Is magnesium aluminum silicate a carcinogen?

Kaolin

Kaolin is a hydrated aluminum silicate with a crystalline structure that allows for a large surface area that adsorbs many times its weight in water. Its use in the treatment of diarrhea is based on its purported ability to absorb fluid, bacteria, toxins, and various noxious materials in the gastrointestinal tract, decreasing stool liquidity and frequency. In the colon, it may act as an adsorbent or protectant, but the adsorption is not selective, and it should not be used in children younger than 12 years without physician approval. If taken together, kaolin may adsorb other medications and reduce their systemic absorption. Few controlled clinical studies showing the efficacy of kaolin have been published, and fewer products containing kaolin as the sole active ingredient are presently available in the United States.

3.2 Silicates

The study of gels with the mullite composition (3Al2O3, 2SiO2) is of particular interest, since the crystallization process of premullite gels is known to be affected drastically by their microstructure, giving a good estimation of the scale of mixing between aluminum and silicon oxides. Thus the formation of mullite (in the so-called pseudotetragonal form) as low as 980 °C for gels obtained by etherolysis of SiCl4/AlCl3 mixtures is indicative of a homogeneous distribution of the components on an atomic scale.

Well-condensed gels with various compositions, SiO2–TiO2 and SiO2–ZrO2, have been prepared by aprotic condensation between chloride and isopropoxide functions at 110 °C. The Si/M ratio of the final oxide is controlled by the composition of the starting mixture. SiO2–TiO2 samples within the stable glass region (5 mol% Ti) appear perfectly homogeneous: they crystallize at 900 °C as single-phase cristobalite, with Ti4+ ions substituting Si4+ ions at random. Conversely, the precipitation of anatase is observed for SiO2–TiO2 samples with a high titanium content (20–50 mol% Ti), which are outside the stable glass region. However, the transformation of anatase to rutile is not observed, even after 2 h at 1300 °C (in pure titania gels this transformation takes place between 600 °C and 1000 °C).

The silicon/zirconium oxides arising from alkyl halide elimination remain amorphous after calcination for 5 h at 600 °C. At this stage infrared and 29Si NMR spectroscopy show a large number of Si–O–Zr bonds. The crystallization of tetragonal zirconia takes place at higher temperature; the transformation of tetragonal to monoclinic zirconia is strongly retarded and does not take place after 2 h at 1300 °C. Despite the above-mentioned homogeneity, the crystallization of zircon, ZrSiO4 (for a sample containing 50 mol% Zr), does not take place at a lower temperature than for conventional xerogels (it starts only at 1500 °C). An explanation for this behavior could lie in the incongruent melting of zircon, if one keeps in mind the fact that, on heating, sol–gel-derived glasses usually follow the same sequence of phase transitions as melts of the same composition on cooling.

M–Cl/M–OPri (M=Ti, Zr) condensations are much faster than Si–Cl/Si–OPri condensation (in the absence of titanium or zirconium). However, the fact that no macroscopic precipitation of titanium or zirconium oxide occurs in the mixed Si/Ti or Si/Zr systems and that homogeneous gels form, suggests a leveling of the condensation rates by catalysis of the condensations around the silicon atoms by the transition metal species.

Aluminium

Aluminium has been implicated in the development of respiratory disease during the refining of its principal ore, bauxite, to yield various aluminium oxides (aluminas), in the preparation of the metal by smelting alumina, in the production of corundum abrasive and in the production of special aluminium powders used in explosives.

The Glass Pipette Microelectrode

Glass micropipettes are produced from rubes of aluminum silicate or borosilicate glass, typically with an outer diameter of 2 mm and an inner diameter of 1 mm. If the two ends are pulled away from each other while the center of the tube is heated, the tube will narrow and separate at the middle and two electrodes will form. With proper choice of heating and strength of pull, the narrowed ends of the separated glass tube – the tips – will retain an open lumen and will be continuous with the lumen at the larger end (the shaft) of the glass tube. Tip diameters as small as 0.045 μm have been reported. The thickness of the wall at the tip is typically one-fifth to one-tenth of the tip diameter.

