bluuebunny/crossref_metadata_2025 huggingface dataset:
Expression, purification, and activities of full‐length and truncated versions of the integral membrane protein Vpu from HIV‐1
Authors: Ma Che , Marassi Francesca M. , Jones David H. , Straus Suzana K. , Bour Stephan , Strebel Klaus , Schubert Ulrich , Oblatt‐Montal Myrta , Montal Mauricio , Opella Stanley J.
Published: July 2002

Vpu is an 81‐residue accessory protein of HIV‐1. Because it is a membrane protein, it presents substantial technical challenges for the characterization of its structure and function, which are of considerable interest because the protein enhances the release of new virus particles from cells infected with HIV‐1 and induces the intracellular degradation of the CD4 receptor protein. The Vpu‐mediated enhancement of the virus release rate from HIV‐1‐infected cells is correlated with the expression of an ion channel activity associated with the transmembrane hydrophobic helical domain. Vpu‐induced CD4 degradation and, to a lesser extent, enhancement of particle release are both dependent on the phosphorylation of two highly conserved serine residues in the cytoplasmic domain of Vpu. To define the minimal folding units of Vpu and to identify their activities, we prepared three truncated forms of Vpu and compared their structural and functional properties to those of full‐length Vpu (residues 2–81). Vpu2–37 encompasses the N‐terminal transmembrane α‐helix; Vpu2–51 spans the N‐terminal transmembrane helix and the first cytoplasmic α‐helix; Vpu28–81 includes the entire cytoplasmic domain containing the two C‐terminal amphipathic α‐helices without the transmembrane helix. Uniformly isotopically labeled samples of the polypeptides derived from Vpu were prepared by expression of fusion proteins in E. coli and were studied in the model membrane environments of lipid micelles by solution NMR spectroscopy and oriented lipid bilayers by solid‐state NMR spectroscopy. The assignment of backbone resonances enabled the secondary structure of the constructs corresponding to the transmembrane and the cytoplasmic domains of Vpu to be defined in micelle samples by solution NMR spectroscopy. Solid‐state NMR spectra of the polypeptides in oriented lipid bilayers demonstrated that the topology of the domains is retained in the truncated polypeptides. The biological activities of the constructs of Vpu were evaluated. The ion channel activity is confined to the transmembrane α‐helix. The C‐terminal α‐helices modulate or promote the oligomerization of Vpu in the membrane and stabilize the conductive state of the channel, in addition to their involvement in CD4 degradation.
An accurate method for computer‐generating tungsten anode x‐ray spectra from 30 to 140 kV
Authors: Boone John M. , Seibert J. Anthony
Published: July 2002

A tungsten anode spectral model using interpolating polynomials (TASMIP) was used to compute x‐ray spectra at 1 keV intervals over the range from 30 kV to 140 kV. The TASMIP is not semi‐empirical and uses no physical assumptions regarding x‐ray production, but rather interpolates measured constant potential x‐ray spectra published by Fewell et al. [Handbook of Computed Tomography X‐ray Spectra (U.S. Government Printing Office, Washington, D.C., 1981)]. X‐ray output measurements (mR/mAs measured at 1 m) were made on a calibrated constant potential generator in our laboratory from 50 kV to 124 kV, and with 0–5 mm added aluminum filtration. The Fewell spectra were slightly modified (numerically hardened) and normalized based on the attenuation and output characteristics of a constant potential generator and metal‐insert x‐ray tube in our laboratory. Then, using the modified Fewell spectra of different kVs, the photon fluence Φ at each 1 keV energy bin over energies from 10 keV to 140 keV was characterized using polynomial functions of the form A total of 131 polynomial functions were used to calculate accurate x‐ray spectra, each function requiring between two and four terms. The resulting TASMIP algorithm produced x‐ray spectra that match both the quality and quantity characteristics of the x‐ray system in our laboratory. For photon fluences above 10% of the peak fluence in the spectrum, the average percent difference (and standard deviation) between the modified Fewell spectra and the TASMIP photon fluence was −1.43% (3.8%) for the 50 kV spectrum, −0.89% (1.37%) for the 70 kV spectrum, and for the 80, 90, 100, 110, 120, 130 and 140 kV spectra, the mean differences between spectra were all less than 0.20% and the standard deviations were less than ∼1.1%. The model was also extended to include the effects of generator‐induced kV ripple. Finally, the x‐ray photon fluence in the units of per mR was calculated as a function of HVL, kV, and ripple factor, for various (water‐equivalent) patient thicknesses (0, 10, 20, and 30 cm). These values may be useful for computing the detective quantum efficiency, of x‐ray detector systems. The TASMIP algorithm and ancillary data are made available on line at http://www.aip.org/epaps/epaps.html.
A scan–rotate geometry for efficient equalization mammography
Authors: Sabol John M.
Published: July 2002

