Recently, there has been much interest in developing bio-analogue composites which have advantages over conventional artificial hard-tissue replacement materials in that their mechanical and biological properties can be tailored in order to meet specific clinical requirements. There is a challenge, however, to manufacture the composite with structure similar to the complex natural bone. Physical processing (e.g. mechanical compounding) is one of the important methods to produce the composites, besides chemical processing (e.g. in situ synthesis) and biological processing (e.g. biominerlization), for the simplicity to be controlled....[ Read more ]

Recently, there has been much interest in developing bio-analogue composites which have advantages over conventional artificial hard-tissue replacement materials in that their mechanical and biological properties can be tailored in order to meet specific clinical requirements. There is a challenge, however, to manufacture the composite with structure similar to the complex natural bone. Physical processing (e.g. mechanical compounding) is one of the important methods to produce the composites, besides chemical processing (e.g. in situ synthesis) and biological processing (e.g. biominerlization), for the simplicity to be controlled.

Although high density polyethylene has been used as a matrix material for hydroxyapatite (HA) reinforced bio-composites, its mechanical properties are inferior to ultrahigh molecular weight polyethylene, especially wear resistance and impact toughness. The main obstacle to use UHMWPE as bio-composite matrix is its difficulties in process due to its high viscosity. The primary aim of this project is to process HA/UHMWPE for orthopaedic applications using innovative processing methods.

An innovative processing developed in this project is briefly described as follows. HA and UHMWPE powders are compounded using ball milling in the presence of a non-toxic organic solvent - ethanol, the mixture was compressed into semisolid disc using hot press and then the compact disc was swollen in paraffin oil. The oil in the swollen sample was removed by extraction and hot press afterwards. Finally the sample was hot pressed again under different conditions.

Various techniques, including thermal analysis, microstructure characterization and mechanical evaluation, were carried out to investigate the effect of HA on UHMWPE and the processing parameters on the performance of the composite. TGA results showed that about 53wt% of HA were compounded with UHMWPE. DSC analysis indicated that HA was inert filler to UHMWPE with slightly affecting of the melting temperature and trivial increasing of crystallinity. SEM micrographs illustrated that HA was dispersed homogeneously in the matrix almost nano-scale. TEM characterization revealed the interface adhesion between the filler and the matrix was fairly good after swelling treated. Tensile testing demonstrated that the composite exhibited Young's modulus and yield strength in the middle of cancellous and cortical bone but more close to cancellous bone.

This work demonstrates that it is promising to use ball milling and swelling process to make HA/UHMWPE composites. The ball milling not only generates good mixture but also reduces HA particles effectively. Swelling further improves penetration of HA in polymer chains and thus interfacial adhesion. Improvements of the composite mechanical properties by such novel processing can extend applications of UHMWPE as biomaterials.