Synthesis and Characterization of Porous Silica Gels for Biomedical Applications

Transcription

1 Trends Biomater. Artif. Organs, Vol 18 (2), January Synthesis and Characterization of Porous Silica Gels for Biomedical Applications U. Vijayalakshmi, A. Balamurugan and S. Rajeswari Department of Analytical Chemistry, University of Madras Guindy Campus, Chennai Abstract: In the past few years, the sol-gel process has been increasingly employed for the processing of bioactive ceramics and gels. The growing interest in these systems is based on their ability to induce in-vitro formation of a surface crystalline Hydroxyapatite (HAP) rich layer on the material surface. This behavior is considered of a bio-apatite layer on to the stainless steel material surface. In the present investigation porous silica gels with high surface areas were prepared from tetraethylorthosilicate (TEOS) and Polyacrylic acid (PAA) of high molecular weight in acid media by a sol-gel method. PAA content and aging temperatures were varied in order to obtain different structures. The synthesized samples were characterised using FT-IR, XRD, and TG-DTA. Keywords : Porous silica gel, sol-gel, FTIR Introduction: Biomaterials play a key role in the replacement of body parts and restoration of human anatomical structures, among these; the metallic materials occupy a leading role besides the availability of ceramics and polymers for hard tissue replacement. The material suited for bone joint replacement in the human body should satisfy features like tissue tolerance, resistance to corrosion or wear, biocompatibility and not producing any adverse body reactions (1-2). Certain materials like metals and alloys, ceramics and polymers are able to satisfy these necessary features. Now days, polymeric materials are currently attracting great interest as new bone substitutes and they provide good combinations of toughness and elasticity. For example, polymers such as ultrahigh molecular weight polyethylene (UHMWPE), poly methyl methacrylate (PMMA), poly lactides, polypropylene, poly acrylic acid, poly acetal and poly sulfone are commonly used. The essential requirement for artificial materials to bond to living bone is believed to be the formation of a biologically active bone like apatite layer on their surfaces in the body. Only limited kinds of materials, such as bioglass, sintered hydroxyapatite, and silica gel are generally known to posses the ability to form bone like apatite in the body environment. An apatite layer can form on pure silica gel soaked in simulated body fluid, the rate of formation depends on solution parameters and aging temperature of the silica gel. Hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2, commonly referred to as HAP, has attracted wide spread interest from both the orthopedic (3) and dental fields due to its excellent biocompatibility and tissue bioactivity properties. Hydroxyapatite 101

2 Synthesis and Characterization of Porous Silica Gels for Biomedical Applications is particularly attractive as a biomedical surface modifier used to increase the rate and longevity of implant fixation. Calcium phosphate ceramics and Hydroxyapatite in particular could be used to manufacture ideal biomaterials due to their biocompatibility and Osseo integration, as well as being the materials most similar to the mineral component of the bone. Calcium phosphate compounds such as HAP or β-tcp Ca 3 (PO 4 ) 2 have recently been synthesized in the form of bulk powders or as coatings by sol gel processes (4). Ceramic materials synthesised by sol gel route exhibit many advantages over the other techniques such as solid state reaction, co-precipitation, hydro thermal method) and the advantages like high product purity, homogeneous composition and low synthesis temperature and its applicability for surface coatings. The sol gel route allows a complete coverage of the exposed surface area by a thin layer for medical devices (5). It was confirmed that a pure silica gel prepared by hydrolysis and condensation of Tetraethylorthosilicate formed the apatite on the surface in simulated body fluid (6). Silanol groups abundant on the silica gel, induce the apatite nucleation in the body environment (7). Therefore, if polymeric materials can be modified with silanol groups on their surfaces, they are expected to posses the apatite forming ability. In the present investigation, the synthesis and evaluation of porous silica gel sample is described. For this purpose, tetra ethyl ortho silicate and acrylic acid were used as silica source and inert polymer respectively. The growth of Hydroxyapatite on silica gel surface was evidenced through the analytical techniques like Fourier transform infra red spectroscopy (FT-IR) to determine the functional groups present, X-Ray diffraction (XRD) to determine the phases of developed surface. Experimental procedure: Porous silica gels were prepared by acid hydrolysis and poly condensation of Tetra ethyl ortho silicate (TEOS) through two routes. Tetra ethyl ortho silicate was hydrolysed with deionsed water and nitric acid to produce gels having 9.5 molar ratio. The synthesis was carried out with an excess of water with respect to the stoichiometric relationship and catalysed with nitric acid at a ph of 2. Strong stirring was applied until homogeneous transparent solutions were obtained. (Route I). Acrylic acid has carbonyl groups, the chelation process was easily produced, and the polymer was capable of bonding or complexing with other materials. Variations in the concentration of acrylic acid could produce a range of different characteristics. Acrylic acid was taken and heated at 70 o C. (Route II). The Tetra ethyl ortho silicate functionalized Acrylic acid was prepared by polymerization using Benzoyl peroxide as an initiator. The carboxyl group from acrylic acid was cross linked through silicate ions (8). The AA content was varied. The solutions Tetra ethyl ortho silicate and Acrylic acida were added to the acidulated water and the recipients were sealed. The solutions were then heat treated at different synthesis temperature (40, 50 and 90 o C) during 20 hours in the sealed recipients in order to avoid evaporation. Gelation and aging of samples were carried out and different structure was obtained. In order to eliminate the organic phase, the gel samples were soaked in a solution of water and ethanol (50:50 in volume) and the solutions were renewed every 2 hours and 102