The electrode is filled with a salt solution and connected via a reference electrode (Ag/AgCl or calomel) inserted into the shaft to the input of an amplifier (Figure 1A). The electrode measures the liquid junction potential between the filling solution and the external solution. In the simple case where the electrode contains the same univalent salt (of concentration C2), as the external solution (of concentration C1), this potential equals approximately 56mVU–V/U+VlogC1/C2at 20°C, where V is the mobility of the cation and U that of the anion.

In the general case where the external solution is unknown, the equation for liquid junction potential takes a more complicated form. Yet, any variation in the potential between various sites of measurements can be suppressed if a filling solution of high concentration and with equal mobilities of the anion and cation (e.g., 1–2 mol l−1 KCl) is employed. Thus, the working principle of the electrode depends on free diffusion of the filling solution from the tip into the sample. This has important implications. KCl will be continuously lost into the sample; if this leakage is prevented, then the electrode does not measure the correct potential. With tip diameters of 0.1 μm (impedance of ∼50 MΩ), this leakage rarely constitutes a problem, even for intracellular recordings.

In addition to the liquid junction potential, which can be suppressed, some electrodes exhibit a so-called tip potential, which is ∼5–10 mV. The origin of this potential is unclear. It is associated with narrow tips and is less frequently encountered if the filling solutions are ultrafiltered before use, with a filter limit of 0.2 μm. The tip potential can be defined as the difference in the electrode potential with the tip intact and the tip broken off. The tip potential varies in an unpredictable manner when the tip is placed in different solutions. Such electrodes should be discharged before use.

Glass exposed to water will hydrate and its electrical resistance will decrease. After 2 h of exposure to aqueous media, the resistance of the glass wall may have decreased from say 1000 to 50 MΩ. If the impedance of the electrode is 10 MΩ, this means that a significant shunting of the measured potential may take place. In practice, this means that electrodes ought to be made on the day of the experiment. It is also a common observation that cell debris and other biological material adheres more stubbornly to the sides of micropipettes that have been hydrated. It is often useful to change the electrode after 3–5 h of application in biological fluids.

The input amplifier for the micropipette should have an input resistance of 108–1010 Ω, easily obtained with operational amplifiers with field-effect transistor inputs. If the input is provided with negative capacitance compensation, both the capacitance of the cable that connects the electrode to the amplifier and the capacitance between the inner solution of the electrode and the external bath can be compensated for. With proper compensation the electrode can follow fast changes in, for example, action potentials from nerves or nerve cells.

3.2 SiC(W)–Other Matrices Composites

Whiskers have been incorporated in silicium nitride, mullite, zirconia, and glass (Becher and Rose 1994). In the case of a Si3N4 matrix, the results were disappointing: the reinforcement is less significant than for alumina. This is due to the different microstructures observed for Si3N4: equiaxed or elongated grains depending on the sintering conditions and sintering aids. Toughness values of about 10 MPam1/2 have been observed. For other matrices, the toughness increase is lower.

In order to improve toughness further, various toughening processes can be combined: whisker reinforcement, transformation toughening, and matrix microstructural-grain size effect. Either additive or coupled contributions can be obtained and multiple toughening mechanisms provide significant increase in fracture resistance: values of 10 MPam1/2 have been observed for mullite reinforced by 20 vol.% of SiC(W) and 20 vol.% of t-ZrO2 and toughness higher than 13 MPam1/2 has been achieved for ZTA reinforced with 20 vol.% of SiC(W).

Ceramics

Clay used in ceramics is usually composed of powdered aluminum silicates. It has been found that long-term exposures from inhaling silica dust can result in silicosis. Potters may handle dry clay and respirable dust may accumulate in areas where clay is routinely used. Ceramic glazing components usually consist of silica and a flux. The flux that is often used in ceramics may include heavy metals such as lead or barium. Glazing components are mixed with water and then brushed onto a piece of pottery prior to firing it. Exposure to the fumes from firing pottery with lead glazes may result in lead poisoning, particularly among children. Glazing components may also contain metal oxides such as arsenic, beryllium, cadmium, chromium, and nickel for color. Arsenic, nickel (dust), and chromium(VI) are all classified as known human carcinogens and beryllium and cadmium are probable human carcinogens.

