Thin hydroxyapatite

In the realm of dentistry, restoration and protection of tooth enamel are of great importance in operative and conservative dentistry. Yamamoto et al. (2013) used PLD to create a freestanding flexible double-layered sheet composed of a 4 pm thin hydroxyapatite (HAp) layer coated with a 0.5 pm thin TCP layer. The adhesive strength between the HAp/TCP sheet and enamel was 5.7 MPa, decidedly higher than that between the monolayered HAp sheet and enamel (1.9 MPa). Electron microscopical observation revealed that the HAp/TCP sheet was largely fused with the enamel. Therefore, the double-layered HAp/TCP sheet can be used as a material to promote the repair of tooth eruption and to maintain healthy dentine. 

Thin hydroxyapatite coatings via sol-gel synthesis. J. Mater. Sci. Mater. Med., 9, 839-843. 

All animals were able fully weight bearing (FWB) until the end of their observation period of 6 months. It should be emphasised that the selected model of an intramedullary cylindrical rod resembles closely the situation of an endopros-thetic stem replacement operation. The cylindrical prosthesis stem was positioned in the medulla with a rather wide distance from the cortical bone wall. Since the establishment of a solid bony bridge between corticalis and implant requires an extended period of time, a thin hydroxyapatite coating with a short resorption time would not have been sufficient to guarantee a bony integration. Hence a coating thickness of about 150 pm was selected. 

Gross KA, Chai CS, Kannangara GSK, Ben-Nissan B, Hanley L. Thin hydroxyapatite... 

Gross, K., Chai, C., Kannangara, G.S.K., Ben-Nissan, B. and Hanley, L. (1998) Thin hydroxyapatite coatings via sol-gel synthesis. Journal of Materials Science Materials in Medicine, 9, 839-43. 

Two capabilities of ISS are important in applications to the analysis of ceramics. One of these is its surface sensitivity. Many catalyst systems use ceramics where the surface chemistry of the outer 50 A or less is extremely important to performance. Comparing the ratio of H and O to AI or Si is equally important for many systems involving bonding operations, such as ceramic detectors, thin films, and hydroxyapatite for medical purposes. 

Figure 5 shows the effect of phosphate concentration on crystallization rate Rg. Crystallization rate was obtained by weighing seeds in the reactor periodically, in which phosphate concentration drop in the bed was low enough, not to produce concentration distribution, by making the depth of the bed thin. Solubility of hydroxyapatite is so low, that the influent phosphate concentration is equal to super saturation dC. As shown in Figure 5, crystallization rate was proportional to 1.1th order of supersaturation. 

Fig. 10. Scanning electron microscope (SEM) micrograph of the surface of vacuum plasma sprayed hydroxyapatite (HA) coating after treatment in the fluoridating solution (KF 0.05 M KH2PO4 0.15 M pH 7, temperature 100 C). A layer of thin needle-like fluorhydroxyapatite crystals (0.5-3 m long, 0.1-0.3 m width) can be observed. (With kind permission of Springer Science and Business Media).

In the presence of fluoride, calcium ions have been found to be more firmly anchored than in pure hydroxyapatite [67]. This enhances the overall resistance to dissolution. Thus, the presence of a thin stable film of fluorapatite on the surface of hydroxyapatite crystals has two effects, namely (i) resistance to diffusion and dissolution of the anion and (ii) firmer binding of calcium ions into the surface. Both of these make the resulting apatite structure more resistant to dissolution, regardless of the pH of the external medium, and they thereby increase the resistance of the mineral phase to the onset of caries. 

Hydroxyapatite (with some carbonate inclusions) is the most stable of the possible calcium phosphate salts that can be formed under physiological conditions. However, it is not the most rapid one to form. Instead, octacalcium phosphate (OCP) will precipitate more readily than hydroxyapatite. This led Brown in 1987 to propose that, as the kinetically favoured compound, OCP precipitates first, and then undergoes irreversible hydrolysis to a transition product OCP hydrolyzate [68]. This hypothesis is consistent with the observation that enamel comprises hydroxyapatite crystals that have the long, plate-like morphology that is generally considered characteristic of OCP crystals [69]. Overall, it seems that enamel crystals, with their elongated form, result from early precipitation of OCP, which forms a template on which hydroxyapatite units grow epitaxially [70,71]. This leads to enamel mineralisation with the observed thin, ribbon-like structure of crystals. 

