Thomas Ellis

Oxygen Inhibition in Dental Resins

Oxygen inhibits free radical polymerization and yields polymers with uncured surfaces. This is a concern when thin layers of resin are being polymerized, or in circumstances where conventional means of eliminating inhibition are inappropriate. In this study, we tested the hypothesis that viscosity, filler content, and polymerization temperature modify oxygen diffusion in the resin or the reactivity of radical species, and affect the degree of conversion near the surface. Confocal Raman micro-spectroscopy was used to measure monomer conversion from the surface to the bulk of cured resins. Increased viscosity was shown to limit oxygen diffusion and increase conversion near the surface, without necessarily modifying the depth of inhibition. The filler material was shown to increase, simultaneously, oxygen diffusivity and the viscosity of the resin, which have opposite effects on conversion. Polymerization at a temperature above ~ 110°C was shown to eliminate oxygen inhibition.

A photoacoustic FTIRS study of the chemical modifications of human dentin surfaces: I. Demineralization

Acids are used to modify the structure and composition of dentin surfaces to improve bonds formed with resins. The purpose of this work is to investigate such chemical modifications using the surface-sensitive technique photoacoustic Fourier transform infrared spectroscopy (PA-FTIRS). Spectra of acid-treated samples (citric, maleic, nitric, and phosphoric at pH = 1.0) were recorded at various time intervals. Analysis of these spectra indicates a gradual increase in sample surface area with treatment time. A decrease of the bands associated with calcium hydroxyapatite (HAP) and carbonate apatite inherent to the mineral phase of dentin are also observed. A comparison of spectra of samples treated for 2 min with each acid also reveals that maleic and phosphoric acids remove more HAP than citric acid. We conclude that citric acid may cause the formation of precipitates at the etching front which inhibit etching.

A tapping mode AFM study of collapse and denaturation in dentinal collagen

OBJECTIVES Tapping mode atomic force microscopy (AFM) was used to investigate the surface changes of collagen exposed to phosphoric acid treatment. We focus on denaturation and collapse following demineralization and exposure to air. METHODS Unpolished dentin disks, obtained from freshly extracted human molars, were etched in 37% phosphoric acid for 15s, rinsed ultrasonically and gently blotted with soft paper; the specimens were then continuously observed using tapping mode AFM. RESULTS Immediately after the removal of bulk water, the surface consisted of a porous network of banded collagen fibrils, having periodicities of 67nm. After approximately 8min of subsequent air-drying, the spacing between fibrils was lost, and the surface was observed to consist of a dense array of closely spaced fibrils. The banding periodicity was still observable. SIGNIFICANCE The air drying of etched dentin results in the collapse of the collagen network, but not in the denaturation of the collagen fibrils. This study indicates that collapse and denaturation are separate phenomena. It further shows that water loss occurs rapidly, and disrupts the native conformation of the collagen network. This would have adverse effects on adhesion.

A photoacoustic FTIRS study of the chemical modifications of human dentin surfaces

This work focuses on the chemical alterations occurring on the dentin surface on treatment with sodium hypochlorite (NaOCl), a known deproteinating agent. In addition, sequential treatment with both acid and NaOCl are characterized. Modifications are evaluated using photoacoustic FTIR (PA-FTIRS), a surface-sensitive spectroscopy. Spectra of NaOCl-treated dentin samples show a slow and heterogeneous removal of its organic phase, leaving calcium hydroxyapatite and carbonate apatite unchanged. Spectra of this deproteinated surface resemble those of apatites synthesized at low temperatures and having very high-surface areas. A combined sequential 2-min treatment of dentin with both maleic acid and NaOCl indicates that this treatment can produce a surface region which is neither significantly demineralized nor deproteinated. This sequential treatment can be used to remove the smear layer and restore the dentin surface to its natural composition.

Quantitative assessment of surface roughness as measured by AFM: application to polished human dentin

Abstract Atomic force microscopy (AFM) has been used to investigate the surface morphology of polished dentin. In the studies of chemical conditioning (acid etching, bleaching), the morphology of polished dentin is of great importance and offers a controlled reference point for the correct and unambiguous assessment of morphological effects induced by subsequent chemical treatments. The characterization of polished dentin is required so as to permit reliable and normalized comparisons of various chemical modifications of dentin. AFM topographic data, over a fixed area of 1 μm ×1 μm , have been quantitatively analyzed and compared, using standard roughness descriptors such as the root mean square (RMS) of the surface height deviations. In addition, for a more general and complete description of surface roughness, Fourier transform analysis has been used to determine characteristic parameter values that fully describe the surface roughness in both vertical and lateral directions, independent of the feature size bandwidth considered. As a result, a confident spectral analysis interval has been assigned which ranges over the specific length scales of fibrous dentin structure. To understand how surface-polishing affect dentin roughness, and to evaluate its spatial extent of influence, AFM experiments were carried out on aluminum test surfaces which were prepared and imaged under the same conditions as the dentin samples.

