A novel strategy to enhance interfacial adhesion in fiber-reinforced calcium phosphate cement
Graphical abstract
Introduction
Calcium phosphate cements (CPCs) are ceramic materials, brittle by definition, with a porosity that can vary between 10% and 50% depending mainly on the liquid to powder ratio used in their preparation (Espanol et al., 2009). The intrinsic brittleness derived from the microstructure and composition of these materials is one of the major limitations of their mechanical performance and has restricted their indication of use to non-load-bearing applications.
The toughness of CPCs ranges from 0.010 to 0.050 kJ/m2 in their work of fracture (WOF) (Canal and Ginebra, 2011), which is far below the work of fracture of bone, reported to be between 1.5 and 15 kJ/m2 (Currey and Butler, 1975). Although the bending strength values reported for CPCs are typically in the range of 5–15 MPa (Martin and Brown, 1995, Ginebra et al., 2001), close to that of trabecular bone (estimated between 10 and 20 MPa) (Barinov, 2010), their strain to failure is much lower (Xu et al., 2002). The brittleness of CPCs has recently been highlighted in a report on their strain-to-crack-initiation, which amounted to a mere 0.2% in compression (Ajaxon et al., 2017). An improvement of the mechanical performance of these materials, and particularly a mitigation of their brittle behavior, would significantly extend the applicability of CPCs.
For the last 15 years, several strategies have been evaluated to reinforce CPCs with fibers (Canal and Ginebra, 2011, Krüger and Groll, 2012). Fibers have been incorporated to the CPC matrix using different lengths (Xu et al., 2000, Pan et al., 2007), aspect ratios (diameter/length) (Xu et al., 2000, Zhang and Xu, 2005, Zuo et al., 2010), orientations and textile constructs (mono/multifilaments, yarns, nonwovens, etc.) as reviewed by Canal and Ginebra (2011). These approaches have allowed either an increase of the mechanical properties (Zhang and Xu, 2005) or to couple good mechanical properties and macroporosity, increasing the degradation rate and allowing cell infiltration in the material (Xu et al., 2006, Xu et al., 2007). Nonetheless, up to now, little attention has been paid to the fiber-matrix adhesion, which is crucial for a successful load transfer, a prerequisite for an effective reinforcement (Nelson et al., 2002). The potential of the strategies based on enhancing the fiber-matrix interface has been highlighted recently by the improvement of the reinforcement of PLA fibers through surface modification of the fibers by cold plasmas (Canal et al., 2014, Maenz et al., 2014).
Polymeric additives have earlier been used in CPCs with the purpose of improving their mechanical properties, injectability, resorption rate and biocompatibility (Dorozhkin, 2009, Neumann and Epple, 2006, Low et al., 2010, Perez et al., 2012, Engstrand et al., 2013). The polymers, which are often biodegradable, can be added to the matrix either solubilized in the liquid phase or as a second phase, as particles or fibers. Among the different polymers, chitosan is of interest mainly because it is biodegradable, biocompatible, and it can be processed into several products including flakes, fine powders, beads, membranes, fibers, and gels (Badawy and Rabea, 2011).
In this work we used a strategy inspired by the acrylic bone cements, which consist of polymethyl methacrylate (PMMA) spheres embedded in a matrix of the same polymer (Ginebra, 2009). The excellent adhesion between the PMMA particles and the PMMA matrix is due to the chemical affinity between the liquid and the solid phase. The methyl methacrylate monomer wets completely the PMMA powder, dissolving and repolymerizing the surface of the particles, creating a perfect continuity between the matrix and the filler. In our case, although it is not possible to induce a partial dissolution of the polymeric fibers due to the hydraulic nature of calcium phosphate cements, an attempt was made to enhance the continuity between fibers and matrix by adding to the cement matrix the same polymer used for the fibers.
Thus, the aim of this work was to develop a biocompatible fiber-reinforced CPC (FRCPC) with improved mechanical properties using chitosan as common polymer in the matrix and in the fibers, with the hypothesis that having an additive of similar nature would increase the chemical interactions between matrix and fibers, which would in turn result in a higher toughness. As chitosan is poorly soluble in water, trimethyl chitosan (TMC), which is a more soluble chitosan derivative (Domard et al., 1986), was added to the cement liquid phase, and chitosan fibers were used as reinforcing agents.
Section snippets
Fiber reinforced calcium phosphate cements
Fiber-reinforced calcium phosphate cements (FRCPCs) were prepared by mixing a solid phase containing α-tricalcium phosphate (α-TCP) and chitosan fibers with a liquid phase. The solid phase consisted of in-house made α-TCP obtained by solid-state reaction of a 2:1 molar mixture of calcium hydrogen phosphate (CaHPO4, Sigma–Aldrich C7263) and calcium carbonate (CaCO3, Sigma–Aldrich C4830) at 1400 °C for 15 h followed by quenching in air. The powder was first milled with 10 balls (d = 30 mm) for 15 min
Physico-chemical characterization
The presence of 1 w/v% TMC in the liquid phase increased the setting time of the cement in comparison with that prepared only with water (Table 2). The pH of a cement slurry (200 ml/g) remained at 7.0–7.5 when 1 w/v% TMC was used as solution. In contrast, when only water was used, the pH increased from ca. 7–9.3 in less than 10 min, followed by a slow decrease to a neutral pH during 24 h (Fig. 1).
The crystalline phases of the end-products of the cementitious reaction were analyzed after 7 days (Fig.
Discussion
Fiber Reinforced Calcium Phosphate Cements (FRCPCs) with improved fiber/matrix adhesion were obtained in this work by introducing a polymeric solution (trimethyl chitosan, TMC) in the cement matrix, with high affinity to the chitosan fibers that were randomly oriented in the matrix.
To the best of our knowledge, only two prior studies aimed to improve the adhesion of the fiber/matrix interface in CPC-fiber composites, but with a different strategy. These studies were based on the activation of
Conclusion
Fiber-reinforced calcium phosphate cements (FRCPCs) have been successfully prepared using trimethyl chitosan as additive in the liquid phase and chitosan fibers as reinforcing agent. The improved wettability of the fibers and its chemical similarity with the liquid phase of the cement enhanced their interfacial adhesion. The FRCPCs had a significantly improved toughness (measured as work of fracture) and at the same time the elastic modulus and bending strength were maintained in comparison to
Acknowledgements
Authors acknowledge the “Generalitat de Catalunya” funding through a FI Scholarship of SG, the MICINN for the Ramon y Cajal fellowship of CC and the financial support in the MAT2015-65601-R project (MINECO/FEDER, EU). The research leading to these results received funding from the European Commission Seventh Framework Programme (FP7/2007–2013) under the Grant agreement no. 241879, through the “Reborne” project and through the Swedish Foundation for International Cooperation in Research and
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Both authors contributed equally.