Advances in Science and Technology Vol. 53 (2006) pp 32-37 Online available since 2006/Oct/01 at www.scientific.net © (2006) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AST.53.32
Size-controlled hydroxyapatite nanoparticles as self-organized organic-inorganic composite materials Jingxian Zhang1,a, Mikio Iwasa2,b and Dongliang Jiang1,c 1
Shanghai Institute of Ceramics, 1295 Dingxi Road, Shanghai 200050, China 2
AIST Kansai, Midorigaoka 1-8-31, Ikeda 563-8577, Japan
a
[email protected]
b
[email protected]
c
[email protected]
Keywords: Hydroxyapatite, cellulose, alignemnt
Abstract: Sodium salt carboxymethyl cellulose (CMC) was used to prepare HAp-CMC composites through co-precipitation process. HAp nanorods with well controlled particle size were welll aligned along the c axis in the final composites. TEM, XRD, FTIR analysis were used to characterized the samples. It was found that the carboxyl groups in cellulose might be the main guiding site for the precipitation and growth of HAp and the formation of the resulting composites. Introduction Human bone is an orgnic-bioorganic composite material consisting mainly of collagen proteins and hydroxyapatite. Collagen molecules exert a remarkable level of control over the nucleation, the size and the orientation of hydroxyapatite crystals and over the assembly of nano crystallites as building blocks into hierarchy complex structures to achieve the extraordinary durability and strength[1-3]. During these crystalline phase transitions, acidic extracellular matrix proteins that are attached to the collagen scaffold play important templating and inhibitory roles[4,5]. Presumably, the acidic groups serve as binding sites for calcium ions and align them in an orientation that matches the apatite crystal lattice [6,7]. This ability to direct the assembly of nanoscale components into controlled and sophisticated structures has motivated intense efforts to develop assembly methods that mimic or exploit the recognition capabilities and interactions found in biological systems [8,9]. To duplicate this high performance of natural bone, artificial bone materials have been produced in which organic substrates such as poly- (lactic acid), poly(L-lactide), peptide-amphiphile nanofibers, reconstituted collagen, and inorganic substrate have been used in the mineralization[10-19]. However, the most common approach to mineralization is to design the organic nanophase so it can control crystal nucleation
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and growth of the inorganic component. Samuel I. Stupp et al designed self-assembling, synthetic substitutes for collagen, which can also act as templates for hydroxyapatite crystallization [20]. In this article, we report substitutes for collagen, sodium salt carboxym ethyl cellulose, which can also act as templates for hydroxyapatite crystallization. Cellulose is the most important renewable and biodegradable macromolecule. More importantly, these CMC molecules could guide the formation of hydroxyapatite crystallites with orientations and sizes similar to those in natural bone. Experimental All chemicals, Ca(NO3)2⋅4H2O, (NH4)2HPO4, NaOH (Analytical, Osaka Kishida Chemicals, Japan) were used without further purification. Initially, the methyl cellulose sodium carboxyl salt (Analytical, Osaka Kishida Chemicals, Japan) was dissolved in deionized water in the presence of 0.0334M Ca(NO3)2⋅4H2O as the Ca source, the solution pH was adjusted to 11 using NaOH. 0.02M aqueous (NH4)2HPO4 solutions were also prepared at pH 11 using NaOH. after mixing, (NH4)2HPO4 solutions was added to Ca(NO3)2⋅4H2O drop wise under stirring, the solution temperature were kept at 80°C using water bath. After finishing, the mixture was aged at room temperature for 0-120 h before filtration. The powder was freeze dried subsequently. The crystal structure of the calcium phosphate phase in the composites was determined by X-ray diffraction using the Rigaku RAD-C system with Cu Ka radiation generated at 40 kV and 20 mA. Interactions between CMC and nano HAp powders were determined using the Fourier transform infrared spectrometer (FT/IR-350, JASCO Corp., Japan) in the 400–4000cm-1 region. The microstructure of the samples was observed by transmission electron microscopy (TEM). Results and discussions XRD characterization. The development of HAp-CMC composite directly depends on the CMC/HAp mass ratio. The higher the CMC/HAp ratio, the more turbid the solution and the less the precipitation after aging. The XRD diffraction pattern of HAp-CMC composite (CMC/HAp ratio 0.38) and the pure HAp grains were shown in Fig. °1. Due to the ultrafine nature of the HAp powder, there is an extensive degree of peak broadening in the X-ray diffraction pattern. The HAp crystals that were synthesized without CMC revealed characteristic peaks in the XRD pattern that were consistent with JCPDS files for pure HAp.
