The influence of pulse duration on the stress levels in ablation of ceramics: A finite element study

We present a finite element model to investigate the dynamic thermal and mechanical response of ceramic materials to pulsed infrared radiation. The model was applied to the specific problem of determining the influence of the pulse duration on the stress levels reached in human dental enamel irradiated by a CO2 laser at 10.6 mm with pulse durations between 0.1 and 100 ms and sub-ablative fluence. Our results indicate that short pulses with durations much larger than the characteristic acoustic relaxation time of the material can still cause high stress transients at the irradiated site, and indicate that pulse durations of the order of 10 ms may be more adequate both for enamel surface modification and for ablation than pulse durations up to 1 ms. The model presented here can easily be modified to investigate the dynamic response of ceramic materials to mid-infrared radiation and help determine optimal pulse durations for specific procedures.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/ESP/37944/2001 e SFRH/BD/4725/2001

Modelling the influence of pore size on the response of materials to infrared lasers – an application to human enamel

We present an analytical model for a ceramic material (hydroxyapatite) containing nanometre-scale water pores, and use it to estimate the pressure at the pore as a function of temperature at the end of a single 0.35 µs laser pulse by Er:YAG (2.94 µm) and CO2 (10.6 µm) lasers. Our results suggest that the pressure at the pore is directly related to pore temperature, and that very high pressures can be generated simply by the thermal expansion of liquid water. Since the temperature reached at the pores at the end of the laser pulse is a strong function of pore size for Er:YAG lasers, but is independent of pore size for CO2 lasers, our present results provide a possible explanation for the fact that the enamel threshold ablation fluences are more variable for Er:YAG lasers than for CO2 lasers, and suggest that experimentalists should analyse their results accounting for factors (like age or type of tooth) that may change the pore size distribution in their samples.Comunidade Europeia (CE) - Fundo Comunitário Europeu (FEDER).Fundação para a Ciência e a Tecnologia (FCT)

Mechanical and thermal response of enamel to IR radiation - a finite element mesoscopic model

We present finite element models of human dental enamel that account for water-pores known to exist in this material, and use them to assess the influence of these pores on the temperature and stress profiles during and after single Er:YAG (2.9 µm) and CO2 (10.6 µm) laser pulses of duration 0.35 µs. Our results indicate that the temperature maximum is reached at the water-pores at the end of the laser pulse; this maximum seems to be independent of pore size for the CO2 laser but appears to be strongly dependent of pore size for the Er:YAG laser. The pressure reached at the water pore seems to be directly related to the temperature at the pore and it is significantly higher that the stress levels reached throughout the modelled structure, which indicates that water pores should play a significant role in the ablation mechanisms, even before water vaporization takes place. These results suggest that researchers conducting enamel ablation by Er:YAG lasers - or other lasers with wavelengths for which the absorption coefficients of the mineral and the water differ significantly - may want to select their samples and analyse their results taking into account factors that may alter the degree of mineralization of a tooth, such as age or type of tooth.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência , Tecnologia, Inovação” – POCTI/ESP/37944/2001, SFRH/BD/4725/200.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)

Boundary conditions for 3-D dynamic models of ablation of ceramics by pulsed midiInfrared lasers

We present and discuss a set of boundary conditions (BCs) to use in three-dimensional, mesoscopic, finite element models of mid-infrared pulsed laser ablation of brittle materials. These models allow the study of the transient displacement and stress fields generated at micrometer scales during and after one laser pulse, where using conventional BCs may lead to some results without physical significance that can be considered an artefact of the calculations. The proposed BCs are tested and applied to a micrometer-scale continuous model of human dental enamel under CO2 radiation (10.6 mm, 0.35 ms pulse, sub-ablative fluence), giving rise to the following results: the highest stress is obtained at the irradiated surface of the model, at the end of the laser pulse, but afterwards it decreases rapidly until it becomes significantly lower than the stress in a region 2.5 mm deep in the model; a thermally induced vibration in the material is predicted. This non-intuitive dynamics in stress and displacement distribution cannot be neglected and has to be considered in dynamic laser ablation models, since it may have serious implications in the mechanisms of ablation.Fundação para a Ciência e a Tecnologia (FCT) – Programa Operacional “Ciência, Tecnologia, Inovação” - POCTI/ESP/37944/2001, SFRH/BD/4725/2001.Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER)

The infuence of glycosidic linkage neighbors on disaccharide conformation in vacuum

Correct description of the free energy of conformation change of disaccharides is important in understanding a variety of biochemical processes and, ultimately, in the manufacture of better food and paper products. In this study, we determine the relative free energy of a series of 12 disaccharides in vacuum using replica exchange molecular dynamics (repMD) simulations. The chosen sugars and the novel application of this method allow the exploration of the role of glycosidic linkage neighbors in conformer stabilization. In line with expectations, we find that hydrogen bonding (and therefore energetically preferred conformations) are determined both by the nature of the glycosidic linkage (i.e., 1 f 2, 1 f 3, or 1 f 4), the C1 epimer of the of the nonreducing monosaccharide, and by the configuration of carbon atoms once removed from the glycosidic linkage. Contrary to suggestions by prior authors for repMD more generally, we also demonstrate that repMD provides enhanced sampling, relative to conventional MD simulations of equivalent length, for disaccharides in vacuum at 300 K.Fundação para a Ciência e a Tecnologia (FCT)SFRH/BPD/20555/2004/0GVLNational Science Foundation under Grant CHE-043132

