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Biomedical subjects

G R Baran

Publications and source records attributed to G R Baran.

15 recordsLinked to original sources
PubMed2001

Filler-coupling agent-matrix interactions in silica/polymethylmethacrylate composites.

The interactions of the silane coupling agent methacryloxypropyltrimethoxysilane (MPS) with both fumed silica and a polymethylmethacrylate (PMMA) resin matrix were investigated using thermogravimetric analysis and Fourier transform infrared spectroscopy. OX 50 fumed silica was silanated with MPS at concentrations of 1% and 5% in aqueous ethanol (95%), acetone, and anhydrous toluene. Methyl methacrylate was polymerized with the silanated fumed silica (5% wt/wt) to form composites. The amount of MPS adsorption on the fumed silica and the amount of PMMA attached to the silanated fumed silica were determined by thermogravimetric analysis. MPS could be removed from the fumed silica after washing with methanol, but not after it underwent a drying process at 25 degrees C under vacuum. After vacuum drying at 25 degrees C, two types of adsorbed silane were found, i.e., firmly adsorbed and loosely adsorbed silane. The loosely adsorbed silane could desorb from silica and be incorporated into the polymer matrix through copolymerization with monomeric methyl methacrylate, resulting in crosslinking of the matrix. When the silanated silica was dried at 110 degrees C for 2 h, the loosely adsorbed silane was removed and the amount of firmly adsorbed silane increased. There was a positive correlation between the amount of firmly adsorbed MPS and the amount of PMMA attachment. The highest efficiency for PMMA attachment was found when MPS was adsorbed as a monolayer, because the loosely adsorbed silane did not contribute to the bonding of PMMA, and this suggested that not all of the double bonds of the MPS were accessible for reaction with the methacrylate monomer. Drying at 110 degrees C may also decrease the number of unsaturated double bonds of MPS.

Absorption↗
PubMed2001

Lifetime predictions for resin-based composites using cyclic and dynamic fatigue.

Because dental restorative materials undergo fatigue in use, testing is often performed in the laboratory to evaluate material responses to cyclic loading. The purpose of this study was to compare the lifetime predictions resulting from two methods of fatigue testing: dynamic and cyclic fatigue. Model composites were made in which one variable was the presence of a silanizing agent, and specimens tested in 4-point flexure. Cyclic fatigue was carried out at a frequency of 5 Hz, while dynamic fatigue testing spanned seven decades of stress rate application. Data were reduced and the crack propagation parameters for each material were calculated from both sets of fatigue data. These parameters were then used to calculate an equivalent static tensile stress for a 5-year survival time. The 5-year survival stresses predicted by dynamic fatigue data were approximately twice those predicted by cyclic fatigue data. In the absence of filler particle silanization, the survival stress was reduced by half. Aging in a water-ethanol solution reduced the survival stresses by a factor of four to five. Cyclic fatigue is a more conservative means of predicting lifetimes of resin-based composites.

Acrylic Resins↗
PubMed1999

Nanoindentation studies of titanium single crystals.

Titanium single crystal planes of different atomic density have been reported to show different oxidation characteristics. The differences in oxide characteristics have further been demonstrated to lead to differences in osteoblast attachment. Investigations of the preferred crystallographic planes of titanium for osteoblast attachment can be used to optimize the surfaces of single crystal and polycrystalline titanium implants for anchoring various prostheses. Nanoindentation techniques were used to determine mechanical properties of two crystallographic planes of titanium of different atomic density. Modulus of elasticity of 128 +/- 10 GPa was obtained for polycrystalline titanium and 123 +/- 5 and 124 +/- 6 GPa for the basal plane and pyramidal planes, respectively. The variation of modulus with crystal orientation was not greater than the statistical variation in the data. Surface hardness values were 2.1 +/- 0.1 GPa for the polycrystalline sample and 1.6 +/- 0.1 and 1.9 +/- 0.1 GPa, respectively, for the basal and pyramidal planes. Curves of hardness as a function of depth (0-2000 nm) obtained from electrochemically polished surfaces showed a sharp increase at shallow depths and may reflect changes caused by oxidation of the titanium surfaces.

Biocompatible Materials↗
PubMed1999

Size effect in resin/glass composite flexure strengths.

