Microstructure, mechanical, and biomimetic properties of fish scales from Pagrus major

The fish scale of Pagrus major has an orthogonal plywood structure of stratified lamellae, 1-2 microm in thickness, consisting of closely packed 70- to 80-nm-diameter collagen fibers. X-ray diffraction, energy-dispersive X-ray analysis, and infrared spectroscopy indicate that the mineral phase in the scale is calcium-deficient hydroxyapatite containing a small amount of sodium and magnesium ions, as well as carbonate anions in phosphate sites of the apatite lattice. The tensile strength of the scale is high (approximately 90 MPa) because of the hierarchically ordered structure of mineralized collagen fibers. Mechanical failure occurs by sliding of the lamellae and associated pulling out and fracture of the collagen fibers. In contrast, demineralized scales have significantly lower tensile strength (36 MPa), indicating that interactions between the apatite crystals and collagen fibers are of fundamental importance in determining the mechanical properties. Thermal treatment of fish scales to remove the organic components produces remarkable inorganic replicas of the native orthogonal plywood structure of the fibrillary plate. The biomimetic replica produced by heating to 873 K consists of stratified porous lamellae of c-axis-aligned apatite crystals that are long, narrow plates, 0.5-0.6 microm in length and 0.1-0.2 microm in width. The textured inorganic material remains intact when heated to 1473 K, although the size of the constituent crystals increases as a result of thermal sintering.