Mechanical Properties of Fibre Reinforced Calcium Phosphate Cements

Fibre reinforced calcium phosphate cements (FRCPCs) have been introduced in an effort to improve their tensile, flexural strength and ductility. Some of the important measuring techniques used to determine the mechanical properties of these reinforced cements have been: uniaxial compression, diametric compression, three point and four point bend and biaxial flexure tests. However, some of these techniques have limitations, e.g. toughness and flaw sensitivity cannot be characterised by compression due to stress concentration [81] and adoption of the diametric test configuration is not useful as this theory can only be applied to brittle materials. The most commonly used test method for determining the mechanical properties of fibre reinforced CPCs is either the three or four point bend test method. The mechanical properties of resorbable and non-resorbable fibre composite data are summarised in Table 3.1.

The mechanical properties of polyamide (linear polymers) fibres into CPCs demonstrated increased compressive strength (30%) [82]. When aramid fibres were added to CPCs, the flexural strength increased from 13 MPa (pristine cement) to 39 MPa and 65 MPa at 2 wt.% and 4 wt.% fibre loading [83, 84]. Inclusion of PLLA in CPC increased the work of fracture, however the flexural strength and Young’s modulus decreased progressively (from 7 MPa to 2.6 MPa and from 49 GPa to 15 GPa). It was suggested that the reduction in flexural strength and Young’s modulus was due to an increase in the pore volume created by inter-fibre porosity from the fibre bundle. Xu and Quin showed that adding 25 wt.% PLGA yarns created long cylindrical macropores within the cement and resulted in a three-fold increase in the flexural strength (25 MPa), the work of fracture increased by twofold (up to 3.4 KJ/m²) - however the Young’s modulus did not change considerably (Fig. 3.6) [88]. Due to fibre degradation under physiological conditions, the mechanical properties of composites decreased over time. Calcium phosphate cements containing electrospun PCL fibres exhibited an increase in the work of fracture. However, the flexural strength and elastic modulus decreased on introduction of the fibre bundles, which were not well integrated within the cement microstructure due to the hydrophobic surface of the fibres [98].

Table 3.1 Overview of composition and mechanical properties of calcium phosphates cements containing resorbable and non-resorbable fibres

• 3 The Mechanical Properties of the Scaffolds Reinforced by Fibres or Tubes...
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Fig. 3.6 (a) Flexural strength vs. days of immersion in saline for CPC control, CPC-Vicryl™ fiber composite, and CPC-Vicryl Rapide™ fiber composite, all at a CPC powder: water ratio of 2:1. Each datum is the mean value of six measurements (n = 6), with the error bar showing one standard deviation (SD). (b) Work-of-fracture for CPC control, CPC-Vicryl™ fiber composite, and CPC- Vicryl Rapide™ fiber composite, all at a CPC powder: water ratio of 2:1. (c). Elastic modulus for CPC control, CPC-Vicryl™ fiber composite, and CPC-Vicryl Rapide™ fiber composite, all at a CPC powder: water ratio of 2: 1 [88] © by Elsevier - reprinted with permission