Femtosecond laser ablation of dentin and enamel for fast and more precise dental cavity preparation.

PURPOSE: The purpose of the present work was to achieve fast and more precise ablation in dentin and enamel by using a commercial femtosecond laser system with high repetition rate, whilst avoiding any collateral irreversible damages in the hard tissue and pulp area. METHODS: We used fluence of the incident laser pulses which was marginally higher than the ablation threshold for dentin and enamel. The study was based on the hypothesis that femtosecond laser operating with a repetition rate in the range of 100-500 kHz can controllably ablate dental tissue obtaining sufficiently high removal rate whilst avoiding any collateral irreversible damages in the hard tissue and pulp area. RESULTS: The ablation yielded the formation of 1 mm3 craters with well-defined precise vertical cavity sides and edges. Advantageous high porosity and numerous interconnected pores were introduced in the ablated zones. Thermal load and hence collateral thermo-mechanical damages were avoided and the crystalline structure of the tooth constituent hydroxyapatite was preserved. CONCLUSION: The ultrafast femtosecond laser used in our work hold the promise of a significant drilling ability without collateral thermomechanical effects. It achieves high processing efficiency, overcomes disadvantages of other laser systems reported and can be used to develop an instrument for cavity preparation based on fast and precise ablation. Our further aim is to exceed the speed of traditional drilling instruments and thus to reduce the treatment time which in turn will bring comfort to the patient.

High Speed Imaging of Cavitation around Dental Ultrasonic Scaler Tips.

Cavitation occurs around dental ultrasonic scalers, which are used clinically for removing dental biofilm and calculus. However it is not known if this contributes to the cleaning process. Characterisation of the cavitation around ultrasonic scalers will assist in assessing its contribution and in developing new clinical devices for removing biofilm with cavitation. The aim is to use high speed camera imaging to quantify cavitation patterns around an ultrasonic scaler. A Satelec ultrasonic scaler operating at 29 kHz with three different shaped tips has been studied at medium and high operating power using high speed imaging at 15,000, 90,000 and 250,000 frames per second. The tip displacement has been recorded using scanning laser vibrometry. Cavitation occurs at the free end of the tip and increases with power while the area and width of the cavitation cloud varies for different shaped tips. The cavitation starts at the antinodes, with little or no cavitation at the node. High speed image sequences combined with scanning laser vibrometry show individual microbubbles imploding and bubble clouds lifting and moving away from the ultrasonic scaler tip, with larger tip displacement causing more cavitation.

Bioactivity of polycrystalline silicon layers.

After oxygen, silicon is the second most abundant element in the environment and is present as an impurity in most materials. The widespread occurrence of siliceous biominerals as structural elements in lower plants and animals suggests that Si plays a role in the production and maintenance of connective tissue in higher organisms. It has been shown that the presence of Si is necessary in bones, cartilage and in the formation of connective tissue, as well as in some important metabolic processes. In this work, polycrystalline silicon layers are tested in terms of bioactivity, i.e., their ability to induce hydroxyapatite formation from simulated body fluid. Hydroxyapatite is a biologically compatible material with chemical similarity to the inorganic part of bones and teeth. Polycrystalline silicon layers are obtained by aluminum induced crystallization of Al and amorphous Si thin films deposited sequentially on glass substrates by radio-frequency magnetron sputtering and subsequently annealed in different atmospheres. The hydroxyapatite formation is induced by applying a method of laser-liquid-solid interaction. The method consists of irradiating the samples with laser light while immersed in a solution that is supersaturated with respect to Ca and P. As a result, heterogeneous porous sponge-like carbonate-containing hydroxyapatite is grown on the polysilicon surfaces. Crystals that are spherical in shape, containing Ca, P and O, Na, Cl, Mg, Al, Si and S, as well as well-faceted NaCl crystals are embedded in the hydroxyapatite layer. Enhancement of the hydroxyapatite growth and increased crystallinity is observed due to the applied laser-liquid-solid interaction.

Calcium phosphate formation on plasma immersion ion implanted low density polyethylene and polytetrafluorethylene surfaces.

