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

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

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. 

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