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PubMed2003

Hydrogenated silicon fullerenes: effects of H on the stability of metal-encapsulated silicon clusters.

Ab initio calculations of H interaction on Si12M, Si18M2 (M=Cr, Mo, and W), and Zr@Si(16) fullerene (f) show relatively weak binding of H in agreement with experimental results of H free Si12M and Si18M2 clusters. Adsorption of H enhances sp(3) bonding between the Si atoms, weakens the M-Si cage interactions, and leads to distortions in the cages. Si12CrH12 has 4 mu(B) magnetic moment in contrast to zero for Si12Cr. Removal of the M atom leads to stable empty cages of Si12H12, f-Si16H16, and f-Si20H20 with large highest occupied-lowest unoccupied molecular orbital gaps of 2.5-3.0 eV, making them attractive for optoelectronic applications.

Journal Article↗
PubMed2003

Radiation enhanced silicon self-diffusion and the silicon vacancy at high temperatures.

We report proton radiation enhanced self-diffusion (RESD) studies on Si-isotope heterostructures. Self-diffusion experiments under irradiation were performed at temperatures between 780 degrees C and 872 degrees C for various times and proton fluxes. Detailed modeling of RESD provides direct evidence that vacancies at high temperatures diffuse with a migration enthalpy of H(m)(V)=(1.8+/-0.5) eV significantly more slowly than expected from their diffusion at low temperatures, which is described by H(m)(V)<0.5 eV. We conclude that this diffusion behavior is a consequence of the microscopic configuration of the vacancy whose entropy and enthalpy of migration increase with increasing temperature.

Journal Article↗
PubMed1998

Intraocular lens changes after short- and long-term exposure to intraocular silicone oil. An in vivo study.

OBJECTIVE: Silicone oil frequently is used as a vitreous substitute after complex vitreoretinal procedures. The authors sought to study the effect of short- and long-term exposure to silicone oil on polymethyl methacrylate (PMMA, MC60BM; Alcon, Ft. Worth, TX), silicone (SI-30NB; AMO, Irvine, CA), and soft acrylic (MA60BM; Alcon) intraocular lenses (IOLs). DESIGN: An experimental animal study. INTERVENTION: Forty-one New Zealand white rabbits underwent lensectomy, vitrectomy, capsulotomy, and placement of one of the three types of IOLs into the ciliary sulcus. All lenses were weighed before implantation and 24 hours after explanation. In the short-term study, an fluid-air exchange was performed followed by the use of silicone oil (1000 centistokes) to coat the posterior lens surface. Immediately thereafter, an air-fluid exchange was performed and the remaining silicone on the posterior lens surface was aspirated or wiped or both for 1 minute using a soft-tipped extrusion cannula for 1 minute. In the long-term study, the posterior segment was filled with 1000 centistokes silicone oil after fluid-air exchange. Animals were observed by slit-lamp biomicroscopy and photographed at 1 week, 1 month, and 3 months after surgery. At 3 months, all animals underwent silicone-fluid exchange, an attempt to manually remove any remaining silicone oil, and lens explanation. RESULTS: In the short-term study, no silicone oil remained after manual wiping and/or aspiration in any of the four rabbits implanted with PMMA or acrylic IOLs. In the animals with silicone IOLs, a significant amount of silicone oil remained on the posterior lens surface of all lenses (P < 0.01 for silicone vs. acrylic and silicone vs. PMMA). No statistically significant difference was found when comparing the lens weights in each group before and after implantation. In the long-term study, aqueous droplet formation was found on the posterior lens surface of six of nine PMMA IOLs and ten of ten silicone IOLs at 3 months. No opacities were observed in the group with acrylic IOLs (P < 0.001 for acrylic vs. silicone, P = 0.0018 for acrylic vs. PMMA, and P = 0.047 for PMMA vs. silicone). Adherent silicone oil remained on two of nine PMMA IOLs and on none of ten acrylic IOLs. In contrast, a significant amount of silicone oil remained on the posterior lens surface of ten of ten silicone IOLs (P < 0.001 for silicone vs. acrylic and silicone vs. PMMA). Furthermore, there was a statistically significant increase in lens weights before and after implantation in the silicone IOL group but not in the PMMA or acrylic group (P < 0.01). CONCLUSIONS: It is extremely difficult or impossible to remove remaining silicone oil from the posterior surface of a silicone IOL after short- or long-term exposure to silicone oil. This oil may interfere with the surgeon's view of the retina and may diminish the patient's visual acuity. In contrast, oil is readily removed from the posterior surface of an acrylic IOL. The authors therefore recommend the use of a soft acrylic or PMMA IOL over a silicone IOL when choosing a lens for implantation in patients who may require vitreoretinal procedures with silicone oil tamponade.

