Fluoridation of hydroxyapatite : a physicochemical view on caries prophylaxis via fluoride application

Since decades the application of fluoride containing dental health care products is a well established method in caries prophylaxis. According to current knowledge, fluoridation supports the remineralization of damaged enamel, inhibits demineralization of enamel by decreasing its demineralization-threshold, and decreases adhesion forces of bacteria. However, the exact mechanisms of fluoride uptake in the mineral component of tooth enamel, i.e., hydroxyapatite (HAp), are not yet completely understood. Therefore, in this work, experiments have been designed in order to enhance our understanding of how fluoride is taken up by HAp and how it alters the behavior of HAp surfaces under external influences, such as acid attacks. Sintered HAp pellets have been used as model systems for the tooth mineral. Fluoridation experiments have shown that the fluoridated layer forming on the HAp surface is only a few nanometers thick and that the layer thickness and the overall amount of fluoride taken up approach a point of saturation on a timescale of about 3 min. Although the layer is very thin and contains only minute amounts of fluoride, a very strong effect on the acid resistance of HAp could be observed: The surface was apparently inert to acid attacks for several minutes. These results give new insight into the mechanisms and especially into the timescale of fluoride uptake by HAp and they show how the incorporated fluoride in HAp correlates with its protective impact.Die Anwendung von Fluoriden in Zahnpflegeprodukten ist seit Jahrzehnten fester Bestandteil der Kariesprophylaxe. Nach heutigem Kenntnisstand unterstützen Fluoride die Remineralisierung beschädigten Zahnschmelzes. Sie mindern die Demineralisierung des Schmelzes und sie verringern die Haftkräfte von Bakterien. Bis heute sind die genauen Mechanismen der Fluoridaufnahme in die mineralische Komponente des Schmelzes, d.h. Hydroxylapatit (HAp), noch nicht vollständig verstanden. In der vorliegenden Arbeit wurden Experimente entwickelt, die zu einem besseren Ver\-ständ\-nis der Fluoridaufnahme beitragen, und die zeigen, wie die Fluorierung das Verhalten der HAp-Oberfläche bei einem Säureangriff verändert. Als Modellsysteme für die mineralische Komponente des Schmelzes wurden gesinterte HAp-Prüfkörper verwendet. Die fluorierte Schicht, die sich an der HAp-Oberfläche bildet, ist nur wenige Nanometer dick. Die Schichtdicke und die Gesamtmenge an Fluorid, die aufgenommen wird, erreichen nach etwa 3 min einen Zustand der Sättigung. Trotz der geringen Dicke und des geringen Fluoridgehalts der Schicht konnte eine starke Auswirkung auf die Säureresistenz von HAp nachgewiesen werden: Die Oberfläche war für einige Minuten immun gegenüber Säureangriffen. Die Ergebnisse dieser Arbeit liefern neue Erkenntnisse über die Mechanismen und insbesondere über den zeitlichen Verlauf der Fluoridaufnahme in HAp und zeigen die schützenden Auswirkungen aufgenommenen Fluorids auf HAp-Oberflächen.DP

Impact of geometry on chemical analysis exemplified for photoelectron spectroscopy of black silicon

For a smooth surface, the chemical composition can be readily evaluated by a variety of spectroscopy techniques; a prominent example is X-ray photoelectron spectroscopy (XPS), where the relative proportions of the elements are mainly determined by the intensity ratio of the element-specific photoelectrons. This deduction, however, is more intricate for a nanorough surface, such as black silicon, since the steep slopes of the geometry mimic local variations of the local emission angle. Here, we explicitly quantify this effect via an integral geometric analysis, by using so-called Minkowski tensors. Thus, we match the chemical information from XPS with topographical information from atomic force microscopy (AFM). Our method provides reliable estimates of layer thicknesses for nanorough surfaces. For our black silicon samples, we found that the oxide layer thickness is on average comparable to that of a native oxide layer. Our study highlights the impact of complex geometries at the nanoscale on the analysis of chemical properties with implications for a broad class of spectroscopy techniques

