JOURNAL OF MATERIALS SCIENCE 37 (2 0 0 2 ) 4425 – 4430 Fracture behavior and biocompatibility evaluation of nylon-infiltrated porous hydroxyapatite A. NAKAHIRA, M. TAMAI, S. MIKI, G. PEZZOTTI Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Gosho-kaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan E-mail: nakahira@ipc.kit.ac.jp Hybrid hydroxyapatite/polymer composites were prepared by the infiltration of nylon into porous hydroxyapatite. Porous hydroxyapatite (HAp) bodies were prepared from a whisker-like powder with high aspect ratio by pressureless-sintering at various temperatures. Pore characteristics, such as the fraction of open porosity and the pore size distribution, were designed and evaluated by mercury porosimeter. Through the in situ polymerization of ε-caprolactam, infiltrated into the porous HAp body, a polymeric secondary phase network interpenetrated with the HAp phase was obtained. The obtained hybrid HAp/nylon composites were evaluated with respect to their fracture behavior, i.e., fracture energy, and in vitro bioactivity in simulated body fluid (SBF) in the present paper. These HAp/nylon hybrid composite have a K IC of 1.65 MPam 1/2 and also a good bioactivity according to the results of SBF immersion tests. C  2002 Kluwer Academic Publishers 1. Introduction Hydroxyapatite (HAp) is a useful bioceramic, because of its superior biocompatibility as well as bioglass and AW-glass [1]. For bone and teeth repairs, biomateri- als are demanded of both good biocompatibility and reliable mechanical properties for long periods. How- ever, the nature of the HAp bonding results in low frac- ture toughness and fracture energy, similar of those of glasses [2]. According to other authors, the frac- ture toughness and fracture energy of monolithic HAp are reported to be approximately 0.7–1.0 MPam 1/2 and 1.0 J/m 2 , respectively [3]. The disadvantage of such poor intrinsic properties makes difficult clinical and or- thopedic applications. Therefore, while the improve- ment of the fracture properties by the addition of a sec- ondary phase is necessary for actual applications, at the same time, to maintain the biocompatibility character- istics of HAp is a mandatory requirement. For example, when the incorporation of tetragonal ZrO 2 to a HAp ma- trix was attempted to improve the fracture toughness through a transformation toughening mechanism, the diffusion of calcium due to decomposition of HAp into tricalcium phosphate led to full stabilization of the ZrO 2 particles after the sintering process [4, 5]. Also, it was reported that the hot-pressed Al 2 O 3 -platelet toughened HAp composites had a toughness of 2.0 MPam 1/2 [6]. However, a drastic improvement of the fracture energy of HAp by the dispersion of a ceramic phase is difficult, because of the low intrinsic fracture toughness and frac- ture energy of ceramic phases. Furthermore, coating of HAp onto metals or Al 2 O 3 implants also have been tried, but the coated layer is also affected by brittleness [7, 8]. A different approach for the achievement of im- proved fracture energy and toughness characteristics is to use an infiltration technique. Infiltration of molten metals, polymers, and ceramics into porous ceramics through various processings is a potentially successful technique [9–11]. Especially, polymer infiltration into porous hydroxyapatite would be advantageous to fabri- cate high-performance composites, because of the va- riety of polymers available and their good mechanical properties. The obtainment of high-performance HAp- polymer hybrid composites much demands on the con- trol of the HAp matrix microstructure. In particular, a well-designed pore structure, pore size and pore distri- bution is imperative [12, 13]. In this paper, the design of the porosity characteristics of HAp is first pursued, and then the hybridization of such a porous body with nylon as a basic study is attempted to prepare a new biomaterial composite for bone replacement. In our experiments, we attempted to control the porous structure by using a starting whisker-like HAp powder with high aspect ratio. It was possible to fab- ricate a porous HAp body with a high open porosity fraction and a well-percolated pore distribution upon sintering. A 6-nylon polymer was selected for the hy- bridization of the porous structure, because of its good mechanical properties and good inertness in the human body. 2. Experimental procedure Whisker-like hydroxyapatite powder was synthesized through the hydrolysis of tricalcium phosphate in a mixture of the binary system of H 2 O and isopropyl 0022–2461 C  2002 Kluwer Academic Publishers 4425