Resistência à fadiga de vitrocerâmicas cristalizadas por energia de micro-ondas / Fadigue resistance of glass-ceramics crystallized by microwave energy

O objetivo deste trabalho foi analisar a resistência à fadiga de duas vitrocerâmicas cristalizadas convencionalmente ou por energia de microondas. Foram confeccionados 120 discos (N) de 1,2 milímetros de espessura e 12 milímetros de diâmetro e estes foram distribuídos em 4 grupos (n=30) ­ Celtra cristalizado convencionalmente (CC), Celtra cristalizado por energia de micro-ondas (CM), e.max cristalizado convencionalmente (EC) e e.max cristalizado por energia de micro-ondas (EM). Foi realizado o cálculo de resistência à flexão biaxial em 5 amostras de cada grupo, servindo como parâmetro para a definição das taxas de carregamento do teste de fadiga. Os demais espécimes foram distribuídos na proporção 9:6:3 e testados sob fadiga acelerada (step stress) nos perfis leve (9), moderado (6) e agressivo (3), variando os incrementos de carga e o número de ciclos. Os espécimes fraturados foram avaliados em estereomicroscópio e microscópio eletrônico de varredura (MEV), para análise da superfície e das características de fratura. Os dados de sobrevivência foram utilizados para calcular o valor beta (ß) de Weibull e a confiabilidade para as missões dadas. Foi calculada a probabilidade de sobrevivência ­ de acordo com a carga e o número de ciclos ­ usando um software para análise de fadiga. Os valores de beta (ß) para os grupos CC, CM, EC e EM foram, respectivamente, 2,65, 0,25, 0,62 e 0,3. Quanto à confiabilidade para missões de 50.000 ciclos, não houve diferença estatística quando a carga aplicada foi de 50 Mpa. Para a carga de 100 Mpa, apenas o grupo CM apresentou diferença estatistica em relação aos outros grupos, com maior probabilidade de falha. O método de cristalização não influenciou no desempenho do dissilicato de lítio com 150 Mpa de carga, enquanto o silicato de lítio reforçado por zircônia apresentou 9% a mais de confiabilidade quando cristalizado convencionalmente. Com 200 e 300 Mpa de carga, o método de cristalização e o tipo de cerâmica influenciaram no resultado, com destaque para o desempenho do dissilicato de lítio cristalizado por energia de micro-ondas (maior confiabilidade; 89% para 200 Mpa e 6% para 300 Mpa). Concluiu-se que a cristalização por energia de micro-ondas é uma alternativa viável, pois apresentou resistência compatível aos grupos controle, com o grande benefício de economia de energia(AU)

The objective of this study was to analyze the fatigue strength of two glass ceramics conventionally crystallized or crystallized by microwave energy. 120 discs (N) of 1.2 mm thickness and 12 mm diameter were made and distributed in 4 groups (n=30) ­ Celtra conventionally crystallized (CC), Celtra crystallized by microwave energy (CM), e.max conventionally crystallized (EC) and e.max crystallized by microwave energy (EM). Biaxial flexural strength was calculated in 5 samples from each group, serving as a parameter to define the loading rates of the fatigue test. The remaining specimens were distributed in a 9: 6: 3 ratio and tested under step stress in light (9), moderate (6) and aggressive (3) profiles, varying the load increments and the number of cycles. Fractured specimens were evaluated by stereomicroscope and scanning electron microscope (SEM) for surface and fracture characteristics analysis. Survival data were used to calculate Weibull's beta (ß) value and reliability for the given missions. The survival probability - according to load and number of cycles - was calculated using software for fatigue analysis. The beta (ß) values for the CC, CM, EC and MS groups were, respectively, 2.65, 0.25, 0.62 and 0.3. For 50,000 cycles missions, there was no statistical difference when the applied load was 50 Mpa. For the 100 Mpa load, only the CM group presented statistical difference with the other groups, with a higher probability of failure. The crystallization method did not influence the performance of the 150 Mpa charged lithium disilicate, while zirconia reinforced lithium silicate showed 9% more reliability when conventionally crystallized. At 200 and 300 Mpa of charge, the crystallization method and the type of ceramic influenced the result, especially the performance of microwave energy crystallized lithium disilicate (higher reliability; 89% for 200 Mpa and 6% for 300 Mpa). It was concluded that microwave energy crystallization is a viable alternative because presented resistance compatible with the control groups, with the with the great benefit of energy saving(AU)

Induction of stress volatiles and changes in essential oil content and composition upon microwave exposure in the aromatic plant Ocimum basilicum.