Mammographic screening is acknowledged as the best method for the reduction of breast cancer mortality. However, breasts containing a significant fraction of dense, fibroglandular tissue produce a range of exposure which exceeds the dynamic range of conventional mammography. Equalization radiography involves the modulation of the incident x‐ray exposure distribution to compensate for variations in x‐ray transmission within the patient, ensuring optimal contrast throughout the image. It has been shown that equalization radiograpy offers the potential for dose‐efficient, improved lesion detection in the dense breast. However, current equalization geometries are not practical due to tube loading and scan duration inefficiencies. I propose a scan–rotate geometry for equalization radiography [rotary scanning equalization radiography (RSER)] in which the image receptor is exposed by repeated scans of a modulated fan beam, oriented at a variety of angles with respect to the patient. The superposition of exposure from appropriately modulated, rotated fan beams will produce an entrance exposure distribution that will efficiently equalize the film exposure. In this thesis, it is shown that less than half of the area of a dense breast is imaged conventionally with high contrast. The RSER geometry is described and its imaging characteristics are shown theoretically to be similar to current equalization geometries. Furthermore, RSER is resistant to exposure artifacts in typical mammographic imaging tasks. Numerical simulations which compared the imaging performance of RSER, current equalization geometries and conventional mammography show that RSER produces images with the same degree of equalization as current equalization geometries, with minimal tube loading, and only four scanning angles. Finally, an experimental prototype RSER system is described and characterized. Experimental images confirm the theoretical and numerical predictions. High contrast, artifact‐free images of anthropomorphic breast phantoms can be obtained with minimal scan times and tube loading. The prototype system increases the fraction of the breast imaged with high contrast from 46% to 80%. These results indicate that RSER is an efficient, simple, and practical means of overcoming the latitude limitations of film screen mammography, and improving the detection of breast cancer.
Clinical brachytherapy with neutron emitting sources and adherence to AAPM TG‐43 dosimetry protocol
Authors: Rivard Mark J. , Wierzbicki Jacek G. , Van den Heuvel Frank , Martin Rodger C. , McMahon Robert R.
Published: July 2002

Using Monte Carlo methods, neutron dosimetry for Applicator Tube (AT) type medical sources available from Oak Ridge National Laboratory (ORNL) has for the first time been determined in terms of TG‐43 formalism. This approach, as compared to previous “along‐away” formalisms, demonstrates the relative angular independence of dose rate data, when the geometry factor has been removed. As the ORNL‐made AT type sources are considerably physically larger than most clinical sources used today, the radial dose function increases for radii less than 3.0 mm due to breakdown of the line source model. A comparison of the neutron radial dose function with those for other medical sources revealed similarities with that from Differences with respect to previous AT source neutron dosimetry data generally increased at increasing distances. This was attributed to differences in the various AT source models and phantom compositions. The current status of medical source fabrication and calibration procedures at ORNL is presented.
Breathing‐synchronized radiotherapy program at the University of California Davis Cancer Center
Authors: Kubo H. Dale , Len Patrick M. , Minohara Shin‐ichi , Mostafavi Hassan
Published: July 2002

In this paper we present a complete description of the breathing synchronized radiotherapy (BSRT) system, which has been jointly developed between the University of California Davis Cancer Center and Varian Associates. BSRT is a description of an emerging radiation oncology procedure, where simulation, CT scan, treatment planning, and radiation treatment are synchronized with voluntary breath‐hold, forced breath‐hold, or breathing gating. The BSRT system consists of a breathing monitoring system (BMOS) and a linear accelerator gating hardware and software package. Two methods, a video camera‐based method and the use of wraparound inductive plethysmography (RespiTrace), generate the BMOS signals. The BMOS signals and the synchronized fluoroscopic images are simultaneously recorded in the simulation room and are later analyzed to define the ideal treatment point (ITP) where organ motion is stationary. The BMOS signals at ITP can be used to gate a CT scanner or a linear accelerator to maintain the same organ configuration as in the simulation. The BSRT system allows breath‐hold or gating. This dual role allows the system to be applicable for a variety of patients, i.e., the breath‐hold method for those patients who can maintain and reproduce the ITP, and the forced breath‐hold or gating method for those who are not ideal for voluntary breath‐hold.
Teaching time-series analysis. II. Wave height and water surface elevation probability distributions
Authors: Whitford Dennis J. , Waters Jennifer K. , Vieira Mario E. C.
Published: July 2002

This paper describes the second of a two-part series of pedagogical exercises to introduce students to methods of time-series analysis. While these exercises are focused on the analysis of wind generated surface gravity waves, they are cross-disciplinary in nature and can be applied to other fields dealing with random signal analysis. Two computer laboratory exercises are presented which enable students to understand many of the facets of random signal analysis with less difficulty and more understanding than standard classroom instruction alone. The first pedagogical exercise, described in the previous article, uses mathematical software on which the students execute the manual arithmetic operations of a finite Fourier analysis on a complex wave record. The results are then compared to those obtained by a fast Fourier transform. This article, the second of this two-part pedagogical series, addresses analysis of a complex sea using observed and theoretical wave height and water surface elevation probability distributions and wave spectra. These results are compared to a fast Fourier transform analysis, thus providing a link back to the first exercise.
Surface phonons and other localized excitations
Authors: Allen Philip B. , Aubin Seth , Doak R. B.
Published: July 2002

The diatomic linear chain of masses coupled by harmonic springs is a textbook model for vibrational normal modes (phonons) in crystals. In addition to propagating acoustic and optic branches, this model is known to support a “gap mode” localized at the surface, provided the atom at the surface has light rather than heavy mass. An elementary argument is given which explains this mode and provides values for the frequency and localization length. By reinterpreting this mode in different ways, we obtain the frequency and localization length for three other interesting modes: (1) the surface vibrational mode of a light mass impurity at the surface of a monatomic chain; (2) the localized vibrational mode of a stacking fault in a diatomic chain; and (3) the localized vibrational mode of a light mass impurity in a monatomic chain.
Acoustic flow perception in cf-bats: Extraction of parameters
Authors: Müller Rolf , Schnitzler Hans-Ulrich
Published: July 2002