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The crystallite size was calculated by Scherrer’s equation. The crystallinity (Xc), corresponds to the fraction of crystalline apatite phase, was deduced according to reference [21]. It was shown that the CMC content had a direct influence on the grain size and crystallinity degree(χc ) of HAp: The higher the CMC/HAp ratio, the lower the HAp grain size and the crystallinity degree. However, the ageing time has no obvious influence on the grain size and χc of HAp, Table 1. Table 1 CMC content on the HAp grain size and cystallinity With CMC
CMC/HAp ratio[g/g] 0.383764 0.354635 0.350128 0.349508
Without CMC
20
25
30
35
40
45
HAp grain Crystallinity de size [nm] -gree of HAp 228.2207 247.2112 292.9483 306.7187
0.113486 0.144239 0.240022 0.275486
50
o
2θ ( C)
Fig. °1 XRD pattern of HAp-CMC composites Microstructure The morphology and microstructure of obtained HAp powder were observed by TEM, see Fig. °2. Nano HAp rods with the length around 22nm were well aligned along the c axis and form big particles.High resolution micrograph also showed the well aligned HAp nanocrystals along c axis, Fig. °2(b). These self-assembly of HAp nano rods with CMC would indicate the existence of interactions between CMC molecules and nano HAp particles.
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(a) HAp–CMC composites
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(b) Well aligned HAp nano rods
Fig. °2 TEM observations of well aligned HAp nano crystals FTIR analysis. Fig. °3 shows the FTIR spectra of various MC/ HAp composites. For pure HAp powder, the band at 1042 cm-1 are the characteristic bands of phosphate stretching vibration, while the bands at 603 and 569 cm−1 are due to phosphate bending vibration. The weak carbonate bands at 1454, 1416, and 877 cm−1 were also observed, HAp crystals prepared by the precipitation
902 877 1641 1454 1416
1328 1630 1616
1423
603 569 1063 1115
CMC HAp-CMC HAp
1385 1040
2000 1800 1600 1400 1200 1000
800
600
400
-1
Wave number (cm ) Fig. °3 FTIR spectra of HAp in the presence and absence of CMC
method are believed to contain a small amount of carbonate ions. As shown in Fig. °3, For pure CMC, the peak at 1616 and 1423 cm-1 can be assigned to the the C–O asymmetric and symmetric stretching vibrations, respectively[22]. However, in the HAp-MC powders, the two peaks moved to 1630 and 1385 cm-1. This might be due to the interaction between CMC and HAp.
Fig. °4 CMC structure Mechanism. The structure of CMC is shown in Fig. °4. This formation of well aligned HAp nano particles is presumably templated by CMC, similar to the case in the natural bone that guided by collagen. The presence of carboxyl in carboxymethylated cellulose affords attractive sites for the Ca2+ ions and subsequently guides the orientation of HAp nano grains developed, as shown in Fig. °5. Initially, the CMC long chains in the solutions will dissociate and assume a stretched configuration at high pH.
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The negatively charged CMC can simultaneously chelate Ca ions in the solution to form a Ca-CMC complex. Then, a cluster of critical size can be formed by adsorbing further phosphates and Ca ions, and/or another CMC on the Ca-CMC complex; the resultant three-dimensional cluster molecules could act as nucleus for HAp crystals. This development of HAp grains will then grow along the chain direction of CMC. In this way, the alignment of HAp grains was resulted. These HAp-CMC composites might be applicable for bottom up approach for the development of artificial human bone structure. This formation of CMC-HAp composite can also find application in a wide range of area such as scaffold for bone growth and drug release.
Ca(NO3)2⋅H2O
(NH4)2HPO4
Ca2+
HAp grains
Fig. °5 Schematic route for formation and alignment of HAp grains along CMC chains Conclusions CMC molecules were found to be effective to control the particle size of HAp and the subsequent alignment of them. This might be due to the carboxyl groups in CMC which can attract Ca2+ ions and thus guide the growth of HAp grains along the chain. Acknowledgement The author was grateful for Dr. Qiang Xu for the help in FTIR measurement. References [1] [2] [3] [4]
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Size-Controlled Hydroxyapatite Nanoparticles as Self-Organized Organic-Inorganic Composite Materials 10.4028/www.scientific.net/AST.53.32 DOI References [5] R.H. Clark, A.A.Campbell, L.A. Klumb, C.J. Long and P.S. Stayton: Calcif. issue Int. Vol.64 (1999), p516 doi:10.1007/s002239900642 [15] M. Kikuchi, S. Itoh, S. Ichinose, K. Shinomiya and J. Tanaka: Biomaterials ol.22 (2001), p1705 doi:10.1055/s-2001-12700
