442 research outputs found
Tunneling Calculation in the Field Ion Microscope
In this work we describe calculations of tunneling rate constants for the Field Ion Microscope (FIM) using one-dimensional model potential that simulates the ionization process in a FIM. We obtain expressions for the ionization rate constant (ionization probability per unit of time) of inert gas atoms as a function of their position above the surface. In order to calculate the probability of barrier penetration we have used the semiclassical (JWKB) approximation. We have also calculated ionization zone widths as the distance between points where ionization rate is a maximum and half of this value. An application to helium as the imaging gas is presented to highlight the power of the method
Supersymmetry and Integrability in Planar Mechanical Systems
We present an N=2-supersymmetric mechanical system whose bosonic sector, with
two degrees of freedom, stems from the reduction of an SU(2) Yang-Mills theory
with the assumption of spatially homogeneous field configurations and a
particular ansatz imposed on the gauge potentials in the dimensional reduction
procedure. The Painleve test is adopted to discuss integrability and we focus
on the role of supersymmetry and parity invariance in two space dimensions for
the attainment of integrable or chaotic models. Our conclusion is that the
relationships among the parameters imposed by supersymmetry seem to drastically
reduce the number of possibilities for integrable interaction potentials of the
mechanical system under consideration.Comment: 20 pages, 3 figure
Caracterização da produção agroecológica do sul do Rio Grande do Sul e sua relação com a mecanização agrícola.
A agricultura agroecológica tem-se destacado como uma das alternativas de renda para os pequenos agricultores devido à crescente busca por parte da população por alimentação mais saudável. No Rio Grande do Sul, a Associação Regional de Produtores Agroecológicos da Região Sul (Arpasul), constituída por 48 famílias, é representativa do processo de produção e de comercialização de produtos agroecológicos, servindo como referencial para o presente estudo. O objetivo principal deste trabalho foi o levantamento das necessidades dos produtores agroecológicos
em relação à mecanização agrícola. As informações obtidas foram baseadas no sistema de produção utilizado, sendo caracterizados aspectos referentes à propriedade, às operações agrícolas e às necessidades específicas de máquinas e implementos. Por meio deste estudo, foi evidenciada a carência no atendimento das demandas específicas, na área de máquinas agrícolas para esse segmento, podendo servir como referencial para o desenvolvimento de novas máquinas e/ou aperfeiçoamento das existentes. A semeadora de milho e feijão foi a máquina que a maioria dos agricultores pesquisados indicou como sua maior necessidade
The Hahn Quantum Variational Calculus
We introduce the Hahn quantum variational calculus. Necessary and sufficient
optimality conditions for the basic, isoperimetric, and Hahn quantum Lagrange
problems, are studied. We also show the validity of Leitmann's direct method
for the Hahn quantum variational calculus, and give explicit solutions to some
concrete problems. To illustrate the results, we provide several examples and
discuss a quantum version of the well known Ramsey model of economics.Comment: Submitted: 3/March/2010; 4th revision: 9/June/2010; accepted:
18/June/2010; for publication in Journal of Optimization Theory and
Application
Scalable production of human mesenchymal stromal cell-derived extracellular vesicles under serum-/xeno-free conditions in a microcarrier-based bioreactor culture system
Copyright © 2020 de Almeida Fuzeta, Bernardes, Oliveira, Costa, Fernandes-Platzgummer, Farinha, Rodrigues, Jung, Tseng, Milligan, Lee, Castanho, Gaspar, Cabral and da Silva. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.Mesenchymal stromal cells (MSC) hold great promise for tissue engineering and cell-based therapies due to their multilineage differentiation potential and intrinsic immunomodulatory and trophic activities. Over the past years, increasing evidence has proposed extracellular vesicles (EVs) as mediators of many of the MSC-associated therapeutic features. EVs have emerged as mediators of intercellular communication, being associated with multiple physiological processes, but also in the pathogenesis of several diseases. EVs are derived from cell membranes, allowing high biocompatibility to target cells, while their small size makes them ideal candidates to cross biological barriers. Despite the promising potential of EVs for therapeutic applications, robust manufacturing processes that would increase the consistency and scalability of EV production are still lacking. In this work, EVs were produced by MSC isolated from different human tissue sources [bone marrow (BM), adipose tissue (AT), and umbilical cord matrix (UCM)]. A serum-/xeno-free microcarrier-based culture system was implemented in a Vertical-WheelTM bioreactor (VWBR), employing a human platelet lysate culture supplement (UltraGROTM-PURE), toward the scalable production of MSC-derived EVs (MSC-EVs). The morphology and structure of the manufactured EVs were assessed by atomic force microscopy, while EV protein markers were successfully identified in EVs by Western blot, and EV surface charge was maintained relatively constant (between −15.5 ± 1.6 mV and −19.4 ± 1.4 mV), as determined by zeta potential measurements. When compared to traditional culture systems under static conditions (T-flasks), the VWBR system allowed the production of EVs at higher concentration (i.e., EV concentration in the conditioned medium) (5.7-fold increase overall) and productivity (i.e., amount of EVs generated per cell) (3-fold increase overall). BM, AT and UCM MSC cultured in the VWBR system yielded an average of 2.8 ± 0.1 × 1011, 3.1 ± 1.3 × 1011, and 4.1 ± 1.7 × 1011 EV particles (n = 3), respectively, in a 60 mL final volume. This bioreactor system also allowed to obtain a more robust MSC-EV production, regarding their purity, compared to static culture. Overall, we demonstrate that this scalable culture system can robustly manufacture EVs from MSC derived from different tissue sources, toward the development of novel therapeutic products.unding received by iBB-Institute for Bioengineering and Biosciences from the Portuguese Foundation for Science and Technology (FCT) (UID/BIO/04565/2020) and through the projects PTDC/EQU-EQU/31651/2017, PTDC/BBB-BQB/1693/2014, and PTDC/BTM-SAL/31057/2017 is acknowledged. Funding received from POR de Lisboa 2020 through the project PRECISE – Accelerating progress toward the new era of precision medicine (Project N. 16394) is also acknowledged. MAF (PD/BD/128328/2017) and FO (PD/BD/135046/2017) acknowledge FCT for the Ph.D. fellowships and DG (SFRH/BPD/109010/2015) for the Post-Doctoral fellowship.info:eu-repo/semantics/publishedVersio
RI'/SMOM scheme amplitudes for quark currents at two loops
We determine the two loop corrections to the Green's function of a quark
current inserted in a quark 2-point function at the symmetric subtraction
point. The amplitudes for the scalar, vector and tensor currents are presented
in both the MSbar and RI'/SMOM renormalization schemes. The RI'/SMOM scheme two
loop renormalization for the scalar and tensor cases agree with previous work.
The vector current renormalization requires special treatment as it must be
consistent with the Slavnov-Taylor identity which we demonstrate. We also
discuss the possibility of an alternative definition of the RI'/SMOM scheme in
the case of the tensor current.Comment: 36 latex pages, 1 figure, 21 tables, anc directory contains txt file
with data in table
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