Hydroxyapatite Pellets as Versatile Model Surfaces for Systematic Adhesion Studies on Enamel : A Force Spectroscopy Case Study

Research into materials for medical application draws inspiration from naturally occurring or synthesized surfaces, just like many other research directions. For medical application of materials, particular attention has to be paid to biocompatibility, osseointegration, and bacterial adhesion behavior. To understand their properties and behavior, experimental studies with natural materials such as teeth are strongly required. The results, however, may be highly case-dependent because natural surfaces have the disadvantage of being subject to wide variations, for instance in their chemical composition, structure, morphology, roughness, and porosity. A synthetic surface which mimics enamel in its performance with respect to bacterial adhesion and biocompatibility would, therefore, facilitate systematic studies much better. In this study, we discuss the possibility of using hydroxyapatite (HAp) pellets to simulate the surfaces of teeth and show the possibility and limitations of using a model surface. We performed single-cell force spectroscopy with single Staphylococcus aureus cells to measure adhesion-related parameters such as adhesion force and rupture length of cell wall proteins binding to HAp and enamel. We also examine the influence of blood plasma and saliva on the adhesion properties of S. aureus. The results of these measurements are matched to water wettability, elemental composition of the samples, and the change in the macromolecules adsorbed over time on the surface. We found that the adhesion properties of S. aureus were similar on HAp and enamel samples under all conditions: Significant decreases in adhesion strength were found equally in the presence of saliva or blood plasma on both surfaces. We therefore conclude that HAp pellets are a good alternative for natural dental material. This is especially true when slight variations in the physicochemical properties of the natural materials may affect the experimental series

Is adhesion superficial? Silicon wafers as a model system to study van der Waals interactions

Adhesion is a key issue for researchers of various fields, it is therefore of uppermost importance to understand the parameters that are involved. Commonly, only surface parameters are employed to determine the adhesive forces between materials. Yet, van der Waals forces act not only between atoms in the vicinity of the surface, but also between atoms in the bulk material. In this review, we describe the principles of van der Waals interactions and outline experimental and theoretical studies investigating the influence of the subsurface material on adhesion. In addition, we present a collection of data indicating that silicon wafers with native oxide layers are a good model substrate to study van der Waals interactions with coated materials

Time Dependence of Fluoride Uptake in Hydroxyapatite

Fluoridation of enamel is believed to provide an effective tool to protect teeth from caries, but there is still little information on the time scale of fluoride uptake. In this study, highly compressed pellets of hydroxyapatite are used as first-order model systems to approximate the mineral component of natural enamel for investigations on the time-dependence of fluoride uptake. We found that both the overall amount of fluoride as well as the mean thickness of the fluoridated surface layer cannot be extended to any values just by increasing the application time of a fluoride containing agent. Instead, both parameters start to become constant on a time scale of about 3 min. The present results as obtained on a synthetic model “tooth” show that the time scale to provide the maximum amount of fluoride possible is of the same order of magnitude as that in usual daily practice in dental care when applying toothpastes or mouth rinses

Synthesis of Hydroxyapatite Substrates: Bridging the Gap between Model Surfaces and Enamel

Hydroxyapatite substrates are common biomaterials, yet samples of natural teeth do not meet the demands for well-defined, highly reproducible properties. Pellets of hydroxyapatite were produced via the field assisted sintering technology (FAST) as well as via pressureless sintering (PLS). The applied synthesis routes provide samples of very high density (95%–99% of the crystallographic density) and of very low surface roughness (lower than 1 nm when averaged per 1 μm²). The chemical composition of the raw material (commercial HAP powder) as well as the crystalline structure is maintained by the sintering processes. These specimens can therefore be considered as promising model surfaces for studies on the interactions of biomaterial with surfaces of biological relevance, as demonstrated for the adsorption of BSA proteins