Biomaterials 23 (2002) 4295–4302 Effect of hydroxyapatite morphology/surface area on the rheology and processability of hydroxyapatite filled polyethylene composites R. Joseph a,1 , W.J. McGregor a,2 , M.T. Martyn b , K.E. Tanner a, *, P.D. Coates b a IRC in Biomedical Materials, Queen Mary University of London, Mile End Road, London, E1 4NS, UK b IRC in Polymer Science & Technology, Department of Mechanical and Medical Engineering, Bradford University, Bradford BD7 1DP, UK Received 20 September 2001; accepted 16 May 2002 Abstract The commercial success of hydroxyapatite (HA) filled polyethylene composite has generated growing interest in improving the processability of the composite. A number of synthetic procedures and post synthesis heat treatment of HA has lead to the availability of powders with widely varying morphological features. This paper addresses the effect of morphological features of HA on the rheology and processability of an injection-moulding grade HA-HDPE composite. The results showed that low surface area HA filled composite exhibited better injection processing characteristics through improved rheological responses. The effect of reducing the surface area of the filler is to require less polyethylene to wet the filler and allows more polyethylene to be involved in the flow processes. These changes reduced the temperatures and pressures required for successful processing. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Hydroxyapatite; Composite; Rheology; Injection moulding; Polyethylene; Morphology; Surface area 1. Introduction The concept of producing bioactive composites for bone replacements by reinforcing a bioinert high-density polyethylene (HDPE) matrix with a bioactive hydro- xyapatite (HA) ceramic was introduced in early 1980s by Bonfield [1]. Downes et al. [2] and Tanner et al. [3] have reported successful clinical applications of this compo- site as orbital implants to treat problems such as post- enucleation socket syndrome and orbital floor fractures. These ophthalmic problems arise when the normal position of an eye or ocular prosthesis, i.e., artificial eye, is disturbed by trauma or a volume deficient socket. Another successful clinical application of HA-HDPE composite is as middle ear implant. The middle ear contains the three auditory bones which conducts sound [4]. A chronic infection of these bones or trauma can result in hearing loss. Dornhoffer [5] designed two ossicular replacement prostheses with a HA head and a HA-HDPE composite shaft. The preliminary clinical results of both designs showed that the majority of the patients have good or excellent post-operative hearing. Furthermore the implant could easily be shaped to fit individual patients. The properties of HA-HDPE composites have been studied extensively [6–11]. Recently injection moulding of middle ear implants has been performed successfully [12] making the whole implant out of HA-HDPE composites. Since the bioactivity of the composite is solely contributed by HA, the amount of HA controls the bioactivity. HA can be produced by a number of techniques, namely, precipitation from a solution [13], microwave treatment of raw materials [14] and sol–gel processing [15]. HA also can be subjected to post production heat treatment processes such as calcina- tion and sintering. All these control the structural and morphological features of the powder obtained [13,16]. This paper addresses the effect of surface area and morphology of HA on the rheology and processa- bility of an injection-moulding grade HA-HDPE com- posite. *Corresponding author. Tel.: +44-20-7882-5318; fax: +44-20-8983- 1799. E-mail address: k.e.tanner@qmul.ac.uk (K.E. Tanner). 1 Now at: Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum-695012, Kerala, India. 2 Now at: Pipeline Developments Ltd., Salford Quays, Salford, Manchester, M5 2XW, UK. 0142-9612/02/$-see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII:S0142-9612(02)00192-8