Materials Science Forum ISSN: 1662-9752, Vol. 888, pp 251-255 doi:10.4028/www.scientific.net/MSF.888.251 © 2017 Trans Tech Publications, Switzerland
Submitted: 2016-05-25 Revised: 2016-08-30 Accepted: 2016-10-25 Online: 2017-03-06
Biomimetic Bone-Like Apatite Coating on Anodised Titanium in Simulated Body Fluid under UV Irradiation T.C. LEE1,a, P. KOSHY2,b, H.Z. ABDULLAH1,c, M.I. IDRIS1,d 1
Department of Materials Engineering and Design, Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Batu Pahat, Johor, Malaysia. 2
School of Materials Science and Engineering, UNSW Australia, NSW 2052, Australia. a
b
c
[email protected],
[email protected],
[email protected], d
[email protected]
Keywords: Biomimetic coating, Anodic oxidation, Simulated body fluid, UV irradiation, Glancing angle X-ray diffraction
Abstract. Low temperature deposition techniques of bioceramics coatings are now being researched and developed to avoid deficiencies inherent in high temperature techniques. Biomimetic coatings are a solution-based method conducted at ambient temperature to deposit bioactive coatings on the surface. The current study aims to investigate the effect of ultraviolet (UV) irradiation on the coating of bone-like apatite on the anodised surface. High purity titanium foils were anodised with an applied voltage of 350 V, current density of 70 mA.cm-2 in mixture of 0.04 M β-glycerophosphate disodium salt pentahydrate (β-GP) and 0.4 M calcium acetate (CA) for 10 min. After anodic oxidation, UV light treatment was conducted in pH-adjusted distilled water for 12 h with ultraviolet light A (UVA) irradiation. Subsequently, the UV-treated anodised titanium foils were soaked in SBF for 7 days with/without UVA irradiation. After SBF immersion for 7 days, anodised titanium with combination of UV light treatment and UV irradiation during in vitro testing was fully covered by highly crystalline bone-like apatite at maximal thickness of 2.8 µm. This occurred mainly due to the formation of large amounts of Ti-OH groups which act as nucleation sites for bone-like apatite. This study also revealed that UV irradiation during in vitro testing is superior in promoting growth of bone-like apatite compared to UV light treatment. The suggested mechanism for bone-like apatite formation on anodised titanium under different UV irradiation conditions is illustrated in this article. The findings of this study indicated that biomimetic bone-like apatite coating with assistance of UV irradiation is an effective method in accelerating the formation of bone-like apatite. Introduction Titanium (Ti) is the most popular implant material due to its superior properties such as biocompatibility, good mechanical properties, low modulus of elasticity, and high corrosion resistance compared to other metals [1]. Enhanced bone bonding can be achieved through a spontaneous formation of bone-like apatite on their surface. Hence, a number of efforts have been undertaken using plasma spraying, physical vapour deposition (PVD) sol-gel deposition, electrodeposition, and biomimetic deposition in order to enhance the bioactivity of the titanium by coating bone-like apatite layer on the surface [2]. High temperature coating methods such as plasma spraying and PVD have drawbacks such as adhesion problem, thermal dehydroxylation and decomposition of hydroxyapatite. Low temperature techniques such as biomimetic deposition are now being researched and developed to overcome the drawbacks [2]. Biomimetic deposition is a solution-based method carried out at ambient temperature to deposit the calcium phosphate coatings on the surface. Typically, the substrates with active surfaces are immersed in a simulated body fluid and crystalline bone-like apatite layer will automatically grow on the surface [3].