Acrylic Resins↗
PubMed1995

Histopathology of rabbit eyes with silicone-fluorosilicone copolymer oil as six months internal retinal tamponade.

Silicone-fluorosilicone copolymer oil has low viscosity (175-185 cSt) and is heavier than water (density, 1.16 g cm-3). Short term retinal tolerance (within 2 months) of the silicone-fluorosilicone copolymer oil has been reported to be the same as that of currently used intraocular silicone oil. Ocular response of the purified silicone-fluorosilicone copolymer oil were examined clinically and histopathologically from 2.5 months to 6 months after vitreous cavity injection in rabbit phakic eyes, and compared the oil tolerance with that of purified silicone oil (0.97 g cm-3, 5000 cSt). The effects in anterior chamber also were examined within 4 weeks of the silicone-fluorosilicone copolymer oil injection in different rabbits. Silicone-fluorosilicone copolymer oil recovered from the vitreous cavity at 6 months was analysed for cholesterol and retinol content by high performance liquid chromatography. Because of its low viscosity, silicone-fluorosilicone copolymer oil was easy to inject and remove from the vitreous cavity with a 20-G needle. After the vitreous injection, discrete droplet formation by the silicone-fluorosilicone copolymer oil occurred more easily than by silicone oil. Medullary ray detachment was seen in a silicone oil-, and some silicone-fluorosilicone copolymer oil-injected eyes at 4-6 months. Histopathologically, after 3-6 months disappearance of outer plexiform layer and disorganization of the photoreceptor layer of silicone oil-, and silicone-fluorosilicone copolymer oil-injected eyes were seen in the superior and the inferior retina, respectively. Migration of the photoreceptor cell nuclei to the photoreceptor layer was found in the inferior retina of silicone-fluorosilicone copolymer oil-injected eyes at 5-6 months. Small droplets ingested by mononuclear cells were found in the vitreous cavity or preretina at 4-6 months in silicone-fluorosilicone copolymer oil-injected eyes. After the anterior chamber injection, silicone-fluorosilicone copolymer oil induced endothelial cell damage in the area where the oil contacted continuously. Retinol and cholesterol were identified in silicone-fluorosilicone copolymer oil removed from the vitreous cavity. Silicone-fluorosilicone copolymer oil may be useful as an intraoperative device in retinal detachment surgery and as a short term (up to about 2 months) retinal tamponade but we do not recommend it for long term retinal tamponade.

Animals↗
PubMed2000

Solvent for removing silicone oil from intraocular lenses: experimental study comparing various biomaterials.

PURPOSE: To evaluate the effectiveness of a perfluorocarbon-perfluorohexyloctane (PFHO)-in removing silicone oil from different intraocular lenses (IOLs) and determine whether any removal technique enhances the removal of silicone oil. METHODS: Six IOL materials were analyzed: standard poly(methyl methacrylate) (PMMA), heparin-surface-modified (HSM) PMMA, AcrySof, 2 polyHEMAs with different water contents, and a second-generation SLM-2 silicone. One or 2 microL of a highly viscous silicone oil (5700 cs) was applied to the center of each IOL optic. Perfluorohexyloctane (2 mL) was then used to remove the silicone oil by 1 of 3 techniques: immersion of the IOL in PFHO; irrigation of the silicone oil with a small, blunt cannula; mechanical wiping with a PFHO-saturated Merocel swab (used only when irrigation failed). Next, each IOL was examined by light and scanning electron microscopy. The expanse of silicone oil residue remaining on the IOLs was objectively quantified using an integrated computerized analysis system combined with energy-dispersive spectrometry. RESULTS: After application of 1 microL silicone oil, immersion in PFHO did not remove silicone oil from any IOL material. Irrigation with PFHO removed the silicone oil from the HSM PMMA and polyHEMA IOLs, but oil remained on the other lens materials. Mechanical wiping was necessary to remove the oil from the PMMA, AcrySof, and silicone IOLs. Attempts to remove 2 microL of silicone oil produced similar results. Although mechanical wiping of the AcrySof IOL removed all silicone oil, a slight residue remained on the PMMA and silicone IOLs. The PFHO irrigation removed significantly more silicone oil from the HSM PMMA than the unmodified PMMA IOL (P =.001). More silicone oil was removed from polyHEMA IOLs than from the AcrySof or silicone IOLs (P =.05). CONCLUSION: Although PFHO can remove silicone oil, it is of limited usefulness. The effectiveness of PFHO depends on the removal technique, IOL material, and amount of silicone oil present. Based on these results, we recommend PFHO irrigation followed by mechanical wiping with a PFHO-saturated swab.

Adhesiveness↗
PubMed2002

Dietary silicon intake and absorption.

