The increased demand for well-being has fueled interest in sleep. Research in technology for monitoring sleep ranges from sleep efficiency and sleep stage analysis to sleep disorder detection, ...centering on wearable devices such as fitness bands, and some techniques have been commercialized and are available to consumers. Recently, as interest in digital therapeutics has increased, the field of sleep engineering demands a technology that helps people obtain quality sleep that goes beyond the level of monitoring. In particular, interest in sleep aids for people with or without insomnia but who cannot fall asleep easily at night is increasing. In this review, we discuss experiments that have tested the sleep-inducing effects of various auditory stimuli currently used for sleep-inducing purposes. The auditory stimulations were divided into (1) colored noises such as white noise and pink noise, (2) autonomous sensory meridian response sounds such as natural sounds such as rain and firewood burning, sounds of whispers, or rubbing various objects with a brush, and (3) classical music or a preferred type of music. For now, the current clinical method of receiving drugs or cognitive behavioral therapy to induce sleep is expected to dominate. However, it is anticipated that devices or applications with proven ability to induce sleep clinically will begin to appear outside the hospital environment in everyday life.
Drowsiness while driving can lead to accidents that are related to the loss of perception during emergencies that harm the health. Among physiological signals, brain waves have been used as ...informative signals for the analyses of behavioral observations, steering information, and other biosignals during drowsiness. We inspected the machine learning methods for drowsiness detection based on brain signals with varying quantities of information. The results demonstrated that machine learning could be utilized to compensate for a lack of information and to account for individual differences. Cerebral area selection approaches to decide optimal measurement locations could be utilized to minimize the discomfort of participants. Although other statistics could provide additional information in further study, the optimized machine learning method could prevent the dangers of drowsiness while driving by considering a transitional state with nonlinear features. Because brain signals can be altered not only by mental fatigue but also by health status, the optimization analysis of the system hardware and software will be able to increase the power-efficiency and accessibility in acquiring brain waves for health enhancements in daily life.
Arterial stiffness is considered an index of vascular aging. The brachial-ankle pulse wave velocity (baPWV) method is widely used because of its proven effectiveness; and the pulse wave velocity ...measurement method using both electrocardiogram (ECG) and photoplethysmogram (PPG) is actively being studied due to the convenience of measurement and the possibility of miniaturization. The aim of this study was to evaluate and compare the effects of age and gender in Korean adults using both the baPWV method and the PWV method with ECG and finger PPG (heart-finger PWV). The measurements have been carried out for 185 healthy subjects of Korean adults, and the results showed that the baPWV was highly correlated with age in both genders (r = 0.94 for both males and females). However, the correlation values in heart-finger PWV measurement were significantly lower than those of baPWV (r = 0.37 for males and r = 0.71 for females). Although the heart-finger PWV method is suitable for mobile applications because it can be easily miniaturized while maintaining its signal quality, these results show that the heart-finger PWV method is not as effective as baPWV at evaluating the arterial stiffness.
The temperature (T) dependence of the performance and cell parameters of photovoltaic (PV) Si solar cells was studied in the 298–353K temperature range and under high illumination conditions (10 and ...15 suns). The PV cell parameters were analytically predicted using the short circuit current density (Jsc), and open circuit voltage (Voc), inverse of the slopes at short circuit and open circuit conditions along with the current density and voltage values at the maximum power point. When T was varied from 298K to 353K, the obtained variations of the performance parameters under illumination intensities of 10 and 15 suns were, respectively: Jsc increased by 5.38% and 2.86%; Voc, on the other hand, decreased approximately by 16.87% and 17.75%; the obtained fill factor losses were 5.84% and 5.34%, and the overall losses in efficiency were approximately 17.52% and 19.91%. The inverse of the slopes in the short and open circuit conditions did also decrease with increasing T. Both current and voltage at the maximum power point decreased with the rise in T, whereas Jph appeared to increase linearly with temperature (5.84% and 3.51% at 10 and 15 suns, respectively). The value of Rsh decreased linearly with T under both illumination intensities (20.42% at 10 suns and 23.73% at 15 suns), while Rs showed an exponential decrease with increasing T (15.38% at 10 suns and 17.73% at 15 suns). The diode ideality factor decreased with T (13.36% at 10 suns and 12.41% at 15 suns); in contrast, J0 increased with the rise in temperature. The losses resulting from the series resistance and surface charge recombination were reduced with the increase in temperature, whereas the losses caused by the shunt resistance and the reverse saturation current increased. The reverse saturation current density was increased by 3583% and 5988% under the 10 and 15 suns illumination levels, respectively. Display omitted
•Dependency of PV cell parameters on T has been investigated.•The PV cell parameters analytically determined using single J-V curve.•The analytically predicted values of Rsh, Rs and n decreased with increases of T.•J0 is increased by 3583% and 5988% under 10 and 15 suns, respectively.•Relations between PV cell parameters and T have also been developed.
