IOP PUBLISHING BIOMEDICAL MATERIALS Biomed. Mater. 3 (2008) 025014 (5pp) doi:10.1088/1748-6041/3/2/025014 Influence of hydroxyapatite crystallization temperature and concentration on stress transfer in wet-spun nanohydroxyapatite- chitosan composite fibres J Z Xie 1 , S Hein 2 , K Wang 3 , K Liao 3 and K L Goh 3 1 School of Engineering, Republic Polytechnic, Woodlands Ave 9, Singapore 738964 2 Interdisciplinary Nanoscience Center, Faculty of Science, University of Aarhus, Ny Munkegade, Building 521, Aarhus C 8000, Denmark 3 School of Chemical and Biomedical Engineering, Nanyang Technological University, 637457 Singapore E-mail: gohkl@ntu.edu.sg Received 12 February 2008 Accepted for publication 15 April 2008 Published 14 May 2008 Online at stacks.iop.org/BMM/3/025014 Abstract Hydroxyapatite possesses appropriate osteoconductivity and biocompatibility for hard-tissue replacement implants but suffers from brittleness. One approach to overcome this problem is to incorporate nanometre hydroxyapatite (nHA) into a polymer matrix, such as chitosan, to yield a hydroxyapatite-chitosan (HC) composite. Here, a novel HC composite was synthesized and its elastic properties were investigated by varying (1) nHA concentration and (2) crystallization temperature (T), where T is a parameter which influences the morphology of the crystals. Crystals of nHA were precipitated at T = 40 ◦ C and 100 ◦ C, blended in a chitosan matrix, and wet-spun to yield fibres of HC composites at 5, 15, 20 and 40% concentrations (mass fraction of nHA). Scanning electron microscopy and energy-dispersive x-ray spectroscopy revealed a uniform distribution of nanocrystallites within the fibre. Tensile testing revealed that HC fibres, which comprised nHA treated at T = 100 ◦ C, possessed low tensile strength, σ 0 , and stiffness, E, at low nHA concentrations but high σ 0 and E at higher concentrations, i.e. beyond a 15% mass fraction of nHA. However, with nHA treated at T = 40 ◦ C, the fibres yielded high σ 0 and E at low nHA concentrations but low σ 0 and E at high concentrations. The results strongly implicate the underlying effect of crystallite morphology on stress transfer at different concentrations. 1. Introduction Hydroxyapatite (Ca 10 (PO 4 ) 6 (OH) 2 ) possesses the appropriate osteoconductivity and biocompatibility for use in hard-tissue replacement implants but as a monolithic material, it suffers from brittleness [1]. This limits its role as a load-bearing material. Fortunately, this brittleness can be overcome by reducing the crystalline material to nanocrystallites [2]; these crystallites can be incorporated into a biogradable polymer matrix such as chitosan, a copolymer of N-acetyl glucosamine and D-glucosamine extracted from crustacean shell wastes [3–5], to yield hydroxyapatite-chitosan (HC) composites from which the desired mechanical property can be tailored. Studies on the HC composite for bone marrow implants revealed no inflammation but more importantly, these reports showed that a new bone could form around the implant [1]. Results from experiments have shown that small concentrations of nHA favour high tensile strength (σ 0 ) and stiffness (E) of HC composites [6] in contrast to predictions from finite element models [7]. Underlying the influence on the mechanical properties of HC composites is the morphology of nHA crystallites; this was not addressed in [6]. Interestingly, 1748-6041/08/025014+05$30.00 1 © 2008 IOP Publishing Ltd Printed in the UK