Mechanical Properties of Nacre as a Model for Implant Material

INTRODUCTION: One of the most important properties of implant materials for joint arthroplasty is their wear resistance. Because of their excellent wear properties, ceramic materials have found application in particular in total hip arthroplasty; however, brittle failure remains a serious problem. Basic research has thus been focused on nacre, a natural composite consisting of imbricated aragonite platelets embedded in a protein matrix [1]. The material structure of nacre has been viewed as a promising model to develop better implant materials, since it possesses favorable strength and toughness properties. Some mechanical properties of nacre have been reported. However, many previous investigations concentrated on the stiffness of the material. This study therefore focused on parameters which are better indicators of the material’s suitability as an implant material. Failure strength and toughness of nacre were investigated under standardized conditions. The method of testing fracture toughness (KIc) is known to significantly influence measured values [2]. We used the single-edge-Vnotched-beam (SEVNB) method, which has been proposed as the most reliable method [2]. To allow direct comparison to latest developed bioceramics, which promise high fracture toughness, samples of BIOLOXdelta were also tested with the same SEVNB protocol. MATERIALS AND METHODS: All nacre tests were conducted on polished samples from the shells of the mollusk Pinctada maxima. All bioceramic tests were carried out with polished BIOLOXdelta samples, a zirconia toughened alumina material (CeramTec AG, Germany). A three-point-bending test was performed according to international standards for Young’s modulus, bending strength, and fracture toughness (EN 843, DIN CEN/ TS 14425-5). Nacre and bioceramic specimens were created with dimensions of 25x2x2.5 mm for fracture toughness (Fig. 1). Further nacre specimens of 20x9.5x1.2 mm for modulus testing, and 25x2.5x2 mm for bending strength testing were created. A total of 60 nacre samples were tested in four states of hydration (dry, distilled water, 0.9% NaCl and sea water), with five samples each. Ten dry bioceramic samples were tested for fracture toughness. RESULTS: The material properties of nacre depend to some degree on the medium in which the samples are conditioned (Table 1). The fracture toughness of nacre tended to be higher for specimens conditioned in 0.9% NaCl than for dry specimens (5.3±0.6 vs. 4.3±0.7 MPa m, p=0.061). The fracture toughness of the BIOLOXdelta bioceramic investigated was observed to be higher than dry nacre (Fig. 1; 5.8±0.4 vs. 4.3±0.7 MPa m, p≤0.001). DISCUSSION: The increase in fracture toughness of hydrated nacre was not as large as would be expected based on the difference in stiffness of the matrix material alone that have been reported [1]. The Young’s modulus of nacre observed was similar to published values (64-73 GPa), fracturetoughness somewhat higher (3.3-4.6 MPa m) [1, 3]. The fracture toughness of nacre observed was higher than reported by the manufacturer for the pure alumina bioceramic BIOLOXforte (3.2 MPa m; CeramTec AG), an impressive result [4] considering the poor mechanical properties of the aragonite inorganic component (KIc, Aragonite = 0.5 MPa m) of nacre [5]. However, it should be noted, that the method of testing can significantly influence measured values of fracture toughness [2]. By applying the same SEVNB protocol to the latest developed BIOLOXdelta, we were able to compare the material directly with nacre and found that this bioceramic material can in fact match nature in terms of fracture toughness (Fig. 2). The high fracture toughness of BIOLOXdelta can be explained by its specific structure, in which the mechanism preventing crack propagation is similar to that conveyed by nature in nacre. The data illustrate that nacre possesses impressive mechanical properties, which are particularly interesting for implant materials. The applied methods will allow us to compare new nacre-like bio-mimetic materials to existing medical ceramics, with the aim of producing implant materials with favourable wear properties in combination with a high degree of fracture toughness. A further exploration of the potential of nacre would be to mimic its structural architecture. One could envision a composite material with a comparable architecture as that found in nacre, but using platelets made of a dense bioceramic embedded in a highly cross-linked polyethylene matrix [6]. Such a material could prove to be an ideal, long lasting material for use in manufacturing implants for joint arthroplasty.