Moisture‐dependent renaturation of collagen in phosphoric acid etched human dentin

We used atomic force microscopy (AFM) to investigate the effects of acidic and aqueous treatments on human dentin. Two basic points were determined: the first is the ability of AFM to discriminate the effect of phosphoric acid (pH approximately equal to 1) on polished dentin, and the second is the demonstrable effect of moisture on fibrous collagen structure. AFM images confirmed that the polishing process led to the removal of both smear layer and smear plugs. Our AFM study of undried dentin, which was then acid treated and kept moist, revealed substantial morphological changes at the dentin surface. Collagen fibers, having a characteristic periodicity of 67 nm, were imaged in situ for the first time; these structures were absent in dentin treated by phosphoric acid and subsequently vacuum dried, even after prolonged reimmersion in water. The AFM technique permitted us to demonstrate the important roles that moisture and etching play in the determination of the structure of collagen fibrils. Such structure may also play an important role in the diffusibility of subsequently applied dental adhesion systems.

High resolution SEM evaluation of dentin etched with maleic and citric acid

OBJECTIVES This study evaluated the ultra-morphological effects of maleic and citric acid on human dentin by means of a field emission in-lens scanning electron microscope (FEISEM). Both acids were tested on human dentin at pH 0.7 and 1.4 in aqueous solutions. METHODS Each of 12 dentin disks were divided into four groups and exposed to either maleic acid at pH 0.7, maleic acid at pH 1.4, citric acid at pH 0.7 and citric acid at pH 1.4. All samples were then fixed and dehydrated in a critical point drying apparatus. Observations were carried out by means of a FEISEM (JEOL 890) after coating with a carbon-platinum film. RESULTS Both acids removed smear layer and partially removed smear plugs. Details of fine structures measuring from 5 to 15 nm were shown on the intertubular demineralized dentin. Maleic acid at pH 0.7 showed the highest depth of demineralization of all the tested samples; citric acid, showed a higher depth of demineralization values when tested at pH 1.4 than at pH 0.7. SIGNIFICANCE The FEISEM reveals ultra-structural aspects of the demineralization process of the dentin tissue of the both acids tested. Differences related to the pH of the acids were found. Images obtained at high magnification clarify the dentin collagen structure of both peritubular and intertubular dentin. Small periodic structures associated with collagen fibrils were also imagined.

The surface structure of Dow Cyclotene 3022, as determined by photoacoustic FTIR, confocal Raman and photoelectron spectroscopies

Abstract The cure of B-staged Cyclotene 3022 has been studied by photoacoustic FTIR and X-ray photoelectron spectroscopy (XPS). The data indicate that the cure reaction is second-order, consistent with a Diels–Alder reaction. The lack of any loss of aromaticity on cure, as observed by both photoacoustic FTIR and XPS, and confirmed by confocal Raman spectroscopy, indicates a reaction mechanism involving the production of tetrahydronaphthalene. The lack of structural differences between surface and bulk, determined by a comparison of photoacoustic and published transmission FTIR data, indicates that the cyclobutane homopolymerization pathway lies at an energy too high to be attained even with an added surface energy. The polymer loses a significant amount of low molecular weight material on curing.

Field emission in-lens SEM study of enamel and dentin.

This investigation used field emission in-lens scanning electron microscopy (FEISEM) for the study of tooth surfaces, with particular reference to adhesive bonding and acid conditioning. Dentin wafers with an intact enamel periphery were treated by either ethylenediaminetetraacetic acid (EDTA) (pH 7.4) or phosphoric acid (pH 0.7). The samples were then fixed, sequentially dehydrated in alcohol, and either air- or critical point-dried. After coating, surfaces were examined by FEISEM. For enamel, intraprismatic crystals were clearly recognizable, with the crystals showing both a longitudinal and parallel orientation to the long axis of the prisms. For dentin, the surface ultrastructure (mineral crystals and collagen banding) for the both untreated and treated samples was observed. Fine structures measuring on the order of 6 nm were also observed on samples treated by EDTA. We conclude that FEISEM can routinely provide high-resolution images of enamel and dentin, and that it has the capability of revealing the defined distribution of crystals and collagen fibers in dental tissues.

Water at surfaces: What can we learn from vibrational spectroscopy?

Recent developments in the vibrational spectroscopy of thin films of water adsorbed on well defined metal surfaces under ultra-high vacuum (UHV) conditions will be discussed. New results will be presented for H 2 O and D 2 O adsorbed on Cu(110), measured using grazing incidence reflection-absorption infrared spectroscopy (RAIRS). These will be discussed in context with previous high-resolution electron energy loss spectroscopy (HREELS) measurements of H 2 O on Pd(110). Both of these results are typical of measurements from other laboratories. Particular attention will be paid to the O-H stretching region of the spectrum. A detailed examination of this region of the spectrum is used to study the amorphous to crystalline phase transition in thin films, the temperature dependence ofthe sticking coefficient of water, and the structure of the first one or two layers of water directly at the solid interface. The spectroscopic signature observed for very thin films is found to be consistently different from that of crystalline ice, and the possible reasons for this difference are discussed.