Biominerals such as seashells, pearls, exoskeletons of crustaceans, and bones are good models for the development of new hybrid synthetic materials with significant properties and environmental benignity. For example, the nacre of the shell has layered structures of calcium carbonate and biopolymers, resulting in their high mechanical toughness and unique optical properties. The exoskeletons of crustaceans are also fascinating biominerals in the view of materials science because they are ductile, tough, and lightweight. The elaborate structures of these biominerals are controlled by the cooperative effects of the biopolymers.

We have focused on synthetic approaches to the preparation of organic/calcium carbonate hybrid materials by mimicking the formation process of the nacre of shells and other biominerals. Polymer/calcium carbonate layered thin films have been successfully obtained by using these self-organization processes. The interactions between the functional group of polymers and calcium ions play key roles in thin film crystallization of calcium carbonate. We have also succeeded in the formation of thin films with selected morphologies of calcite, aragonite, and vaterite by tuning assembled structures of synthetic polymer templates. In addition, patterned thin-film composites of calcium carbonate have been developed, whose structures have not yet been seen in nature. Furthermore, we have succeeded in preparing uniaxially oriented calcium carbonate thin-film crystals on a polymer matrix by using a natural peptide (CAP-1) isolated from the exoskeleton of a crayfish.

(The study using the natural peptide (CAP-1) is conducted by collaboration with Prof. H. Nagasawa at Graduate School of Agricultural and Life Sciences.)

We have developed hydroxyapatite liquid crystals that respond to external stimuli by controlling the morphologies of hydroxyapatite nanocrystals using acidic polymers. The obtained hydroxyapatite nanorods show liquid-crystalline properties in condensed aqueous colloidal dispersions and macroscopicallsy align by mechanical shearing, spin-coating and megnetic fields. Thes new biocompatible liquid crystals have great potentials for their application to various functional materials.

1. Stimuli-Responsive Hydroxyapatite Liquid Crystal with Macroscopically Controllable Ordering and Magneto-Optical Functions
   M. Nakayama, S. Kajiyama, A. Kumamoto, T. Nishimura, Y. Ikuhara, M.i Yamato, and T. Kato
   Nature Commun., 9, 568 (2018).
2. One-Dimensional Supramolecular Hybrids: Self-Assembled Nanofibrous Materials Based on a Sugar Gelator and Calacite Developed along an Unusual Axis
   H. Sukegawa, T. Nishimura, M. Yoshio, S. Kajiyama, and T. Kato
   CrystEngComm, 19, 1580-1584 (2017) (Back Cover).
3. Rapid and Topotactic Transformation from Octacalcium Phosphate to Hydroxyapatite (HAP): A New Approach to Self-Organization of Free-Standing Thin-Film HAP-Based Nanohybrids
   Satoshi Kajiyama, Takeshi Sakamoto, Moe Inoue, Tatsuya Nishimura, Taishi Yokoi, Chikara Ohtsuki, and Takashi Kato
   CrystEngComm, 18, 8388-8395 (2016) (Back Cover).
4. Biomineralization-Inspired Preparation of Zinc Hydroxide Carbonate/Polymer Hybrids and Their Conversion to Zinc Oxide Thin-Film Photocatalysts
   Shunichi Matsumura, Yoshimasa Horiguchi, Tatsuya Nishimura, Hideki Sakai, and Takashi Kato
   Chem. Eur. J., 22, 7094-7101 (2016) (Hot Paper) (Frontispiece).
5. Liquid-Crystalline Calcium Carbonate: Biomimetic Synthesis and Alignment of Nanorod Calcite
   Masanari Nakayama, Satoshi Kajiyama, Tatsuya Nishimura, and Takashi Kato
   Chem. Sci., 6, 6230-6234 (2015).
6. Formation of Helically Structured Chitin/CaCO₃ Hybrids through an Approach Inspired by the Biomineralization Processes of Crustacean Cuticles
   Shunichi Matsumura, Satoshi Kajiyama, Tatsuya Nishimura, and Takashi Kato
   Small, 11, 5127-5133 (2015).
7. Liquid-Crystalline Biomacromolecular Templates for the Formation of Oriented Thin Film Hybrids Composed of Ordered Chitin and Alkaline-Earth Carbonate
   Tatsuya Nishimura, Ken Toyoda, Takahiro Ito, Yuya Oaki, Yukiko Namatame, and Takashi Kato
   Chem. Asian J., 10, 2356-2360 (2015).
8. Tuning of Morphology and Polymorphs of Carbonate/Polymer Hybrids Using Photoreactive Polymer Templates
   Takeshi Sakamoto, Yosuke Nishimura, and Takashi Kato
   CrystEngComm, 17, 6947-6954 (2015).
9. Macromolecular Templates for the Development of Organic/Inorganic Hybrid Materials
   Tatsuya Nishimura
   Polym. J., 47, 235-243 (2015). (Focus Review, Open Access)
10. Hydroxyapatite Formation on Oxidized Cellulose Nanofibers in a Solution Mimicking Body Fluid
    Seira Morimune-Moriya, Sakina Kondo, Ayae Sugawara-Narutaki, Tatsuya Nishimura, Takashi Kato and Chikara Ohtsuki
    Polym. J., 47, 158-163 (2015). (Special Issue: Fusion Materials)

