Morphology and structure of polymer layers protecting dental enamel against erosion.

OBJECTIVES: Human dental erosion caused by acids is a major factor for tooth decay. Adding polymers to acidic soft drinks is one important approach to reduce human dental erosion caused by acids. The aim of this study was to investigate the thickness and the structure of polymer layers adsorbed in vitro on human dental enamel from polymer modified citric acid solutions. METHODS: The polymers propylene glycol alginate (PGA), highly esterified pectin (HP) and gum arabic (GA) were used to prepare polymer modified citric acids solutions (PMCAS, pH 3.3). With these PMCAS, enamel samples were treated for 30, 60 and 120s respectively to deposit polymer layers on the enamel surface. Profilometer scratches on the enamel surface were used to estimate the thickness of the polymer layers via atomic force microscopy (AFM). The composition of the deposited polymer layers was investigated with X-ray photoelectron spectroscopy (XPS). In addition the polymer-enamel interaction was investigated with zeta-potential measurements and scanning electron microscopy (SEM). RESULTS: It has been shown that the profilometer scratch depth on the enamel with deposited polymers was in the range of 10nm (30s treatment time) up to 25nm (120s treatment time). Compared to this, the unmodified CAS-treated surface showed a greater scratch depth: from nearly 30nm (30s treatment time) up to 60nm (120s treatment time). Based on XPS measurements, scanning electron microscopy (SEM) and zeta-potential measurements, a model was hypothesized which describes the layer deposited on the enamel surface as consisting of two opposing gradients of polymer molecules and hydroxyapatite (HA) particles. SIGNIFICANCE: In this study, the structure and composition of polymer layers deposited on in vitro dental enamel during treatment with polymer modified citric acid solutions were investigated. Observations are consistent with a layer consisting of two opposing gradients of hydroxyapatite particles and polymer molecules. This leads to reduced erosive effects of citric acid solutions on dental enamel surfaces.

Acids with an equivalent taste lead to different erosion of human dental enamel.

OBJECTIVES: The consumption of acidic soft drinks may lead to demineralization and softening of human dental enamel, known as dental erosion. The aims of this in vitro study were to determine: (i) if different acids with a similar sensorial acidic taste lead to different hardness loss of enamel and (ii) if the fruit acids tartaric, malic, lactic or ascorbic acid lead to less hardness loss of enamel than citric or phosphoric acid when their concentration in solution is based on an equivalent sensorial acidic taste. METHODS: Enamel samples of non-erupted human third molars were treated with acidic solutions of tartaric (TA), malic (MA), lactic (LA), ascorbic (AA), phosphoric (PA) and citric (CA) acids with a concentration that gave an equivalent sensorial acidic taste. The acidic solutions were characterized by pH value and titratable acidity. Atomic force microscopy (AFM) based nanoindentation was used to study the nano mechanical properties and scanning electron microscopy (SEM) was used to study the morphology of the treated enamel samples and the untreated control areas, respectively. RESULTS: The investigated acids fell into two groups. The nano hardnesses of MA, TA and CA treated enamel samples (group I) were statistically significantly greater (p<0.05) than the nano hardnesses of PA, AA and LA treated enamel samples (group II). Within each group the nano hardness was not statistically significantly different (p>0.05). The SEM micrographs showed different etch prism morphologies depending on the acid used. SIGNIFICANCE: In vitro, the acids investigated led to different erosion effects on human dental enamel, despite their equivalent sensorial acidic taste. This has not been reported previously.

Snapshots of cooperative atomic motions in the optical suppression of charge density waves.

Macroscopic quantum phenomena such as high-temperature superconductivity, colossal magnetoresistance, ferrimagnetism and ferromagnetism arise from a delicate balance of different interactions among electrons, phonons and spins on the nanoscale. The study of the interplay among these various degrees of freedom in strongly coupled electron-lattice systems is thus crucial to their understanding and for optimizing their properties. Charge-density-wave (CDW) materials, with their inherent modulation of the electron density and associated periodic lattice distortion, represent ideal model systems for the study of such highly cooperative phenomena. With femtosecond time-resolved techniques, it is possible to observe these interactions directly by abruptly perturbing the electronic distribution while keeping track of energy relaxation pathways and coupling strengths among the different subsystems. Numerous time-resolved experiments have been performed on CDWs, probing the dynamics of the electronic subsystem. However, the dynamics of the periodic lattice distortion have been only indirectly inferred. Here we provide direct atomic-level information on the structural dynamics by using femtosecond electron diffraction to study the quasi two-dimensional CDW system 1T-TaS(2). Effectively, we have directly observed the atomic motions that result from the optically induced change in the electronic spatial distribution. The periodic lattice distortion, which has an amplitude of ∼0.1 Å, is suppressed by about 20% on a timescale (∼250 femtoseconds) comparable to half the period of the corresponding collective mode. These highly cooperative, electronically driven atomic motions are accompanied by a rapid electron-phonon energy transfer (∼350 femtoseconds) and are followed by fast recovery of the CDW (∼4 picoseconds). The degree of cooperativity in the observed structural dynamics is remarkable and illustrates the importance of obtaining atomic-level perspectives of the processes directing the physics of strongly correlated systems.