The narrow-band portions of the echolocation pulses seen in cf-bats are a hypothetical substrate for target localization. This localization could be based on estimates of echo envelope amplitude and carrier frequency together with their derivatives. Evaluation of these parameters is referred to as “acoustic flow” in loose analogy to optic flow. It is assessed whether the requirements for this task may be reconciled with known principles of auditory function. For the evaluation of a single echo, this seems to be the case: auditory filter shapes provide sufficient frequency resolution; at the same time envelopes are preserved well and some noise removal is achieved. Nevertheless, should bats not be endowed with additional capabilities for noise removal, analysis of acoustic flow would be limited to favorable signal-to-noise ratios. Multiple, temporally overlapping echoes are probable in any realistic echolocation scenario. In this case, additional auditory processing steps have to be postulated, which allow simultaneous estimation of multiple carrier frequencies and reduction of demodulation distortions.
The single sonic muscle twitch model for the sound-production mechanism in the weakfish, Cynoscion regalis
Authors: Sprague Mark W.
Published: July 2002

A model of the weakfish, Cynoscion regalis, sound-production mechanism based on damped driven oscillators is presented. The weakfish “purr” consists of several short pulses of sound separated by intervals of no sound. Each pulse is produced by an individual simultaneous twitch of each sonic muscle causing swimbladder oscillations that radiate sound into the surrounding water. The sonic muscles are modeled as a stretched string with a time-varying tension force. The swimbladder is modeled as a highly damped, driven oscillator undergoing radial oscillations. Although the swimbladder functions as an impedance-matching device between the sonic muscles and the surrounding fluid, its transient response to the sonic muscle pulses includes frequencies that are not a part of the spectrum of the sonic muscle excitations. Differential equations of motion for the sonic muscles and swimbladder are given and are numerically solved to produce predicted waveforms matching those of measured weakfish sounds. The model leads to better understanding of the weakfish sound-producing mechanism and the effects of environmental and physiological factors on sound production. This model may also lead to a better understanding of sound production in other species, particularly other members of Family Sciaenidae, with similar mechanisms for sound production.
Vocal intensity in falsetto phonation of a countertenor: An analysis by synthesis approach
Authors: Tom Kenneth , Titze Ingo R.
Published: July 2002

An analysis by synthesis paradigm was implemented to model glottal airflow and vocal tract acoustics for the falsetto phonation of a trained countertenor. Changes in vocal intensity were measured as a function of subglottal pressure, open quotient of the time-varying glottal airflow pulse, and formant tuning. The contributions of laryngeal adduction (open quotient of the glottal flow pulse) and of formant tuning to intensity change were derived from modeled data. The findings were: (1) Subglottal pressure accounted for almost 90% of the variation in SPL in falsetto phonation. (2) The open quotient of the glottal flow pulse was remarkably constant in these falsetto phonations, and thus did not affect vocal intensity significantly. (3) Formant tuning occurred in two out of nine possibilities for the vowel /ɑ/. These instances did not support the concept of systematic exploitation of formant tuning.
The use of distortion product otoacoustic emission suppression as an estimate of response growth
Authors: Gorga Michael P. , Neely Stephen T. , Dorn Patricia A. , Konrad-Martin Dawn
Published: July 2002

Distortion product otoacoustic emission (DPOAE) levels in response to primary pairs (f2=2 or 4 kHz, L2 ranging from 20 to 60 dB SPL, L1=0.4L2+39 dB) were measured with and without suppressor tones (f3), which varied from 1 octave below to 12 octave above f2, in normal-hearing subjects. Suppressor level (L3) varied from −5 to 85 dB SPL. DPOAE levels were converted into decrements by subtracting the level in the presence of the suppressor from the level in the absence of a suppressor. DPOAE decrement vs L3 functions showed steeper slopes when f3f2. This pattern is similar to other measurements of response growth, such as direct measures of basilar-membrane motion, single-unit rate-level functions, suppression of basilar-membrane motion, and discharge-rate suppression from lower animals. As L2 increased, the L3 necessary to maintain 3 dB of suppression increased at a rate of about 1 dB/dB when f3 was approximately equal to f2, but increased more slowly when f3<f2. Functions relating L3 to L2 in order to maintain a constant 3-dB reduction in DPOAE level were compared for f3<f2 and for f3≈f2 in order to derive an estimate related to “cochlear-amplifier gain.” These results were consistent with the view that “cochlear gain” is greater at lower input levels, decreasing as level increases.
Robust maximum-likelihood source localization in an uncertain shallow-water waveguide
Authors: Tabrikian J. , Krolik J. L. , Messer H.
Published: July 2002

This paper addresses the problem of matched-field source localization in the presence of uncertainties in the ocean environment. Because signal wavefront mismatch can cause anomalous source location estimates, development of robust localization methods is critically important. In this paper, a robust maximum-likelihood estimator is proposed. It is based on a decomposition of the field into predictable and unpredictable subspaces of the acoustic normal mode representation. The estimator uses the predictable subspace for source localization. Identification of the predictable modes is made according to the second-order joint statistics of the horizontal wave numbers. The performance of the method is evaluated and compared to other matched-field methods using simulations and acoustic array data from the Mediterranean Sea. In the presence of mismatches, the algorithm has superior probability of correct localization than the maximum-likelihood, matched-mode-processing, and Bartlett methods.
Dialect variation and formant frequency: The American English vowels revisited
Authors: Hagiwara Robert
Published: July 2002

