23 research outputs found
Flexibility of in vitro cortical circuits influences resilience from microtrauma
BackgroundSmall clusters comprising hundreds to thousands of neurons are an important level of brain architecture that correlates single neuronal properties to fulfill brain function, but the specific mechanisms through which this scaling occurs are not well understood. In this study, we developed an in vitro experimental platform of small neuronal circuits (islands) to probe the importance of structural properties for their development, physiology, and response to microtrauma.MethodsPrimary cortical neurons were plated on a substrate patterned to promote attachment in clusters of hundreds of cells (islands), transduced with GCaMP6f, allowed to mature until 10â13 days in vitro (DIV), and monitored with Ca2+ as a non-invasive proxy for electrical activity. We adjusted two structural factorsâisland size and cellular densityâto evaluate their role in guiding spontaneous activity and network formation in neuronal islands.ResultsWe found cellular density, but not island size, regulates of circuit activity and network function in this system. Low cellular density islands can achieve many states of activity, while high cellular density biases islands towards a limited regime characterized by low rates of activity and high synchronization, a property we summarized as âflexibility.â The injury severity required for an island to lose activity in 50% of its population was significantly higher in low-density, high flexibility islands.ConclusionTogether, these studies demonstrate flexible living cortical circuits are more resilient to microtrauma, providing the first evidence that initial circuit state may be a key factor to consider when evaluating the consequences of trauma to the cortex
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Method for the prediction of the hydriding thermodynamics of ternary PD-based alloys.
A method has been developed to calculate the hydriding thermodynamics of ternary Pd-X-Y systems, where X and Y are substitutional alloying elements, by using the properties of the binary Pd-X and Pd-Y systems. Experimental data was collected on the Pd-Rh-Co system to test the validity of this method. Hydrogen pressure-composition isotherms of several binary Pd-Rh and Pd-Co alloys and Pd-Rh-Co ternary alloys were measured to determine the thermodynamics of hydrogen absorption, hydride formation and decomposition, and hydrogen capacity. Good agreement between the calculated and measured values for the ternary Pd-Rh-Co system, in the dilute alloying regime (< 10 at.% total alloying additions), was obtained using our method. Examining literature results on other ternary Pd-X-Y systems checked the universality of this method. Again, the method succeeds in predicting the hydriding thermodynamics for both lattice contracted and lattice expanded alloy systems, Pd-Ni-Rh and Pd-Ag-Y respectively
Anomalous enhancement of tetragonality in PbTiO3 induced by negative pressure
Using a first-principles approach based on density-functional theory, we find
that a large tetragonal strain can be induced in PbTiO3 by application of a
negative hydrostatic pressure. The structural parameters and the dielectric and
dynamical properties are found to change abruptly near a crossover pressure,
displaying a ``kinky'' behavior suggestive of proximity to a phase transition.
Analogous calculations for BaTiO3 show that the same effect is also present
there, but at much higher negative pressure. We investigate this unexpected
behavior of PbTiO3 and discuss an interpretation involving a phenomenological
description in terms of a reduced set of relevant degrees of freedom.Comment: 9 pages, with 9 postscript figures embedded. Uses REVTEX and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/st_pbti/index.htm
Theory of structural response to macroscopic electric fields in ferroelectric systems
We have developed and implemented a formalism for computing the structural
response of a periodic insulating system to a homogeneous static electric field
within density-functional perturbation theory (DFPT). We consider the
thermodynamic potentials E(R,eta,e) and F(R,eta,e) whose minimization with
respect to the internal structural parameters R and unit cell strain eta yields
the equilibrium structure at fixed electric field e and polarization P,
respectively. First-order expansion of E(R,eta,e) in e leads to a useful
approximation in which R(P) and eta(P) can be obtained by simply minimizing the
zero-field internal energy with respect to structural coordinates subject to
the constraint of a fixed spontaneous polarization P. To facilitate this
minimization, we formulate a modified DFPT scheme such that the computed
derivatives of the polarization are consistent with the discretized form of the
Berry-phase expression. We then describe the application of this approach to
several problems associated with bulk and short-period superlattice structures
of ferroelectric materials such as BaTiO3 and PbTiO3. These include the effects
of compositionally broken inversion symmetry, the equilibrium structure for
high values of polarization, field-induced structural phase transitions, and
the lattice contributions to the linear and the non-linear dielectric
