Sol gel preparation of bioactive apatite films

Transcription

1 Surface and Coatings Technology 167 (2003) Sol gel preparation of bioactive apatite films a, b a a a a a Wenjian Weng *, Sam Zhang, Kui Cheng, Haibo Qu, Piyi Du, Ge Shen, Jun Yuan, Gaorong Han a a Department of Materials Science and Engineering, Zhejiang University, Hangzhou , PR China b School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore , Singapore Abstract The surface of biomedical metallic implants covered by a bioactive apatite film can create bioactivity of the implant and shorten healing time. In this work, apatite films on Ti6Al4V were prepared by sol gel route using Ca(NO 3) 2, P2O5 and HPF6 as the precursors, in vitro evaluations of the resulting hydroxyapatite (HA) and fluorapatiteyhydroxyapatite solid solution (FHA) films were done in Kokubo s simulated body fluid and citric acid modified phosphate buffer solution (CPBS). HPF6 showed a good reagent for the incorporation of fluorine into apatite films. The FHA film demonstrated to have good bioactivity, and to have better stability in CPBS and higher adhesion strength than the HA film. When fluorine is incorporated into the film, an increase in crystallinity of the apatite film and a decrease in intrinsic solubility of the FHA film could make significant contributions to the improvement in the stability; the thermal expansion coefficient of the FHA film getting closer to that of Ti6Al4V could be responsible for the increase in adhesion strength Elsevier Science B.V. All rights reserved. Keywords: Sol gel; Fluorapatiteyhydroxyapatite; Film; Dissolution 1. Introduction Sol gel technique has been well developed and employed to prepare various materials because the technique has the main advantages of the easy control of chemical composition and low temperature synthesis w1x that are very important for film preparation. Recently, the technique has been used to prepare bioactive apatite powders and films w2 4x. The sol gel derived apatite powders have fine particle size, resulting in a low densification temperature and a ceramic with a good microstructure that can be accepted by host tissues. The sol gel derived apatite films or coatings show to have a tailored chemical composition, crystallinity, microstructure and a good adhesion strength. Sol gel technique shows a great promising potential for preparing bioactive materials. For medical applications, bioactive apatite films are of great importance because the bioactive film coated titanium alloy implants can combine the mechanical advantages of the metal with biological affinity of apatite *Corresponding author. Tel.: q ; fax: q address: wengwj@zju.edu.cn (W. Weng). to natural tissue, as a result, healing time can be shortened w5x. However, an apatite film functioned as a bioactive film on metallic implants should have a reasonably good stability to withstand dissolution of the film while in contact with the body fluid w6x, rough or porous surface morphology to increase joint area with natural bone w7x, and good adhesion strength to maintain a strong bridge to connect the implant with bone tissue w8x. Hence, it is a prior task to make the films meet the requirements for any film preparation method. Sol gel technique is versatile and capable of controlling preparation process, eventually high quality bioactive films are able to be obtained. Many factors can affect the morphology, solubility and adhesion strength of the sol gel derived apatite film. The surface morphology of the films can be controlled by sol gel processing parameters such as different precursors w9x, post heat-treatment w10x and addition of chemical additives w11x. The film solubility can be lowered by enhancing the apatite phase to develop well w12x and incorporating fluorine into the apatite phase w13x. To roughen surface w4x and to add another oxide w8x can improve adhesion strength between films and substrates. Chemical bonding w8x and /03/$- see front matter 2002 Elsevier Science B.V. All rights reserved. doi: /s (02)