Pectin, alginate and gum arabic polymers reduce citric acid erosion effects on human enamel.

OBJECTIVES: Consumption of acidic soft drinks may lead to the dissolution and softening of human enamel, known as erosion. The first aim of this in vitro study was to test the hypothesis that food-approved polymers added to citric acid solutions (CAS) decrease the erosion of human dental enamel compared to citric acid solutions without these polymers. The second aim was to test the hypothesis that these polymers added to CAS form a polymer layer directly on the eroded enamel surface. METHODS: Enamel samples were obtained by embedding pieces of non-erupted human third molars in resin, grinding, and polishing them. CAS with pH values (pH: 2.3, 3.3 and 4.0) of typical soft drinks were prepared and modified by adding one of the following food-approved polymers (1%, w/w): highly esterified pectin (HP), propylene glycol alginate (PGA) and gum arabic (GA). The enamel samples were exposed to these polymer-modified citric acid solutions (PMCAS) or CAS not containing polymers, respectively, for different time periods (30, 60 and 120s). Atomic force microscopy (AFM)-based nanoindentation was used to analyze the nanomechanical properties of the treated enamel samples and the control samples. The enamel nanohardness and the reduced elastic modulus of the samples treated with PMCAS were statistically analyzed (ANOVA, t-test) and compared to the mechanical properties of the samples treated with unmodified CAS. Thus treated enamel samples were imaged by scanning electron microscopy (SEM) to investigate the surface morphology of the different enamel samples. RESULTS: Enamel samples treated with PMCAS containing GA or PGA showed statistically significantly higher nanohardness (p<0.05) compared to samples treated with CAS. PMCAS containing HP did not reduce the enamel nanohardness loss significantly compared to the CAS treated enamel samples. The enamel samples eroded with PMCAS show generally a smoother surface compared to the enamel surfaces of samples treated only with CAS as detected by SEM. Therefore, it is hypothesized that the polymers possibly adsorb on the eroded enamel surface. SIGNIFICANCE: The present in vitro erosion study shows that some of the polymers used in this study may possibly adsorb like a protective layer directly onto the human enamel surface. For GA and PGA this possibly formed polymer layer reduces the erosive effects of citric acid solutions as shown by nanoindentation measurements.

Quantification of dental erosion--a comparison of stylus profilometry and confocal laser scanning microscopy (CLSM).

OBJECTIVES: Since stylus profilometry applies a force on the sample surface, it is logical to hypothesize that the profilometer penetrates the surface of the enamel softened by acid solutions. The aims of the present study were, therefore, to test the hypothesis that surface profilometry measurements of eroded enamel alter the surface of the enamel, to quantify the potential effect of the surface alteration (scratches) on the measured values of enamel erosion by atomic force microscopy and to compare the values of enamel loss caused by erosion as measured by profilometry and non-contact confocal laser scanning microscopy (CLSM). METHODS: Enamel samples, cut from unerupted human third molars were treated with Volvic Mineral Water and citric acid solutions of different pH values. The enamel material loss was measured by two different contact profilometers and a reflection mode CLSM. The scratches depth was analyzed by atomic force microscopy. RESULTS: Our study demonstrated that the tip of the profilometer penetrated the surface of eroded enamel during the profilometry measurements, leading to clearly visible surface scratches on the enamel samples. The profilometers created surface scratches of a depth ranging from 57.6 (47.1)nm to 577.1 (157.6)nm on the surface of the eroded enamel and led, therefore, to a larger measured value of erosion. It was shown that the depth of the scratches depends on the pH value, the erosion time and the profilometer used. SIGNIFICANCE: With few exceptions profilometers deliver reliable values of erosive enamel material loss, although they create surface scratches on eroded enamel. Reflection mode CLSM is a non-tactile, fast and precise method for analyzing enamel erosion quantitatively in vitro.