Jian Wen, Ill Yong Kim*, Koichi Kikuta and Chikara Ohtsuki
Graduate School of Engineering, Nagoya University, B2-3(611) Furo-cho, Chikusa-ku, Nagoya, 464-8603 Japan
Received: 25 December, 2015; Accepted: 25 January, 2016; Published: 27 January, 2016
Ill Yong Kim, Assistant Professor, Graduate School of Engineering, Nagoya University, B2-3(611) Furo-cho, Chikusa-ku, Nagoya, 464-8603 Japan, Tel: +81-52-789-3183; Fax: +81-52-789-3182; E-mail:
Wen J, Kim IY, Kikuta K, Ohtsuki C (2016) Optimization of Sintering Conditions for Improvement of Mechanical Property of a-Tricalcium Phosphate Blocks. Glob J Biotechnol Biomater Sci 1(1): 010-016.
© 2016 Wen J et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Bioactive ceramic materials have been under research as bone substitute for several decades. To repair the high-load bearing bones, mainly cortical bones, there is a need for the substitute to possess comparable mechanical strength to cortical bone, of which the compressive strength ranges between 100 and 230 MPa. Two prevailing bone repairing material, β-tricalcium phosphate (β-TCP, β-Ca3(PO4)2) and hydroxyapatite (HAp: Ca10(PO4)6(OH)2) have been widely researched and sintered into dense blocks to meet the mechanical requirements. α-tricalcium phosphate (α-TCP, α-Ca3(PO4)2), a high temperature polymorph of β-TCP, received relatively less attention and α-TCP dense sintered blocks have not been reported yet. In this research, we fabricated α-TCP dense blocks by sintering under various temperatures (1150-1400 °C) and the highest compressive strength was around 230 MPa. Intermediate porous blocks (porosity: 33%) were also fabricated from mixed powder of α-TCP and starch. In vitro properties variation of the intermediate porous blocks were investigated by soaking samples in simulated body fluid (SBF) and the compressive strength was maintained above 100 MPa after soaking for 14 d.
Calcium orthophosphates materials have been under research and applied to clinical therapy for several decades. In terms of different types of human bones, various bone substitutes have been developed for the corresponding bones [1-3]. As is well known, human bones consist of two different types of bones, cortical bone and trabecular bone. The hard outer layer of bone is cortical bone (also referred as compact bone), which mainly exists in the shafts of long bones and as a shell around trabecular bone, containing around 80% of bone mass and facilitating bone's main functions. Trabecular bone (also referred as cancellous or spongy bone) can be found in the ends of long bones, in vertebrae and in flat bones like the pelvis. These two types of bones are classified on the basis of porosity and the unit microstructure. According to a variety of research results, cortical bone has a porosity ranging between 2% and 27 % [4,5], while trabecular bone is much more porous and the porosity can be as large as 95% . As for the mechanical properties of the bones, trabecular bone has been reported to exhibit a compressive strength of 2-10 MPa , and cortical bone possesses much higher compressive strength, ranging from 100-230 MPa .
Among a variety of calcium phosphates, there are three materials attracting great attention as bone repairing materials, referred as α-tricalcium phosphate (α-TCP, α-Ca3(PO4)2), β-tricalcium phosphate (β-TCP, β-Ca3(PO4)2) and hydroxyapatite (HAp: Ca10(PO4)6(OH)2). Macroporous scaffolds that separately consist of these three types of materials attempting to repair porous bone have been reported in plenty of papers [9-11] and both of porosity and mechanical strength were comparable to those of trabecular bone. However, when it comes to the cortical bone, requirements of repairing material become more complicate. Cortical bone mainly exists in the high load-bearing bones, like tibia and fibula. To repair the damaged sites in these bones, a compressive strength above 150 MPa of the repairing scaffold is generally demanded . Meanwhile, as a candidate to repair bone, the substitute should be porous, or able to provide porosity after implantation . Nevertheless, ceramic scaffolds get quite brittle when they become porous, making it difficult to reach a balance between the mechanical property and the porosity. More crucially, rapid return of function (formation of new bones) is also a requisite evaluation criteria for bone substitutes, which could bring the patients back to their normal daily life as quickly as possible, minimizing the negative influence to their quality of life. Based on the above three factors, mechanical strength, moderate porosity, biodegradability and osteoconduction are pivotal properties required for an optimal cortical bone substitute.