Vowel production data collected from 15 southern Californian English-speaking monolinguals is compared with data reported by Hillenbrand et al. [J. Acoust. Soc. Am. 97, 3099–3111 (1995)] and Peterson and Barney [J. Acoust. Soc. Am. 24, 175–184 (1952)]. Recordings were made of nine women and six men producing multiple repetitions of [i, ɪ, e, ɛ, æ, u, ʊ, o, ɑ, ʌ, ɹˌ] in three consonant contexts. The frequencies of the first three formants were measured by simultaneous comparison of wideband spectrograms, narrow-band FFT spectral slices, and LPC spectra taken at vowel center, or steady state where available. The Southern Californian data are seen to differ greatly from that described by Peterson and Barney (1952) and Hillenbrand et al. (1995).
Acoustics of a fluid-loaded plate with attached oscillators. Part I. Feynman rules
Authors: Photiadis Douglas M.
Published: July 2002

The “fuzzy structure” paradigm developed by Soize and others provides a framework within which the acoustic properties of systems with complex internal structure can be explored. In this two part paper, the acoustic properties of one of the simplest such systems, a fluid-loaded plate with attached internal oscillators, are explored. In Part I, the theoretical tools needed in this investigation, the representation of the Neumann series in terms of Feynman diagrams, is presented. In most ways, the formal aspects of this development are identical to similar results from other disciplines, but the presence of damping in the system, either internal or due to radiation into the fluid, requires one to make some modifications to existing theories. The rules for evaluating averaged quantities such as G⋯GG*⋯G* involving higher-order products of the Green’s function are given. These quantities are necessary to determine a number of relevant properties of the system including the scattering cross section and the size of typical fluctuations. The Feynman rules, detailed mathematical expressions which enable averaged quantities to be evaluated for particular systems, are explicitly given for the case in which the internals are simple oscillators distributed uniformly and independently in both space and frequency. In Part II of this paper, the theoretical approach will be applied to predict a number of acoustic properties of the system.
From geology to geoacoustics—Evaluation of Biot–Stoll sound speed and attenuation for shallow water acoustics
Authors: Badiey Mohsen , Cheng Alexander H-D. , Mu Yongke
Published: July 2002

A procedure for estimating acoustic wave velocity and attenuation in ocean sediment using a minimum amount of geological and geotechnical data is demonstrated. First, the Biot–Stoll theory is presented. Next, various asymptotic formulae for the attenuation coefficient are derived for high, low, and intermediate frequencies. These expressions clearly isolate the effects of intergranular Coulomb friction and fluid viscous dissipation on the attenuation of shear and compressional waves. Under the constraint of a minimum amount of geological and geotechnical information, a sequence of empirical equations is compiled to convert basic data, such as blow count number from a Standard Penetration Test or shipboard density, into sediment geoacoustic properties. As a demonstration, two well-known field cases, the Atlantic Generating Station (AGS) site and the Atlantic Margin Coring (AMCOR 6010) site, are examined. By incorporating the uncertainty involved in the data collection, the estimated geoacoustical parameters are provided with a standard deviation.
Using sound to map fish spawning: Determining the seasonality and location of spawning for weakfish and red drum (Family Sciaenidae) within Pamlico Sound, NC
Authors: Luczkovich Joseph J. , Johnson Stephen E. , Sprague Mark W.
Published: July 2002

Fish produce species-specific sounds during courtship, during aggressive encounters with other fish of the same species, and as a response to threats from predators. Particular sounds are species-specific and have been recorded from fish during spawning in captivity. Such acoustical data may be used to identify and locate fish that are reproductive in the sea. In this paper, nocturnal sound production and planktonic egg production are correlated for weakfish (Cynoscion regalis) and red drum (Sciaenops ocellatus) (Family Sciaenidae), two species for which there are concerns over declining stocks. Recordings of underwater sounds and ichthyoplankton net tows were made after sunset at ten sites in Pamlico Sound, NC. Sites were relocated with a differentially corrected Global Positioning System receiver on a biweekly basis from May through October 1997. Based on laboratory studies, weakfish ‘‘purring’’ sounds and red drum ‘‘knocking’’ sounds only occur during spawning. ‘‘Purring’’ was recorded along with sciaenid eggs at stations near Ocracoke and Hatteras Inlets during May through July. ‘‘Knocking’’ was recorded in September and October at sites away from the inlets, also occurring with sciaenid eggs in one case. Mapping of sciaenid spawning areas may be possible using acoustical data alone, but may overestimate egg production.
A least-squares approximation for the delays used in focused beamforming
Authors: Trucco Andrea
Published: July 2002

A novel approximation for the delays used in focused beamforming is presented that is based on weighting the terms of the conventional Fresnel approximation. The objective is to enlarge the validity region of the Fresnel approximation in order to investigate wider scanning regions. A least-squares solution provides the exact weight values by the minimization of the mean-square error computed over the desired scanning region and on the basis of the array geometry. The attractive results obtained by applying the proposed approximation to a linear array are presented.
Numerical evaluation of the far-field directivity pattern using the fast Fourier transform
Authors: Assaad Jamal , Rouvaen Jean Michel
Published: July 2002