The objective of this study was to test the hypothesis that composite restorative materials possess an elastic-brittle nature and therefore will exhibit a size effect for flexure strength data. The experimental material consisted of 20 wt% 60:40 BISGMA:TEGDMA, 10 wt% colloidal silica, and 70 wt% Sr glass and was cured by light irradiation. Two sizes of flexure specimens were fabricated: 3.2x1.6x35 mm, and 6.25x3.1x35 mm. Half of the specimens made were soaked to equilibrium weight gain in 50:50 ethanol:water. The fracture strengths were measured in four-point bending tests. The beams under load were modelled by the finite element package ABAQUS. A statistical fracture mechanics methodology embodied in a public domain computer program called CARES/LIFE, developed by NASA, utilized the ABAQUS input and the fracture strengths of the smaller specimens to predict the fracture strengths of the larger specimens. In making the computation it used an approach that combines a Weibull distribution of flaw size with Batdorf's fracture mechanical model for failure at a material flaw. Both the soaked and unsoaked specimens exhibited Weibull behaviour, with shape parameters ranging from 4.04 to 8.15. Soaking had a clearly detrimental effect on the strengths of specimens of both sizes, and produced a comparable percentage reduction in the estimated scale parameter of the fracture strength distribution. Both the soaked and unsoaked specimens also exhibited a clear and comparable size effect, i.e. the larger specimens had a fracture strength that was lower than that of the smaller specimens by roughly the same percentage. Moreover, the magnitude of the size effect was well predicted by the CARES/LIFE methodology for both the soaked and the dry specimens. The elastic-brittle character of both soaked and unsoaked composite specimens was validated by load-deflection data, the magnitude of the Weibull shape parameters of the observed fracture strength data (<10), and the observed effect of specimen size. The accuracy of CARES/LIFE in predicting the magnitude of the observed size effect in beams of two different sizes strongly suggests that CARES/LIFE will be useful for computation of failure probabilities for clinically relevant structures.

Bisphenol A-Glycidyl Methacrylate↗
PubMed1998

Weibull models of fracture strengths and fatigue behavior of dental resins in flexure and shear.

In estimating lifetimes of dental restorative materials, it is useful to have available data on the fatigue behavior of these materials. Current efforts at estimation include several untested assumptions related to the equivalence of flaw distributions sampled by shear, tensile, and compressive stresses. Environmental influences on material properties are not accounted for, and it is unclear if fatigue limits exist. In this study, the shear and flexural strengths of three resins used as matrices in dental restorative composite materials were characterized by Weibull parameters. It was found that shear strengths were lower than flexural strengths, liquid sorption had a profound effect on characteristic strengths, and the Weibull shape parameter obtained from shear data differed for some materials from that obtained in flexure. In shear and flexural fatigue, a power law relationship applied for up to 250,000 cycles; no fatigue limits were found, and the data thus imply only one flaw population is responsible for failure. Again, liquid sorption adversely affected strength levels in most materials (decreasing shear strengths and flexural strengths by factors of 2-3) and to a greater extent than did the degree of cure or material chemistry.

Composite Resins↗
PubMed1994

Morphometric assessment of surface conformation as an estimate of roughness.

One of the standard methods for assessing the roughness of a material subjected to wear uses the surface arithmetic means and root-mean-square deviation. However, these parameters often do not provide a qualitative assessment of the difference in materials worn under the same conditions of load and elapsed time. The profile and surface roughness parameters are frequently inconsistent. Such measurements are also required to determine the wear characteristics of various materials under different conditions. A morphometric assessment of wear characteristics, based on the surface area fraction of localized deviations in the surface texture and stress fractures, is provided, and clearly indicates the observed difference.

Biocompatible Materials↗
PubMed1993

Determination of the degree of cure of dental resins using Raman and FT-Raman spectroscopy.

FT-IR spectroscopy has traditionally been used to determine the degree of conversion of dental resins. FT-Raman scattering provided an alternate method of obtaining degrees of conversion for these systems and was particularly useful for measuring spectra of materials without any sample preparation. Raman and FT-Raman spectroscopy gave identical results, but the latter technique was preferred for the highly fluorescent samples often encountered in commercial composites. Linear calibration curves were obtained for the aromatic mixtures Bis-GMA/TEGDMA and Bisphenol-A/TEGDMA using C = C/phi, and for the wholly aliphatic mixture EGDMA/EGDA using C = C/C = O, over a wide range of mole ratios. If both the mole and intensity ratios [C = C/phi or C = C/C = O] were known for an uncured dental resin, then the degrees of conversion could be obtained for the cured materials using Raman spectroscopy. However, if the mole ratios for the uncured resin were unknown, then the degree of conversion depended on the calibration curve, since the Raman scattering cross section of the vibrational modes depended on the molecules to which they were attached.

Benzhydryl Compounds↗
PubMed1985

Selection criteria for base metal alloys for use with porcelains.