Exposure to sustained low intensity microwaves can constitute a stress for the plants, but its effects on plant secondary chemistry are poorly known. We studied the influence of GSM and WLAN-frequency microwaves on emissions of volatile organic compounds and content of essential oil in the aromatic plant Ocimum basilicum L. hypothesizing that microwave exposure leads to enhanced emissions of stress volatiles and overall greater investment in secondary compounds. Compared to the control plants, microwave irradiation led to decreased emissions of ß-pinene, α-phellandrene, bornyl acetate, ß-myrcene, α-caryophyllene and benzaldehyde, but increased emissions of eucalyptol, estragole, caryophyllene oxide, and α-bergamotene. The highest increase in emission, 21 times greater compared to control, was observed for caryophyllene oxide. The irradiation resulted in increases in the essential oil content, except for the content of phytol which decreased by 41% in the case of GSM-frequency, and 82% in the case of WLAN-frequency microwave irradiation. The strongest increase in response to WLAN irradiation, >17 times greater, was observed for hexadecane and octane contents. Comparisons of volatile compositions by multivariate analyses demonstrated a clear separation of different irradiance treatments, and according to the changes in the volatile emissions, the WLAN-frequency irradiation represented a more severe stress than the GSM-frequency irradiation. Overall, these results demonstrating important modifications in the emission rates, essential oil content and composition indicate that microwave irradiation influences the quality of herbage of this economically important spice plant.

Enhancement of solubility and dissolution rate of poorly water soluble raloxifene using microwave induced fusion method

The objective of the present work was to enhance the solubility and dissolution rate of the drug raloxifene HCl (RLX), which is poorly soluble in water. The solubility of RLX was observed to increase with increasing concentration of hydroxypropyl methylcellulose (HPMC E5 LV). The optimized ratio for preparing a solid dispersion (SD) of RLX with HPMC E5 LV using the microwave-induced fusion method was 1:5 w/w. Microwave energy was used to prepare SDs. HPMC E5 LV was used as a hydrophilic carrier to enhance the solubility and dissolution rate of RLX. After microwave treatment, the drug and hydrophilic polymer are fused together, and the drug is converted from the crystalline form into an amorphous form. This was confirmed through scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and powder X-ray diffraction (PXRD) studies. These results suggested that the microwave method is a simple and efficient method of preparing SDs. The solubility and dissolution rate of the SDs were increased significantly compared with pure RLX due to the surfactant and wetting properties of HPMC E5 LV and the formation of molecular dispersions of the drug in HPMC E5 LV. It was concluded that the solubility and dissolution rate of RLX are increased significantly when an SD of the drug is prepared using the microwave-induced fusion method.

O objetivo do presente trabalho foi aumentar a solubilidade e taxa de dissolução do cloridrato de raloxifeno (RLX), que é pouco solúvel em água. A solubilidade do RLX aumentou com o aumento da concentração de hidroxipropilmetilcelulose (HPMC E5 LV). A proporção otimizada para a preparação de uma dispersão sólida (DS) de RLX com HPMC E5 LV utilizando o método de fusão induzida por microondas foi de 1:5 (p/p). A energia do microondas foi usada para preparar DS. O HPMC E5 LV foi utilizado como veículo hidrofílico para aumentar a solubilidade e a taxa de dissolução de RLX. Após o tratamento por microondas, o polímero hidrofílico e o fármaco são fundidos em conjunto, sendo o fármaco convertido da forma cristalina para a amorfa. Confirmou-se por meio de microscopia eletrônica de varredura (MEV), calorimetria exploratória diferencial (DSC) e difração de raios X do pó (PXRD). Estes resultados sugerem que o método de microondas é simples e eficiente para a preparação de DS. A solubilidade e taxa de dissolução de DS foram aumentadas, significativamente, em comparação com RLX puro devido às propriedades tensoativas e umectantes de HPMC E5 LV e à formação de dispersões moleculares do fármaco em HPMC E5 LV. Concluiu-se que a solubilidade e a taxa de dissolução de RLX foram significativamente aumentadas quando a DS do fármaco é preparada utilizando o método de fusão induzida por microondas.

[Determination of trace zinc, manganese, cadmium and lead in aloe by microwave-digestion atomic absorption spectrometry].

A microwave digestion procedure was developed for the determination of trace Zn, Mn, Cd and Pb in aloe-leaf cuticle and aloe-leaf gelatin, using the obturated vessel microwave digestion system with a pressure controlling part, and the amounts of these trace metallic elements were determined by atomic absorption spectrometry after microwave digestion. The effects of the composition of digestion solution, the ratio of the sample to digestion solution, and the digestion time were studied. It is satisfactory to apply the microwave digestion procedure to the determination of Zn, Mn, Cd and Pb under the optimized condition with the recovery of 95.0% to 110.0% and RSD of 0.3% to 6.2%. The results show that this method is rapid and simple with low environmental contamination and complete digestion of the sample.

