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MIT Open Access Articles

Mostrando recursos 41 - 60 de 20,907

  1. Estimating the Number of Stable Configurations for the Generalized Thomson Problem

    Calef, Matthew; Griffiths, Whitney; Schulz, Alexia
    Given a natural number N, one may ask what configuration of N points on the two-sphere minimizes the discrete generalized Coulomb energy. If one applies a gradient-based numerical optimization to this problem, one encounters many configurations that are stable but not globally minimal. This led the authors of this manuscript to the question, how many stable configurations are there? In this manuscript we report methods for identifying and counting observed stable configurations, and estimating the actual number of stable configurations. These estimates indicate that for N approaching two hundred, there are at least tens of thousands of stable configurations.

  2. Microfluidic fabrication of cell adhesive chitosan microtubes

    Lee, Sang-Hoon; Lee, Kwang Ho; Lee, Dong Hwan; Oh, Jonghyun; Kim, Keekyoung; Won, Sung Wook; Selimovic, Seila; Bae, Hojae; Cha, Chaenyung; Gaharwar, Akhilesh; Khademhosseini, Alireza
    Chitosan has been used as a scaffolding material in tissue engineering due to its mechanical properties and biocompatibility. With increased appreciation of the effect of micro- and nanoscale environments on cellular behavior, there is increased emphasis on generating microfabricated chitosan structures. Here we employed a microfluidic coaxial flow-focusing system to generate cell adhesive chitosan microtubes of controlled sizes by modifying the flow rates of a chitosan pre-polymer solution and phosphate buffered saline (PBS). The microtubes were extruded from a glass capillary with a 300 μm inner diameter. After ionic crosslinking with sodium tripolyphosphate (TPP), fabricated microtubes had inner and outer...

  3. A contactless electrical stimulator: application to fabricate functional skeletal muscle tissue

    Ahadian, Samad; Ramón-Azcón, Javier; Ostrovidov, Serge; Kaji, Hirokazu; Ino, Kosuke; Shiku, Hitoshi; Matsue, Tomokazu; Camci-Unal, Gulden; Khademhosseini, Alireza
    Engineered skeletal muscle tissues are ideal candidates for applications in drug screening systems, bio-actuators, and as implantable constructs in tissue engineering. Electrical field stimulation considerably improves the differentiation of muscle cells to muscle myofibers. Currently used electrical stimulators often use direct contact of electrodes with tissue constructs or their culture medium, which may cause hydrolysis of the culture medium, joule heating of the medium, contamination of the culture medium due to products of electrodes corrosion, and surface fouling of electrodes. Here, we used an interdigitated array of electrodes combined with an isolator coverslip as a contactless platform to electrically stimulate...

  4. Model approximation for batch flow shop scheduling with fixed batch sizes

    Warnick, Sean; Weyerman, W. Samuel; Rai, Anurag
    Batch flow shops model systems that process a variety of job types using a fixed infrastructure. This model has applications in several areas including chemical manufacturing, building construction, and assembly lines. Since the throughput of such systems depends, often strongly, on the sequence in which they produce various products, scheduling these systems becomes a problem with very practical consequences. Nevertheless, optimally scheduling these systems is NP-complete. This paper demonstrates that batch flow shops can be represented as a particular kind of heap model in the max-plus algebra. These models are shown to belong to a special class of linear systems...

  5. On the Distribution of Atkin and Elkies Primes

    Shparlinski, Igor E.; Sutherland II, Andrew Victor
    Given an elliptic curve E over a finite field F[subscript q] of q elements, we say that an odd prime ℓ∤q is an Elkies prime for E if t[superscript 2][subscript E]−4q is a square modulo ℓ, where t[subscript E]=q+1−#E(F[subscript q]) and #E(F[subscript q]) is the number of F[subscript q]-rational points on E; otherwise, ℓ is called an Atkin prime. We show that there are asymptotically the same number of Atkin and Elkies primes ℓ0 and a sufficiently large q. We use this...