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UV light has been employed to accelerate bone-like apatite formation on the surface of anodised titanium. This is because UV irradiation can alter the chemical composition of the surface by producing greater amounts of hydroxyl groups (•OH) which in turn will improve the bioactivity of the surface and promote the formation of bone-like apatite. TiO2 is an attractive photocatalyst which generates electron-hole pairs during UV irradiation. The electrons tends to reduce Ti4+ to Ti3+ while the holes are trapped by surface H2O to yield •OH groups [4, 5]. The purpose of this paper is to evaluate the effect of UV irradiation on the deposition of bone-like apatite coating on the anodised surface. Materials and Method Sample Preparation. High-purity titanium foils with dimensions 25 mm x 10 mm x 0.05 mm were hand-polished with 1200 grit SiC abrasive paper. Anodic oxidation was carried out using a programmable power supply (Genesys 600-1.3, Densei-Lambda, Japan) in a 400 mL electrolytic solution of 0.04 M β-GP (Sigma, ≥ 98.0%) and 0.4 M CA (HmbG, ≥ 90.0%) at a voltages of 350 V and a current density of 70 mA.cm-2 for 10 min at room temperature. The UV light treatment was conducted after the anodic oxidation in pH-adjusted distilled water for 12 h. The pH of distilled water was adjusted to pH 1 by adding hydrochloric acid (QRëC, ≥ 37%). UVA lamp (Philips TL-D, 15 W with peak intensity of 365 nm) was used for irradiating the specimens. In Vitro Testing. The SBF solution was prepared according to Kokubo’s recipe [6]. The foils were kept at 36.5 °C for 7 days under UVA illumination. The distance between UV lamp and samples was maintained at 15 cm to avoid heat transfer to the SBF and to ensure the temperature was stable. List of specimens and parameters are tabulated in Table 1.
Denotation UV1 UV2 UV3 UV4
Table 1 Parameters used in study Role of UV irradiation UV irradiation during in vitro UV light treatment testing X X √ X X √ √ √
Characterisation. The surface morphology of the foils was examined using field emission scanning electron microscopy (FESEM) (JFM-7600F, JEOL) at an accelerating voltage of 2.0 kV. The mineralogical composition was determined using glancing angle X-ray diffraction (GAXRD) (X'pert-Pan Analytical) at 40 kV and 40 mA, angle of incidence of 1° and a scanning step of 0.02°. The cross-sections of the sample were examined using focused ion beam (FIB) (FEI Helios NanoLab 650). Platinum was deposited on the surface of samples to avoid charging during the investigation. Gallium ions beam were accelerated at voltage 30 kV. The angle between gallium ion beam and electron beam was 50° to allow vertical cutting with FIB. Results and Discussion Fig. 1 shows the FESEM images of anodised titanium after soaking in SBF for 7 days under different UV irradiation conditions. Without UV light treatment and UV irradiation during in vitro testing (sample UV1), it was noticed that no bone-like apatite was formed owing to the lack of nucleation sites. By comparing samples UV2 and UV3, it could be inferred that UV irradiation during in vitro testing is superior in promoting growth of bone-like apatite compared to UV light treatment. For sample UV2, bone-like apatite partially covered the surface of anodised titanium after 7 days. In contrast, bone-like apatite fully covered the surface of sample UV3. It is necessary to mention that a combination of UV light treatment and UV irradiation during in vitro testing
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(sample UV4) accelerated the bone-like apatite formation on the anodised titanium in a short time. Large amounts of dense, spherical and island-like agglomerated bone-like apatite were formed on the surface of anodised titanium.
UV1
UV2 Specimen Partially covered with bone-like apatite
No formation of bone-like apatite
1µm
UV3
Fully covered with bone-like apatite
1µm
UV4 Fully covered by large amount of dense and spherical bone-like apatite
1µm
1µm
Fig. 1 FESEM images of anodised titanium after soaking in SBF for 7 days under different UV irradiation conditions The cross sectional images sample UV4 is shown in Fig. 2. It can be clearly seen that the specimens after in vitro testing are comprised of three major layers: titanium, TiO2, and bone-like apatite. The maximal thickness of bone-like apatite layers for sample UV4 after soaking in SBF for 7 days with UV irradiation is 2.8 µm.