Comparative effects of extremely high power microwave pulses and a brief CW irradiation on pacemaker function in isolated frog heart slices.

The existence of specific bioeffects due to high peak power microwaves and their potential health hazards are among the most debated but least explored problems in microwave biology. The present study attempted to reveal such effects by comparing the bioeffects of short trains of extremely high power microwave pulses (EHPP, 1 micros width, 250-350 kW/g, 9.2 GHz) with those of relatively low power pulses (LPP, 0.5-10 s width, 3-30 W/g, 9.2 GHz). EHPP train duration and average power were made equal to those of an LPP; therefore both exposure modalities produced the same temperature rise. Bioeffects were studied in isolated, spontaneously beating slices of the frog heart. In most cases, a single EHPP train or LPP immediately decreased the inter-beat interval (IBI). The effect was proportional to microwave heating, fully reversible, and easily reproducible. The magnitude and time course of EHPP- and LPP-induced changes always were the same. No delayed or irreversible effects of irradiation were observed. The same effect could be repeated in a single preparation numerous times with no signs of adaptation, sensitization, lasting functional alteration, or damage. A qualitatively different effect, namely, a temporary arrest of preparation beats, could be observed when microwave heating exceeded physiologically tolerable limits. This effect also did not depend on whether the critical temperature rise was produced by LPP or EHPP exposure. Within the studied limits, we found no indications of EHPP-specific bioeffects. EHPP- and LPP-induced changes in the pacemaker rhythm of isolated frog heart preparation were identical and could be entirely attributed to microwave heating.

Low-level exposure to pulsed 900 MHz microwave radiation does not cause deficits in the performance of a spatial learning task in mice.

There is some concern that short-term memory loss or other cognitive effects may be associated with the use of mobile cellular telephones. In this experiment, the effect of repeated, acute exposure to a low intensity 900 MHz radiofrequency (RF) field pulsed at 217 Hz was explored using an appetitively-motivated spatial learning and working memory task. Adult male C57BL/6J mice were exposed under far field conditions in a GTEM cell for 45 min each day for 10 days at an average whole-body specific energy absorption rate (SAR) of 0.05 W/kg. Their performance in an 8-arm radial maze was compared to that of sham-exposed control animals. All behavioral assessments were performed without handlers having knowledge of the exposure status of the animals. Animals were tested in the maze immediately following exposure or after a delay of 15 or 30 min. No significant field-dependent effects on performance were observed in choice accuracy or in total times to complete the task across the experiment. These results suggest that exposure to RF radiation simulating a digital wireless telephone (GSM) signal under the conditions of this experiment does not affect the acquisition of the learned response. Further studies are planned to explore the effects of other SARs on learned behavior. Bioelectromagnetics 21:151-158, 2000. Published 2000 Wiley-Liss, Inc.

Cardiovascular and thermal effects of microwave irradiation at 1 and/or 10 GHz in anesthetized rats.

Relatively large thermal gradients may exist during exposure of an animal to microwaves (MWs), particularly at high frequencies. Differences in thermal gradients within the body may lead to noticeable differences in the magnitude of cardiovascular changes resulting from MW exposure. This study compares the thermal distribution and cardiovascular effects of exposure to a single MW frequency with effects of simultaneous exposure to two frequencies. Ketamine-anesthetized male Sprague-Dawley rats (n = 58) were exposed individually to one of three conditions: 1-GHz, 10-GHz, or combined 1- and 10-GHz MWs at an equivalent whole-body specific absorption rate of 12 W/kg. The continuous-wave irradiation was conducted under far-field conditions with animals in E orientation (left lateral exposure, long axis parallel to the electric field) or in H orientation (left lateral exposure, long axis perpendicular to the electric field). Irradiation was started when colonic temperature was 37.5 degrees C and was continued until lethal temperatures were attained. Colonic, tympanic, left and right subcutaneous, and tail temperatures, and arterial blood pressure, heart rate, and respiratory rate were continuously recorded. In both E and H orientations, survival time (i.e., time from colonic temperature of 37.5 degrees C until death) was lowest in animals exposed at 1-GHz, intermediate in those exposed at 1- and 10-GHz combined, and greatest in the 10-GHz group (most differences statistically significant). At all sites (with the exception of right subcutaneous), temperature values in the 1- and 10-GHz combined group were between those of the single-frequency exposure groups in both E and H orientations. During irradiation, arterial blood pressure initially increased and then decreased until death. Heart rate increased throughout the exposure period. The general, overall patterns of these changes were similar in all groups. The results indicate that no unusual physiological responses occur during multi-frequency MW exposure, when compared with results of single-frequency exposure. Bioelectromagnetics 21:159-166, 2000. Published 2000 Wiley-Liss, Inc.