The flexible structure of polymers has enabled them to be useful in a wide variety of medical applications due to the possibility to tailor their properties to suit desired applications. For a long time, there has been an increasing interest in utilizing polymers as matrices for calcium phosphate-based composites with applications in hard tissue implants. On the other side, polymers with application as heart valves, urea catheters and artificial vessels present a case where the formation of minerals (namely calcification) should be avoided. The modification of polymer surfaces by various ion beam treatments for reducing the calcification, as for example plasma immersion ion implantation (PIII), is well known and has a long time effect. This work is part of a wider investigation of the ability of plasma immersion ion implanted polymers to induce calcium phosphate formation from an aqueous solution resembling the human blood plasma. In the experiment described in this paper, topographical and chemical changes were inserted on the surfaces of two conventional polymers (low density polyethylene and polytetrafluorethylene) by PIII with nitrogen ions, and under conditions mimicking the natural mineral formation processes. The effect of the plasma modification on the calcium phosphate nucleation and growth from the aqueous solution was ambiguous. We suppose that the complex combination of surface characteristics influenced the ability of the plasma treated polymer films to induce the formation of a calcium phosphate layer.

Hydroxyapatite grown on a native extracellular matrix: initial interactions with human fibroblasts.

Proteins are known to modulate the physical properties of minerals, and thus we anticipate that they will strongly influence the structure and the biological properties of biomimetically prepared carbonate-containing hydroxyapatite. This study was designed to learn more about the main morphological characteristics of hydroxyapatite layer grown on different substrates coated with an extracellular matrix, a biological matrix that was produced by cultured osteoblast-like cells. The hydroxyapatite growth was carried out in a simulated body fluid, a solution that resembles the human blood plasma. It was found that the extracellular matrix may serve as a template for the mineralization of biomimetic hydroxyapatite on the surface of materials like stainless steel, silicon, and silica glass, leading to the formation of a homogeneous layer. The latter was consisting of nanometer-sized hydroxyapatite crystals grouped in particles with regular sphere shape and with a significantly higher average diameter in comparison to samples without extracellular matrix coating. Subsequent in vitro studies with living fibroblasts showed that the cellular behavior depended on the type of underlying substrate used for the hydroxyapatite growth, as well as on the immersion time of the samples in the simulated body fluid. Increasing the thickness of the hydroxyapatite layer altered visibly the cellular response, and the fibroblasts developed stellate morphology on the samples with a hydroxyapatite-extracellular matrix coating. Preadsorption with fibronectin significantly improved the initial cell adhesion and spreading to all surfaces. Thus, such an approach may contribute to the development of surfaces with better tissue compatibility.

White light scanning interferometry adapted for large-area optical analysis of thick and rough hydroxyapatite layers.

Understanding the mechanisms of biomineralization continues to be an important area of research in physics, chemistry, materials science, medicine, and dentistry due to its importance in the formation of bones, teeth, cartilage, etc. Stimulated by these fascinating natural examples, as well as by certain others such as shells and corals, attempts are being made to develop synthetic, biomimetic nanocomposites by simulating the basic principles of biomineralization. We have grown bio-like hydroxyapatite layers in vitro on substrates of stainless steel, silicon, and silica glass by using a biomimetic approach (i.e., immersion in a supersaturated simulated body fluid). Hydroxyapatite is one of the most common natural biomaterials and an important structural component of bones and teeth. Metal substrates are of interest for hard tissue implants, while semiconductors and glasses are under investigation for their use as biosensors. Using classical techniques such as stylus profiling, atomic force microscopy (AFM), and scanning and transmission electron microscopy (SEM and TEM), it was found difficult, ambiguous, destructive, or time-consuming to measure the topography, thickness, and profile of the grown heterogeneous, thick, and rough hydroxyapatite layers. On the other hand, coherence probe microscopy based on white light scanning interferometry and image processing provides rapid, contactless measurements of surface roughness and does not need any sample preparation. The results obtained have shown a typical layer thickness of up to 20 microm and an average root-mean-square (rms) roughness of about 4 mum. The hydroxyapatite investigated in this work presents nonetheless a challenge for this technique because of its semi-translucency, high surface roughness, and the presence of cavities formed throughout its volume. This results in a variable quality of fringe pattern, ranging from classical fringes (on a smooth surface) to complex fringes displaying properties of white light speckle (on a rough surface), together with multiple fringe signals along the optical axis in the presence of buried layer interfaces, which in certain configurations affect the axial and lateral precision of the measurement. In this paper we present the latest results for optimizing the measurement conditions in order to reduce such errors and to provide additional useful information concerning the layer.