Fig. 2 FIB micrographs of the cross sectional images for sample UV4 after soaking in SBF for 7 days The GAXRD patterns of anodised titanium after soaking in SBF for 7 days under different UV irradiation conditions are shown in Fig. 3. It is evident that the greater number and higher intensity of bone-like apatite peaks were detected on the sample with the combination of UV light treatment and UV irradiation during in vitro testing (sample UV4) and this could be attributed to the
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formation of large amounts of Ti-OH groups which act as nucleation sites to trigger and accelerate the growth of bone-like apatite on the anodised titanium surface compared to other methods [4, 5]. This mechanism is depicted in Fig. 4. UV light treatment and UV irradiation induced the photocatalysis activity on the surface of anodised titanium which activated and increased the hydroxyl •OH radicals. The •OH groups combine with Ti to form Ti-OH groups. Han et al. [4] claimed that UV-irradiated coatings contain acidic TiOH groups as well as basic TiOH groups. Acidic TiOH groups are neutralised in SBF with pH 7.4. As a result, the surface of anodised titanium becomes negatively charged due to the presence of basic Ti-OH groups. The negatively charged surface attracts Ca2+ ions in SBF and combines with Ti-OH groups to form amorphous calcium titanate. After an extended soaking time, amorphous calcium titanate reacts with phosphate ions (PO43-) and forms amorphous calcium phosphate with low Ca/P ratio of 1.40. Bone-like crystalline apatite is formed at the end of the process. The apatite nuclei grow after consuming the calcium and phosphate ions from SBF [4,5].
Fig. 3 GAXRD patterns of anodised titanium after soaking in SBF for 7 days under different UV irradiation conditions
Fig. 4 Schematic diagram of mechanism for bone-like apatite formation on anodised titanium under influenced of UV treatment and UV irradiation after soaking in SBF for 7 days
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Summary In summary, combination of UV light treatment and UV irradiation during in vitro testing is an effective method to accelerate the bone-like apatite formation on the anodised titanium within short time. Highly crystalline bone-like apatite with maximal thickness of 2.8 µm was obtained after immersion in SBF for 7 days. This is due to the fact that large amount of Ti-OH groups were formed during UV light treatment and immersion in SBF under UV irradiation. Therefore, the nucleation sites of bone-like apatite were increased and activated. Acknowledgement The authors gratefully acknowledge to Universiti Tun Hussein Onn Malaysia, Ministry of High Education Malaysia for the Research Acculturation Collaborative Effort (RACE Vot 1442) and Fundamental Research Grant Scheme (FRGS Vot 1212). References [1] [2] [3] [4] [5] [6]
R.B. Osman, M.V. Swain, A critical review of dental implant materials with an emphasis on titanium versus zirconia, Mater. 8(3) (2015) 932-958. X. Wei, C. Lindahl, J. Lausmaa, H. Engqvist, Biomimetic hydroxyapatite deposition on titanium oxide surfaces for biomedical application, in: A. George (Ed.), Advances in Biomimetics, InTech Open Access Publisher, Rijeka, 2011, 429-452. Y. Zhou, R.M. Huang, Z.D. Cui, X.J. Yang, Preparation of bone-like apatite coating on surface of Ti-25Nb-2Zr alloy by biomimetic growth method, Trans. Tianjin Univ. 15(6) (2009) 423-427. Y. Han, K. Xu, Photoexcited formation of bone apatite-like coatings on micro-arc oxidized titanium, J. Biomed. Mater. Res. 71(4) (2004) 608-614. Y. Gao, Y. Liu, L. Zhou, Z. Guo, M. Rong, X. Liu, C. Lai, X. Ding, The effects of different wavelength uv photofunctionalization on micro-arc oxidized titanium, Chi. J. Stom. 47 (2012) 359-363. T. Kokubo, H. Takadama, How useful is SBF in predicting in vivo bone bioactivity?, Biomater. 27 (2006) 2907-2915.
