生物陶瓷英文文献

Fabrication and Mechanical Properties of Dense/Porous β-Tricalcium
Phosphate Bioceramics
Faming Zhang
1, a , Jiang Chang 1, b*, Jianxi Lu 1, 2, c , Kaili Lin 1, d 1 Biomaterials and Tissue Engineering Research Center, Shanghai Institute of Ceramics, Chinese
Academy of Sciences, Shanghai 200050, China 2 Shanghai Bio-lu Biomaterials Company, Shanghai 200335, China a star.zhang@, b* jchang@,c ir2bberck@,d lklsic@
Keywords: Bioceramics, calcium phosphate, bone regeneration, weight bearing sites
Abstract: Attempt t o increase the mechanical properties of porous bioceramics, a dense/porous structured β-TCP bioceramics that mimic the characteristics of nature bone were fabricated. Experimental results show that the dense/porous structured β-TCP bioceramics demonstrated excellent mechanical properties with compressive strength up to 74 MPa and elastic modulus up to 960 MPa, which could be tailored by the dense/porous cross-sectional area ratio obeying the rule of exponential growth. The interface between the dense and porous bioceramics is connected compactly and tightly with some micropores distributed in the matrix of both porous and dense counterparts. The dense/porous structure of β-TCP bioceramics may provide an effective way to increase the mechanical properties of porous bioceramics for bone regeneration at weight bearing sites.
Introduction
Various methods for bone defect treatments have been developed using biological or synthetic grafts. The synthetic alternatives are promising grafts for their unlimited availability and without risk of disease transmission [1]. Calcium phosphate bioceramics, especially hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP), have been extensively explored as bone grafts due to their compositions are similar to the inorganic components of nature bone [2]. The β-TCP bioceramics is well known as a biodegradable material demonstrated clinical efficacy. The porous β-TCP bioceramics is a structurally biomimetic of the cancellous bone, whose porous network could allow tissue to ingrowth exhibiting nicer osteoconductive properties. However, the porous β-TCP shows weak mechanical properties, which limit its application as bone grafts. The macrostructure feature of nature bone is porous cancellous bone inside with dense compact bone surrounding outside, which provides excellent biomechanical properties. Carotenuto et al [3] have prepared dense/porous layered HA bioceramic for orthopedic device coating by tape casting technique, whereas the bulk dense/porous bioceramics were rarely reported. Therefore in present study, a dense/porous structured β-TCP bioceramics that mimics the characteristics of nature bone were fabricated, and the microstructure and mechanical properties of such bioceramics were studied.
Experimental
The β-TCP powders were synthesized by chemical precipitation reaction. The dense/porous structured β-TCP bioceramics were prepared by injected molding and subsequently pressureless sintering. The shrinkage rate of both porous and dense parts during sintering process was measured at different temperatures. X-ray diffraction (XRD) with Cu K α radiation was used to characterize the phase composition of the ceramics. The microstructures observation of the bioceramic samples was performed on a scanning electron microscopy (SEM).The compressive strength was conducted with a mechanical tester at 0.5 mm/min crosshead speed. The elastic modules were reanalyzed from the slope of the compressive strength-strain curve.
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Results and Discussion
The major problem in preparation of the dense/porous bioceramics is the interface adhesion between the dense and porous parts because of their different shrinkage rate during sintering process. The shrinkage rate of dense and porous bioceramics at different temperatures was measured and the results are shown in Fig.1. It can be noticed that the porous β-TCP bioceramics exhibit much higher shrinkage rate than the dense counterpart. The porous bioceramics shows about 23% shrinkage in radial direction; in contrast, the dense bioceramics presents about 17% shrinkage. It can be calculated that from 850 o C to 1100 o C, the porous β-TCP bioceramics shows about 17% shrinkage rate and almost the same with that of the dense counterpart from 600 o C to 1100 o C. So as to avoiding the shrinkage differences, the porous β-TCP bioceramics were pre-sintered at 850 o C, then the dense bioceramics were injected surrounding the porous ceramics, finally the composites were pressureless sintered at 1100 o C for 5 hours and the dense/porous structured β-TCP bioceramics were obtained.
Fig.1 The radial shrinkage rate of the porous and dense β-TCP bioceramics
The phase composition of the as prepared bioceramics was analyzed by X-ray diffraction. The XRD results show that the high temperature sintered β-TCP preserved their original β phase without transform into their α-TCP phase, as shown in Fig.2. Because the α-TCP though bioactive, have proven less useful as bone regeneration materials due to their excessively high resorption rate than the β-TCP phase. And none of the other impurity phases can be detected in the XRD patterns; resultantly, high purity β-TCP bioceramics were prepared.