11. Organic/Inorganic Fusion Materials: Cyclodextrin-Based Polymer/CaCO₃ Hybrids Incorporating Dye Molecules through Host-Guest Interactions
    Fangjie Zhu, Tatsuya Nishimura, and Takashi Kato
    Polym. J., 47, 122-127 (2015). (Special Issue: Fusion Materials)

12. Biomineralization-Inspired Approach to the Development of Hybrid Materials: Preparation of Patterned Polymer/Strontium Carbonate Thin Films Using Thermoresponsive Polymer Brush Matrices
    Yulai Han, Tatsuya Nishimura, and Takashi Kato
    Polym. J., 46, 499-504 (2014). (Special Issue: Biorelated Polymers and Materials)
13. Bioinspired Stiff and Flexible Composites of Nanocellulose-Reinforced Amorphous CaCO₃
    Tsuguyuki Saito, Yuya Oaki, Tatsuya Nishimura, Akira Isogai, and Takashi Kato
    Materials Horizons, 1, 321-325 (2014).
14. Morphology Tuning in the Formation of Vaterite Crystal Thin Films with Thermoresponsive Poly(N-isopropylacrylamide) Brush Matrices
    Yulai Han, Tatsuya Nishimura, and Takashi Kato
    CrystEngComm, 16, 3540-3547 (2014).
15. Aragonite Nanorods in Calcium Carbonate/Polymer Hybrids Formed through Self-Organization Processes from Amorphous Calcium Carbonate Solution
    Satoshi Kajiyama, Tatsuya Nishimura, Takeshi Sakamoto, and Takashi Kato
    Small, 10, 1634-1641 (2014).
16. Supramolecular Effects on Formation of CaCO₃ Thin Films on a Polymer Matrix
    Fangjie Zhu, Tatsuya Nishimura, Hiroki Eimura, and Takashi Kato
    CrystEngComm, 16, 1496-1501 (2014).
17. Tuning the Stability of CaCO₃ Crystals with Magnesium Ions for Formation of Aragonite Thin Films on Organic Polymer Templates
    Fangjie Zhu, Tatsuya Nishimura, Takeshi Sakamoto, Hidekazu Tomono, Hiroki Nada, Yasushi Okumura, Hirotsugu Kikuchi, and Takashi Kato
    Chem. Asian J., 8, 3002-3009 (2013).
18. CaCO₃/Chitin Hybrids: Effects of Recombinant Acidic Peptides Designed Based on a Peptide Extracted from an Exoskeleton of a Crayfish on Morphologies of the Hybrids
    Hiromu Kumagai, Ryou Matsunaga, Tatsuya Nishimura, Yuya Yamamoto, Satoshi Kajiyama, Yuya Oaki, Kei Akaiwa, Hirotaka Inoue, Hiromichi Nagasawa, Kohei Tsumoto, and Takashi Kato
    Faraday Discuss., 159, 483-494 (2012).
19. Photoimaging of Self-Organized CaCO₃/Polymer Hybrid Films by Formation of Regular Relief and Flat Surface Morphologies
    Takeshi Sakamoto, Yosuke Nishimura, Tatsuya Nishimura, and Takashi Kato
    Angew. Chem. Int. Ed., 50, 5856 (2011).
20. CaCO₃/Chitin-Whisker Hybrids: Formation of CaCO₃ Crystals in Chitin-Based Liquid-Crystalline Suspension
    Yuya Yamamoto, Tatsuya Nishimura, Tsuguyuki Saito, and Takashi Kato
    Polym. J., 42, 583 (2010).
21. Three-Dimensional Relief Structures of CaCO₃ Crystal Assemblies Formed by Spontaneous Two-Step Crystal Growth on a Polymer Thin Film
    Takeshi Sakamoto, Akira Oichi, Yuya Oaki, Tatsuya Nishimura, Ayae Sugawara, and Takashi Kato
    Cryst. Growth Des., 9, 622 (2009).
22. Nanosegregated Amorphous Composites of Calcium Carbonate and an Organic Polymer
    Yuya Oaki, Satoshi Kajiyama, Tatsuya Nishimura, Hiroaki Imai, and Takashi Kato
    Adv. Mater., 20, 3633 (2008).
23. Macroscopically Ordered Polymer/CaCO₃ Hybrids Prepared by Using a Liquid-Crystalline Template
