Macromolecular Templating for the Formation of Inorganic-Organic Hybrid Structures
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Templating for the Formation of Inorganic-Organic Hybrid Structures
Takashi Kato, Takeshi Sakamoto, and Tatsuya Nishimura Abstract Biominerals such as the nacre of shells, spicules of sea urchins, teeth, and bones are inorganic-organic hybrids that have highly controlled hierarchical and complex structures. These structures are formed in mild conditions, and the processes are controlled by macromolecular templates of proteins, peptides, and polysaccharides. Materials scientists can obtain ideas from the structures, properties, and formation processes of biominerals for use in creating synthetic, biomimetic materials. This article highlights bioinspired synthetic approaches to the development of organic/CaCO3 hybrids using macromolecular templates. These hybrids have oriented, patterned, and 3D complex structures, as well as thin films with smooth surfaces. The structures are formed by templating synthetic and semisynthetic macromolecules. These materials have great potential for new functional materials.
Introduction Living organisms form a variety of inorganic-organic hybrids.1 These hybrids have highly controlled hierarchical and complex structures, which are formed at ambient temperature. The endoskeletons of mammals, such as bones, teeth, and ivory, are hybrids of hydroxyapatite (Ca10(PO4)6(OH)2) and organic macromolecules. Calcium carbonate (CaCO3) is used for the exoskeletons of mollusks and crustaceans, such as the nacre lining of shells and the spines of sea urchins, which are hybrids with macromolecules.2 Silica and iron oxides are also employed for biominerals.1,2 In the biomineralization process, biomacromolecules such as proteins, peptides, and polysaccharides form organized structures, which serve as templates for inorganic crystallization and hybrid formation. For example, the nacre of shells takes the form of layered hybrid structures (Figure 1), which resemble
bricks and mortar.1–7 During nacre formation, pre-organized layered sheets consisting of chitin, a natural polysaccharide (Figure 2a, left), and hydrophobic macromolecules are covered with acidic hydrophilic proteins (Figure 1b). Oriented aragonite crystals (CaCO3) are formed on and between the pre-organized macromolecular sheets.7 Although we have not yet fully understood the mechanism, materials scientists can obtain some ideas from biomineralization.1,2,8–16 Biominerals are environmentally friendly and ubiquitous on the earth. Mineralization proceeds under mild conditions, which is energy saving. Moreover, highly organized structures are formed by macromolecularinduced processes, resulting in the induction of significant mechanical properties. In the biomineralization process, the interactions between the acidic moieties and inorganic substances are critical.
MRS BULLETIN • VOLUME 35 • FEBRUARY 2010 • www.mrs.org/bulletin
The soluble biomacromolecules are adsorbed on the insoluble matrices and constitute the nucleation sites of controlled crystals. In the bioinspired synthetic systems, use of designed soluble macromolecular templates i
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