Hydroxyapatite bone-like

Nano-hybrid Consisting of Bone-like Hydroxyapatite and Polymer... 

There are several reports on the coating of bone-like hydroxyapatite onto natural polymer substrates. Kawashita et at. [57] reported that carboxymethylated chitin and gellan gum gels, which have carboxyl groups, can form hydroxyapatite on their surfaces in SBF if they are treated with a saturated Ca(OH)2 solution in advance, while curdlan gel, which has no carboxyl group, does not form hydroxyapatite in SBF, even if it is treated with Ca(OH)2 solution. These results support the hypothesis that carboxyl groups induce hydroxyapatite nucleation. Kokubo et at. [58,59] reported that non-woven fabrics of carboxymethylated chitin and alginate fibers also form hydroxyapatite on their surfaces in SBF if they are treated with Ca(OH)2 solution. 

Chapter 24 Synthesis of Bone-Like Hydroxyapatite/Collagen... 

Kikuchi, M. Ikoma, T. Itoh, S. Matsumoto, H.N. Koyama, Y. Takakuda, K. Shinomiya, K. Tanaka, J. Biomimetic synthesis of bone-like nanocomposites using the self-organization mechanism of hydroxyapatite and collagen. Composites Sciences and Technology 2004, 64, 819-825. 

The substitution by other ions reduces the theoretical stoichiometric Ca/P ratio of 1.67 of hydroxyapatite to values for bone-like apatite of less than 1.6 (LeGeros, 1991), even as low as 1.4 (Weng etal., 1994). This non-stoichiometry of biological apatite can be described by the approximate formula (Young and Holcomb, 1982 Hattori and Iwadate, 1990 Liu et al., 2001). 

T. (1996) Acceleration and deceleration of bone-like crystal growth on ceramic hydroxyapatite by electric poling. Chem. Mater., 8, 2697 - 2700. 

Rambo et al. (2006) produced highly porous biomorphous alumina scaffolds by pyrolysis of natural cellulosic sponges that afterwards were infiltrated by aluminium vapour, and subsequently oxidised and sintered. These alumina scaffolds were immersed in highly supersaturated SBF for 4 days to yield a thin (2 pm) calcium phosphate layer with a Ca/P ratio of 1.62, indicating the formation of a Ca-deficient bone-like hydroxyapatite layer. Kim et al. (2003a) had performed similar work on biomorphous zirconia scaffolds previously. 

A supersaturated bioinspired solution was used to coat alumina and zirconia substrates with a thin, poorly crystalline layer of OCP that after heat treatment at 1050 °C for 1 h was converted to hydroxyapatite with particle size of 300 nm (Pribosic, Beranic-Klopcic and Kosmac, 2010). Stefanic et al. (2012) applied a related method to rapidly deposit an OCP layer by a two-step process onto yttria-stabilised tetragonal zirconia polycrystal (Y-TZP). 80vol% Mg-PSZ/20 vol% alumina substrates were used by Nogiwa and Cortes (2006) to deposit biomimetically by immersion in 1.4 SBF a bone-like apatite coating of 15-30 pm thickness, using a bed of either wollastonite ceramics or bioactive glass as an additional source of Ca2+ ions. 

Hence in coatings incubated for only up to 28 days the newly formed layer of secondary hydroxyapatite is decoupled from the stress state of the original layer showing essentially zero stress. Increasing the incubation time may also increase the compressive stress state again caused by reorganisation, consolidation and densification of the biomimetically precipitated Ca-deficient bone-like apatite layer (Gotze etal., 2001). 

These contradictory results about the sequence of calcium phosphate phase nucleation and growth demonstrate vividly how complex and in many important details not yet understood mechanisms appear to govern biomimetic formation of bone-like hydroxyapatite. In particular, the transformation of OCP to HAp was shown to be crystallographically controlled (Fernandez et al., 2003) because hydroxyapatite and octacalcium phosphate can form an epitaxial interface. A new OCP-HA interface model based on an earlier configuration model (Brown, 1962) and using the minimum interface free-energy optimisation was presented. In this new model a structure is formed that consists of half a unit cell of HAp and one unit cell of OCP whereby [0001]HAp is parallel to [001]OCp and [1210]HAp is parallel to [010]Ocp (Figure 7.66). It was shown by self-consistent field methods that the atoms of this model possess similar environments as in the HAp and OCP unit cells and that, as a result of the differences between HAp and OCP unit cell parameters, this interface displays misfit dislocation-like features. 

M, Kikuchi, T. Ikoma, S. Itoh, H, N, Matsumoto, Y, Koyama, K, Takakuba, K. Shinomiya and J, Tanaka, Biomimetic Synthesis of Bone-like Nanocomposites using the Self-organization Mechanism of Hydroxyapatite and Collagen, Composites Science and Technology, 64, 819-825(2004)... 

The use of this class of materials is based on the premise that a more natural hydroxyapatite (HA-like) could act as a scaffold for enhanced bone response — osseointegration — and thereby minimize the long-term healing periods currently required for uncoated metal implants. 

Kikuchi, M., Itoh, S., Ichinose, S., Shinomiya, K., Tanaka, J. Self-Organization Mechanism in a Bone-like Hydroxyapatite/Collagen Nanocomposite Synthesized in vitro and Its Biological Reaction in vivo. Biomaterials. 22, 1705—1711 (2001)... 

Dentin is a bone-like tissue, consisting of collagen fibrils reinforced with carbonated hydroxyapatite particles. It constitutes the body of teeth and is covered by a hard enamel layer. It has long been used by humans as a material in the form of ivory. Given the function of teeth, the design of the dentine structure needs to sustain many years of extreme loads without any repair mechanism similar to bone remodeling. As a consequence, the dentin stmcture is likely to be very well adapted... 

Kikuchi M, Itoh S, Ichinose S, Shinomiya K, Tanaka J. Self-organization mechanism in a bone-like hydroxyapatite/collagen nanocomposite synthesized in vitro and its biological reaction in vivo. Biomaterials 2001 22 1705-11. 

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