1,364 research outputs found
Thermoelastic Damping in Micro- and Nano-Mechanical Systems
The importance of thermoelastic damping as a fundamental dissipation
mechanism for small-scale mechanical resonators is evaluated in light of recent
efforts to design high-Q micrometer- and nanometer-scale electro-mechanical
systems (MEMS and NEMS). The equations of linear thermoelasticity are used to
give a simple derivation for thermoelastic damping of small flexural vibrations
in thin beams. It is shown that Zener's well-known approximation by a
Lorentzian with a single thermal relaxation time slightly deviates from the
exact expression.Comment: 10 pages. Submitted to Phys. Rev.
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Analyzing Heart Rate Variability Using a Photoplethysmographic Signal Measuring System
A heart rate variability (HRV) measuring system and its analysis method have been developed in this study. It is composed of a hardware measuring system based on a noninvasive photoplethysmographic (PPG) signal measuring device to acquire oxyhemoglobin saturation using pulse oximetry (SpO2) signals and a further software package including the methods used to filter and analyze the signals for heart rate variability. Firstly, an experiment is designed for measuring heartbeat using the system to observe whether the empirical mode decomposition(EMD) can really inhibit noise or not on one volunteer with 10 minutes repeated for 10 times. Then, the hardware system and analysis method are tested on another 10 volunteers before and after receiving cold face immersion. The results of the first experiment have no significant difference with commercial instrument (p > 0.05), but the results using EMD perform better when signals are contaminated by artifacts. The second part experiment is subdivided into two stages. The results show that HR values at each stage have no significant difference with commercial instrument (p > 0.05). The LF significantly decreases from 0.33 0.03 to 0.31 0.03, while HF significantly increases from 0.41 0.07 to 0.43 0.07 indicating cold face immersion can increase parasympathetic and decrease sympathetic actions. Hence, LF/HF changes significantly from high (0.85 0.17) to low (0.74 0.17) before and after adding stimulation. Due to the reasons above, it confirms that the developed system can measure heartbeat and observe the heart rate variability. So the findings of this research may be useful for developing a low-cost and a miniaturized pulse oxymeter system to continuously measure HR and HRV for the purpose of convenience, portability, and operability
Selective N,N-Dibenzylation of primary aliphatic amines with dibenzylcarbonate in the presence of phosphonium salts
In the presence of catalytic amounts of tetraalkylphosphonium
salts and under solventless conditions,
primary aliphatic amines (RNH2: R ) PhCH2, Ph(CH2)2,
n-decyl, and 1-naphthylmethyl) are efficiently N-benzylated
to the corresponding RN(CH2Ph)2, using dibenzyl carbonate
as the benzylating reagent. Compared to the reaction run
without salt, where the competitive formation of the benzyl
carbamate is favored, the phosphonium salt promotes high
selectivity toward the benzylated amine and an increase of
the reaction rate as well. However, in a single case explored
for an amino acidic compound, namely 4-(aminomethyl)benzoic
acid [4-(NH2CH2)C6H4CO2H], both N,N-dibenzylation
and esterification of the acid group were observed. Analysis
of the IR vibrational modes of benzylamine in the presence
of tetrabutylphosphonium bromide supports the hypothesis
that this enhanced selectivity may be due to an acid-base
interaction between the salt and the amine, which increases
the steric bulk of the amine and favors attack of the
nucleophile on the less hindered alkyl terminus of dibenzyl
carbonate
Pressure-induced structural and electronic transition in KTb(MoO<sub>4</sub>)<sub>2</sub> through Raman and optical studies
Raman and optical absorption studies under pressure have been conducted on KTb(MoO4)2 up to 35.5 GPa. A phase transformation occurs at 2.7 GPa when the crystal is pressurized at ambient temperature in a hydrostatic pressure medium. The sample changes to a deep yellow color at the transition and visibly contracts in theÎą-axis direction. The color shifts to red on further pressure increase. The Raman spectral features and the X-ray powder pattern change abruptly at the transition indicating a structural change. The pressure-induced transition appears to be a property of the layer-type alkali rare earth dimolybdates. However, the color change at the transition in KTb(MoO4)2 is rather unusual and is attributed to a valence change in Tb initiated by the structural transition and consequent intervalence charge transfer between Tb and Mo.In situ high pressure X-ray diffraction data suggest that phase II could be orthorhombic with a unit cell having 3 to 4% smaller volume than that of phase I
Corneal perforation in ocular graft-versus-host disease
PURPOSE: Corneal perforation is a rare, vision-threatening complication of ocular graft-versus-host disease (GVHD) and is not well understood. Our objective was to examine the clinical disease course and histopathologic correlation in patients who progressed to this outcome.