Benefits in cost and reduced discomfort of new techniques of minimally invasive cavity treatment

The concept of minimally invasive dentistry is appealing to more and more dentists. Patients share this enthusiasm. Three basic principles underlie minimally invasive dentistry: prevention of dental caries, less intrusive treatment for early lesions, and conservation of tissue when deeper lesions are restored. Together, these principles improve patient well-being by prolonging the life of teeth and by reducing the need for uncomfortable and costly dental treatments

Influence of methylene fluorination and chain length on the hydration shell structure and thermodynamics of linear diols

The interplay between the local hydration shell structure, the length of hydrophobic solutes, and their identity (perfluorinated or not) remains poorly understood. We address this issue by combining Raman–multivariate curve resolution (Raman-MCR) spectroscopy, simulation, and quantum-mechanical calculations to quantify the thermodynamics and the first principle interactions behind the formation of defects in the hydration shell of alkyl–diol and perfluoroalkyl–diol chains. The hydration shell of the fluorinated diols contains substantially more defects than that of the nonfluorinated diols; these defects are water hydroxy groups that do not donate hydrogen bonds and which either point to the solute (radial-dangling OH) or not (nonradial-dangling OH). The number of radial-dangling OH defects per carbon decreases for longer chains and toward the interior of the fluorinated diols, mainly due to less favorable electrostatics and exchange interactions; nonradial-dangling OH defects per carbon increase with chain length. In contrast, the hydration shell of the nonfluorinated diols only contains radial-dangling defects, which become more abundant toward the center of the chain and for larger chains, predominantly because of more favorable dispersion interactions. These results have implications for how the folding of macromolecules, ligand binding to biomacromolecules, and chemical reactions at water–oil interfaces could be modified through the introduction of fluorinated groups or solvents

Modelling of the human enamel laser ablation process at the mesoscopic scale

Comunidade Europeia (CE). Fundo Europeu de Desenvolvimento Regional (FEDER).A mesoscopic simulation of the process of human enamel laser ablation by Er:YAG and CO2 lasers is being developed using the finite element method, taking into account the complex structure and chemical composition of this material. A geometric model that allows studying in detail the temperature, stress and displacement distribution within a few enamel rods is presented. The heat generation that takes place inside the enamel at the centre of the laser spot, caused by a non-ablative laser pulse emitted by CO2 and Er:YAG lasers, was simulated. The sensitivity of our model to the estimated material parameters was studied. Temperature, displacement and stress distribution maps obtained for both lasers are presented. These preliminary results suggest that the temperature distribution across the enamel rods is different in the two situations considered; thermally induced stresses in the material are higher in the regions that are richer in hydroxyapatite (HA), and the higher displacements are observed in the regions that are rich in water. The rod tails inside enamel present higher stresses in the direction perpendicular to the surface of enamel than the ones that are created at the surface of our simulated structure. We conclude that the mesostructure plays a crucial role in the accurate modelling of dental laser ablation.Fundação para a Ciência e a Tecnologia (FCT) - Programa Operacional “Ciência , Tecnologia, Inovação” -POCTI/ESP/37944/2001, SFRH/BD/4725/2001

Seizure activity and brain damage in a model of focal non-convulsive <i>status epilepticus</i>

Aims: Focal non-convulsive status epilepticus (FncSE) is a common emergency condition that may present as the first epileptic manifestation. In recent years, it has become increasingly clear that de novo FncSE should be promptly treated to improve post-status outcome. Whether seizure activity occurring during the course of the FncSE contributes to ensuing brain damage has not been demonstrated unequivocally and is here addressed. Methods: We used continuous video-EEG monitoring to characterise an acute experimental FncSE model induced by unilateral intrahippocampal injection of kainic acid (KA) in guinea pigs. Immunohistochemistry and mRNA expression analysis were utilised to detect and quantify brain injury, 3-days and 1-month after FncSE. Results: Seizure activity occurring during the course of FncSE involved both hippocampi equally. Neuronal loss, blood-brain barrier permeability changes, gliosis and up-regulation of inflammation, activity-induced and astrocyte-specific genes were observed in the KA-injected hippocampus. Diazepam treatment reduced FncSE duration and KA-induced neuropathological damage. In the contralateral hippocampus, transient and possibly reversible gliosis with increase of aquaporin-4 and Kir4.1 genes were observed 3 days post-KA. No tissue injury and gene expression changes were found 1-month after FncSE. Conclusions: In our model, focal seizures occurring during FncSE worsen ipsilateral KA-induced tissue damage. FncSE only transiently activated glia in regions remote from KA-injection, suggesting that seizure activity during FncSE without local pathogenic co-factors does not promote long-lasting detrimental changes in the brain. These findings demonstrate that in our experimental model, brain damage remains circumscribed to the area where the primary cause (KA) of the FncSE acts. Our study emphasises the need to use antiepileptic drugs to contain local damage induced by focal seizures that occur during FncSE