Mineralized tissue is biological tissue in which mineral forms a substantial structural component of an organic matrix. Bone, dentine and enamel are familiar examples, but they differ markedly in composition and architecture. In bone, mineral based on hydroxyapatite is associated with a collagen-rich extracellular matrix; biological cells form and maintain that matrix. Mineralization describes a material feature, not the absence of living activity.[ref]
A mineralized tissue is therefore also a biological composite material. Mineral amount, organic matrix, water, porosity and orientation can each affect the response. The Young's modulus of an entire specimen cannot be inferred from mineral content alone: constituent geometry changes load sharing, just as it does in the isostress rule of mixtures and the longitudinal rule of mixtures. These ideal mixture models clarify the role of geometry without being exact models of a real bone.
Even the phrase “mineral fraction” needs a specified measurement. Consider a hypothetical dry two-component specimen with of mineral of density and of organic material of density . Its mineral mass fraction is , but its mineral volume fraction isThese chosen numbers describe a bookkeeping example, not a measured tissue composition. Adding water or pores changes which total volume belongs in the denominator. A mechanical mixture calculation needs the appropriate volume fractions, not a mass percentage silently substituted for them.
An instructive primary structural study examined elephant ivory, a form of dentine, with complementary X-ray scattering, polarized-light microscopy and scanning electron microscopy. The investigators found that mineralized collagen orientations differed between the outer and inner parts of the tusk, including an outer wrapping orientation and a predominantly axial, plywood-like arrangement farther inside.[ref]
The three-dimensional arrangement of the mineralized collagen fibers in elephant ivory and its relation to mechanical and optical properties by M. Albéric and others. Primary abstract, complementary methods and region-dependent fibre orientations
The result motivates combining methods: a bright pattern in polarized light, an electron-microscope image and a scattering pattern provide different evidence about the same organized mineralized tissue. Observing orientation is not by itself a direct measurement of its full mechanical constitutive equation. New to topics? Read the docs here!