Volume hydroxyapatite

The outer layer of a tooth is composed of calcified tissue, enamel, the hardest substance in the body. Enamel consists of 92%, by volume, hydroxyapatite laid down as a series of interlocking prisms aligned perpendicular to the tooth surface. The remaining constituents are 6% water and 2% organic material (mainly collagen). The bulk of the inside of the tooth consists of dentine, which is a vital tissue perforated throughout by tubules. These tubules are continuous from the pulp of the tooth to the outer surface and allow liquid flow. The approximate composition of dentine is 48% hydroxyapatite, 29% collagen and 23% water. 

S. Overgaard, K. Soballe, M. Lind, C. Biinger, Resorption of hydroxyapatite and fluorapatite coatings in man. An experimental study in trabecular bone, J. Bone Joint Surg. (British Volume) 79 (1997) 654-659. 

The various findings about fluoride and its interaction with the hydroxyapatite at the molecular level show that the relationship is complicated and multifaceted. The broad conclusion from the enormous volume of work that has led to our current understanding of the role of fluoride is that it is overwhelmingly beneficial. It promotes numerous desirable properties in tooth mineral, reducing solubility through action in both the saliva and in the mineral phase, it shifts the demineralisation/remineralisation equilibrium in favour of remineralisation, and through its actions in the solid state, ensures that the kinetically favoured OCP is transformed into the more thermodynamically stable hydroxyapatite. Research continues, and there is no doubt that there is still more to learn about the complexities of the interaction of fluoride with hydroxypatite under physiological conditions. 

Purification of Monoclonal Antibody. Immunoglobulins were precipitated from the pooled ascites by addition of an equal volume of saturated ammonium sulfate [50% (NH4)2S04]. The precipitate was collected by centrifugation (20 min 10,240 X g), dissolved in 0.01 M sodium phosphate (pH 6.8), and reprecipitated. After the second ammonium sulfate precipitation, the pellet was dissolved in a minimum volume of 0.01 M sodium phosphate (pH 6.8) and centrifuged for 10 min at 10,600 X g). The resulting supemate was applied to a P6G, gel filtration polyacrylamid, column (Bio-gel Biorad, Rockville Center, NY 1.5 X 40 cm). Fractions containing protein were pooled and applied to a hydroxyapatite column that had been equilibrated with 0.01 M sodium phosphate (pH 6.8). Proteins were eluted with a linear gradient of 0.01 to 0.3 M sodium phosphate. 

The CCC fractions, HDL-LDL and VLDL-serum proteins, were each separately dialyzed against distilled water until the concentration of the potassium phosphate was decreased to that in the starting buffer used for the hydroxyapatite chromatography. These two fractions were concentrated separately by ultrafiltration. The concentrates of both fractions were chromatographed on the hydroxyapatite column. Fig. 4 shows the elution profile on hydroxyapatite obtained from the HDL-LDL fraction. A 1.4-mL volume of the concentrate was loaded onto a Bio-Gel HTP DNA-grade column (5.0 x 2.5 cm I.D.)... 

Figure 10.32 TG curves of hydroxyapatite (HA)-ultra-high molecular weight polyethylene (UHMWPE) composites with different content HA particles. The legends indicate the nominal volume fraction of HA in the UHMWPE matrix.

The mineral phase constimtes about 43% of the volume of bone and mostly contains calcium and phosphate, with small, but highly significant amounts of carbonate (and a few other impurities). The bone mineral is not hydroxyapatite, but rather can be classified as carbonated apatite [Ca5(P04)C03]3. Within the mammalian skeleton, bone mineralisation depends on the organisation of the cross-linked collagen network. Initially, water fills the void space within the organic framework of the collagen matrix of the osteoid. Crystal nucleation occurs... 

Most material studies reported in medical journals are of interest to those involved in mainstream plastic applications. Some medical plastics must perform under constant water immersion. It was reported that absorption of 1 % water reduces the fatigue life of PMMA by a factor of four, since bone cement can only be replaced by a surgical operation such a performance is clearly unacceptable. The use of silane to treat the hydroxyapatite filler in this material reduced water uptake. The water uptake increased with increased concentration of hydroxyapatite. In applications, such as dental fillings, increased water uptake is considered helpful since it compensates for the loss of volume due to shrinkage of the filling during curing. 

Strontium distributes relatively uniformly within the bone volume where it exchanges with calcium in hydroxyapatite (see Section 3.5.1), although small differences in the calcium and strontium distributions within bone have been reported. The Sr Ca concentration ratio in bone increases with age from approximately 0.3 mg strontium/g Ca at birth to a value of 0.5 in adults (Papworth and Vannart 1984 Tanaka et al. 1981). The Sr Ca ratio in bone also has been shown to vary with the bone type ratios in cortical bone were approximately 10-20% higher than in trabecular bone (Tanaka et al. 1981). 

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. 

Alternatively, the volume of the critical nucleus may favour the amorphous state. The number of ions in the critical nucleus will be of the order 10-1000 resulting in a cluster size 5-20 A. If the stable critical nucleus is smaller than the dimensions of the unit cell of a potential crystalline phase, for example, the large unit cell of hydroxyapatite, then since the nucleus grows without reference to the basic structural unit of the crystalline phase the less-ordered atomic arrangement of the amorphous phase may be favoured. 

Typically, the oil phase contained 78% monomer/co-monomer, 8% divinyl benzene (cross-linking agent), and 14% non-ionic surfactant Span 80 (Sorbitan monooleate), while the aqueous phase contained 1% potassium persulfate as the initiator. In most cases studied here, monomer is styrene and when elasticity of the polymer is required, 2-ethylhexyl acrylate (2EHA) was used (styrene/2EHA ratio is 1 4). Whenever additives/fillers are placed in the aqueous phase their amounts are stated as weight percent while the phase volume of the aqueous phase remains constant. In some cases, the aqueous phase contains 0.5% hydroxyapatite and 15% phosphoric acid which is used to dissolve the hydroxyapatite, or alternatively, the aqueous phase may contain varying amounts of water-soluble polymer, such as polyethylene glycol or polyethylene oxide. If the styrene-based PHP is to be sulfonated to obtain ionic-hydrophilic foam, the pre-dispersion of sulfuric acid within the pores is useful, if not essential, and in that case, acids (typically 10%) can be used as the internal phaseP . ... 

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