1.10.3 Apatite–Mullite Glass-Ceramics

Hill and coworkers24–26 developed castable FAP-mullite glass-ceramics from the SiO2–Al2O3–P2O5–CaO–CaF2 system that bulk-nucleated, via prior amorphous phase separation. The needle-like apatite crystals in these materials give these glass-ceramics high fracture toughness and high strength.24,27 The high aspect ratio of the FAP crystals which are thought to grow by a screw dislocation mechanism in the c-axis is thought to result in high fracture surface energies. Fig. 5 shows a fracture surface showing the elongated nature of the hexagonal FAP crystals. The mechanical properties are superior to the AW system. These glass-ceramics can be readily cast to shape using a lost-wax casting process and can be used to form complex parts with near-net shapes, overcoming the drawbacks of the AW system. The second phase to form in these glass-ceramics is mullite (2SiO2⋅3Al2O3) which is chemically very stable and serves to lock away the aluminum in a chemically stable inert form. The glass-ceramics are chemically stable following appropriate heat treatment; the residual glass phase has a high network connectivity and low aluminum content, and is chemically stable. They do not release any aluminum ions into solution.

The glass-ceramics do not form HCA in Kokubo׳s SBF test but they do osseointegrate in vivo without any apparent formation of a fibrous capsule layer. In contrast, some of the glasses are degradable and do form HCA in SBF but do not osseointegrate in vivo. An example of an apatite–mullite (AM) glass-ceramic implanted into the femur of a rat and the equivalent amorphous glass of exactly the same chemical composition are shown in Fig. 6.

The AM glass-ceramic osseointegrates28 while the amorphous glass does not osseointegrate and has been fibrous encapsulated. Glass-ceramics, which crystallized to FAP, but which had not crystallized to mullite in addition to FAP, osseointegrated poorly. Fig. 7 shows the XRD patterns for three samples that were implanted; LG112a is the amorphous glass. This sample did not osseointegrate at all. LG112b has been heat treated to give FAP and LG112c has been heat treated to give mullite and a small amount of aluminum phosphate. LG112b osseointegrated poorly while LG112c exhibited better osseointegration and evidence of osteoconduction from the marrow cavity. It is clear that the presence of FAP can give rise to osseointegration provided there is no release of aluminum.

Goodridge et al.29 investigated both AM glass-ceramics and AW glass-ceramics in the same in vivo implant study in the tibae of rabbits. In addition, they also studied the porous selectively laser sintered (SLS) samples for use as tissue-engineered scaffolds. The AM glass-ceramics also did not form HCA after immersion in SBF, nor was there any pH change following immersion and no significant ion release into the SBF solution. Any Al release was below the background levels found in the SBF solution of ~0.2 ppm. Fig. 8 shows scanning electron micrographs of the samples. The cast and heat treated AM glass-ceramic osseointegrates, as does the porous selectively sintered AM glass-ceramic.

The in vivo response of apatite glass-ceramics and their ability to stimulate bone formation and to osseointegrate can be summarized as follows:

The in vivo response depends on the presence of the apatite phase and the residual glass phase. Increasing the apatite volume fraction in the glass appears to increase the osseointegration. The residual glass phase may add to the osseointegration by promoting apatite formation or may hinder osseointegration if the glass phase degrades and releases ions such as Al3+ that are known to inhibit biological mineralization. In the case of aluminum-containing glass-ceramic compositions, it is important that all the aluminum is either locked away in either a chemically inert glass phase or a chemically inert crystalline phase as even trace amounts of the order of 1 ppm of aluminum are known to inhibit the mineralization of the newly forming osteoid.30 The SBF test developed by Kokubo is widely used as a predictor of bioactivity and the ability of a bioceramic to osseointegrate. While this test is of value for bioactive glasses and AW glass-ceramics, its value with regard to aluminum-containing glasses and glass-ceramics must be questioned.

Oxygen concentrators

Oxygen concentrators, also known as pressure swing adsorption systems, extract oxygen from air by differential adsorption. These devices may be small, designed to supply oxygen to a single patient (Fig. 1.22), supply oxygen to an anaesthetic machine (Fig. 1.23) or they can be large enough to supply oxygen for a medical gas pipeline system (Fig. 1.24).