JOURNAL OF MATERIALS SCIENCE: MATERIALS IN MEDICINE 11 (2000) 621±628 Dynamic mechanical characterization of hydroxyapatite reinforced polyethylene: effect of particle size S. N. NAZHAT, R. JOSEPH, M. WANG, R. SMITH, K. E. TANNER, W. BONFIELD IRC in Biomedical Materials, Queen Mary and West®eld College, University of London, Mile End Road, London, E1 4NS, UK E-mail: s.n.nazhat@mds.gmw.ac.uk Dynamic mechanical analysis (DMA) was used to characterize biomedical composites consisting of synthetic hydroxyapatite (HA) particulate reinforced polyethylene (PE). The effects of the HA volume fraction, temperature and HA particle on the storage modulus E I  and damping (tan d) were investigated. Increasing HA volume fractions increased E I and decreased tan d. E I was found to be linearly related to the Young's modulus values obtained from quasi-static tensile tests. Relative modulus and damping studies showed that the viscoelastic behavior of un®lled PE was different to that of the ®lled matrix due to the presence of thermally induced tensile stresses in the matrix at the ®ller-matrix interface. # 2000 Kluwer Academic Publishers 1. Introduction Bone, at an ultra-structural level, can be considered as a composite material consisting mainly of a mineral phase, hydroxyapatite, and a protein phase, collagen [1]. Bone is viscoelastic within the range of physiological loading, providing an important energy dissipating mechanism during deformation. Bon®eld [2] stated ``that the existence of viscoelasticity should be recognized as an essential feature of an optimized prosthetic material for bone''. Over the past decade, there has been considerable interest in the development of polymer-based composites as biomaterials for bone and joint replacement. HA reinforced polyethylene (PE) composites (HA/PE) have been designed as bone analog materials [3] and have established successful clinical uses in ophthalmic [4] and otologic applications [5]. Dynamic mechanical analysis (DMA) measures the response of a material to a sinusoidal stress over a range of temperature and frequencies and is sensitive to chemical and physical structure of polymers and their composites. The main variables obtained from DMA are the storage modulus E I , which represents the elastic component of a system and is equivalent to the energy stored through deformation, the loss modulus E II , which represents the viscous component and is equiva- lent to the energy dissipated through deformation, and tan d, which is the ratio of E II =E I and provides a measurement of the damping of the material. However, limited work has been performed previously relating the viscoelastic properties of particulate-®lled polyethylenes to those of the un®lled polymer. A particular HA/PE composite, at the 0.4 volume fraction of HA level (HAPEX TM ) has been shown to be mechanically compatible to bone, with a modulus approaching the lower bound of that for cortical bone [6]. However, composites based on thermoplastic polymers are viscoelastic and the mechanical properties are temperature and strain-rate dependent. As the response to dynamic loading can not be predicted by quasi-static loading alone, evaluation of the viscoelastic properties using DMA is required for the overall mechanical characterization. The objective of this work was to stimulate physiological loading by applying small strain cyclic deformations by DMA on HA/PE compo- sites, so as to determine the associated viscoelastic variables. 2. Materials The matrix investigated was high-density PE (Rigidex HM4560XP, BP Chemicals Ltd, UK) (Table I). Two forms of morphologically similar synthetic HA particles ± HA P88 and HA P81B (Plasma Biotal Ltd, UK) ±were used as reinforcing agents in PE (Table II). Compression molded HA/PE composites were manu- factured with 0.1, 0.2, 0.3, 0.4 and 0.45 HA volume fractions of each HA type and un®lled PE [6]. Synthetic HA particles were mixed with PE and subsequently processed in a twin screw compounder-extruder (Betol BTS40L). The compounded material was extruded into distilled water at room temperature, so as to produce rapid cooling of the extrudate to prevent oxidation and degradation of the polymer matrix. After cooling, the 0957±4530 # 2000 Kluwer Academic Publishers 621