Evaluation of the sintering temperature on the mechanical behavior of β-tricalcium phosphate/calcium silicate scaffolds obtained by gelcasting method
Graphical abstract
Introduction
Scaffolds are temporary support structures that provide propitious conditions to cells proliferation, migration, and differentiation in 3D, allowing the formation of a specific tissue with appropriate functions. Some common properties for an ideal scaffold for tissue engineering are biocompatibility, porosity with interconnected pores and an adequate mechanical strength (Ikada, 2006, Katari et al., 2014, Fisher and Mauck, 2013, Yang et al., 2001). For an optimal performance, bone scaffolds should also be biodegradable, or bioresorbable, providing mechanical support while a new tissue is formed to replace it (Katari et al., 2014; Black et al., 2015; Amini et al., 2012; Bose et al., 2012).
There are many ceramics which have been used for bone tissue engineering as calcium phosphates, especially hydroxyapatite – HA (Ikada, 2006, Dash et al., 2015, Baradararan et al., 2012, Asif et al., 2014) and β-tricalcium phosphate – β-TCP (Ikada, 2006, Black et al., 2015, Baradararan et al., 2012), bioactive glasses (Fu et al., 2011, Chen et al., 2006), titania (Cunha et al., 2013) and alumina (Sarhadi et al., 2016, Song et al., 2013). Tricalcium phosphate (TCP) is a bioresorbable, bioactive and osteoconductive bioceramic and β-TCP, a TCP polymorph, stands out due to its solubility and degradation rate (Dorozhkin, 2010, Ratner et al., 2013).
Gel casting stands out among the many methods commonly employed to produce scaffolds (Ikada, 2006, Amini et al., 2012) such as three-dimensional printing (Zhang et al., 2015), replication technique (Black et al., 2015, Baradararan et al., 2012), and freeze casting (Rosset et al., 2014, Chen et al., 2015). A foaming agent is added into a ceramic suspension containing the ceramic powder, organic monomers and dispersant. After vigorous stirring a foam is formed and it became rigid after monomers polymerization. Scaffolds obtained are highly porous (40 – 90%) with spherical geometry, sizes between 50 and 800 µm and thick walls with homogeneous microstructure which improve mechanical properties, leading to a great performance for cellular growth and tissue engineering. Furthermore, this method is more feasible and cheaper when compared to others and does not require atmospheric control (Elliot, 1994, Janney et al., 1998, Young et al., 1991, Yang et al., 2011).
An ideal porosity of the scaffold is required to maximize the space for cellular adhesion, growth, revascularization, adequate nutrition and other factors that can influence cellular and tissue growth (Ikada, 2006, Katari et al., 2014, Fisher and Mauck, 2013, Yang et al., 2001). Unfortunately, porosity and mechanical resistance are indirectly related and as higher the porosity lower is the compressive strength and the reproducibility of the scaffold manufacturing (Ratner et al., 2013, Carter and Norton, 2007).
In order to improve the mechanical properties some recent researchers have been discussing the use of additives in scaffolds composition such as fibers (Panzavolta et al., 2012, Abdullah et al., 2012) and whiskers (Fang et al., 2013). Fibers and whiskers can be characterized by their aspect ratio (L/D, where L is the length of the fiber and D is its diameter) and as higher the value the better is the interaction between fiber and matrix (Dorozhkin, 2016, Motisuke et al., 2014).
As studied in previous works, molten salt synthesis is a method that may be used to produce fibers and whiskers since it is a low cost process with high control of properties (Motisuke et al., 2014, Hayashi et al., 2000). Calcium silicate (wollastonite – CaSiO3) is highly suitable to produce fibers for bone tissue scaffolds reinforcement due to its outstanding biocompatibility, bioactivity and biodegradability (Motisuke et al., 2014, Fei et al., 2012, De Aza et al., 1994). Moreover, its positive effect on bone formation processes has been recognized and debated in the literature (Zhou et al., 2017).
To increase the accessibility of the technology and improve quality of life, the research on methods and materials that are feasible and efficient for bone tissue therapy is necessary. Therefore, the aim of this work was to produce by gel-casting β-TCP scaffolds with calcium silicate fibers with appropriated porosity, interconnectivity and mechanical strength for application in tissue engineering, as scaffolds.
Section snippets
Synthesis and characterization of β-TCP and CaSiO3 fibers
β-TCP powder was obtained by solid state reaction of calcium carbonate (CaCO3 – Synth, Brazil) and calcium phosphate (CaHPO4 – Synth, Brazil) in the molar ratio of 1:2 (CaCO3:CaHPO4). The powder was calcined at 1050 °C for 6 h and afterwards it was dry milled in a horizontal ball-mill (MA500, Marconi, Piracicaba, SP, Brazil) for 48 h.
The of calcium silicate (CaSiO3) fiber used in this work were prepared as published elsewhere (Motisuke et al., 2014) by molten salt method using an alkaline flux
Results
Fig. 1 shows the XRD pattern of the β-TCP powder and CaSiO3 fibers. From the analysis of Fig. 1-a it was possible to identify only and exclusively XRD lines of β-TCP (JCPDS 09–0169; 2θ = 22.20°, 27.76°, 31.02°, 34.37°). The XRD diffraction of the CaSiO3 fibers is shown in Fig. 1-b. The analysis reveals that the fibers have a considerable degree of crystalline phase purity (JCPDS 043–1460), except by the peak (2θ = 31.03°) characteristic of substance Ca6(SiO4)(Si3O10) (JCPDS 029–0370), whose
Conclusions
The presence of CaSiO3 fibers, associated with a higher sintering temperature, resulted in an increase of the mechanical properties of the β-TCP scaffolds. The best mechanical performance (1.16 ± 0.16 MPa) was achieved by the composition containing 5 wt% fibers sintered at 1300 °C, representing an increase of 64.65% of the maximum load under compression stress. Besides of the β →α polymorphic transformation which have happened at this temperature and should led to some structural defects, there
Acknowledgements
The authors would like to thank the São Paulo Research Foundation – FAPESP - (Grant IDs: 2010/00863-0, 2011/09240-9 and 2012/07897-3) and the National Council for Scientific and Technological Development (CNPq/PIBITI, Grant: 456461/2014-0) for the financial support. We also thank the LNNano/CNPEM by the X ray microtomograph facility.
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