The far-field directivity pattern can be computed using the fast Fourier transform (FFT) algorithm. Numerical implementation of the angular spectrum approach (ASA) is generally used to compute the pressure field. The aim of this paper is to demonstrate that the discrete far-field pressure can be only computed by the calculation of the DFT of the normal velocity. In fact, using the asymptotic expression of the Rayleigh’s integral, which is also the solution of Helmholtz equation, it will be shown that the analytical far-field pressure is given by the Fourier transform of the normal velocity. Guidelines for the selection of sampling interval and the size of the baffle in which the source is mounted will be given. Then this paper shows that the size of baffle influences the angular resolution at which the far field is computed and when the source is oversampled the computed discrete pressure becomes better. Numerical results concerning a transducer that exhibits harmonic oscillations will be considered and discussed.
Electrical resonance in the cuticle of hornets (Hymenoptera, Vespinae)
Authors: Ishay Jacob S.
Published: July 2002

Evidence of electrical resonance in the abdominal cuticle was investigated in 16 dead hornets, eight juvenile and eight adult. Electrical measurements were consistent with high-impedance (low current, wide bandwidth) series resonance in the 500-Hz region with some significant differences between juveniles and adults. These findings cannot be explained on a solely electrical basis, but rather suggest a combination of piezoelectric effects and a mechanically resonant system, both of which are known to exist in insect cuticle. It is tempting to contemplate a hypothetical sensor role for these sounds and vibrations in hornets and their colonies, but the data do not yet support such a conclusion.
On the role of envelope fluctuation processing in spectral masking
Authors: Derleth Ralph P. , Dau Torsten
Published: July 2002

This study examines the role of temporal cues in spectral masking, such as beats and intrinsic envelope fluctuations. Predictions from the modulation-filterbank model developed by Dau et al. [J. Acoust. Soc. Am. 102, 2906–2919 (1997)] are compared to average masking patterns from Moore et al. [J. Acoust. Soc. Am. 104, 1023–1038 (1998)]. In these experiments, tones and narrow-band noises have been used as the signal and the masker, so that all four signal–masker combinations are considered. In addition, model predictions are compared with new experimental data in conditions of notched-noise masking, where the masker consisted of two narrow-band noises whose bandwidth and frequency separation were varied systematically. The model uses a peripheral filtering stage with linear and symmetric Gammatone filters, an adaptation stage that includes a static compressive nonlinearity for stationary input stumuli and a higher sensitivity for envelope fluctuation, and a modulation filterbank that analyzes the output for each peripheral channel. For low and medium masker levels, the model accounts very well for the masking patterns in all signal-masker conditions, as well as for the notched-noise conditions. In contrast, predictions from a version of the model that acts like an energy detector account for only some of the notched-noise data, and generally do not account for the shape of the masking patterns. For a high masker level, the simulations suggest the use of asymmetric filters, with a steeper high-frequency slope than is used in the linear model, consistent with results from previous studies. In addition, several nonlinear effects become apparent at this masker level, which cannot be accounted for by the current model.
Rheo-optics of liquid-crystalline polymers in complex geometries
Authors: Baleo Jean-Noël , Navard Patrick
Published: July 2002

The flow of isotropic and liquid-crystalline hydroxypropylcellulose-water solutions is studied in a transparent slit die with several geometries including divergent and convergent configurations. The flow is observed between crossed polars. In addition to the two different flow regimes that are known in shear (zones I and II), a new flow behavior is observed (zone III). Zone I is characterized by a large macroscopic orientation, as seen between crossed polars. It is localized where either shear rates or elongation rates are large. The extinction angle is a function of the position in the die. It is similar to the flow-aligning regime observed at high shear rates. Zone II is much less oriented, due probably to a large density of defects. It is typical of the low shear rate region observed in simple shear. Zone III exhibits a peculiar behavior with long-range orientation instabilities. It is always localized after a diverging zone. Flow around obstacles and colliding streams are also studied.
An Alternative Explanation for the Apparent Passivation of Molybdenum in 1 mol/L Hydrochloric Acid
Authors: Falkenberg F. , Raja V. S. , Ahlberg E.
Published: July 2002

A key question arising from studies on the corrosion behavior of molybdenum has been whether the passivation of molybdenum originates from a three-dimensional oxide film or from chemisorbed oxygen. In the present paper an alternative explanation for the apparent passivation behavior of molybdenum is given based on XPS measurements in combination with electrochemical techniques. Below the transpassive potential region, anodic current is obtained only if the sample has been cathodically activated and not after activation by ion etching. The experiments also show that the commonly used cathodic activation is insufficient to reduce the air-formed film. Thus, for such samples the anodic current in the apparent passive region could be due to oxidation of low valent oxides on the surface. However, a transient anodic current is observed also for ion-etched samples that have been subjected to cathodic polarization. In addition, impedance measurements show an unexpectedly large capacitance in the apparent passive region, indicating some kind of adsorption equilibrium at the metal-solution interface. This leads to the suggestion that hydrogen formed in the reduction process can diffuse into the molybdenum metal and cause a limiting oxidation of hydrogen upon positive polarization. Thus, in the apparent passive region the current is mainly due to oxidation of hydrogen and an extension of the transpassive corrosion yields a very low corrosion current for molybdenum. This is in agreement with the XPS experiments, where no more than a monolayer of oxidized molybdenum can be detected in the apparent passive region. © 2001 The Electrochemical Society. All rights reserved.
Structural Stability of Hydrogen Forming Gas Annealed  ( Ba , Sr ) RuO3 Oxide Electrodes
Authors: Choi Eun-Suck , Yoon Soon-Gil
Published: July 2002

(
Ba
,
Sr
)

RuO

3

(BSR) electrodes were deposited onto Si(100) substrates by liquid delivery metallorganic chemical vapor deposition and investigated for stability under hydrogen forming condition. The BSR films showed structural and morphological stability in hydrogen forming gas (4%

H

2

/
balance

N

2

)

anneal up to

500
°
C
.

