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Dy) to react and form an insoluble phosphate.* When calciumcontaining borate glasses were reacted a semicrystalline or gel calcium phosphate formed that had a composition very similar to hydroxyapatite. Although early work by Hench and colleagues has indicated the need for the formation of a silica gel surface layer for silicate glasses to be bioactive, the work of Day and colleagues has indicated that a silica gel is not always necessary for bioactivity.
6.4 LEAD-CONTAINING GLASSES
Yoon [6.71] found that lead release was a linear function of pH when testing lead-containing glasses in contact with various beverages. Low pH beverages such as orange juice or colas, leached lead more slowly than did neutral pH beverages such as milk. This dependence upon pH was also reported by Das and Douglas [6.16] and by Pohlman [6.72]. Later, Yoon [6.73] reported that if the ratio of moles of lead plus moles of alkali per moles of silica were kept below 0.7, release in 1 hr was minimized. If this ratio was exceeded, lead release increased linearly with increasing PbO content. Lehman et al. [6.74] reported a slightly higher threshold for more complex compositions containing cations of Ca2+ and
A13+ or B3+, in addition to the base Na2O–PbO–SiO2 composition. The lead release in these complex compositions was not linear but increased upward with increased moles of modifiers. Lehman et al. related the mechanism of release or corrosion to the concentration of nonbridging oxygens. A threshold concentration was necessary for easy diffusion of the modifier cations. This threshold was reported to be where the number of nonbridging oxygens per mole of glass-forming cations equaled 1.4.
Krajewski and Ravaglioli [6.75] correlated the release of Pb2+ by acid attack to the site coordination of the network modifiers. The presence of cations with cubic coordination produced increased Pb2+ release, whereas cations with
* Phosphorus is from a phosphate-buffered saline simulated physiological liquid.
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antiprismatic coordination produced a decreased Pb2+ release.
In general, it has been determined that mixed alkalies lower the release of lead by attack from acetic acid below that of a single alkali-PbO-silicate glass; lead release increased with increasing ionic radius of the alkaline earths; however, combinations of two or more alkaline earths exhibited lower lead release; A12O3 and ZrO2 both lowered the lead release; and B2O3 increased the lead release. Thinner glaze coatings on clay-based ceramic bodies decreased lead release because of interaction of the glaze and the body, providing higher concentration of A12O3 and SiO2 at the glaze surface [6.76].
Haghjoo and McCauley [6.77] found that small substitutions (0.05–0.15 mol%) of ZrO2 and TiO2 to a lead bisilicate glass lowered the solubility of lead ion in 0.25% HCl by an order of magnitude. Additions of A12O3 had a lesser effect, while additions of CaO had essentially no effect.
The mechanism of release or corrosion for these glasses containing lead is similar to those proposed by Charles [6.6] for alkali-silicate glasses. The rate of this reaction depends upon the concentration gradient between the bulk glass and the acid solution and the diffusion coefficient through the reacted layer.
In general, maximum durability can be related to compact, strongly bonded glass structures, which in turn exhibit low thermal expansion coefficients and high softening points [6.78].
6.5 PHOSPHORUS-CONTAINING GLASSES
The study of phosphate glass corrosion has shown that the glass structure plays a very important role in the rate of dissolution. Phosphate glasses are characterized by chains of PO4 tetrahedra. As the modifier (alkalies or alkaline earths) content of these glasses is increased, there is increased cross linking between the chains. When very little cross linkage exists, corrosion is high. When the amount of cross linkage is high, corrosion is low. Similar phenomena should exist for other glass-forming cations that form chain structures (B3+ and V5+).
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During the study of aqueous attack of soda-lime-silica glasses containing P2O5, Clark et al. [6.79] found that a double reaction layer was formed, consisting of a silica-rich region next to the glass and a Ca-P-rich reaction next to the water solution. This Ca-P film eventually crystallized into an apatite structure and provided a good mechanism to bond the glass to bone in implant applications. In order for these compositions to be highly active toward aqueous media, the bioactive glass composition must contain less than 60 mol% SiO2, a high content of Na2O and CaO, and a high CaO/P2O5 ratio [6.80]. When the SiO2 content was greater than 60 mol%, the hydroxyapatite reaction layer did not form within 2–4 weeks. For a glass to be beneficial as an implant, the reactions leading to the formation of the CaO–P2O5-rich surface film must occur within minutes of implantation. The dependency of bioactivity upon the structure of the glass is thus a very important concern in the development of these materials. When the silica content exceeds 60 mol%, the glass structure changes from one of twodimensional sheets containing chains of polyhedra to a threedimensional network common to the high silica glasses. The two-dimensional structure being a more open structure allows more rapid ion exchange and thus faster hydroxyapatite film formation.
