Cortical bone accounts for 80% of the total bone mass of an adult skeleton and it plays a load-bearing role in human body, with over 150 MPa in tensile strength, more than 17 GPa in tensile modulus and around 2.0% in tensile strain. Currently, metal alloys are used for cortical bone replacement due to their good mechanical properties. However, the mismatch between bone and metal alloy can cause “stress shielding” of the bone. Besides, those metal alloys are also too heavy. Since Bonfield introduced hydroxyapatite (HA) reinforced high density polyethylene (HDPE), there have been numerous works done to develop polymer composites with bioactivity, but these composites are just suitable for soft bone substitution applications due to their relatively low strength and stiffness....[ Read more ]

Cortical bone accounts for 80% of the total bone mass of an adult skeleton and it plays a load-bearing role in human body, with over 150 MPa in tensile strength, more than 17 GPa in tensile modulus and around 2.0% in tensile strain. Currently, metal alloys are used for cortical bone replacement due to their good mechanical properties. However, the mismatch between bone and metal alloy can cause “stress shielding” of the bone. Besides, those metal alloys are also too heavy. Since Bonfield introduced hydroxyapatite (HA) reinforced high density polyethylene (HDPE), there have been numerous works done to develop polymer composites with bioactivity, but these composites are just suitable for soft bone substitution applications due to their relatively low strength and stiffness.

Hydroxyapatite (HA) filled Ultra-high Molecular Weight Polyethylene (UHMWPE) nanocomposites were developed for cortical bone replacement in this research thesis. Nanocomposites containing three different concentrations of HA 5.9 wt%, 10.2 wt% and 15.4 wt% (or 1.8%, 3.3%, and 5.1% by volume, respectively) were prepared by a novel process protocol where twin-screw swelling extrusion, compression molding and hot drawing was applied systematically. A silane coupling agent was used to enhance the compatibility between HA particles and UHWMPE matrix. Samples with HA weight concentration of 9.9 wt% (3.2 vol%), but without silane coupling agent treatment were also prepared as control. Results showed that a highly oriented fibrous structure of UHMWPE inside the oriented composite samples was achieved after hot drawing in the semi-solid state. Uniform and nanoscale dispersion of HA particles in the UHMWPE matrix was observed in all composite samples prepared with the use of silane coupling agent by TEM analysis. The hot drawn nanocomposites exhibited tensile strength significantly higher than (>500%) natural cortical bones and better than all other reported bone replacement biomaterials. Moreover, these nanocomposites showed good ability of inducing Ca-P layer precipitation on their surfaces by in vitro studies using simulated body fluids (SBF), with a record low of 3.3 vol% of HA in the composite. Cell culture tests using MC3T3-E1 mouse pre-osteoblast cells indicate the nanocomposites have good biocompatibility and can facilitate cell proliferation.