Novel substituted hydroxyapatites

Abstract

Hydroxyapatite (HAp) has been widely used as a bone replacement material for biomedical applications, in particular as a bone graft and coating for implants, because of its similarity in composition and crystal structure to natural bone. The present study aims to develop of a novel method to prepare (unsubstituted and substituted) HAp by hydrolysis of monocalcium phosphate (MCP) using calcium hydroxide (Ca(OH)₂). Different techniques were used to characterize the prepared samples such as SEM, FTIR, XRD and ICP-MS. The presence of an apatitic phase in the sintered unsubstituted powders prepared by the hydrolysis of MCP/Ca(OH)₂ was confirmed by FTIR spectra, through detecting the fundamental vibrational modes of PO₄³⁻ group as well as by observation of the typical HAp lattice OH vibrations at 3572 and 631 cm⁻¹, so indicating formation of crystalline material. The XRD diffraction patterns of the sintered unsubstituted HAp powders were in a good agreement with the HAp patterns reported in the current literature, the standard HAp (reference card number 01-074-9780) as well as commercial HAp (Fluka) and showed the characteristic peaks of the HAp phase with a slight presence of the β-TCP phase that would have formed from the sintering process used to crystallise the HAp powders at 900 ˚C. The ICP-MS analyses of unsubstituted HAp powders showed that the Ca/P mole ratios of the unsubstituted HAp materials produced was 1.40, which is lower than the theoretical value of stochiometric HAp (1.67). This result can be ascribed to the substitution process of calcium ions by sodium ions in the HAp crystal lattice as a result of using sodium hydroxide to adjust pH in the reactions leading to the formation of HAp. The value of the (Ca+Na)/P when computed using elemental data obtained from the ICP-MS analyses was found to be 1.65 which is very close in value to of the value expected for stoichiometric HAp (1.67). SEM images showed that the sintered samples of unsubstituted HAp powders consist of particles with fine grains, spheroidal in shape associated with an irregular distribution of particle sizes. The degree of crystallinity and the numerical value of the crystallite size of the sintered unsubstituted HAp materials were 82.6% and 549.8 Å, respectively. The Rietveld refined values of lattice constants of the sintered unsubstituted HAp materials were a= 9.421±0.003 and c= 6.882±0.005 Å. By means of the novel hydrolysis route (hydrolysis of MCP and Ca(OH)₂), three substituted “MHAp” materials were prepared with the following chemical formula: Ca₁₀₋ₓMₓ(PO₄)₆(OH)₂ (M=Zn, Sr and Cu, x=0.5, 1.0 and 1.5). FTIR spectra showed that there was: • a presence of apatite phase in the prepared powders. • a clear relation between the amount of M ions, and the intensities of OH⁻ vibration modes at 3572 cm⁻¹ (stretching) and 630 cm⁻¹. • A clear shift of phosphate bands in the unsubstituted HAp at 572 and 604 cm⁻¹ to a lower wave number due to substitution of Sr²⁺ ions. This shift increases as the amount of Sr ions increases. Another shift of the OH librational band from 630 to 612 cm⁻¹ was also observed with an increase in the percentage of Sr²⁺ ions in the substituted HAp. The phase purity, crystallinity, crystallite size and lattice parameters of the prepared MHAp materials varied with increasing substitution levels. SEM images showed that ZnHAp and CuHAp materials consisted of agglomerated crystals which are spheroidal in shape. Spheroidal particles were seen in 0.5 SrHAp and 1.0 SrHAp samples, but a combination of spheroidal and rod like particles was recorded in the case of the 1.5 SrHAp materials. The results of ICP-MS measurements displayed clearly the presence of these ions (Zn²⁺, Sr²⁺ and Cu²⁺ ions) in the HAp samples but did not offer explicit proof of substitution. The measured (actual) wt.% of Zn²⁺, Sr²⁺ and Cu²⁺ ions that that were detected by ICP-MS analysis was lower than the theoretical value indicating how difficult it is for the small ions such as Zn²⁺ and Cu²⁺as well as the large ions such as Sr²⁺to be introduced and hosted by the HAp sample. Also, in the present study different systems of cationic substituted HAp, namely 1% RbHAp (1 wt.% Rb⁺), 1%EuHAp (1 wt.% Eu³⁺) and 1,3 and 5% ScHAp (1, 3 and 5 wt.% Sc³⁺), were prepared by using two different methods (precipitation and hydrolysis of MCP/Ca(OH)₂). The results of ICP-MS analysis showed that the presence of Rb⁺, Eu³⁺ and Sc³⁺ ions in the HAp crystal was very low. In the case of the Sc-substituted HAp, the level of substitution was negligible. Various systems of anionic and co substituted HAp, namely 1, 3 and 5% NbHAp, 1% B₄O₇HAp, bromapatite, sulfoapatite and NaClHAp were also prepared by using different preparation methods (precipitation, hydrolysis of MCP/Ca(OH)₂ and ion exchange routes). There was found to be a reduction of the intensity of the stretching mode of the OH⁻ group at 3572 cm⁻¹ due to the replacement process of phosphate and/or hydroxyl groups by monovalent and bivalent ions( Br⁻, S²⁻ and B₄O₇²⁻). The mechanical strength and the particle size of specific systems of unsubstituted HAp, substituted and co substituted HAp powders prepared by the hydrolysis process of MCP and Ca(OH)₂ were also studied. The results of the mechanical properties in the present investigation showed clearly that particle size plays a fundamental rule in the value of mechanical strength, since an enhancement in the mechanical strength was recorded because of the reduction in particle size that occurred in some preparations.