Several attempts have been made to investigate the PV cell parameters’ dependence on temperature under normal illumination conditions, although there is no report available in the literature providing information about this dependence in the case of high illumination conditions. We therefore investigate analytically the dependence on temperature of five PV cell parameters of Si solar cells under high illumination conditions. The obtained performance parameters under illumination intensities of 10 and 15 suns were, respectively: the short circuit current density, Jsc values increased by 5.38% and 2.86%; the open circuit voltage, Voc values, on the other hand, decreased approximately by 16.87% and 17.75%; the obtained fill factor, FF losses were 5.84% and 5.34%, and the overall losses in efficiency, η were approximately 17.52% and 19.91%. The losses resulting from the series resistance and surface charge recombination are reduced with an increase in temperature within the 298–353K range, whereas the losses caused by the shunt resistance and reverse saturation current are increased. The reverse saturation current density increases by 3583% and 5988% under illumination intensities of 10 and 15 suns (1sun=1kW/m2), respectively. The analytically predicted values of Voc, FF, and η showed good agreement with the experimentally measured values for the several values of Pin and T.
Abstract
Various large-area growth methods for two-dimensional transition metal dichalcogenides have been developed recently for future electronic and photonic applications. However, they have not ...yet been employed for synthesizing active pixel image sensors. Here, we report on an active pixel image sensor array with a bilayer MoS
2
film prepared via a two-step large-area growth method. The active pixel of image sensor is composed of 2D MoS
2
switching transistors and 2D MoS
2
phototransistors. The maximum photoresponsivity (
R
ph
) of the bilayer MoS
2
phototransistors in an 8 × 8 active pixel image sensor array is statistically measured as high as 119.16 A W
−1
. With the aid of computational modeling, we find that the main mechanism for the high
R
ph
of the bilayer MoS
2
phototransistor is a photo-gating effect by the holes trapped at subgap states. The image-sensing characteristics of the bilayer MoS
2
active pixel image sensor array are successfully investigated using light stencil projection.
Objective: Wearable health monitoring devices have recently become popular, but they can still only measure the average heart rate. Heart rate variability (HRV) is known to represent changes in the ...autonomic nervous system and analysis of HRV has the potential to be used for monitoring various wellness-related parameters such as sleep or stress. HRV analysis requires accurate measurement of the heartbeat interval. In wearable devices, it is difficult to accurately measure the heartbeat interval due to motion noise. In this paper we propose a new method for performing HRV analysis on photoplethysmographic (PPG) signals corrupted by motion artifacts measured at the wrist. Approach: A frequency-tracking algorithm based on the oscillator-based adaptive notch filter was used to measure instantaneous heart rate. The algorithm consists of a time-varying bandpass filter for enhancing the heartbeat signal and an adaptive mechanism for tracking heart rate frequency. By optimizing the filter bandwidth and forgetting factor of the adaptive mechanism, the frequency-tracking algorithm better reflects the variability of instantaneous heart rate. The new HRV index was calculated as the standard deviation of the heartbeat interval data converted using the heart rate estimated by the frequency-tracking algorithm. In order to verify the effectiveness of the proposed index, the new HRV index calculated for each sleep stage was compared with SDNN, the standard deviation of the heartbeat interval, which was calculated using simultaneous electrocardiogram measurements. In addition, changes in SDNN and the new index were compared during a socially evaluated speech task. Finally, the relationship between the new index and SDNN was compared with the data collected during daily activities over a 24 h period. Main results: Experimental results showed that statistically significant changes in HRV could be monitored in different sleep stages using the proposed method. In addition, when subjects were stressed by a socially evaluated speech task, significant reduction in HRV was observed using the proposed method. Finally, HRV values measured during daily activities over a 24 h period showed a high correlation coefficient of 0.812 with reference HRVs. Significance: The new HRV index calculated by the proposed method is expected to be an effective new solution for noisy PPG signals.