3 U. Vijayalakshmi, A. Balamurugan and S. Rajeswari repeating the washing step three times. After this solvent-exchange treatment, the gel was dried at 80 o C in an oven for 2 hours. Finally, the dried gel was heat treated at temperature 400 o C at a heating rate of 1 o C/min and held for 2 hours and then allowed to cool to room temperature. Figure 1 shows a schematic of the experimental procedure followed to produce silica films. Characterisation of silica gel Fourier transform infrared spectroscopy (FT-IR): The FT-IR spectral studies were conducted using Hitachi spectrophotometer by KBr disks in the range of cm -1 X-Ray Diffraction (XRD): Sintered silica gel granules were powdered and XRD spectrum was recorded using SEIFERT JSO-DEBYEREX-2002 using a step size of 0.02 o, scan rate of 1 o Per minute and a scan range between 0-60 o 2 theta in flat plate geometry with Cu radiation. Soaking in simulated body fluid (SBF): The silica gel obtained by the above method was soaked in simulated body fluid with ion concentrations nearly equal to those of human blood plasma in order to examine the apatite formation on its surface. The SBF was prepared by dissolving reagent grade chemicals like NaCl, KCl, CaCl 2, NaHCO 3, glucose, MgCl 2.6H 2 O, Na 2 HPO 4, KH 2 PO 4, MgSO 4.7H 2 O into double distilled water and buffering at ph 7.3 at 37 o C. It has been confirmed that this fluid can produce apatite formation on the surfaces of silica gel precisely in in-vitro.the sample surface was analysed by FT-IR, XRD. Results: Fourier transform infrared spectroscopy: Fig 2 (a,b,c) shows FT-IR spectra of silica gel at various concentrations of Acrylic acid and various ageing temperatures (40,50 and 90 o C). The peaks at 470 and 810cm -1 are ascribed to the Si-O-Si bending vibration, that at 1100 cm -1 to the Si-O stretching vibration and that at 960 cm -1 to the Si-OH stretching vibration and a strong peaks at about cm -1 after hydrolysis & polycondensation of TEOS with water are ascribed to hydroxyl groups on the surface of the silica. Fig 1: Flow chart of silica gel preparation by the sol gel route: 2(a) 103