Pulsed microwave induced light, sound, and electrical discharge enhanced by a biopolymer.

Intense flashes of light were observed in sodium bicarbonate and hydrogen peroxide solutions when they were exposed to pulsed microwave radiation, and the response was greatly enhanced by a microwave-absorbing, biosynthesized polymer, diazoluminomelanin. A FPS-7B radar transmitter, operating at 1.25 GHz provided pulses of 5.73 +/- 0.09 micros in duration at 10.00 +/- 0.03 pulses/s with 2.07 +/- 0.08 MW forward power (mean +/- standard deviation), induced the effect but only when the appropriate chemical interaction was present. This phenomenon involves acoustic wave generation, bubble formation, pulsed luminescence, ionized gas ejection, and electrical discharge. The use of pulsed microwave radiation to generate highly focused energy deposition opens up the possibility of a variety of biomedical applications, including targeting killing of microbes or eukaryotic cells. The full range of microwave intensities and frequencies that induce these effects has yet to be explored and, therefore, the health and safety implications of generating the phenomena in living tissues remain an open question.

Non-uniformity of surface temperatures after microwave heating of poultry meat.

Microwave energy has the potential to raise the surface temperatures of meat rapidly for a short period of time sufficient to reduce bacterial numbers significantly without causing physical changes to the meat. Studies have investigated the ability of a standard domestic microwave oven (2450 MHz; IEC 1191 W), an experimental repeatable microwave cavity (2450 MHz; IEC 1139 W) and a number of shielding techniques to achieve uniform surface temperature distributions on pieces of poultry meat. In the domestic oven temperature differences of up to 60 and 80 degrees C were found between different points on the surface of the same sample after 30 s and 3 minutes of heating respectively. The use of a standard cavity and shielding resulted in a difference of less than 5 degrees C between the average surface temperature on the edge and middle of regular slabs of chicken after 30 s exposure. Results show that microwave heating, using 2450 MHz, is unlikely to produce consistently uniform enough surface temperatures on meat to reduce bacterial numbers without surface damage.

Effect of continuous-wave and amplitude-modulated 2.45 GHz microwave radiation on the liver and brain aminoacyl-transfer RNA synthetases of in utero exposed mice.

Investigations have been carried out concerning the effects of microwave (MW) exposure on the aminoacyl-transfer ribonucleic acid (tRNA) synthetase of the progeny of females that were exposed during their entire period of gestation (19 days). The changes caused by continuous-wave (CW) and amplitude-modulated (AM) MW radiation have been compared. CFLP mice were exposed to MW radiation for 100 min each day in an anechoic room. The MW frequency was 2.45 GHz, and the amplitude modulation had a 50 Hz rectangular waveform (on/off ratio, 50/50%). The average power density exposure was 3 mW/cm2, and the whole body specific absorption rate (SAR) was 4.23 +/- 0.63 W/kg. The weight and mortality of the progeny were followed until postnatal day 24. Aminoacyl-tRNA synthetase enzymes and tRNA from the brains and livers of the offspring (461 exposed, 487 control) were isolated. The aminoacyl-tRNA synthetase activities were determined. The postnatal increase of body weight and organ weight was not influenced by the prenatal MW radiation. The activity of enzyme isolated from the brain showed a significant decrease after CW MW exposure, but the changes were not significant after 50 Hz AM MW exposure. The activity of the enzyme isolated from liver increased under CW and 50 Hz modulated MW.

Alteration of life span of mice chronically exposed to 2.45 GHz CW microwaves.

Female CD1 mice were exposed from the thirty-fifth day of age for the remainder of their lives to 2.45 GHz, CW-microwave radiation at a power density of 3 or 10 mW/cm2 (SAR = 2.0 or 6.8 W/kg). Exposures took place 1 h/day, 5 day/week in an anechoic chamber at an ambient temperature of 22 degrees C and a relative humidity of 50%. There were 25 animals in each exposure group, and an equal number of controls were concurrently sham exposed. The average life span of animals exposed at 10 mW/cm2 was significantly shorter than that of sham-exposed controls (572 days vs. 706 days; P = .049; truncation > 20%). In contrast, the average lifespan of the animals exposed at 3 mW/cm2 was slightly, but not significantly, longer (738 days) than that of controls (706 days).