Self-reinforced bioceramic and polylactide based composites

Abstract

Some bioceramics are known to have osteoconductive or osteoinductive potential and excellent bone bonding properties. However, they do not have the required mechanical properties, especially bending properties, to replace bone tissue due to their brittle behavior. Polymeric biomaterials have in many cases more acceptable mechanical behavior, but most of the polymers do not have the properties to facilitate bone tissue healing. Composites of osteoconductive component and polymeric biomaterial could be ideal for bone tissue implant material due to the option to tailor their mechanical properties and osteoconductivity to meet the specific requirements of each application.

The main objective of this thesis was to study self-reinforced osteoconductive bioabsorbable composites composed of either bioactive glass 13-93 or -tricalcium phosphate as a filler material and different copolymers of polylactide and polyglycolide as a matrix material. The composites studied were compounded using a twin-screw extruder. The samples containing 0 – 50 weight-% of filler material were further selfreinforced by die-drawing to achieve better mechanical properties and porous structure. By self-reinforcing, for example, the shear strength of the plain matrix polymer poly-L/D-lactide 96/4 could be increased by 150 % at best (from 43 MPa to 120 MPa). Selfreinforcing also made initially brittle samples ductile. Compounding and selfreinforcing together turned out to be a suitable method for achieving a homogeneous filler distribution. Different composite compositions were characterized by determining the changes in mechanical properties, thermal properties, molecular weight, mass loss and water absorption in phosphate buffered saline (PBS) at 37 °C for up to 104 weeks. The bioactivity of certain samples and the changes in pH of the buffer solution were also defined.

The results showed that the addition of the osteoconductive filler material impaired the initial mechanical properties of the composites. Osteoconductive filler addition was also observed to modify the degradation kinetics and material morphology of the matrix material. The overall degradation rate of the matrix polymer was observed to be delayed when filler material was added. This was thought to be due to the neutralizing capability of the osteoconductive filler materials.

All composites studied exhibited certain degradation behavior depending on the filler and matrix material used and their proportions as well as on microstructural, macrostructural and environmental factors. Thus it is difficult to predict the exact degradation profile for a certain composite composition. The optimal combination of the matrix polymer and the filler material will be largely dependent on the application planned. When the optimal composition of the composite is found, self-reinforced osteoconductive bioabsorbable composites can be considered as a potential implant material for small bone fracture fixations.