2,254 research outputs found
Interlayer screening effect in graphene multilayers with ABA and ABC stacking
We study the effect of perpendicular electric fields on the band structures
of ABA and ABC graphene multilayers, and find that the electronic screening
effect is significantly different between them. In ABA multilayers, the field
produces a band overlap and gives a linear screening, while in ABC multilayers,
in contrast, it opens an energy gap in the surface-state band at low energy,
leading to a strong screening effect essentially non-linear to the field
amplitude. The energy gap of a large ABC stack sharply rises when the external
field exceeds a certain critical value.Comment: 8 pages, 6 figure
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Direct Freeform Fabrication of Spatially Heterogeneous Living Cell-Impregnated Implants
The objectives of this work are the development of the processes, materials, and tooling to
directly ā3-D printā living, pre-seeded, patient-specific implants of spatially heterogeneous
compositions. The research presented herein attempts to overcome some of the challenges to
scaffolding, such as the difficulty of producing spatially heterogeneous implants that require
varied seeding densities and/or cell-type distributions. In the proposed approach, living implants
are fabricated by the layer-wise deposition of pre-cell-seeded alginate hydrogel. Although
alginate hydrogels have been previously used to mold living implants, the properties of the
alginate formulations used for molding were not suitable for 3-D printing. In addition to changing
the formulation to make the alginate hydrogels āprintable,ā we developed a robotic hydrogel
deposition system and supporting CAD software to deposit the gel in arbitrary geometries. We
demonstrated this technologyās capabilities by printing alginate gel implants of multiple materials
with various spatial heterogeneities, including, implants with completely embedded material
clusters. The process was determined to be both viable (94Ā±5% n=15) and sterile (less than one
bacterium per 0.9 ĀµL after 8 days of incubation). Additionally, we demonstrated the printing of a
meniscus cartilage-shaped gel generated directly from a CT Scan. The proposed approach may
hold advantages over other tissue printing efforts [5,9]. This technology has the potential to
overcome challenges to scaffolding and could enable the efficient fabrication of spatially
heterogeneous, patient-specific, living implants.Mechanical Engineerin
Geometries and energetics of methanolāethanol clusters: a VUV laser/time-of-flight mass spectrometry and density functional theory study
Hydrogen-bonded clusters, formed above liquid methanol (Me) and ethanol (Et) mixtures of various compositions, were entrained in a supersonic jet and probed using 118 nm vacuum ultraviolet (VUV) laser single-photon ionization/time-of-flight mass spectrometry. The spectra are dominated by protonated cluster ions, formed by ionizing hydrogen-bonded MemEtn neutrals, m = 0ā4, n = 0ā3, and m + n = 2ā5. The structures and energetics of the neutral and ionic species were investigated using both the all-atom optimized potential for liquid state, OPLS-AA, and the density functional (DFT) calculations. The energetic factors affecting the observed cluster distributions were examined. Calculations indicate that the large change in binding energy going from trimer to tetramer can be attributed more to pair-wise interactions than to cooperativity effects
Prediction of the Size Distributions of Methanol-Ethanol Clusters Detected in VUV Laser/Time-of-flight Mass Spectrometry
The size distributions and geometries of vapor clusters equilibrated with methanolāethanol (MeāEt) liquid mixtures were recently studied by vacuum ultraviolet (VUV) laser time-of-flight (TOF) mass spectrometry and density functional theory (DFT) calculations (Liu, Y.; Consta, S.; Ogeer, F.; Shi, Y. J.; Lipson, R. H. Can. J. Chem. 2007, 85, 843ā852). On the basis of the mass spectra recorded, it was concluded that the formation of neutral tetramers is particularly prominent. Here we develop grand canonical Monte Carlo (GCMC) and molecular dynamics (MD) frameworks to compute cluster size distributions in vapor mixtures that allow a direct comparison with experimental mass spectra. Using the all-atom optimized potential for liquid simulations (OPLS-AA) force field, we systematically examined the neutral cluster size distributions as functions of pressure and temperature. These neutral cluster distributions were then used to derive ionized cluster distributions to compare directly with the experiments. The simulations suggest that supersaturation at 12 to 16 times the equilibrium vapor pressure at 298 K or supercooling at temperature 240 to 260 K at the equilibrium vapor pressure can lead to the relatively abundant tetramer population observed in the experiments. Our simulations capture the most distinct features observed in the experimental TOF mass spectra: Et3H+ at m/z = 139 in the vapor corresponding to 10:90% MeāEt liquid mixture and Me3H+ at m/z = 97 in the vapors corresponding to 50:50% and 90:10% MeāEt liquid mixtures. The hybrid GCMC scheme developed in this work extends the capability of studying the size distributions of neat clusters to mixed species and provides a useful tool for studying environmentally important systems such as atmospheric aerosols
Universal Robotic Gripper based on the Jamming of Granular Material
Gripping and holding of objects are key tasks for robotic manipulators. The
development of universal grippers able to pick up unfamiliar objects of widely
varying shape and surface properties remains, however, challenging. Most
current designs are based on the multi-fingered hand, but this approach
introduces hardware and software complexities. These include large numbers of
controllable joints, the need for force sensing if objects are to be handled
securely without crushing them, and the computational overhead to decide how
much stress each finger should apply and where. Here we demonstrate a
completely different approach to a universal gripper. Individual fingers are
replaced by a single mass of granular material that, when pressed onto a target
object, flows around it and conforms to its shape. Upon application of a vacuum
the granular material contracts and hardens quickly to pinch and hold the
object without requiring sensory feedback. We find that volume changes of less
than 0.5% suffice to grip objects reliably and hold them with forces exceeding
many times their weight. We show that the operating principle is the ability of
granular materials to transition between an unjammed, deformable state and a
jammed state with solid-like rigidity. We delineate three separate mechanisms,
friction, suction and interlocking, that contribute to the gripping force.
Using a simple model we relate each of them to the mechanical strength of the
jammed state. This opens up new possibilities for the design of simple, yet
highly adaptive systems that excel at fast gripping of complex objects.Comment: 10 pages, 7 figure
Weight change and ovarian steroid profiles in young women
Objective: To investigate possible short-term effects of voluntary weight loss on ovarian steroid proļ¬les in young women, in light of better established long-term effects in older women.
Design: We tested for an association of voluntary weight change over the course of a menstrual cycle with salivary E2 and P proļ¬les in the same menstrual cycle.
Setting: Students were recruited in a college residence hall, and they provided daily saliva samples to a researcher living nearby.
Patient(s): The 65 women who participated were all college students and ranged in age between 18 and 23 years.
Intervention(s): None.
Main Outcome Measure(s): Weight was assessed in the ļ¬rst week of the menstrual cycle and ļ¬rst week of the following menstrual cycle. Estradiol and P were measured by radioimmunoassay in daily saliva samples.
Result(s): We did not detect a suppressive effect of weight loss on the overall level of either hormone. However, we did ļ¬nd evidence for more distinct follicular and luteal E2 peaks in women who gained weight. Peak luteal P also arrived about 2 days earlier in women who gained weight.
Conclusion(s): This ļ¬nding adds to evidence that short-term response of ovarian function to weight loss in young women is less pronounced than long-term response in older women.AnthropologyHuman Evolutionary Biolog
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