Today's chromium alloys constitute a distinct alloy group that, because of favorable physical and chemical properties, is widely used for porcelain fused to metal restorations. In making a selection from within this alloy group, the individual should first be aware of how to modify casting techniques in order to produce complete and well-fitting castings. Differences in important mechanical characteristics, such as modulus of elasticity, hardness, and an alloy's formability, do exist between various Ni-Cr and Co-Cr alloys, and one needs to be cognizant of these differences if an optimal restoration design is to result. Because chromium alloys do not represent a homogeneous group, gross differences in castability and porcelain bonding behavior will be encountered between alloys with nominally similar compositions and mechanical properties. No universally accepted test for alloy-porcelain compatibility or bond strength exists, but the interested individual can perform some preliminary tests on new or untried alloy-porcelain combinations prior to fabrication of actual restorations. Although soldering the chromium alloys continues to be technique-sensitive, optimal results can be achieved by proper selection of high- and low-fusing solders for specific alloys.

Chromium Alloys↗
PubMed1984

Oxide compounds on Ni-Cr alloys.

Five Ni-Cr alloys were studied in order to identify the compounds formed on the alloy surface during oxidation under conditions similar to those encountered during dental laboratory procedures prior to application of porcelain. After the alloys were oxidized, the films covering the surfaces were removed with the aid of a Br-methanol solution. X-ray diffraction was used to analyze the compounds formed. Oxides of nearly all elements contained by the alloys were found after low-temperature (650 degrees C) oxidation, while NiO and particularly Cr2O3 were predominant after oxidation at high temperatures (1000 degrees C).

Aluminum Oxide↗
PubMed1983

The metallurgy of Ni-Cr alloys for fixed prosthodontics.

Ni-Cr alloys continue to be an important part of dental restorative materials. It behooves those interested to study these alloys and develop an understanding of their chemical and physical characteristics. Introduction of new alloys almost monthly presents an educational challenge to the entire profession. Many dental schools do not use Ni-Cr alloys for fixed prosthodontics because questions exist about the efficacy and performance of such alloys. These doubts may be resolved by continued research. Concern about mechanical properties can be alleviated if an understanding of the requirements for successful use, for example, casting, burnishing, polishing, and stresses encountered in service, is achieved. Information on the nature of corrosion intraorally is needed, particularly with regard to correlating in vitro testing with in vivo performance. Further studies of porcelain-alloy compatibility are indicated to understand properties affecting compatibility. Performance of high-temperature porcelain-alloy systems could influence processing techniques so that specific materials could be used more appropriately. In addition, the nature of porcelain-to-alloy bonding may be revealed by use of advanced chemical analysis techniques to identify processes occurring at the atomic level, with identification of oxides and reacted zone produces may also provide data for design of new alloys for porcelain-metal systems. Finally, factors related to the clinical performance of nickel-based alloys, including their allergenic potential, have not been considered in this report.

Chemical Phenomena↗
PubMed1979

Phase changes in base metal alloys along metal-porcelain interfaces.

Five commercial, base metal alloys used with porcelain were investigated. After oxidizing heat treatments and the porcelain bake, phase changes occurred in some alloys near the porcelain interface. These changes were due to the depletion of the matrix in alloying elements as a result of the oxidizing treatments.

Acid Etching, Dental↗
PubMed1979

[Changes in the metallic phase at the metal-ceramic margin of baked alloys of nonprecious metals].

Five base metal alloys were investigated to determine if phase changes in the alloy microstructure occurred as a result of the porcelain baking procedure. It was found that during the oxidation treatments concentration changes in the alloy surface led to a depletion of certain elements in four of the alloys investigated. It was also found that phase changes could occur in a region 20 micrometers wide next to the alloy surface. Phase changes and alloying element depletion occurs as a result of the oxidation treatments prior to the fusion of porcelain and not as a result of or during porcelain fusion.

Acid Etching, Dental↗
PubMed1977

Colored emission of rare earth ions in a potassium feldspar glass.

The viability of rare earth ions as fluorescent coloring additives for dental porcelain was investigated. The data presented allow the formulation of porcelain with specific color properties under illumination by ultraviolet light. Attention is drawn to the possibility of energy transfer occurring with the result that prediction of color may not be done by application of the additive color-mixing scheme.

Aluminum Silicates↗
PubMed1977

[Fluorescence of dental ceramics by means of non-radioactive materials].

In order to improve the aesthetics of dental ceramics under all lighting conditions, uranium and cerium are added as fluorescent additives. However, studies have shown that the fluorescent behavior of dental ceramics is still not identical to that of natural teeth in all cases. A further disadvantage of the use of uranium are its radioactive and poisonous properties. This study was undertaken in order to investigate the possibility of using elements of the rare earth family as fluorescent additives for dental ceramics. The rare earths were added to a potassium feldspar glass host and emission spectra were measured. As a result it is recommended that divalent europium and trivalent dysprosium be further investigated as possible fluorescent additives for dental ceramics, as they show intense blue-white and yellow fluorescence respectively.

Dental Porcelain↗