2 W. Weng et al. / Surface and Coatings Technology 167 (2003) Table 1 The nominal compositions of the dipping sols Sol No. CayP HPF6yCa Remark (molar ratio) (molar ratio) S The dipping sol derived and 600 8C heat-treated film, called HA film SF y10 The dipping sol derived and 600 8C heat-treated film, called FHA film low thermal stress residue w14x in interfaces are considered to make great contributions to obtaining high adhesion strength. This work aims to prepare high quality bioactive films on Ti6Al4V by sol gel preparation in Ca(NO ) Ø4H O P O based system and by incorporation of fluorine into 2 5 the films, the resulting films were evaluated in simulated fluids for bioactivity and solubility. 2. Experimental 1 M ethanol solutions of Ca(NO 3) 2Ø4H2O (AR) and PO 2 5 (AR) were prepared as Ca and P precursors, respectively w15x. HPF 6 (40 wt.% H2O, AR) was chosen as a fluorine containing reagent. For preparation of a dipping sol to form fluorine incorporated films, HPF 6 was added into the mixed solution of Ca and P precursors, a slightly cloudy mixture was formed, and then the mixture was refluxed for 12 h. The refluxed mixture was stable, no sedimentation in the sol appeared until 6 months. The nominal compositions of two dipping sols are listed in Table 1. For preparation of apatite films, the cleaned Ti6Al4V substrates were put into the dipping sols and withdrawn at a rate of 8 cmymin. The films were dried at 150 8C for 15 min and heated at 600 8C for 15 min for each run. The dipping-coat procedure was repeated for 5 times. The films underwent in vitro evaluations for characterizing the influences of fluorine incorporation on bioactivity and stability. Kokubo s SBF w16x was used to test bioactivity with refreshing the fluid every 24 h, and citric acid modified phosphate buffer solution (CPBS) w17x was used for solubility or stability, the composition of CPBS is listed in Table 2. The weights of the films before and after soaking were obtained in an electronic balance (Mettler-Toledo PB602-N) with accuracy of " g. The film phase was measured by X-ray powder diffractoscopy (Rigaku, DyMAX-RA), the existence of fluorine element was checked by X-ray photoelectron spectroscopy (VG ESCALAB MKII), and the adhesion strength of the films was determined by a Kokubo s method w18x. 3. Results and discussions The 600 8C heat-treated films (HA and FHA) had a pure apatite phase (Fig. 1a and c), however, FHA film had stronger XRD intensities; The lattice parameters of the apatite films are calculated to be as9.4197a and cs A for HA film, as A and cs A for FHA film, i.e. the fluorine incorporation decreases a value. The XPS profiles of the films show the existence of fluorine element in FHA film (Fig. 2), and the FyCa molar ratio is 0.16"0.02. These data obviously indicate that the apatite in FHA film is FHA solid solution, while the apatite in HA film has been proven to be HA w15x. In addition, HPF6 demonstrates a new and effective reagent for incorporation of fluorine into Table 2 Composition of CPBS Constituent amount (gyl) NaCl KCl Na2HPO4Ø2H2O KH2PO4 Citric acid ph Fig. 1. XRD patterns of 600 8C heated-films before and after 14 day soaking: (a) unsoaked HA film; (b) SBF soaked HA film; (c) unsoaked FHA film; (d) SBF soaked FHA film.

3 294 W. Weng et al. / Surface and Coatings Technology 167 (2003) Fig. 2. XPS profiles of the apatite films: (a) HA film; (b) FHA film. sol gel derived phosphate materials compared with other regents such as trifluoroacetic acid w19x. When HA and FHA films were soaked in SBF, the reaction between the films and the fluid led to weight gain with time (Fig. 3) and increase in apatite XRD intensities (Fig. 1b and d). This demonstrates an increase in film thickness because of an extra apatite layer formation as the usual behavior in SBF w20x, and also implies that the partial substitution of fluorine for OH in HA does not affect the ability to induce apatite growth, which is regarded as a criterion for bioactivity. When HA and FHA films were soaked in CPBS, the films tended to dissolve (Fig. 4), the dissolution amount depended on both soaking time and ph value of CPBS. HA film had a larger dissolution amount than FHA film for any time and at any ph value. When the ratio (DHA filmyd FHA film) of the 14-day soaked HA and FHA film dissolution amount is used to measure the difference between the two films after soaking, the ratio varies with ph value in CPBS. CPBS with a high or low ph value causes relatively small DHA filmydfha film value, 1.4 at ph 6.95 (Fig. 4a) and 1.1 at ph 4.70 (Fig. 4c), while CPBS with a moderate ph 6.05 results in a large DHA filmydfha film value of 5. Since the apatite films tend to be stabilized in an aqueous medium with a high ph value w13x, the soaking at ph 6.95 does not dissolve the films much, leading to having low dissolution amounts for the both films and small DHA filmy D value (Fig. 4a); When soaked in acidic solu- FHA film tion, apatite dissolves because it is a basic oxide, CPBS with ph 4.70 seems acidic enough and dissolves the films almost completely for 14 day soaking, resulting in large dissolution amount for the two films (Fig. 4c) and small DHA filmydfha film value; CPBS with a moderate ph 6.05 makes HA film dissolution amount obviously much larger than FHA film (Fig. 4b) and a consequentially large DHA filmydfha film value. Therefore, the CPBS soaking may be a suitable condition to compare the dissolution ability of the films by avoiding the influences from acidic and basic conditions. The better stability of FHA film in CPBS than HA film could be attributed to that the incorporation of fluorine improves crystallinity (Fig. 1c), and FHA solid solution has a low intrinsic solubility due to a more compact apatite lattice structure. In addition, the two films had reasonably good adhesion strength, 20.6 MPa for FHA film and 15.0 MPa for HA film, which could be ascribed to the formation of chemical bonding in interfaces because the films have passed 600 8C heat-treatment. It is well known that smaller difference of thermal expansion coefficients between films and metallic substrates leads to lesser thermal stress residue and higher adhesion strength. The incorporation of fluorine into apatite can decrease the thermal expansion coefficient from 15=10 for HA to 9.1=10 for fluorapatite (the complete substitution of F for OH in HA), therefore, the better adhesion strength of FHA film could arise from lesser thermal stress residue since the FHA film has a closer thermal coefficient to that of Ti6Al4V (8.8=10 ) than HA film. 4. Conclusion HPF with Ca(NO ) and P O ethanol system can be used to prepare FHA solid solution films on Ti6Al4V, Fig. 3. Weight gains of the films against soaking time in SBF: (a) HA film; (b) FHA film.

