Figure 4. The upper (dashed line) and lower (solid line) theoretical bounds for the Young’s modulus of a composite of hydroxyapatite mineral and type I collagen fibrils as calculated from Eqs. 1 and 2. The experimental data are shown as the solid bar at a volume percent mineral phase (45%) corresponding to the known composition of dentin. The graph emphasizes the difficulties of modeling mineralized tissues with bounds; The large difference in moduli between the two phases separates the upper and lower bounds by too great a magnitude to be useful.