Among the three materials, β-TCP and HAp have already been widely used, both in macroporous and dense forms [14,15]. Some previous research has reported dense sintered blocks separately fabricated from β-TCP and HAp, of which the porosity was less than 1% and the compressive strength was far higher than that of human cortical bone . Recently, H.O. Mayr et al. , reported an intermediate microporous β-TCP scaffold, which has less porosity (around 43.5%) than the macro porous scaffold and the pore size is merely around 5.40 µm. After seeded with chondrocytes and implanted into a sheep model for one year, 80% of the scaffold was replaced by new bone. This research provides us a new possible approach to reach a balance between the mechanical strength and porosity suggesting that it may be feasible to fabricate relatively dense scaffolds that match both the mechanical properties and porosity of human cortical bone, and in spite the pores are micro porous, the ability to be resorbed and replaced by human body may not be hindered.
As α-TCP is relatively newly discovered, it is receiving growing attention because of α-TCP has higher dissolution rate than β-TCP. Besides α-TCP transforms easily into HAp by the reaction with H2O. So, α-TCP can be applied to bone cements or an additive in bone cements, although not investigated and applied as widely as the other two materials. The priority of α-TCP to β-TCP or HAp has been reported by several research groups. In regard to the in vitro properties, at 37 °C and physical pH (7.2 ~ 7.4), α-TCP releases relatively more Ca and P and the concentration ranks in the order of α-TCP > β-TCP >> HAp [10,17], implying better ability of providing requisite resources for bone formation. Hisham Rojbani et al. , conducted an in vivo experiment of these three materials and confirmed that although α-TCP, β-TCP and HAp were osteoconductive and successfully acted as space maintainer for bone formation when applied to a bone defect and α-TCP showed the advantage of higher rate of degradation and more bone formation was observed. Both in vitro and in vivo tests manifested the better potential of α-TCP to be resorbed by human body, whereby the return-of-function process may be promoted, especially when a dense structure retards the dissolving of an implant.
Up to present, α-TCP are mainly applied in forms of cements and macroporous scaffolds , while α-TCP-based sintered dense blocks have rarely been investigated or reported. Additionally, the stable temperature of α-TCP (1125-1430 °C) differs from that of β-TCP (< 1250 °C), and HAp is stable in a wide range of temperature between 800 °C and 1400°C  according to various published literature data. Hence, it can be assumed that after sintered into dense forms, the properties of these three materials would also differ with each other. Therefore, the purpose of this research is to fabricate dense α-TCP sintered blocks and investigate the variation of properties under different sintering temperatures, followed by the fabrication of less denser blocks, which have biggest possible porosity meanwhile match with that of cortical bone. Then the samples with both moderate porosity and high compressive strength (> 100 MPa) will be chosen for in vitro test in simulated body fluid (SBF), which has ion concentrations nearly equal to those of human blood plasma and was proposed by Kokubo et al. , to predict the osteoconduction of the sintered blocks.
Materials and Methods
Fabrication of sintered blocks from α-TCP powder by cold isostatic pressing (CIP) method
A cold isostatic pressing (CIP) method , was utilized to fabricate the dense sintered blocks. α-TCP powder (α-TCP-B, Taihei Chem. Inc., Osaka, Japan) was poured into stainless mold (Ø=7.00 mm) and hand-pressed for preforming. The preformed green cylinders were subsequently encapsulated into rubber finger-cot, evacuated and sealed. The sealed green cylinders were then pressed under pressure of 20 MPa in a pre-established water condition for about 30 s. Afterwards, both ends of the green α-TCP cylinders were rubbed to parallel with the horizontal axis. The received green cylinders (Ø=6.15 mm; length 13.50 mm) were sintered under different conditions and the details are given in Table 1.