  6. The Effect of Ear Canal Orientation on Tympanic Membrane Motion and the Sound Field Near the Tympanic Membrane

    Ravicz, Michael; Guignard, Jérémie; Furlong, Cosme; Cheng, Jeffrey Tao; Rosowski, John J
    The contribution of human ear canal orientation to tympanic membrane (TM) surface motion and sound pressure distribution near the TM surface is investigated by using an artificial ear canal (aEC) similar in dimensions to the natural human ear canal. The aEC replaced the bony ear canal of cadaveric human temporal bones. The radial orientation of the aEC relative to the manubrium of the TM was varied. Tones of 0.2 to 18.4 kHz delivered through the aEC induced surface motions of the TM that were quantified using stroboscopic holography; the distribution of sound in the plane of the tympanic ring PTR...

  7. Low level laser therapy/photobiomodulation in the management of side effects of chemoradiation therapy in head and neck cancer: part 1: mechanisms of action, dosimetric, and safety considerations

    Elad, Sharon; Barasch, Andrei; Genot, Marie-Thérèse; Lansaat, Liset; van der Brink, Ron; Arnabat-Dominguez, Josep; van der Molen, Lisette; Jacobi, Irene; van Diessen, Judi; de Lange, Jan; Bensadoun, René-Jean; Zecha, Judith A. E. M.; Raber-Durlacher, Judith E.; Nair, Raj G.; Epstein, Joel B.; Sonis, Stephen T.; Migliorati, Cesar A.; Milstein, Dan M. J.; Smeele, Ludi E.; Schubert, Mark M.; Hamblin, Michael R
    Purpose: There is a large body of evidence supporting the efficacy of low level laser therapy (LLLT), more recently termed photobiomodulation (PBM), for the management of oral mucositis (OM) in patients undergoing radiotherapy for head and neck cancer (HNC). Recent advances in PBM technology, together with a better understanding of mechanisms involved, may expand the applications for PBM in the management of other complications associated with HNC treatment. This article (part 1) describes PBM mechanisms of action, dosimetry, and safety aspects and, in doing so, provides a basis for a companion paper (part 2) which describes the potential breadth of potential...

  8. Coloring intersection graphs of x-monotone curves in the plane

    Suk, Andrew
    A class of graphs G is χ-bounded if the chromatic number of the graphs in G is bounded by some function of their clique number. We show that the class of intersection graphs of simple families of x-monotone curves in the plane intersecting a vertical line is χ-bounded. As a corollary, we show that the class of intersection graphs of rays in the plane is χ-bounded, and the class of intersection graphs of unit segments in the plane is χ-bounded.

  9. Vulnerability models for environmental risk assessment. Application to a nuclear power plant containment building

    Crusells-Girona, M.; Egozcue, J. J.; Musolas Otano, Antoni Maria
    Environmental risk management consists of making decisions on human activities or construction designs that are affected by the environment and/or have consequences or impacts on it. In these cases, decisions are made such that risk is minimized. In this regard, the forthcoming paper develops a close form that relates risk with cost, hazard, and vulnerability; and then focuses on vulnerability. The vulnerability of a system under an external action can be described by the conditional probability of the degrees of damage after an event. This vulnerability model can be obtained by a simplicial regression of those outputs, as a response...

  10. Multiscale modelling framework for the fracture of thin brittle polycrystalline films: application to polysilicon

    Vayrette, Renaud; Raskin, Jean-Pierre; Pardoen, Thomas; Galceran, Montserrat; Godet, Stéphane; Noels, Ludovic; Mulay, Shantanu S.; Becker, Gauthier
    Micro-electro-mechanical systems (MEMS) made of polycrystalline silicon are widely used in several engineering fields. The fracture properties of polycrystalline silicon directly affect their reliability. The effect of the orientation of grains on the fracture behaviour of polycrystalline silicon is investigated out of the several factors. This is achieved, firstly, by identifying the statistical variation of the fracture strength and critical strain energy release rate, at the nanoscopic scale, over a thin freestanding polycrystalline silicon film having mesoscopic scale dimensions. The fracture stress and strain at the mesoscopic level are found to be closely matching with uniaxial tension experimental results. Secondly,...