BACKGROUND: Increasing evidence suggests that silicon is important in bone formation. The main source of silicon for humans is the diet, but the bioavailability of silicon from solid foods is not well understood. OBJECTIVE: We estimated the dietary intake of silicon by adults, separately for men and women and for different age groups. Foods that were major contributors to silicon intake were identified. We then estimated the gastrointestinal uptake of silicon from major food sources and studied how uptake correlated with the silicon contents of the foods. DESIGN: Silicon intakes were determined in cohorts from the original Framingham Study and the Framingham Offspring Study by using a 126-item food-frequency questionnaire. Gastrointestinal uptake of silicon from foods was estimated in 3-8 healthy subjects by using urinary silicon excretion as a surrogate measure of silicon uptake. RESULTS: Mean silicon intakes in men (30 and 33 mg/d in the original Framingham and Framingham Offspring cohorts, respectively) were significantly higher than those in women (24 and 25 mg/d in the 2 cohorts, respectively; P = 0.0001). Silicon intake decreased with age (P < 0.001, adjusted for sex). The major food sources were beer and bananas in men and bananas and string beans in women. Silicon was readily available from foods; a mean of 41% of the ingested silicon was excreted in urine. The silicon content of the foods consumed was significantly correlated with urinary silicon excretion (P = 0.019). CONCLUSIONS: Solid foods are a major source of available silicon. The association between dietary silicon intake and bone health should now be investigated.

Absorption↗
PubMed1998

Using a colorimeter to develop an intrinsic silicone shade guide for facial prostheses.

PURPOSE: To determine if using CIE L*a*b* color measurements of white facial skin could be correlated to those of silicone shade samples that visually matched the skin. Secondly, to see if a correlation in color measurements could be achieved between the silicone shade samples and duplicated silicone samples made using a shade-guide color formula. MATERIALS AND METHODS: A color booth was designed according to ASTM specifications, and painted using a Munsell Value 8 gray. A Minolta colorimeter was used to make facial skin measurements on 15 white adults. The skin color was duplicated using custom-shaded silicone samples. A 7-step wedge silicone shade guide was then fabricated, representing the commonly encountered thicknesses when fabricating facial prostheses. The silicone samples were then measured with the Minolta colorimeter. The readings were compared with the previous L*a*b* readings from the corresponding patient's skin measurements, and the relative color difference was then calculated. Silicone samples were fabricated and analyzed for three of the patients to determine if duplication of the visually matched silicone specimen was possible using the silicone color formula, and if the duplicates were visually and colorimetrically equivalent to each other. The color difference Delta E and chromaticity was calculated, and the data were analyzed using a coefficient-of-variation formula expressed by percent. A Pearson Product Moment Correlation Coefficient was performed to determine if a correlation existed between the skin and the silicone samples at the p < or = .05 level. RESULTS: The highest correlation was found in the b* dimension for silicone thicknesses of 1 to 4 mm. For silicone thicknesses of 6 to 10 mm, the highest correlation was found in the L* dimension. All three dimensions had positive correlations (R2 > 0), but only the 1-mm and 4-mm b* readings were very strong. Patient and silicone L*a*b* measurement results showed very little change in the a* axis, while the L* and b* measurements showed more change in their numbers, with changes in depth for all patient silicone samples. Delta E numbers indicated the lowest Delta E at the 1-mm depth and the highest Delta E at the 10-mm depth. All duplicated samples matched their original silicone samples to a degree that visual evaluation could not distinguish any color differences. Using volumetric measurements, a shade guide was developed for all 15 patients. CONCLUSIONS: There was good correlation between the patient's colorimeter measurements and the silicone samples, with the b* color dimension the most reproducible, followed by the L* and the a*. Silicone samples at 6, 8, and 10 mm matched the patient the best, and this study showed that silicone samples can be duplicated successfully if a good patient-silicone match is obtained. Rayon flocking fibers and liquid makeup are effective at matching facial prostheses and can be used to develop a simple shade guide for patient application.

Color↗
PubMed1995

Analysis of silicon levels in capsules of gel and saline breast implants and of penile prostheses.