In the field of metallic powder applications, a method of plasma spray coating suitable for biomedical materials has been developed using titanium and calcium phosphate composite powder. By means of the mechanical shock process, the appropriate composite powder was prepared, and plasma sprayed on Ti substrate under a low-pressure argon atmosphere. A porous Ti coating layer was obtained in which the surface and the inside of the pores were covered thinly with hydroxyapatite. This surface coating is expected to show excellent bone ingrowth and fixation with bone (21). 

Hontsu, S., T. Matsumoto, J. Ishii, M. Nakamori, H. Tabata, and T. Kawai, Electrical properties of hydroxyapatite thin films grown by pulsed laser deposition. Thin Sol. Films, 295, 214-217 (1997). 

Despite these excellent in vitro results, little progress has been made in vivo. For instance, under clinical conditions, blood proteins have been demonstrated to adsorb extensively to a PEO coated polymer, in contrast to in vitro results [69], Also, in vivo research on PEO coatings in the oral cavity showed poor results [70], despite excellent in vitro reductions in salivary protein adsorption and oral bacterial adhesion to PEO coatings on glass and hydroxyapatite [70,71], Possibly, the durability of the thin layer of grafted PEO chains in the oral cavity was not sufficient over a clinically relevant time scale. 

Cementum is a thin layer of hard material that covers the root of a tooth. Its composition is close to that of skeletal bone, with about 65% hydroxyapatite, 23% collagen, and 12% water. 

During plasma spraying of hydroxyapatite onto titanium alloy substrate surfaces minor amounts of highly reactive CaO are being formed by thermal decomposition that react with the native titanium oxide layer to form calcium titanate, CaTiOs. This thin reaction layer has been implicated to be an important... 

Reactions (1) and (2) have been postulated by Kokubo (1997) and subsequently electrochemically confirmed. The conversion of amorphous calcium titanate to calcium phosphate (Figure 5.2d,e) may be initiated by the formation of a thin layer of calcium phosphate with the configuration of the Posner s cluster, Ca3(P04)2 3 (reaction (5)) that has been shown to have the energetically most stable configuration (Onuma et al., 2000). This amorphous layer takes up more calcium and phosphate ions from the SBF to form OCP (reaction (6)) with Ca/P = 1.33 and finally stoichiometric hydroxyapatite (reaction (7)) with... 

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. 

Some work has been reported on deposition of hydroxyapatite under hydrothermal conditions, that is much above 100 °C. This includes a study by Liu, Savino and Yates (2011) who coated hydroxyapatite on titanium, stainless steel, aluminium and copper substrates by a seeded hydrothermal deposition method. The deposition strategy included an electrochemical reaction to form quickly a thin layer of HAp seed crystals. Subsequent hydrothermal crystal growth from the seed layer resulted in dense and durable HAp films. In a typical hydrothermal synthesis, a solution of Na2EDTA (0.20 M) and Ca(NOs)2 (0.20 M) was prepared in 15 ml water and a solution of (NH4)2HP04 (0.12 M) in 15 ml water was prepared in a second container. The two source solutions were mixed together after the pH of each solution was raised to 10.0 with ammonium hydroxide. The resulting combined solution was stirred at room temperature for about 20 min and then transferred to a Teflon-lined stainless steel pressure vessel of 40 ml internal volume. 

Current trends in coating biomedical implants with osseoconductive nanostruc-tured hydroxyapatite coatings lean towards thin (<50 pm) homogeneous layers that are able to maintain the roughened microstructure of the implant substrate conducive to cell attachment and to minimise residual stresses and thus reduce the risk of coating delamination. 

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