Fig.2 X-ray diffraction pattern of the prepared bioceramics.
Fig.3 shows the optical and SEM micrographs of the prepared dense/porous β-TCP bioceramics samples. It is clear to see that the inner porous structure mimics the cancellous bone to some extent, and outer side dense structure mimics the compact bone, as shown in Fig.3(a) and indicated by the
S h i n k a g e (%)Temperature (o C)1020304050607080
100200300
400500600 2theta (deg.)
I n t e n s i t y (c p s )
arrows. Fig.3 (b) shows the interface of the dense/porous β-TCP bioceramic, it can be found that the interface between the dense and porous bioceramics is connected compactly and tightly. In the porous part, the macropore size is about 500 μm in diameter; the diameter of the interconnected pores is about 100 μm. Additionally, the porosity of the porous parts is about 72%, and the interconnectivity is more than 95%. The microstructure of the macroporous wall was shown in Fig.3(c); it is obvious that there are some micropores with diameter of 1 μm distributed uniformly in the porous wall. As the results, the microstructure of porous part of the bioceramics is a combination of macroporous and microporous. Contrastively, the microstructure of the dense bioceramics shows refined particle size and few micropores, as exhibited in Fig.3(d). The dense compact part is much denser than the porous cancellous part.
Fig.3 The dense/porous β-TCP bioceramic sample (a), the microstructure of dense/porous interface
(b), the macroporous wall (c) and dense compact bone (d).
The variation of the compressive strength and Elastic modulus of the bioceramics with different dense/porous cross-sectional area ratio (S dense /S porous ) was illustrated in Fig 4. It is exhibited that the compressive strength increases from 10 MPa to 74 MPa with the dense/porous ratio from 0.1 to 4.7 obeying rule of exponential growth. And the elastic modulus has been increased form 180 MPa to 960 MPa with the dense/porous ratio increment, also following exponential growth. Evidently, the value of the porous bioceramics is only about 2.0 MPa and the elastic modulus is about 20 MPa, indicated by the square in Fig.4. It has been achieved about 5 to 37 times increment in the mechanical properties by the dense/porous structure design. The mechanical properties of the dense/porous bioceramics could be tailored by the dense/porous cross-sectional area ratio.
Porous materials always have poor mechanical properties. Applications of calcium phosphates in the body have been limited by their low strength and numerous techniques have been investigated in attempts to retain their useful bioactive properties whilst providing more suitable mechanical properties for particular applications. These include the reinforcement of β-TCP using HA fiber or
bioglass additives [4, 5]; however these techniques are limited for the porous calcium phosphate Compact bone Cancellous
bone (b)(c) (d)
using in the load bearing sites’ bone regeneration. In this study, excellent mechanical properties of the porous β-TCP bioceramics have been achieved by the dense/porous structured design. The compressive strength of human femoral cancellous bone, weight bearing sites, is in the range of 25~90 MPa, so the dense/porous structured β-TCP is comparable to the strength of human femoral cancellous bone. The high interconnective porous structure of the dense/porous β-TCP bioceramics could allow the tissue ingrowths, and the dense structure could bear the load to some extent. The dense/porous structure of β-TCP bioceramics may provide a simple but effective way to increase the mechanical properties of porous bioceramics for the bone regeneration applications at weight bearing sites.
Fig.4 The variation of the compressive strength and elastic modulus of the bioceramics with
different dense/porous cross-sectional area ratio. Conclusions
The dense/porous structured β-TCP bioceramics were prepared and revealed excellent mechanical properties with compressive strength from 10 to 74 MPa and elastic modulus from 180 to 960 MPa, which is 5 to 37 times higher than that of the pure porous β-TCP and comparable to the strength of human femoral cancellous bone. The interface between the dense and porous bioceramics is connected compactly and tightly. The dense/porous structure of β-TCP bioceramics may provide a simple but effective way to increase the mechanical properties of porous bioceramics for weight bearing site’s bone regeneration.
Acknowledgement
Financial supports from the Shanghai Postdoctoral Scientific Key Program and the Science & Technology Commission of Shanghai Municipality of China (No.04DZ52043) are greatly acknowledged.
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E l a s
t i c M o d u l u s (M P a ) C o m p r e s s i v e S t r e n g h (M P a )S dense /S porous。