4 Synthesis and Characterization of Porous Silica Gels for Biomedical Applications 2(b) 2(c) Fig 2: FTIR spectra of silica gel at various acrylic acid and aging temperatures (2a-40 C, 2b- 50 C and 2c-90 C) Fig 4: XRD pattern of silica gel surfaces at various aging temperatures before soaking in SBF Fig 3 : FT-IR spectra of the silica gel sample surfaces soaked in SBF showed apatite formation after 168 hours of immersion. X-Ray Diffraction: Figure 4 shows thin film X-Ray diffraction patterns of the silica gel samples at various ageing temperatures (40, 50 and 90 o C) before soaking in SBF and Figure 5 shows the same after soaking in SBF for 168 hours. From figure, it can be seen that sintered silica gels form an apatite phase on their surfaces in the simulated body fluid. Fig 5: XRD pattern of silica gel surfaces soaked in SBF for 168 hours Discussion: The synthesis is based on the preparation of silica gels as an inorganic polymer from the hydrolysis and polycondensation of silicone alkoxides with the presence of inert polymer. In this study, various ageing temperatures like 40, 50 and 90 o C were studied and the acrylic 104

5 U. Vijayalakshmi, A. Balamurugan and S. Rajeswari acid content was also varied in order to obtain different characteristic. At lower temperature 40 and 50 o C, the acrylic acid content was low compared with the higher temperature 90 o C (AA-High). Sol gel process is a low temperature process and it can be able to form a gel at lower temperature itself, but at higher temperature the gel formation was low. So by increasing the acrylic acid polymer at higher temperature, the formation of gel was fast compared with the low temperature because of low acrylic acid content. So acrylic acid play a major role in this study, the yield and gel formation was high. Li et al proposed that the high concentration of Si-OH groups on the sample surface could promote HAP nucleation. Pereire et al reported that those silica gel samples with OH - groups are suitable for inducing invitro HAP formation in SBF. The HAP nucleation rate could be related to the total amount of hydroxyl concentration on the surface (9). Finally, it was observed that the higher acrylic acid increases the polymerization rate and shorter the gelation time. Conclusion: The composite of Acrylic acid/tetra ethyl ortho silicate was synthesised by sol gel method. Its activity was assessed using SBF and crystalline HAP was successfully formed on its surface after soaking for 168 hours at ph 7.3. The samples were sintered at 400 o C for different ageing temperature. No XRD peaks for HAP were ascribed to the silica gel before soaking. A strong peak at about 11 o was attributed to original silicone. The peaks due to crystalline apatite were formed on the surface of silica gel after immersion in SBF for 168 hours with higher amount of Acrylic acid at ageing temperature (90 o C). Further experimental study was needed for identifying the silanol groups responsible for the apatite nucleation. References: 1. L.L. Hench, J. Am. Ceram. Soc., Vol. 74, 1487 (1991). 2. K. E. Tanner, R. N. Downer, W. Bonfield, Br. Ceramic.Trans, Vol. 93, 104 (1994). 3. R.J. Furlong, J. F. Osborn, Fixation of hip prosthesis by hydroxyapatite ceramic coatings, J. Bone. Joint. Surg., Vol. 5, 741 (1991) 4. S. W. Russell, K. A. Luptak, T. Carlos, A. Suchicital, T. L. Alford, V. B. Pizziconi, Chemical and structural evolution of sol-gel derived hydroxyapatite thin films under rapid thermal processing, J. Am. Ceram. Soc., Vol. 79(4), 837 (1996). 5. D. M. Liu, Ageing effect on the phase evolution of water based sol-gel hydroxyapatite, Biomaterials, Vol. 23,1227 (2002). 6. L. P. Ohtsukic, T. Kokubo, K. Nakanishi, N. Soga, T. Nakamura, T. Yamamuro, Apatite formation induced by silica gel in a simulated body fluid, J. Am. Ceram. Soc., Vol. 75, 2094 (1992). 7. A. Oyane, K. Nakanishi, H. M. Kim, F. Miyaji, T. Kokubo, N. Soga, T. Nakamura, Sol-gel modification of silicone to induce apatite forming ability, Biomaterials Vol. 20,79 (1999). 8. J.H.U. Brown, J. F. Dickson, III, Advances in Biomedical engineering, Vol. 73(3), 141 (1973). 9. M.M. Pereira, A.E. Clark, L.L. Hench, Effect of texture on the rate of hydroxyapatite on gel-silica surface, J. Am. Ceram. Soc., Vol. 78(9), 2463 (1995). 105