Original Research PaperStrengthening bioceramic through an approach of powder processing
Introduction
With the increase of aged population, the number of patients with bone defects is also increased. Four bioceramics, alumina, zirconia, calcium phosphate and calcium sulfate, are frequently used in clinics [1], [2]. Though these bioceramics are safe in terms of their toxicity, their functions within human body are very much different. The alumina and zirconia are chemically inert (bio-inert), and they maintain their shape and strength throughout their service lifespan. Nevertheless, they remain foreign within biological environment due to their chemical inertness. Calcium phosphate and calcium sulfate are not stable in terms of their shape. They would be degraded within human body. However, their degradation rates are very much different. The complete degradation of calcium phosphate may take months even years [3]. In contrast, the complete degradation of calcium sulfate may take only days.
These bioceramics differ significantly for their strength. Among these bioceramics, the strength of zirconia is the highest; its strength is one order of magnitude higher than that of calcium sulfate. Due to the difference in their strength, zirconia can be used in load-bearing locations, such as the ball in hip-joint [4]. The phosphate and sulfate can only be used as the fillers for bone void defects [5], the locations without external loads. Although they could not withstand large loads, their bio-conduction and bio-induction functions could reduce the recovery time from bone defects [5]. The bone void fillers are usually in the shape of pellet or rectangular rod [6]. In clinics, surgeon needs to use tweezers to insert these pellets or rods into bone voids. During this insertion, no debris is allowed. A minimum handling strength for these pellets and rods is therefore required. In order to enhance the interactions between these fillers and osteoblast cells, pores are preferred to be present within these fillers [6]. A recent study [7] demonstrated that the porosity and pore size are the key factors for the synthetic bone graft. Though the size of pores affects little on strength, the increase of porosity decreases the strength of porous bioceramics. For porous bone graft, the degradation may take place within the pore channels [8], the strength of bioceramics is therefore important.
Apart from strength, one more concern is the elastic modulus of these bioceramics. The elastic modulus of alumina is the highest (~380 GPa) among all bioceramics. As this value is far too high than that of bone, its presence may induce stress shielding at the location near bone defect [9]. Such effect enhances the degradation of the nearby hard tissue. The elastic modulus of calcium phosphate and calcium sulfate (~80 GPa) is close to that of bone [10]. The use of phosphate and sulfate for load-bearing locations is thus worth noting, provided their strength is improved.
The calcium sulfate has been used as bone void filler for more than 100 years [3]. Its safety and bioactivity have been well received. The degradable rate of calcium sulfate is much faster than that of phosphates [3]. To be demonstrated later, the strength of calcium sulfate is much lower than that of tricalcium phosphate. To increase the strength of calcium sulfate without affecting its degradation rate is thus of interest.
The objectives of this study are listed in the following:
- a.
A microstructure design methodology is proposed to enhance the strength of degradable bioceramic.
- b.
The microstructure design can be achieved through proper powder processing techniques.
Since the strength of calcium sulfate is the lowest among all bioceramics, the calcium sulfate is chosen to demonstrate the effectiveness of the approach. Based on the concepts of microstructure engineering, the strategy involves the use of nano-particles to enhance the strength of calcium sulfate. To avoid affecting the bio-degradation behavior, the amount of second phase particles used should be as low as possible.
Section snippets
Microstructure engineering
Many approaches have been employed to enhance the strength of ceramics. The most effective approaches are through the refinement of microstructure [11], [12] and the addition of nano-particles [13]. For brittle ceramics, the crack frequently initiates from their grain boundary; furthermore, the propagation of crack is usually not stable at the grain boundary either [14]. The fracture of brittle ceramics thus shows strong dependence on their grain size. The Orowan-Petch relationship has been
Preparation of SiO2-CaSO4 composites
The size of as-received calcium sulfate hemihydrate (CaSO4 1/2H2O, JT Baker, USA) particles is relatively large. A pre-grinding treatment was adopted to reduce the size of as-received calcium sulfate hemihydrate powder. The powder was ball milled in a tumbling mill for 24 h. The milling media was ethyl alcohol and zirconia balls. The diameter of zirconia milling balls was 10 mm. The slurry was dried in an oven at 120 °C overnight.
The dried calcium sulfate hemihydrate was then mixed with various
Results and discussion
In the present study, a biaxial load was applied to break the specimen discs to obtain their biaxial strength. For brittle ceramics, cracks are frequently formed at their edges during handling the specimens. The possible presence of edge cracks of testing bars may affect a lot on the strength value. Extra care is therefore needed to avoid the formation of such cracks. For the measurement of biaxial strength, the edge of the discs is outside the loading area. The possible presence of edge cracks
Conclusions
Two mechanical properties, strength and elastic modulus, of bioceramics are essential for their biomedical applications. Two bioceramics, alumina and zirconia, exhibit both high strength and high modulus. The strength of calcium phosphate and calcium sulfate is low; nevertheless, their moduli are close to bone. In the present study, a microstructure engineering approach is adopted to improve the strength of calcium sulfate. This approach indicates that a small amount of second phase particles
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The present study is supported by the Ministry of Science and Technology, Taiwan, through the contract of MOST 107-2221-E-002-073-MY2, MOST 106-2218-E-002-032-MY2 and by the international collaboration project of the Joining and Welding Research Institute of Osaka University.
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