Potassium phosphate glasses containing various oxide additions were tested for water solubility by Minami and Mackenzie [6.81], with Al2O3 and WO3 additions yielding the greatest improvement. In alkali phosphate glasses containing Al2O3 or WO3, the durability increased as the ionic radius of the alkali cation decreased, a trend that was common in most glasses.
Reis et al. [6.82] investigated the durability of zinc-iron phosphate glasses in distilled water at 90°C for up to 32 days. They found the durability to be 100 times better than window glass and the dissolution rate to decrease with increasing iron content. Excellent durability of glasses containing more than 30 mol% Fe2O3 was related to the presence of the Fe–O–P bond.
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According to Hench [6.83] in his discussion of bioactive glasses, the dissolution kinetics are a function of the following variables:
1.Composition
2.Particle size
3.Pore size distribution, average size, and volume %
4.Surface area
5.Thermal stabilization temperature
6.Chemical stabilization temperature
The alumina content of bioactive glasses is very important in controlling the durability of the glass surface. The bioactivity, although dependent upon the bulk composition of the glass, decreased beyond acceptable levels once the alumina content increased above 1.0–1.5 wt% [6.84]. This same phenomenon was present for glass compositions containing cations such as Ta2O5 except higher levels were tolerable (1.5–3.0 wt.%).
Avent et al. [6.85] studied the dissolution of Na-Ca- phosphate glasses containing small amounts of silver in an attempt to develop biocompatible controlled release glasses for applications in medical equipment such as urological catheters. It has been known for a long time that traces of silver have bactericidal properties. With that in mind, Avent et al. investigated the dissolution of several glass compositions in distilled water and two different simulated urine solutions at 25 and 35°C. They found that silver release was dependent upon the Na/Ca ratio of the glass and that silver release was double in simulated urine compared to distilled water. They concluded that these glass compositions dissolved by destruction of the links between polyphosphate species with the dominant polyphosphate specie being cyclo— hexophosphate.
6.6 FLUORIDE GLASSES
The corrosion of fluoride glasses has become rather important recently because of their potential application as optical
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components because of their excellent IR transmission properties [6.86] and their application as membranes in fluoride-ion-selective electrodes [6.87]. The corrosion of these glasses is generally characterized by a double interfacial layer, an inner portion of hydrated species and an outer nonprotective layer of crystalline precipitates, generally ZrF4 [6.88], except when highly soluble compounds are present [6.87,6.89]. The reaction:
(6.5)
reported by Ravaine and Perera [6.87] depicts the exchange reaction that forms this interfacial hydrated layer.
Simmons and Simmons [6.89] studied the corrosion of fluorozirconate glasses in water (pH=5.6). A direct correlation was found between the solubility of the modifier additive and the glass durability. Those additives with the greatest water solubility (AlF3, NaF, LiF, and PbF2) were determined to cause the greatest solubility of the glasses. ZrF4, BaF2, and LaF3 exhibited lower solubilities. The corrosion behavior of all the glasses was controlled by the Zr and Ba contents and the pH drift of the solution. The other modifier additives had only a limited effect upon corrosion. The order of leach rate for ZBL glass was Zr>Ba La. The order when Al was added changed to Al>Zr Ba>La, and when Li was added changed to Li> Al>Zr>Ba La. When Na replaced Li, the Al leach rate was lower than the Na, and the others remained the same. The addition of Pb had the greatest effect by not exhibiting the marked decrease in the leach rate with time for the various components.
The major difference between fluorozirconate and silicate glasses was the drift in pH during the corrosion process. The fluorozirconate exhibited a solution pH drift toward acidic values. The equilibrium solution pH for a ZrBaLaAlLi-fluoride glass was found to be 2.6. Additional studies upon crystalline forms of the various additives indicated that the main cause of the drop in pH was the hydrolysis of ZrF4 forming the complex species:
Copyright © 2004 by Marcel Dekker, Inc.