Alignable liquid-crystalline hydroxyapatite as new key for development of teeth-inspired synthetic materials

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Written by Takashi Kato and Masanari Nakayama

Our paper in Nature Communications is here:

We use teeth in our mouth to chew food throughout our lives. Have you thought about why our teeth are so strong and durable? Teeth mainly consist of hydroxyapatite [Ca10(PO4)6(OH)2], which is known as biomineral. The remarkable mechanical properties of teeth are caused by the highly organized hybrid structures of hydroxyapatite nanorods that serve as stiff components and a small amount of organic biomolecules such as proteins that function as soft glue to assemble the hydroxyapatite nanorods. What can we learn from the structures of the biominerals for the design of new bio-friendly materials? One of the characteristics of the tooth nanostructure is uniform alignment of hydroxyapatite nanorods at nanoscale. The details of the formation process are still uncertain and it is not an easy task to synthetically mimic such high-strength and ordered inorganic/organic hybrids. We are challenging this task from the view point of materials chemists, and started the research project to pioneer a new technology to develop synthetic biodegradable mechanically tough nanomaterials.

 Figure 1. Synthesized liquid-crystalline hydroxyapatite/polymer hybrid nanorods.

Our idea here is to use liquid-crystalline formation for hydroxyapatite nanorods for processing to obtain highly aligned materials. It is known that acidic proteins play the key role in the formation of the biominerals. Inspired by the processes, we have been working on the preparation of a variety of organic/inorganic hybrid materials through crystallization control of biominerals using acidic organic polymers under mild conditions (Matsumura, S. et al. Small 11, 5127–5133, 2015; Kato, T. et al. MRS Bulletin 35, 127–132, 2010). Based on these researches, we developed an unprecedented hydroxyapatite nanorods optimized for formation of liquid crystals by using acidic polymers (Figure 1). The atomic-scale structures of the liquid-crystalline hydroxyapatite/polymer hybrid nanorods were disclosed through TEM observation in collaboration with Prof. Yuichi Ikuhara and Dr. Akihito Kumamoto at the University of Tokyo.

The liquid-crystalline hydroxyapatite nanorods spontaneously align in water within a particular concentration range and the alignment can be controlled over centimeter scales with external physical stimuli. For example, unidirectionally aligned structures resembling the nanostructure of tooth enamel are formed by rubbing this liquid crystal of hydroxyapatite/polymer hybrid nanorods. We also achieved the alignment control of the liquid-crystalline hydroxyapatite using magnetic fields in a non-contact manner in collaboration with Prof. Masafumi Yamato in Tokyo Metropolitan University. We believe that these liquid-crystalline hydroxyapatite/polymer hybrids provide us a promising approach to the formation of new biomineral-based synthetic hybrid materials. 

Takashi Kato

Professor , The Unviersity of Tokyo

Takashi Kato is a professor at Department of Chemistry and Biotechnology, School of Engineering, the University of Tokyo since 2000. He received his Ph.D. degree at the University of Tokyo in 1988 under supervision of Professor Toshiyuki Uryu. After his postdoctoral research at Cornell University, Department of Chemistry with Professor Jean M. J. Frechet on supramolecular liquid crystals and polymers (1988-1989), he joined the University of Tokyo. He is currently appointed as Research Supervisor of PRESTO research “Molecular Technology” of Japan Science and Technology Agency (JST) (2012-present). He was the Project Leader of Grant-in-Aid for Scientific Research “Fusion Materials” on Innovative Areas of MEXT (2010FY-2014FY). His research focuses on design, synthesis and functionalization of self-assembled materials including supramolecular soft materials such as liquid crystals, stimuli- and environment-responsive self-organized materials, nanostructured ion- and electron-, and photo- active materials, liquid-crystalline physical gels, functional polymers, and biomineralization-inspired organic/inorganic hybrids. He is the recipient of The Chemical Society of Japan Award for Young Chemists (1993), The Sakurada Memorial Award of the Society of Fiber Science and Technology Japan (1993), The Paper Award of the Japanese Liquid Crystal Society (2000), The Wiley Polymer Science Award (Chemistry) 2001, the 17th IBM Japan Science Award (Chemistry) (2003), the 1st JSPS (Japan Society for the Promotion of Science) Prize (2005), the Award of Japanese Liquid Crystal Society (2008), Molecular Science Forum Lecture Professorship of Institute of Chemistry, Chinese Academy of Science (2009), and The Award of the Society of Polymer Science, Japan (2010), The Fast Breaking Paper of Thomson Reuters (2012), The Paper Award of the Japanese Liquid Crystal Society (2012), The Chemical Society of Japan Award (2017). He is a fellow of Royal Society of Chemistry (2014-present). He has published about 450 papers including original papers, reviews, and chapters of books. He is the editor in chief of "Polymer Journal" (2012-present), advisory boards of "Advanced Materials" (2004-present)," Journal of Materials Chemistry" (2008-present) "Chemical Science" (2010-present), "The Chemical Records" (2004-present), "Small" (2012 - present), “New Journal of Chemistry (2012-present)”, "ChemPlusChem" (2012 - present) and executive board of "Advanced Science" (2014-present). He was a co-editor of “Handbook of Liquid Crystals, 2nd Edition" (2014).