Although a potential link between silicone-gel breast implants and autoimmune connective tissue disease has been suggested, none has been proven. The potential role of silicone as an immune adjuvant remains very controversial. Currently available techniques do not allow precise measurements of silicone in tissues. However, all compounds containing silicon (including silicone) can be measured accurately. The present study was designed to measure silicon levels in the fibrous capsules of patients with silicone-gel breast implants, saline breast implants, and silicone inflatable penile prostheses. Baseline control silicon levels were obtained from the breast tissue of patients undergoing breast reduction, who had no exposure to breast implants. All silicon measurements were carried out using atomic absorption spectrometry with a graphite furnace. Silicon was measured in a normal heptane extract of silicone from dried tissue. The mean silicon levels in 16 breast tissue control samples from 8 patients undergoing breast reduction varied from 0.025 to 0.742 micrograms/gm with the median mean being 0.0927. The median silicon level in capsules from six patients with saline implants was 7.7 micrograms/gm (range, 1.9-36.6 micrograms/gm). The median silicon level in capsules from five patients with silicone inflatable penile prostheses was 19.5 micrograms/gm (range, 1.9-34.8 micrograms/gm). Although the levels of silicon in capsules of patients with saline breast prostheses and penile implants were higher than in control samples, they were much lower than those from the capsules of the 58 gel implants (median, 9,979 micrograms/gm; range, 371-152,000 micrograms/gm).(ABSTRACT TRUNCATED AT 250 WORDS)

Breast↗
PubMed1992

Tissue compatibility of collagen-silicone composites in a rat subcutaneous model.

Collagen-silicone composites were fabricated and tested for biocompatibility by subcutaneous implantation in rats. The silicone component consisted of addition cure or condensation cure sheets. The collagen component was either (a) a sponge layer 2 mm thick, (b) a thin film 12-20 microns thick, or (c) residual collagen bonded to or incorporated in the silicone rubber. Collagen sponges were mechanically bonded to silicone sheets, and collagen thin films and residual collagen were physically and chemically attached to epoxy-derivatized silicone sheets. Analysis of implanted samples showed that reduced capsule formation occurred around collagen sponge-silicone, compared to control silicone sheets. Only where the underlying silicone sheet, or interpenetrating silicone, was exposed to the tissue, did limited capsule formation occur. In contrast, thin capsule developed completely around silicone coated with a thin collagen film and around silicone bonded to residual collagen. Sponge-silicone composites and control silicone sheets were free of acute and chronic inflammation, except for occasional foreign body giant cells in sponge adjacent to silicone. Silicone coated with micron-thick collagen films exhibited some inflammation, but residual collagen-silicone did not. This study suggests that, to prevent capsule formation, a collagen coat must be of minimum thickness and surface coverage sufficient to prevent any contact between silicone and tissue.

Animals↗
PubMed1997

Determination of silicon in breast and capsular tissue from patients with breast implants performed by inductively coupled plasma emission spectroscopy. Comparison with tissue histology.

A method for analysis of silicon in tissue was developed to determine silicon content in breast parenchymal and periprosthetic capsular tissues of patients with silicone or saline implants and to compare levels in tissues from normal (nonaugmented) breasts. It is of interest to determine whether increased silicon content in tissues can be associated with morbidity in patients who have received silicone implants. This manuscript addresses the issues involved in analysis of breast tissue samples for silicon and compares silicon levels with tissue histologic findings and patient morbidity. One hundred sixty tissue samples were obtained for silicon analysis from 72 patients during augmentation, capsulectomy with or without replacement mammoplasty, mastectomy, or biopsy procedures and were frozen in acid-washed polystyrene tubes at 220 degrees C until analysis. Samples were thawed, sectioned to approximately 0.1 g (dry weight), and digested in nitric acid before analysis by inductively coupled plasma emission spectroscopy, monitoring emission intensity at 251.6 nm. Tissue silicon levels (breast parenchymal and periprosthetic capsular tissue) in patients with silicone gel implants were much higher (mean, 9,287 micrograms/g, n = 106) than in patients with saline implants (mean, 196 micrograms/g, n = 37) or nonaugmented breasts (mean, 64 micrograms/g, n = 17). Histologic examination was performed on 54 tissue samples stained with hematoxylin-eosin. Tissue samples were rated as to degree of inflammation and calcification, and amount of giant cells, foamy histiocytes, and vacuoles containing a colorless refractory material. Vacuolization and foamy histiocyte ratings correlated significantly with tissue silicon concentration. No correlations were found between tissue silicon concentration and inflammation, calcification, or giant cell rating. Implant age (number of years an implant was in place before sampling) correlated with capsular tissue silicon concentration in patients with intact implants but not in those with ruptured implants. No difference in tissue silicon concentration was found between patients with or without signs or symptoms of morbidity. Using 0.1 g of tissue, the method was linear to 1,000 micrograms/g, and sensitivity was 3.7 micrograms/g. Precision between runs (mean, 5.1 micrograms/g; coefficient of variance, 13.7%; n = 13) was calculated from multiple analyses of a bovine liver standard (National Bureau of Standards, reference material 1577a). Significant biologic variability (21.4% to 52.5%) was seen in tissues with high silicon levels. Paraffin-embedded, formalin-fixed tissues are not amenable to silicon analysis by this method, because of leaching of silicone from the tissues during preparation. Thus only fresh frozen tissue samples were used.

Age Factors↗