The abrupt increase of resistivity with increasing hydrogen anneal temperature was attributed to the oxygen loss in BSR films without phase change and was completely recovered by annealing at

700
°
C

in

O

2

ambient. The BSR electrodes, with structural and morphological stability in hydrogen forming gas anneal conditions at high temperature, are suitable for integration of high dielectric constant materials. © 2002 The Electrochemical Society. All rights reserved.
LiMn2 O 4 Core Surrounded by LiCo
x
Mn2 − x
O 4 Shell Material for Rechargeable Lithium Batteries: Synthesis and Characterization
Authors: Hwang B. J. , Santhanam R. , Huang C. P. , Tsai Y. W. , Lee J. F.
Published: July 2002

We demonstrate a novel method for the synthesis of

LiMn

2

O

4

core surrounded by

LiCo

x

Mn

2
−
x

O

4

shell material for rechargeable lithium batteries. Surface and composition analysis by electron spectroscopy for chemical analysis and inductively coupled plasma method, indicated that cobalt mostly distributed on the surface of the

LiMn

2

O

4

particles. X-ray diffraction studies revealed that cobalt was doped into the lattice of the

LiMn

2

O

4

particles. X-ray absorption fine structure analysis further demonstrated that cobalt was doped mainly on the surface of the

LiMn

2

O

4

particles.

LiCo

x

Mn

2
−
x

O

4

solid solution thus formed on the surface of the

LiMn

2

O

4

particles greatly enhanced the initial capacity, capacity retention, and rate capability of the spinel

LiMn

2

O

4

.

The enhanced initial capacity of the

LiMn

2

O

4

core
/

LiCo

x

Mn

2
−
x

O

4

shell material may be due to the involvement of Co in the charging/discharging behavior and the improved capacity retention and the rate capability were most probably due to less cation disordering during charge/discharge cycling experiments at low as well as at high C-rates. © 2002 The Electrochemical Society. All rights reserved.
A Gelation‐Assisted Approach for Versatile MXene Inks
Authors: Chen Derong , Long Yu , Wu Zhitan , Dong Ximan , Wang Ning , Yu Jinyang , Han Daliang , Tao Ying , Yang Quan‐Hong
Published: July 2022

Due to outstanding electrical conductivity and excellent solution processing ability, MXenes show great potential as inks for printed/coated electronic devices. To meet the rheological requirements, an ultra‐high concentration is usually involved to formulate MXenes into versatile inks, however, low efficiency in rheology regulation, and restacking issues of MXene sheets hinder their further applications. Herein, a gelation‐assisted approach is proposed to prepare versatile MXene inks with a 3D interlinked network to well address above issues. With an effective combination of cross‐linking as well as the swelling of interlinked network, the gel ink exhibits tunable rheological properties by simply tuning the water amount within the sol‐gel system. Among various applicable printing/coating techniques ranging from painting to extrusion printing, blade coating is selected to demonstrate the advantage of MXene gel ink. The smooth coating layer with interlinked 3D structures is greatly beneficial to the uniform and dense zinc deposition with high reversibility, and endows the aqueous zinc ion batteries with a remarkable rate capability. This work provides an alternative approach to prepare versatile MXene inks and would offer MXenes new opportunities for their application in advanced printable electronic devices.
Directed Evolution Enables Simultaneous Controlled Release of Multiple Therapeutic Proteins from Biopolymer‐Based Hydrogels
Authors: Teal Carter J. , Hettiaratchi Marian H. , Ho Margaret T. , Ortin‐Martinez Arturo , Ganesh Ahil N. , Pickering Andrew J. , Golinski Alex W. , Hackel Benjamin J. , Wallace Valerie A. , Shoichet Molly S.
Published: July 2022

With the advent of increasingly complex combination strategies of biologics, independent control over their delivery is the key to their efficacy; however, current approaches are hindered by the limited independent tunability of their release rates. To overcome these limitations, directed evolution is used to engineer highly specific, low affinity affibody binding partners to multiple therapeutic proteins to independently control protein release rates. As a proof‐of‐concept, specific affibody binding partners for two proteins with broad therapeutic utility: insulin‐like growth factor‐1 (IGF‐1) and pigment epithelium‐derived factor (PEDF) are identified. Protein–affibody binding interactions specific to these target proteins with equilibrium dissociation constants (KD) between 10−7 and 10−8 m are discovered. The affibodies are covalently bound to the backbone of crosslinked hydrogels using click chemistry, enabling sustained, independent, and simultaneous release of bioactive IGF‐1 and PEDF over 7 days. The system is tested with C57BL/6J mice in vivo, and the affibody‐controlled release of IGF‐1 results in sustained activity when compared to bolus IGF‐1 delivery. This work demonstrates a new, broadly applicable approach to tune the release of therapeutic proteins simultaneously and independently and thus the way for precise control over the delivery of multicomponent therapies is paved.
Engineering MOFs‐Derived Nanoarchitectures with Efficient Polysulfides Catalytic Sites for Advanced Li–S Batteries
Authors: Tao Xuefeng , Yang Zhao , Yan Rui , Cheng Menghao , Ma Tian , Cao Sujiao , Bai Mingru , Ran Fen , Cheng Chong , Yang Wei
Published: July 2022

