514 research outputs found
De conventionele RCT voor trauma- en orthopedisch chirurgen: geen heilige graal
Abstract:
Conventionele RCT’s voor trauma- en orthopaedisch chirurgisch onderzoek zijn moeilijk uitvoerbaar door chirurg- en patiënt gerelateerde redenen. Grote regionale cohortstudies en (quasi-) experimentele designs met vooraf gedefinieerde uitkomst parameters en een fulltime onderzoeker bieden een oplossing. De conventionele RCT kan dan worden ingezet voor specifiek gedefinieerde problemen die voortkomen uit de resultaten van deze studies
Multiscale Analysis of the Stress State in a Granular Slope in Transition to Failure
By means of contact dynamics simulations, we analyze the stress state in a
granular bed slowly tilted towards its angle of repose. An increasingly large
number of grains are overloaded in the sense that they are found to carry a
stress ratio above the Coulomb yield threshold of the whole packing. Using this
property, we introduce a coarse-graining length scale at which all stress
ratios are below the packing yield threshold. We show that this length
increases with the slope angle and jumps to a length comparable to the depth of
the granular bed at an angle below the angle of repose. This transition
coincides with the onset of dilatation in the packing. We map this transition
into a percolation transition of the overloaded grains, and we argue that in
the presence of long-range correlations above the transition angle, the
granular slope is metastable.Comment: 11 pages, 14 Fig, submitted to PR
Unilateral interactions in granular packings: A model for the anisotropy modulus
Unilateral interparticle interactions have an effect on the elastic response
of granular materials due to the opening and closing of contacts during
quasi-static shear deformations. A simplified model is presented, for which
constitutive relations can be derived. For biaxial deformations the elastic
behavior in this model involves three independent elastic moduli: bulk, shear,
and anisotropy modulus. The bulk and the shear modulus, when scaled by the
contact density, are independent of the deformation. However, the magnitude of
the anisotropy modulus is proportional to the ratio between shear and
volumetric strain. Sufficiently far from the jamming transition, when
corrections due to non-affine motion become weak, the theoretical predictions
are qualitatively in agreement with simulation results.Comment: 6 pages, 5 figure
Memory of the Unjamming Transition during Cyclic Tiltings of a Granular Pile
Discrete numerical simulations are performed to study the evolution of the
micro-structure and the response of a granular packing during successive
loading-unloading cycles, consisting of quasi-static rotations in the gravity
field between opposite inclination angles. We show that internal variables,
e.g., stress and fabric of the pile, exhibit hysteresis during these cycles due
to the exploration of different metastable configurations. Interestingly, the
hysteretic behaviour of the pile strongly depends on the maximal inclination of
the cycles, giving evidence of the irreversible modifications of the pile state
occurring close to the unjamming transition. More specifically, we show that
for cycles with maximal inclination larger than the repose angle, the weak
contact network carries the memory of the unjamming transition. These results
demonstrate the relevance of a two-phases description -strong and weak contact
networks- for a granular system, as soon as it has approached the unjamming
transition.Comment: 13 pages, 15 figures, soumis \`{a} Phys. Rev.
Force transmission in a packing of pentagonal particles
We perform a detailed analysis of the contact force network in a dense
confined packing of pentagonal particles simulated by means of the contact
dynamics method. The effect of particle shape is evidenced by comparing the
data from pentagon packing and from a packing with identical characteristics
except for the circular shape of the particles. A counterintuitive finding of
this work is that, under steady shearing, the pentagon packing develops a lower
structural anisotropy than the disk packing. We show that this weakness is
compensated by a higher force anisotropy, leading to enhanced shear strength of
the pentagon packing. We revisit "strong" and "weak" force networks in the
pentagon packing, but our simulation data provide also evidence for a large
class of "very weak" forces carried mainly by vertex-to-edge contacts. The
strong force chains are mostly composed of edge-to-edge contacts with a marked
zig-zag aspect and a decreasing exponential probability distribution as in a
disk packing
Громадська робота як чинник повсякденного життя вчителя
Treatment and reconstruction of large bone defects, delayed unions, and nonunions is challenging and has resulted in an ongoing search for novel tissue-engineered therapies. Bone morphogenetic protein-2 (BMP-2) gene therapy is a promising strategy to provide sustained production of BMP-2 locally. Alginate polymer-based nonviral gene therapy with BMP-2 plasmid DNA (pBMP-2) in constructs with multipotent mesenchymal stromal cells (MSCs) has resulted in prolonged gene expression and bone formation in vivo. To further translate this technology toward larger animal models, important issues remain to be investigated, such as the necessity of seeded cells as a target for gene therapy. For that purpose, a large animal-screening model in an orthotopic location, with fully separated chambers, was investigated. Four cylinder-shaped implants were placed in the iliac crests of ten goats. Polycaprolactone tubes around each implant allowed bone ingrowth from the underlying bone and bone marrow and ensured separation of the experimental conditions. An empty tube showed low levels of spontaneous bone ingrowth, and implantation of autologous bone indicated proper bone function with respect to remodeling and resorption. Control ceramic scaffolds were compared to scaffolds containing pBMP-2 either or not combined with seeded MSCs. Fluorochrome incorporation evaluated at 3, 6, and 9 weeks and histomorphometry at 12 weeks after implantation revealed clear differences between the groups, with pBMP-2 combined with MSCs being the most effective. The BMP-2 was demonstrated in a variety of bone-residing cells through immunohistochemistry. Further analysis indicated that multinucleated giant cells might have an important role in transgene expression. Taken together, this work introduces a large animal model for studying bone formation at multiple sites simultaneously in an orthotopic location. The model appeared robust, showed no neighboring effects, and demonstrated effectivity of combined cell and gene therapy
Схемотехническое моделирование и синтез активных СВЧ-фильтров на полевых транзисторах Шоттки
Разработаны схемы активных СВЧ-фильтров, пригодных для исполнения в виде гибридной или полупроводниковой микросхемы
Frictionless bead packs have macroscopic friction, but no dilatancy
The statement of the title is shown by numerical simulation of homogeneously
sheared packings of frictionless, nearly rigid beads in the quasistatic limit.
Results coincide for steady flows at constant shear rate γ in the
limit of small γ and static approaches, in which packings are equilibrated
under growing deviator stresses. The internal friction angle ϕ, equal to
5.76 0.22 degrees in simple shear, is independent on the average pressure
P in the rigid limit. It is shown to stem from the ability of stable
frictionless contact networks to form stress-induced anisotropic fabrics. No
enduring strain localization is observed. Dissipation at the macroscopic level
results from repeated network rearrangements, like the effective friction
of a frictionless slider on a bumpy surface. Solid fraction Φ remains
equal to the random close packing value ≃ 0.64 in slowly or statically
sheared systems. Fluctuations of stresses and volume are observed to regress in
the large system limit, and we conclude that the same friction law for simple
shear applies in the large psystem limit if normal stress or density is
externally controlled. Defining the inertia number as I = γ m/(aP),
with m the grain mass and a its diameter, both internal friction
coefficient ∗ = tan ϕ and volume 1/Φ increase as
powers of I in the quasistatic limit of vanishing I, in which all mechanical
properties are determined by contact network geometry. The microstructure of
the sheared material is characterized with a suitable parametrization of the
fabric tensor and measurements of connectivity and coordination numbers
associated with contacts and near neighbors.Comment: 19 pages. Additional technical details may be found in v
- …