Use of focused ion beam milling for investigating the mechanical properties of biological tissues: A study of human primary molars

In this paper, the usefulness of the specimen shaping ability of focused ion beam (FIB) milling in the micrometer scale and the high force resolution of the nanoindentation technique are demonstrated on human primary teeth. Micro-cantilevers, with a triangular cross-section <5 μm in width and 10 μm in length, were produced within 50 μm of the dentin–enamel junction (DEJ) using FIB milling, and were point-loaded at their free ends at 20 μN/s until failure using a nanoindenter. The elastic modulus and flexural strength of such micro-samples of human enamel, and their variation with respect to prism orientation, were studied and compared to data from bulk enamel measured using nanoindentation and three-point bend tests. The elastic modulus of the micro-cantilever samples was found to be comparable to that obtained by nanoindentation on bulk samples, but it demonstrated significant anisotropy commensurate with the microstructure of enamel which was not measurable using nanoindentation on bulk samples. The flexural strength of the enamel micro-cantilevers also exhibited strong anisotropy, and was about one order of magnitude higher than that of bulk specimens measured by three-point bending. Through a Weibull analysis, this size dependence of the strength was found to be similar to the normal behaviour in brittle materials. The flexural strength of the enamel samples was also found to be sensitive to changes in the degree of mineralization of the samples.