Fabrication of sintered blocks from α-TCP/starch powder by CIP method
Mixture powder containing α-TCP and potato starch (Potato starch, Nacalai tesque, Inc., Kyoto, Japan) was prepared by milling α-TCP and starch powders for 20 minutes. Then green cylinders were received by the above-mentioned CIP method and sintered at different temperatures. Detailed information of the fabrication process are listed in Table 2.
Porosity and shrinkage of the sintered blocks
The shrinkage of the dense sintered blocks were calculated as:
Shrinkage (%): ,
Where Vg is the volume of the green cylinder and Vs refers to that of sintered block. The volumes of samples were calculated from diameter and length after sintering. Shrinkages of samples are averaged from the values of 6 samples.
The total porosity of all sintered blocks were calculated by Equation:
Total porosity (%) : ,
Where ρb is the density of the porous scaffold calculated by weight and volume, ρs is the theoretical density of α-TCP (2.83g/cm3). While apparent porosity of some samples (n=6) were measured through an Archimedes method (according to ASTM C373-88) with kerosene used as a liquid medium and the porosity was expressed as:
Apparent porosity (%): ,
Where W1 is the weight of sample in air, W2 is the weight of sample suspended in kerosene and W3 is the saturated weight, and calculated from the density and sample size.
The 91/1150 samples were soaked in 40 cm3 SBF solutions for 0, 1, 3, 7 and 14 d, respectively. Composition of SBF with ion concentrations nearly equal to those of human blood plasma as shown in Table 1 . The SBF was prepared by dissolving reagent grade chemicals of NaCl, NaHCO3, KCl, K2HPO4·3H2O, MgCl2·6H2O, CaCl2 and Na2SO4 into distilled water. It was buffered at pH 7.4 with 50 mmol tris(hydroxymethyl)aminomethane ((CH2OH)3CNH2) and 45 mM hydrochloric acid (HCl) and was kept at 36.5°C. After removal of samples from SBF, the variation of Ca, P and Mg concentrations in the solutions (n=5) were measured by induced coupled plasma atomic emission spectroscopy (ICP-AES; Optima 2000DV, PerkinElmer Japan Co. Ltd., Japan). After soaking, the blocks were dried at 100 °C for 12 h, followed by a vacuum-drying process at 100 C for 24 h to calculate the weight changes before and after soaking.
Mechanical strength measurement
Compressive strength and Young's modulus of all samples were measured by an Instron Model 5566 system with a crosshead speed of 1 mm/min and the load cell was 10 kN. Each value used in this study was the average data of 6 specimens.
Crystal phases and morphologies
After sintering process and immersing process, crystal phases of the sintered blocks were tested by powder X-ray diffraction (XRD; RINT2100,Rigaku Co., Japan) using CuKα radiation with a scanning speed of 2°/min at 40 kV and 20 mA after ground into fine powder using a mortar. Morphologies of all the specimens were observed by scanning electron microscopy (SEM; JSM-5600, JEOL Ltd, Tokyo, Japan) at a 15 kV operating voltage after gold sputter-coating.
Dense sintered blocks prepared from α-TCP
The images of the green cylindrical bodies and sintered blocks are shown in Figure 1. The porosity and shrinkage of the dense sintered blocks are shown in Table 1. It is plain to see that as the sintering temperature increased, the blocks shrank and the porosity dropped from 19.2% to 8.3%, implying that sintered blocks became denser. The compressive strength are given in Figure 2. A turning point of the compressive strength can be found around 1300 °C. The average compressive strength of the blocks sintered between 1150 and 1300 °C exceeded 100 MPa, however, decreased precipitously at 1300 °C-from about 230 MPa to 110 MPa, and fell further down below 100 MPa when the temperature came to 1350 °C.