Novel HDPE-based bio-composites for bones replacement application

Abstract

In the present thesis, Alumina toughened Zirconia (ATZ) and Chitosan have been studied as filler of high density polyethylene (HDPE) with the purpose to achieve novel bio-composites intended for bone replacement applications. The composites have been prepared by melt extrusion technique.

Several composites were prepared by melt extrusion technique using different amount of ATZ (1, 2, 3, 5, 7 and 12wt%), obtaining a good dispersion in the polymer matrix, especially at low concentrations. Big aggregates were never formed during the melt processing, suggesting that the procedure was suitable to obtain a good dispersion of the filler in the composites. ATZ presence up to 3wt% leads to an increase of the mechanical properties of the matrix (young modulus and tensile strength), whereas for higher amount a decrease of tensile strength was recorded. ATZ was also effective in reducing the COF and enhancing the wear resistance of the polymer matrix. Among the many interesting properties that chitosan can give to a biomaterial, its proven antibacterial activity can play a fundamental role to avoid possible complication due to infections onset in the surgical site. Chitosan in form of both microparticles and nanoparticles (chitosan M and N) was considered. Two set of composites (i.e. a set for each type of chitosan) were then prepared by melt extrusion technique using different amount of filler. Both chitosan M and N lead to an improving of the composites mechanical properties. In particular, the storage modulus at 40°C for the HDPE/chitosan 98/2 strongly increases with respect to the neat HDPE. In the last part of this thesis work, radiation-induced grafting of HDPE-ATZ composites with chitosan was studied. The induced surface modification was aimed at promoting the interaction between chitosan and composites, in order to match the reinforcing effect of ATZ with the antimicrobial activity of chitosan. The grafting was initiated by irradiation with a low energy electron beam system. Three different type of samples were selected for grafting process (1/99, 2/98 and 7/93 ATZ/HDPE), based on their promising mechanical properties, under the most efficient reaction time assessed on the HDPE (1 hours and 3 hours). Contact angle and ATR measurements have shown evidence of successful grafting process only on the 1/99 ATZ/HDPE composite. In the end, among the materials prepared in this work, the most interesting composites in terms of mechanical properties were subjected to fibroblastic cell adhesion and viability, in order to evaluate the influence of ATZ, chitosan and radiation processing on the biological behavior of the HDPE. Although further research is needed to assess the interaction with more specific cell lines such as osteoblasts and chondrocytes (in order to simulate bones and cartilages), results indicate that both ATZ and chitosan are promising fillers for composites intended for biomedical applications. Indeed, materials with a few amount of filler (2%), were able to improve both cell adhesion and cell viability of the neat HDPE. However, a content of chitosan higher than 2% resulted in a viability lower than the pure polymer. This phenomenon could be related to the ascertained antimicrobial activity of chitosan, that, probably, can results in a detrimental effect on the cells only when the percentage overcomes a threshold value.