Many researchers have put much effort in developing organic-inorganic hybrid materials but only to a limited extent on the hybridization process. Crystallization of hydroxyapatite on cavities formed between collagen fibrils is the main process under which bio-ceramic materials are formed. The formed materials are of superior characteristics, light in weight, and environmentally friendly. Teeth and bones are excellent examples of outcomes of these processes in nature that many researchers are interested in biomimicking. For preparation of organic-inorganic hybrid materials, of superior attributes, attention has to be paid to non-collagenous proteins bearing a lot of anionic functional groups which heavily influence the hybridization process. To prepare tough mechanical hybrid materials, several features should be adequately reproduced: (1) stable organic-inorganic interface under mechanical stress, (2) high contents of inorganic phase to lead to high elasticity, (3) inorganic crystals with nano size and a high aspect ratio, rendering wide contact area with organic polymer, and (4) aligned inorganic crystals as exemplified by the brick and mortar model.
Researchers under the leadership of Professor Tadashi Mizutani, from Doshisha University in Japan proposed to enhance the synthesis of tough mechanical hybrid materials using the crystallization of hydroxyapatite in phosphorylated poly(vinyl alcohol) process. They developed hybrid materials of tough mechanical attributes using this process. The work is now published in the journal Materials Science and Engineering C.
Foremost, they prepared partially phosphorylated poly(vinyl alcohol). Calcium element was then added to the aqueous solution of phosphorylated poly(vinyl alcohol) which in turn yielded a transparent gel. The gel was then treated to a five cycle of alternate soaking process in aqueous calcium chloride solution and (NH4)2HPO4 so as to crystallize the hydroxyapatite in the network of phosphorylated poly(vinyl alcohol) gel. These processes resulted in the above mentioned features such as stable organic-inorganic interface, high inorganic content, and nano-sized hydroxyapatite crystals.
During the synthesis, problems with development of densification process had to be resolved. Presence of small organic content in the hybrid material made high temperatures unsuitable for the use of ceramic powders. To solve this problem warm isostatic pressing densification process was effectively used to densify the phosphorylated poly(vinyl alcohol)-hydroxyapatite hybrid powder.
A rectangular parallelepiped specimen was then prepared by uniaxial pressing of compacted hybrid powder. A three-point bending test was conducted which showed that specimens exposed to cold isostatic pressing were brittle compared to those treated using warm isostatic pressing. The researchers found that warm isostatic pressing process improved mechanical properties such as the bending strength of the hybridized material. Toughness and flexibility of the obtained materials were displayed by using specimens containing 10% phosphorylated poly(vinyl alcohol) which portray the largest displacement and rapture at the largest fracture energy. Control of the density of phosphate groups in polymer resulted in nano-sized hydroxyapatite crystals with stable organic-inorganic interface.
This study demonstrated the advantages of using phosphorylated poly(vinyl alcohol) in the synthesis of tough hybrid mechanical materials by crystallization of hydroxyapatite. It has confirmed a promising method of using warm isostatic pressing to develop hybridized material of greater toughness and flexibility. It is now clear that the appropriate density of phosphate groups in organic polymer regulates the crystallization of hydroxyapatite to help formation of stable organic-inorganic interface under mechanical stress.
Crystallization of hydroxyapatite (HAP) controlled by phosphorylated poly(vinyl alcohol), followed by three point bending test of HAP-PPVA hybrid densified with WIP at 700 MPa.
(a) HAP-20%PPVA hybrid compact prepared by 5 times alternate soaking cycles
(b) HAP-10%PPVA hybrid compact prepared by 5 times alternate soaking cycles
(c) HAP-5%PPVA hybrid compact prepared by 5 times alternate soaking cycles
(d) HAP-10%PPVA hybrid compact prepared by 4 times alternate soaking cycles
(e) HAP-5%PPVA hybrid compact prepared by 4 times alternate soaking cycles
Akane Kusakabe, Ken Hirota, Tadashi Mizutani. Crystallization of hydroxyapatite in phosphorylated poly (vinyl alcohol) as a synthetic route to tough mechanical hybrid materials. Materials Science and Engineering C volume 70 (2017), pages 487–493.
Department of Molecular Chemistry and Biochemistry, Faculty of Science and Engineering, Doshisha University, Kyotanabe, Kyoto 610-0321, Japan.Go To Materials Science and Engineering C