4 W. Weng et al. / Surface and Coatings Technology 167 (2003) Fig. 4. Weight loss of the films against soaking time in CPBS with different ph values: (a) at 6.95; (b) at 6.05; (c) at and works well for the incorporation of fluorine. The resulting FHA film with FyCa ratio of 0.16 shows to have good bioactivity, better stability in CPBS and adhesion strength than HA film. CPBS with ph of ;6 could be suitably used to evaluate, to a maximum degree, the difference in intrinsic solubility between FHA and HA films because the ph value is located at a range at which neither apatite stability nor dissolution is enhanced. Acknowledgments The authors would like to thank Zhejiang provincial natural science foundation of China (598061), the research fund of the doctoral program of higher education ( ) and Nanyang Technological University (Singapore) research fund (SDS17y2000) for supporting the work. References w1x L.L. Hench, J.K. West, Chem. Rev. 90 (1990) w2x C.M. Lopatin, V. Pizziconi, T.L. Alford, T. Laursen, Thin Solid Films 326 (1998) w3x E. Tkalcec, M. Sauer, R. Nonninger, H. Schmidt, J. Mater. Sci. 36 (2001) w4x D. Liu, Q. Yang, T. Troczynski, Biomaterials 23 (2002)

5 296 W. Weng et al. / Surface and Coatings Technology 167 (2003) w5x S. Stea, M. Visentin, L. Savarino, et al., J. Mater. Sci. Mater. Med. 6 (1995) w6x S. Overgaard, M. Lind, K. Josephsen, A.B. Maunsbach, C. Bunger, K. Soballe, J. Biomed. Mater. Res 39 (1998) w7x K. Hayashi, T. Mashima, K. Uenoyama, Biomaterials 20 (1999) w8x L.D. Piveteau, B. Gasser, L. Schlapbach, Biomaterials 21 (2000) w9x W. Weng, J.L. Baptista, Biomaterials 19 (1998) w10x C. You, S. Oh, S. Kim, J. Sol Gel Sci. Tech. 21 (2001) w11x W. Weng, G. Han, P. Du, G. Shen, Mater. Chem. Phys. 77 (2) (2002) w12x P.L. Tranquilli, A. Merolli, O. Palmacc, C. Gabbi, A. Cacchioli, G. Gonizzi, J. Mater. Sci. Mater. Med. 5 (1994) w13x J.C. Elliot, Sturcture and Chemistry of the Apatites and Other Calcium Orthophosphates, Elsevier Science, Amsterdam, 1994, pp w14x Y.C. Yang, E. Chang, Biomaterials 22 (2001) w15x W. Weng, J.L. Baptista, J. Am. Ceram. Soc. 82 (1999) w16x T. Kokubo, H. Kushitani, S. Sakka, T. Kitsugi, T. Yamamuro, J. Biomed. Mater. Res. 24 (1990) w17x H. Robson, D.Q.M. Craig, D. Deutsch, Int. J. Pharmaceutics 195 (1995) w18x M. Tanahashi, T. Yao, T. Kokubo, et al., J. Am. Ceram. Soc. 77 (1994) w19x K. Cheng, G. Shen, W. Weng, G. Han, J.M.F. Ferreira, J. Yang, Mater. Lett. 51 (2001) w20x S. Rhee, J. Tanaka, Biomaterials 20 (1999)