    Tatsuya Nishimura, Takahiro Ito, Yuya Yamamoto, Masafumi Yoshio, and Takashi Kato
    Angew. Chem. Int. Ed., 47, 2800 (2008).
24. Effects of Peptides on CaCO₃ Crystallization: Mineralization Properties of an Acidic Peptide Isolated from Exoskeleton of a Crayfish and Its Derivatives
    Yuya Yamamoto, Tatsuya Nishimura, Ayae Sugawara, Hirotaka Inoue, Hiromichi Nagasawa, and Takashi Kato
    Cryst. Growth Des., 8, 4062 (2008).
25. Self-Organization of Oriented Calcium Carbonate/Polymer Composites: Effects of a Matrix Peptide Isolated from the Exoskeleton of a Crayfish
    Ayae Sugawara, Tatsuya Nishimura, Yuya Yamamoto, Hirotaka Inoue, Hiromichi Nagasawa, and Takashi Kato
    Angew. Chem. Int. Ed., 45, 2876 (2006).
26. Assembled Structures of Nanocrystals in Polymer/Calcium Carbonate Thin-Film Composites Formed by Cooperation of Chitosan and Poly(Aspartate)
    Ayae Sugawara, Akira Oichi, Hiroko Suzuki, Yuzo Shigesato, Toshihiro Kogure, and Takashi Kato
    J. Polym. Sci. Part A: Polym. Chem., 44, 5153 (2006).
27. Self-Organization of Patterned Caco3/Polymer Composite Films: Tuning of Their Morphologies by the Change of Molecular Weights of Acidic Polymers
    Takeshi Sakamoto, Akira Oichi, Ayae Sugawara, and Takashi Kato
    Chem. Lett., 35, 310 (2006).
28. Calcium Carbonate/Polymer Composites: Polymorph Control for Aragonite
    Ayae Sugawara and Takashi Kato
    Compos. Interfaces, 11, 287 (2004).
29. Self-Organized Calcium Carbonate with Regular Surface-Relief Structures
    Ayae Sugawara, Takeshi Ishii, and Takashi Kato
    Angew. Chem. Int. Ed., 42, 5299 (2003).
30. Template Effect of Crystalline Poly(Vinyl Alcohol) for Selective Formation of Aragonite and Vaterite Caco3 Thin Films
    Naoya Hosoda, Ayae Sugawara, and Takashi Kato
    Macromolecules, 36, 6449 (2003).
31. Thin-Film Formation of Calcium Carbonate Crystals: Effects of Functional Groups of Matrix Polymers
    Naoya Hosoda and Takashi Kato
    Chem. Mater., 13, 688 (2001).
32. Aragonite CaCO₃ Thin-Film Formation by Cooperation of Mg2+ and Organic Polymer Matrices
    Ayae Sugawara and Takashi Kato
    Chem. Commun., 2000, 487.
33. Layered Thin-Film Composite Consisting of Polymers and Calcium Carbonate: A Novel Organic/Inorganic Material with an Organized Structure
    Takashi Kato, Takuo Suzuki, and Taku Irie
    Chem. Lett., 2000, 186.
34. A New Approach to Organic/Inorganic Composites. Thin Film Coating of CaCO₃ on a Chitin Fiber in the Presence of Acid-Rich Macromolecules
    T. Kato and T. Amamiya
    Chem. Lett., 1999, 199.
35. Effects of Macromolecules on the Crystallization of CaCO₃ the Formation of Organic/Inorganic Composites
    Takashi Kato, Takuo Suzuki, Takahiro Amamiya, Taku Irie, Makoto Komiyama, and Hiroshi Yui
    Supramol. Sci., 5, 411 (1998).

Selected review papers on this subject:

1. Macromolecular Templating for the Formation of Inorganic-Organic Hybrid Structures
   Takashi Kato, Takeshi Sakamoto, and Tatsuya Nishimura
   MRS Bulletin, 35, 127 (2010).
2. Calcium Carbonate-Organic Hybrid Materials
   Takashi Kato, Ayae Sugawara, and Naoya Hosoda
   Adv. Mater., 14, 869 (2002).
3. Polymer/Calcium Carbonate Layered Thin-Film Composites
   Takashi Kato
   Adv. Mater., 12, 1543 (2000).