  11. Tight bounds for parallel randomized load balancing

    Wattenhofer, Roger; Lenzen, Christoph
    Given a distributed system of n balls and n bins, how evenly can we distribute the balls to the bins, minimizing communication? The fastest non-adaptive and symmetric algorithm achieving a constant maximum bin load requires Θ(loglogn) rounds, and any such algorithm running for r∈O(1) rounds incurs a bin load of Ω((logn/loglogn)1/r). In this work, we explore the fundamental limits of the general problem. We present a simple adaptive symmetric algorithm that achieves a bin load of 2 in log∗n+O(1) communication rounds using O(n) messages in total. Our main result, however, is a matching lower bound of (1−o(1))log∗n on the time...

  12. EmilyCrawford_Special issue on DISC 2010

    Shvartsman, Alexander A.; Lynch, Nancy Ann
    This special issue of Distributed Computing is based on papers that originally appeared as extended abstracts in the Proceedings of the 24th International Symposium on Distributed Computing (DISC2010), held in Cambridge, Massachusetts on August 13–15, 2010. The papers for the Special Issue were chosen by the Program Committee from the 32 regular papers presented at the Symposium, based on their quality and representation of the spectrum of topics encompassed by the Symposium. In addition to being reviewed, in preliminary form, by the Program Committee, the full papers submitted for the Special Issue were refereed according to the standard practices of...

  13. A leader election algorithm for dynamic networks with causal clocks

    Ingram, Rebecca; Shields, Patrick; Viqar, Saira; Walter, Jennifer E.; Welch, Jennifer L.; Radeva, Tsvetomira T.
    An algorithm for electing a leader in an asynchronous network with dynamically changing communication topology is presented. The algorithm ensures that, no matter what pattern of topology changes occurs, if topology changes cease, then eventually every connected component contains a unique leader. The algorithm combines ideas from the Temporally Ordered Routing Algorithm for mobile ad hoc networks (Park and Corson in Proceedings of the 16th IEEE Conference on Computer Communications (INFOCOM), pp. 1405–1413 (1997) with a wave algorithm (Tel in Introduction to distributed algorithms, 2nd edn. Cambridge University Press, Cambridge, MA, 2000), all within the framework of a height-based mechanism...

  14. Failure detectors encapsulate fairness

    Pike, Scott M.; Welch, Jennifer L.; Sastry, Srikanth
    Failure detectors have long been viewed as abstractions for the synchronism present in distributed system models. However, investigations into the exact amount of synchronism encapsulated by a given failure detector have met with limited success. The reason for this is that traditionally, models of partial synchrony are specified with respect to real time, but failure detectors do not encapsulate real time. Instead, we argue that failure detectors encapsulate the fairness in computation and communication. Fairness is a measure of the number of steps executed by one process relative either to the number of steps taken by another process or relative...

  15. Adipose tissue macrophages in non-rodent mammals: a comparative study

    Ampem, Grace; Bacsadi, Árpád; Balogh, Lajos; Schmidt, Susanne; Thuróczy, Julianna; Röszer, Tamás; Azegrouz, Hind
    The stromal vascular fraction (SVF) of adipose tissue in rodents and primates contains mesenchymal stem cells and immune cells. SVF cells have complex metabolic, immune and endocrine functions with biomedical impact. However, in other mammals, the amount of data on SVF stem cells is negligible and whether the SVF hosts immune cells is unknown. In this study, we show that the SVF is rich in immune cells, with a dominance of adipose tissue macrophages (ATMs) in cattle (Bos primigenius taurus), domestic goat (Capra aegagrus hircus), domestic sheep (Ovis aries), domestic cat (Felis catus) and domestic dog (Canis familiaris). ATMs of...

