Fracture characterization of novel bioceramic microbeads filled polymer composite

The biomaterial poly(l-lactic acid) (PLLA) is commonly used for bone fixation devices and dental surgeries; however its natural osseointegrating ability is generally poor, which can lead to dislocation and partial fractures. Recently, calcium phosphate bioceramics have been incorporated into PLLA in order to redress this issue and improve the osseointegrating ability of the material. This study incorporated calcium phosphate microbeads in PLLA at various concentrations and measured their effect on the critical stress intensity factor, K_IC and critical energy release rate at crack initiation, G_in, as well as the unusual fracture mechanic they induced. The inclusion of microbeads into the polymer matrix reduced the materials’ fracture toughness, from a K_IC of 34 ± 7 Pa m^−1/2 for blank PLLA, to 18 ± 1 Pa m^−1/2 for the strongest bead containing group; and from a G_in of 1030 ± 150 J m⁻² for blank PLLA to 200 ± 18 J m⁻² for the same microbead containing group. Importantly however, the microbead containing groups fractured by a different mechanism, which was identified by observing fracture surface morphologies, electron probe microanalysis and finite element analysis. It was seen that polymer intruded into the porous microbeads, and resulted in regions of increased stiffness in the polymer matrix around each bead. This prevented void formation at the polymer/microbead interface, but it allowed the strain energy density to increase rapidly under load in microbead containing groups. This energy concentration in turn caused fractures to occur sooner, and resulted in the brittle fracture surfaces seen around microbeads. The identification of this fracture mechanism is important to understand, as it suggests a way to greatly improve the fracture toughness of the material, reducing the difference in stiffness of the two components. This fracture mechanism may also be useful in explaining other observed fracture events containing two materials of greatly varying stiffness.