Lithium–sulfur batteries (LSBs) have received dramatically increased attention because of their manifest advantages. Nevertheless, due to the severe shuttling of lithium polysulfides (LiPSs), sluggish reaction kinetics, and the insulation of sulfur species, the practical application of LSBs is far away. To overcome the abovementioned issues, metal–organic frameworks (MOFs)‐derived nanoarchitectures have emerged as one of the most promising cathode materials in designing advanced Li–S batteries. This timely progress report highlights and comments on the most recent advances in designing MOFs‐derived nanoarchitectures with diverse electrocatalytic centers as cathode materials for catalyzing the oxidation/reduction reactions of LiPS in LSBs. The molecular/atomic‐level electrocatalytic centers, tunable porosities, large specific surface area, controlled components, and versatile structures of MOFs‐derived nanoarchitectures are carefully discussed for designing favorable electrocatalysts for LiPS. In particular, the catalytic mechanisms of LiPS based on different types of MOFs‐derived materials ranging from single atoms to nanoparticles are systematically summarized. Ultimately, the prospects and challenges of engineering MOFs‐derived catalysts for LiPS in LSBs are also suggested. It is believed that this progress report will offer potential guidance and instructive perspectives for designing future state‐of‐the‐art LiPS catalytic materials in LSBs.
Hierarchical Carbon Nanotube‐Supported Conductive Metal–Organic Framework Nanosheet toward High‐Strain Ionic Soft Actuator
Authors: Wu Yue , Shi Yi‐Xiang , Wang Ying‐Yi , Wang Yong‐Feng , Li Lian‐hui , Feng Si‐Min , Liu Lin , Li Li‐Li , Li Tie , Zhang Ting
Published: July 2022

Conductive metal–organic frameworks (MOFs) have recently been applied in electroactive ionic actuators due to their high surface areas and fast ion migration. However, their actuation performance needs to be promoted in terms of high conversion efficiency and large strain. Here, a soft ionic actuator is assembled by designing a hierarchical Cu‐MOFs‐based active material, which is composed of conductive Cu‐catecholate (Cu‐CAT) nanosheets covalently bridged by carboxylic multiwall carbon nanotubes (Cu‐CAT@MWCNT). Benefited from the large electromechanical deformation and rapid response rate of the Cu‐CAT@MWCNT electrodes, the assembled soft actuator exhibits large displacement of 16.6 mm with high bending strain of 0.52% (AC of ±3 V) and a high energy conversion efficiency (3.02%) with cycling stability over 10 000 cycles (in frequency range of 0.1–10 Hz). In addition, it demonstrates the capacity of gripping objects when assembled on a robot. The Cu‐CAT@MWCNT hybrid material‐based electrode points out a feasible pathway to construct soft actuators with improved performance and broadens their applications.
Investigation of Slip Sensing Technology Based on the Conductive Sponge‐PVDF
Authors: Wang Zhihua , Leng Hansong , Li Na , Yao Tao
Published: July 2022

Tactile sensation is one of the most important senses in living things and smart devices to obtain the required data from the outside world, and slip is one of the main tactile information. In the present study, a slip sensor made of conductive sponge and polyvinylidene fluoride (PVDF) is proposed to sense the magnitude and direction of the sliding force. The structure and working principle of the sensor are analyzed using the finite element method. Compared with the conventional room temperature vulcanized (RTV) silicone rubber and polydimethylsiloxane (PDMS) substrates, the conductive sponge not only provides good support for PVDF film and increases its output voltage but also can be used as an electrode to transfer electric charges. The performed experiments show that the response time of the tactile unit using the conductive sponge substrate is 48 ms, and the detection range of the tactile unit is 0.1–15 N. It is found that the designed sensor has a response time of 80 ms, a recovery time of 160 ms, and a measurement error of the angle perception is 4.54° ± 1.53°. Moreover, its detection error for shear force is 8.04% ± 5.8%. Accordingly, this method can be effectively applied to distinguish the magnitude and direction of the shear force and detect the object sliding in real‐time.
Mechanical Mode Engineering with Orthotropic Metamaterial Membranes
Authors: Conte Gloria , Vicarelli Leonardo , Zanotto Simone , Pitanti Alessandro
Published: July 2022

Metamaterials are structures engineered at a small scale with respect to the wavelength of the excitations they interact with. These structures behave as artificial materials whose properties can be chosen by design, mocking and even outperforming natural materials, and making them the quintessential tool for manipulation of wave systems. In this paper it is shown how the acoustic properties of a silicon nitride membrane can be affected by nanopatterning. The degree of asymmetry in the pattern geometry induces an artificial anisotropic elasticity, resulting in the splitting of otherwise degenerate mechanical modes. The artificial material introduced has a maximum Ledbetter–Migliori anisotropy of 1.568, favorably comparing to most bulk natural crystals. With an additional freedom in defining arbitrary asymmetry axes by pattern rotation, the described approach can be useful for fundamental investigation of material properties as well as for devising improved sensors of light, mass, or acceleration based on micromechanical resonators.
Tailoring the Distinctive Chiral‐Polar Perovskites with Alternating Cations in the Interlayer Space for Self‐Driven Circularly Polarized Light Detection
Authors: Wu Wentao , Li Lina , Li Dong , Yao Yunpeng , Xu Zhijin , Liu Xitao , Hong Maochun , Luo Junhua
Published: July 2022