METHODS: This study is a retrospective case series from four academic centers in the United States. All patients received a hematopoietic stem cell transplant (HSCT) prior to developing ocular GVHD. Variables of interest included patient demographics, time interval between HSCT and ocular events, visual acuity throughout clinical course, corticosteroid and infection prophylaxis regimens at time of corneal perforation, medical/surgical interventions, and histopathology.
RESULTS: Fourteen eyes from 14 patients were analyzed. Most patients were male (86%) and Caucasian (86%), and average age at time of hematopoietic stem cell transplant was 47 years. The mean interval between hematopoietic stem cell transplant and diagnosis of ocular graft-versus-host disease was 9.5 months, and between hematopoietic stem cell transplant and corneal perforation was 37 months. Initial best-corrected visual acuity was 20/40 or better in 9 eyes, and all eyes had moderate or poor visual outcomes despite aggressive management, including corneal gluing in all patients followed by keratoplasty in 8 patients. The mean follow-up after perforation was 34 months (range 2-140 months). Oral prednisone was used prior to perforation in 11 patients (79%). On histopathology, representative specimens in the acute phase demonstrated ulcerative keratitis with perforation but minimal inflammatory cells and no microorganisms, consistent with sterile corneal melt in the setting of immunosuppression; and in the healed phase, filling in of the perforation site with fibrous scar.
CONCLUSIONS: In these patients, an extended time interval was identified between the diagnosis of ocular graft-versus-host disease and corneal perforation. This represents a critical window to potentially prevent this devastating outcome. Further study is required to identify those patients at greatest risk as well as to optimize prevention strategies
Strings on Bubbling Geometries
We study gauge theory operators which take the form of a product of a trace
with a Schur polynomial, and their string theory duals. These states represent
strings excited on bubbling AdS geometries which are dual to the Schur
polynomials. These geometries generically take the form of multiple annuli in
the phase space plane. We study the coherent state wavefunction of the lattice,
which labels the trace part of the operator, for a general Young tableau and
their dual description on the droplet plane with a general concentric ring
pattern. In addition we identify a density matrix over the coherent states on
all the geometries within a fixed constraint. This density matrix may be used
to calculate the entropy of a given ensemble of operators. We finally recover
the BMN string spectrum along the geodesic near any circle from the ansatz of
the coherent state wavefunction.Comment: 41 pages, 12 figures, published version in JHE
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Impact of COVID-19-induced academic stress on insomnia and suicidal ideation among Taiwanese health trainees and junior doctors
BACKGROUND: Clinical training for allied health trainees (AHTs) and postgraduate-year (PGY) doctors needed to go online during the outbreak of coronavirus disease 2019 (COVID-19), which may have caused academic stress and consequent outcomes among this cohort.
MATERIAL AND METHODS: To evaluate academic-related stress, clinical confidence, psychological distress, and insomnia, an online survey-based study was conducted among Taiwanese AHTs and PGY doctors between July and December, 2022, during the COVID-19 pandemic. The survey included the 21-item Depression, Anxiety, and Stress Scale (DASS-21), the Insomnia Severity Index (ISI), and self-designed questions. It was distributed using convenience sampling and snowball sampling and was completed by 522 participants.
RESULTS: Structural equational modelling showed that academic stress was negatively associated with clinical confidence (standardized coefficient [β]=-0.382, p<0.001). Clinical confidence was negatively associated with psychological distress (β=-0.397, p<0.001), which was associated with insomnia (β=0.648, p<0.001). Additionally, clinical confidence and psychological distress were the significant mediators. Results indicated that higher academic stress was associated with higher level of insomnia via the mediation of clinical confidence and psychological distress.
CONCLUSIONS: Academic stress related to changes in clinical training may have led to insomnia among AHTs and PGY doctors during the pandemic. Factors to reduce academic stress should be investigated to promote good mental health while providing sufficient clinical training, especially during events that can cause increased stress (eg, epidemics, pandemics)
Ischemic neurons recruit natural killer cells that accelerate brain infarction
Brain ischemia and reperfusion activate the immune system. The abrupt development of brain ischemic lesions suggests that innate immune cells may shape the outcome of stroke. Natural killer (NK) cells are innate lymphocytes that can be swiftly mobilized during the earliest phases of immune responses, but their role during stroke remains unknown. Herein, we found that NK cells infiltrated the ischemic lesions of the human brain. In a mouse model of cerebral ischemia, ischemic neuron-derived fractalkine recruited NK cells, which subsequently determined the size of brain lesions in a T and B cell-independent manner. NK cell-mediated exacerbation of brain infarction occurred rapidly after ischemia via the disruption of NK cell tolerance, augmenting local inflammation and neuronal hyperactivity. Therefore, NK cells catalyzed neuronal death in the ischemic brain
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