Anti‐reflection (AR) characteristics on the display surface and light outcoupling efficiency are significant factors for controlling external and internal light in organic light‐emitting diodes ...(OLEDs), respectively. This paper presents the fabrication of moth‐eye nanostructured parylene‐C films that simultaneously achieve optimized AR characteristics and enhanced OLED performance. AR performance is optimized by varying the nanostructure dimensions, resulting in a nanostructured film with a reflectance of <1%. Furthermore, applying this nanostructured film to OLEDs improved the light outcoupling efficiency by 40% compared with the reference device. Therefore, employing nanostructured parylene‐C films to enhance the optical characteristics of OLEDs is proposed.
The moth‐eye nanostructured parylene‐C films simultaneously achieve optimized anti‐reflection characteristics and enhanced OLED performance. Anti‐reflection performance is optimized by varying the nanostructure dimensions, resulting in a reflectance of <1%. Furthermore, applying this nanostructured film to OLEDs improves the light outcoupling efficiency by 40%.
We have developed an analytical method to determine the diode parameters of concentrator solar cells under high illumination conditions. The determined values of diode parameters have been used to ...compute the theoretical values of performance parameters. The computed values of the open circuit voltage, curve factor, and efficiency obtained using diode parameters determined with this method showed good agreement (<2% discrepancy) with their experimental values in the temperature range 298–323K. Display omitted
•An analytical method to extract the diode parameters of concentrated Si solar cells.•This method uses single I–V curve under high illumination conditions.•The theoretical values of performance parameters have been computed.•Theoretical values of parameters matched within 2% discrepancy limit.•This method gives best results among the methods used in this work.
An analytical method has been developed to extract all four diode parameters, namely the shunt resistance, series resistance, diode ideality factor, and reverse saturation current density, using a single J–V curve, based on one exponential model of silicon solar cells under high illumination conditions. The slope of the J–V curve (dV/dJ) at a short circuit condition is used to determine the value of the shunt resistance. The slope of the J–V curve at an open circuit condition together with the short circuit current density, open circuit voltage, current density, and voltage at maximum power point have been used to determine the values of the series resistance, diode ideality factor, and reverse saturation current density. The determined values of the diode parameters have been used to compute the theoretical values of the open circuit voltage, curve factor, and efficiency of the solar cell. The theoretical J–V curves matched well with the corresponding experimental curves. This method is applied to determine the diode parameters of concentrator silicon solar cells at different illumination conditions in a temperature range of 298–323K. The computed values of the open circuit voltage, curve factor, and efficiency obtained using diode parameters determined with this method showed good agreement (<2% discrepancy) with the experimental values.
Herein, we suggest a facile and cost-effective metal-assisted chemical etching (MACE) method to synthesize uniformly Ag-coated silicon nanowires (SiNWs) using redox reaction, which provides ...structural and electrical effects on Si negative electrodes at the same time. The combinations of SiNW geometry and Ag nanoparticle (AgNP) coatings are beneficial in preventing pulverization and increasing electrical conductivity of Si negative electrode during cycling, as a result, excellent electrochemical performances are obtained. Transmission electron microscopy (TEM) images reveal that Ag-coated SiNW arrays have been successfully prepared in one-pot MACE process. The electrochemical performances show that the rate capability and cycleability of Ag-coated SiNWs is greatly improved compare to bare NWs. Electrochemical impedance spectroscopy (EIS) measurements show that the electrical conductivity of Ag-coated SiNW electrode is significantly enhanced by Ag coating.
Display omitted
•We demonstrate the facile MACE method to synthesize Ag-coated SiNW.•The electrical conductivity was greatly improved by AgNPs on SiNW.•Ag-coated SiNW electrode shows a capacity of over 500 mAh g−1 at 2C rate.
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