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Table I. Structural and mechanical properties of bone and nacre compared to selected bioinspired thin films, porous scaffolds, and bulk composites. Material composition Total porosity (%) Young's modulus (GPa) Ultimate strength* (MPa) Ultimate strain (%) Fracture toughness (MPa·m1/2) Natural Materials Bone (cancellous) 45–47 HA/collagen > 30 0.001-0.5 0.2-116C 0.3-3 --- Bone (cortical) 45,48,49 HA/collagen < 30 6-28 10-172T 0.9-2 2-11 106-283C 157-238B Abalone nacre 17,50,51 CaCO3/chitin --- 10-147 3-170T 0.2-2 3-9 235-540C 177-197B Thin filmsa Podsiadla et al.26 MTM/PVA --- 106 400T 0.33 --- Bonderer et al.21 Al2O3/chitosan --- 9.6 315T 21 --- Walther et al.27 MTM/PVA --- 45.6 248T 0.9 --- Porous Scaffoldsb Almirall et al.28 HA 51-66 --- 1.2-4.3C --- --- Kim et al.29 ZrO2 74-92 --- 1.6-35C --- --- Tampieri et al.32 HA 70-85 --- 2.5-4C --- --- Fu et al.31 Glass 60-80 --- 40-136C --- --- Deville et al.30 HA 47-64 --- 16-145C --- --- Bulk Compositesc Estili et al.44 Al2O3/CNT --- --- 404B --- 4.62 Munch et al.33 Al2O3/PMMA --- --- 210B --- 5.1 Launey et al.34 Al2O3/Al/Si --- --- 328B --- 8.3 Bouville et al.35 Al2O3/SiO2/CaO --- 290 470B --- 6.2 Libanori et al.37 Al2O3/Epoxy --- 16.6 180B --- 2.56 aHighest reported values for Young's modulus, ultimate tensile strength, and ultimate strain to failure; bRange of values for the total porosity and ultimate compressive strength; cHighest reported values for Young's modulus (flexure), ultimate bend strength, and fracture toughness for crack initiation; *Legend: C: compressive; T: tensile; B: bend. (g), (k)), and organic matrices (Figure 4(d), (h), (l)).33,39,40 The artificial mineral bridges and surface asperities were fabricated by freeze casting,33 and the artificial nacre was produced by sequential deposition of ZrN and PMMA.41 Although all features occur on different length scales in the natural and artificial materials (on the order of ~50 nm for the natural and 500–5000 nm for the artificial), their mechanical functions are the same. The mineral bridges and surface asperities add strength and stiffness, resistance to tensile fracture, and intertile shearing. The organic matrix adds toughness and dissipates energy that accumulates between adjacent lamellae under stress. The micrographs in Figures 3 and 4 show different processing techniques used to synthesize bioinspired materials that mimic structural features of bone and abalone nacre, leading to outstanding mechanical properties that either match or surpass those of their natural counterparts. Ceramic microstructures resulting from freeze casting can be further manipulated by applying an external magnetic or electric field. Figure 5 provides an overview of magnetic-field-assisted freeze casting, and Figure 6 shows an example of a rotating magnetic field used to make helix-reinforced structures inspired by narwhal tusks. Freeze casting efficiently fabricates porous ceramic scaffolds with unidirectionally aligned pores, perpendicular to the direction of ice growth (Figure 5(a)). Applying magnetic fields during freeze casting imposes a second order of microstructural alignment, parallel to the magnetic field direction and perpendicular to the ice growth direction (Figure 5(b)). Finally, infiltrating the bialigned porous scaffolds with a second phase, such as a polymer, yields bulk hybrid composite materials with designer architectures and enhanced mechanical properties (Figure 5(c)). With a static magnetic field, the resulting scaffolds have more than two times the strength in the transverse (magnetic field) direction, without significantly affecting the strength in the longitudinal (ice growth) direction.42 With a rotating magnetic field, the polymer-infiltrated composites have enhanced torsional properties over those produced without the field.43 Summary Structural bioinspired materials development is an active area of investigation and has led to ceramic thin films, porous scaffolds, and composites with unique and superior mechanical properties. Thin films are applied as hard coatings, and the most successful films are synthesized with self assembled monolayers of organic molecules that act as catalysts and templates for the nucleation and growth of the inorganic phase. Porous scaffolds have application as biomedical implants, especially in bone repair and replacement. Because living bone tissue adapts its structure to maximize strength and stiffness in the load bearing direction, synthetic scaffolds must have some degree of anisotropy. The most successful scaffolds are formed from particle infused sacrificial templates, 3D printing, and freeze casting. Fabrication of composites that mimic the structure of nacre is achieved mainly by freeze casting and subsequent polymer or metal infiltration of the aligned porous scaffolds, which leads to extremely strong and tough laminates, with fracture toughness values that exceed any monolithic ceramic. New developments involve field-assisted freeze casting, which can strengthen the scaffold in multiple directions. It is not enough to simply examine a biological material and attempt to duplicate its structure. Rather, functional aspects of the constituents and the microstructural details that result in strength and toughness must be understood. Successful duplication of the important features results in new materials with exceptional properties. Acknowledgements We thank Marc Meyers at University of California, San Diego, and Antoni Tomsia at the Lawrence Berkeley American Ceramic Society Bulletin, Vol. 93, No. 5 | www.ceramics.org 23


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