  16. Imaginary geometry I: interacting SLEs

    Miller, Jason P.; Sheffield, Scott Roger
    Fix constants and θ∈[0,2π), and let h be an instance of the Gaussian free field on a planar domain. We study flow lines of the vector field e[superscript i(h/χ+θ)] starting at a fixed boundary point of the domain. Letting θ vary, one obtains a family of curves that look locally like SLE[subscript κ] processes with κ∈(0,4) (where χ=2[sqrt]κ−[sqrt]κ2), which we interpret as the rays of a random geometry with purely imaginary curvature. We extend the fundamental existence and uniqueness results about these paths to the case that the paths intersect the boundary. We also show that flow lines of different...

  17. The conformal loop ensemble nesting field

    Wilson, David B.; Miller, Jason P.; Watson, Samuel Stewart
    The conformal loop ensemble CLE[subscript κ]with parameter 8/3<κ<8 is the canonical conformally invariant measure on countably infinite collections of non-crossing loops in a simply connected domain. We show that the number of loops surrounding an ε-ball (a random function of z and ε) minus its expectation converges almost surely as ε→0 to a random conformally invariant limit in the space of distributions, which we call the nesting field. We generalize this result by assigning i.i.d. weights to the loops, and we treat an alternate notion of convergence to the nesting field in the case where the weight distribution has mean...

  18. Multilevel Dyson Brownian motions via Jack polynomials

    Shkolnikov, Mykhaylo; Gorin, Vadim
    We introduce multilevel versions of Dyson Brownian motions of arbitrary parameter β>0, generalizing the interlacing reflected Brownian motions of Warren for β=2. Such processes unify β corners processes and Dyson Brownian motions in a single object. Our approach is based on the approximation by certain multilevel discrete Markov chains of independent interest, which are defined by means of Jack symmetric polynomials. In particular, this approach allows to show that the levels in a multilevel Dyson Brownian motion are intertwined (at least for β≥1) and to give the corresponding link explicitly.

  19. Modeling the hydroclimatology of the midwestern United States. Part 1: current climate

    Winter, Jonathan Mark; Eltahir, Elfatih A. B.
    An ensemble of six 22-year numerical experiments was conducted to evaluate the ability of Regional Climate Model version 3 (RegCM3) to simulate the energy and water budgets of the midwestern United States. RegCM3 was run using two surface physics schemes: Integrated Biosphere Simulator (IBIS) and Biosphere-Atmosphere Transfer Scheme 1e (BATS1e), and two convective closure assumptions: Fritsch & Chappell (FC80) and Arakawa & Schubert (AS74). Boundary conditions were provided by the National Centers for Environmental Prediction-Department of Energy Reanalysis 2 dataset and the ECHAM5 general circulation model. A companion paper examines the American Midwest under future climate scenarios. Overall, the model...


    Rana, Vikram; Loh, Alan; Corbel, Stephane; Tomsick, John A.; Walton, Dominic J.; Barret, Didier; Boggs, Steven E.; Christensen, Finn E.; Craig, William; Fuerst, Felix; Gandhi, Poshak; Grefenstette, Brian W.; Hailey, Charles; Harrison, Fiona A.; Madsen, Kristin K.; Rahoui, Farid; Stern, Daniel; Tendulkar, Shriharsh; Zhang, William W.; Chakrabarty, Deepto
    We present results from multi-wavelength simultaneous X-ray and radio observations of the black hole X-ray binary V404 Cyg in quiescence. Our coverage with NuSTAR provides the very first opportunity to study the X-ray spectrum of V404 Cyg at energies above 10 keV. The unabsorbed broadband (0.3–30 keV) quiescent luminosity of the source is 8.9 × 10³² erg s⁻¹ for a distance of 2.4 kpc. The source shows clear variability on short timescales (an hour to a couple of hours) in the radio, soft X-ray, and hard X-ray bands in the form of multiple flares. The broadband X-ray spectra obtained from...

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