25 (2004) 2901. [13] P. Li, J. Biomed. Mater. Res. 66 (2003) 79. [14] J.P. Zitelli, P. Higham, Materials Research Symposium 599 (Mineralization in Natural and Synthetic Biomaterials), Materials Research Society, Warrendale, 1999, p. 117. [15] S.H. Li, Q. Liu, J. de Wijn, J. Woke, B. Zhou, K. de Groot, J. Mater. Sci., Mater. Med. 8 (1997) 543. [16] X. Lu, Y. Leng, Biomaterials 26 (2005) 1097. [17] Soten, G.A. Ozin, J. Mater. Chem. 9 (1999) 703.