Micro- and macromechanical characterization of the influence of surface-modification of poly(vinyl alcohol) fibers on the reinforcement of calcium phosphate cements.

Calcium phosphate cements (CPCs) are frequently used as synthetic bone substitute materials due to their favorable osteocompatibility and handling properties. However, CPCs alone are inherently brittle and exhibit low strength and toughness, which restricts their clinical applicability to non-load bearing sites. Mechanical reinforcement of CPCs using fibers has proven to be an effective strategy to toughen these cements by transferring stress from the matrix to the fibers through frictional sliding at the interface. Therefore, tailoring the fiber-matrix affinity is paramount in designing highly toughened CPCs. However, the mechanistic correlation between this interaction and the macromechanical properties of fiber-reinforced CPCs has hardly been investigated to date. The aim of this study was to tailor the fiber-matrix interface affinity by modifying the surface of poly(vinyl alcohol) (PVA) fibers and correlate their interfacial properties to macromechanical properties (i.e. fracture toughness, work-of-fracture and tensile strength) of CPCs. Results from single fiber pullout tests reveal that the surface modification of PVA fibers increased their hydrophilicity and improved their affinity to the CPC matrix. This observation was evidenced by an increase in the interfacial shear strength and a reduction in the critical fiber embedment length (i.e. maximum embedded length from which a fiber can be pulled out without rupture). This increased interface affinity facilitated energy dissipation during fracture of CPCs subjected to macromechanical three-point flexure and tensile tests. The fracture toughness also significantly improved, even for CPCs reinforced with fibers of lengths greater than their critical fiber embedment length, suggesting that other crack-arresting mechanisms also play an important role in mechanically reinforcing CPCs. Overall, these basic insights will improve the understanding of the correlation between micro- and macromechanical characteristics of fiber-reinforced CPCs.