Chiral hybrid perovskites are recently considered for circularly polarized light (CPL) detection owing to the integration of chiral activity and prominent photoelectric characteristics. However, chiral hybrid perovskites are limited because of the restricted chiral cations; efficient self‐driven CPL detection without external power sources required for halide perovskite photodetectors is desirable but remains scarce. Herein, two alternating cations in the interlayer space of chiral‐polar hybrid perovskites, (R‐β‐MPA)EAPbBr4 and (S‐β‐MPA)EAPbBr4 (MPA = methylphenethylammonium and EA = ethylammonium), are developed via a mixed‐cation approach. These perovskites exhibit chiral characteristics featuring CPL detection with high detectivity. Notably, the chiral‐polar structure gives rise to a built‐in electric field to realize self‐driven CPL detection without an applied voltage, and the photocurrent difference reaches 19% under the right and left CPL irradiation. Moreover, a prominent response time of 300 µs is obtained, which is among the fastest for CPL detection. This work highlights the significance of chemical design in the exploration of new chiral perovskites, which will broaden the scope of chiral materials for high‐performance applications.
Acceptor Interlocked Molecular Design for Solution‐Processed Stable Deep‐Blue TADF and Hyper Fluorescence Organic LED Enabling High‐Efficiency
Authors: Alam Md Intekhab , Nagar Mangey Ram , Nayak Sandhya Rani , Choudhury A. , Jou Jwo‐Huei , Vaidyanathan Sivakumar
Published: July 2022

Solution processed deep‐blue organic light emitting diodes (OLEDs) with high external quantum efficiency (EQE) and a long operational lifetime are still constrained. In this context, two thermally activated delayed fluorescence (TADF) emitters are synthesized utilizing a new design strategy of twisted interlocked acceptor core integrated with carbazole (KCCz) and tert‐butylcarbazole (KCTBC) as donors, respectively, for solution processed deep‐blue TADF OLEDs. Twisting of the acceptor core by two methyl groups results in complete separation of highest occupied molecular orbital and lowest unoccupied molecular orbital, along with cyanide group facilitating the generation of low‐lying triplet excited states as suggested by theoretical simulation. The combined effect of both results in tuning of emission in ultradeep blue region through the efficient population of triplet excitons and concurrently reverse intersystem crossing to produce highly efficient devices. A doped device based on KCTBC shows EQEmax of 9.0% along with low efficiency roll‐off with long operational device half lifetime of 72 min at initial brightness of 1000 cd m−2, and Commission Internationale de L'Eclairage (CIE) coordinates of (0.17, 0.13). In addition, with 12.5 wt% of 4CzFCN as assistant dopant/cohost the performance of the KCTBC‐based device is enhanced to an EQEmax of 13.9% and CIE coordinates of (0.18, 0.13). Further, a high‐efficiency warm white OLED adopting the TADF hybrid approach is realized with EQEmax of 9.0%.
Mesenchymal Stromal Cells Overexpressing Farnesoid X Receptor Exert Cardioprotective Effects Against Acute Ischemic Heart Injury by Binding Endogenous Bile Acids
Authors: Xia Yunlong , Xu Xinyue , Guo Yongzhen , Lin Chen , Xu Xiaoming , Zhang Fuyang , Fan Miaomiao , Qi Tingting , Li Congye , Hu Guangyu , Peng Lu , Wang Shan , Zhang Ling , Hai Chunxu , Liu Rui , Yan Wenjun , Tao Ling
Published: July 2022

Bile acid metabolites have been increasingly recognized as pleiotropic signaling molecules that regulate cardiovascular functions, but their role in mesenchymal stromal cells (MSC)‐based therapy has never been investigated. It is found that overexpression of farnesoid X receptor (FXR), a main receptor for bile acids, improves the retention and cardioprotection of adipose tissue‐derived MSC (ADSC) administered by intramyocardial injection in mice with myocardial infarction (MI), which shows enhanced antiapoptotic, proangiogenic, and antifibrotic effects. RNA sequencing, LC‐MS/MS, and loss‐of‐function studies reveal that FXR overexpression promotes ADSC paracrine angiogenesis via Angptl4. FXR overexpression improves ADSC survival in vivo but fails in vitro. By performing bile acid‐targeted metabolomics using ischemic heart tissue, 19 bile acids are identified. Among them, cholic acid and deoxycholic acid significantly increase Angptl4 secretion from ADSC overexpressing FXR and further improve their proangiogenic capability. Moreover, ADSC overexpressing FXR shows significantly lower apoptosis by upregulating Nqo‐1 expression only in the presence of FXR ligands. Retinoid X receptor α is identified as a coactivator of FXR. It is first demonstrated that there is a bile acid pool in the myocardial microenvironment. Targeting the bile acid‐FXR axis may be a novel strategy for improving the curative effect of MSC‐based therapy for MI.