constants.Comment: 19 pages, with 15 postscript figures embedded. Uses REVTEX4 and epsf
macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/sai_pol/index.htm
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Unanticipated results in the uranium niobium alloy system
The uranium niobium binary alloy system exhibits a rich collection of phenomena for study. The composition range from 0 wt.% Nb to 10 wt.% Nb exhibits multiple crystallographic phases with interesting properties such as superconductivity, charge density waves and shape memory effects. We have measured the resistivity and heat capacity as a function of temperature from 2 to 325K in the above composition range in an effort to map out the phase boundaries of interest. Surprisingly the temperature dependence of the resistivity transitions from metallic (decreasing with decreasing temperature) to nonmetallic (increasing with decreasing temperature). It is not clear if the nonmetallic resistivity is caused by strongly correlated electronic effects or is the result of some other effect such as disorder driven scattering
The distribution of Heterotrissocladius oliveri Saether (Diptera: Chironomidae) in Lake Michigan
Fifty one chironomid species were identified from 504 samples collected at depths ranging 8 to 267 m in Lake Michigan, U.S.A. Heterotrissocladius oliveri Saether occurred in 32% of these samples and had an average abundance of 22 m â2 which was similar to other estimates from the Great Lakes. Maximum average lake-wide density was at 30 to 60 m (41 m â2 ). At depths â„60 m, H. oliveri was the dominant chironomid species comprising 75% of total Chironomidae. The substrate preference of H. oliveri differed within each depth regime considered: at 30â60 m, 2â3 Ï; at 60â120 m, 3â5 Ï, 7â9 Ï; and at 120â180 m, 6â8 Ï. Abundance was notably reduced at all depths in substrates characterized as medium silt (5â6 Ï). On a lake-wide basis, the distribution pattern suggested H. oliveri was most numerous from 30 to 60 m along the southwestern, eastern, and northern shorelines and at 60â120 m depths along the southern and eastern shorelines. Increased abundance in the South Basin was concurrent with evidence of increased sedimentation at 60 to 100 m. However, in several other areas of the lake, high densities were associated with medium to very fine sands relatively free of silts and clays. This observation suggested occurrence of H. oliveri was minimally affected by sediment type.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42871/1/10750_2004_Article_BF00008856.pd
Recent developments in the study of hydrogen embrittlement at the University of Illinois
This paper summarizes recent work at the University of Illinois on the fundamental mechanisms of hydrogen embrittlement. Our approach combines experimental and theoretical methods. We describe the theoretical work on hydride formation and its application to hydrogen embrittlement of titanium alloys through the stress-induced hydride formation and cleavage mechanism, the localization of shear due to solute hydrogen, and finally, we present experimental evidence that favors the decohesion mechanism of hydrogen embrittlement in a ??-Ti alloy.published or submitted for publicationis peer reviewe
The effects of hydrogen on the deformation and fracture behavior of the metastable beta-titanium alloy, TIMETAL(RTM) 21S
The metastable -titanium alloy, TIMETAL\sp\circler 21S, exhibits a sharp ductile-to-brittle transition when the hydrogen concentration is increased slightly above H/M 0.22. To understand this sharp transition, a series of experiments was devised to test for the possible hydrogen embrittlement mechanisms. In situ straining experiments in an environmental cell TEM showed that hydrogen enhances the mobility of dislocations. However, this mechanism cannot account for the abrupt transition that is observed. No evidence for the formation of hydrides on fracture surfaces or in the stress fields of active cracks was found suggesting that the stress-induced hydride mechanism is not responsible for the observed transition. Therefore, the most viable mechanism is hydrogen-induced decohesion. Bulk testing showed that internal hydrogen reduces the yield strength of ductile specimens and decreases the fracture stress of the brittle specimens. All of the observed phenomena are consistent with a decohesion mechanism.U of I OnlyETDs are only available to UIUC Users without author permissio
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Uranium Hydride Nucleation Kinetics: Effects of Oxide Thickness and Vacuum Outgassing
Many factors such as impurities in the oxide and metal, microstructure, gas impurities, and oxide thickness may influence the rate and location of the nucleation of hydride on uranium. This work has concentrated on isolating one of these variables, the oxide thickness, and measuring the effect of the oxide thickness on uranium hydride nucleation. Uranium samples, all from the same lot, were prepared with different oxide thicknesses. The oxide thickness was measured using Rutherford Backscattering Spectroscopy. Oxidized uranium samples were then exposed to ultra-high purity hydrogen gas under constant volume conditions. Decreases in pressure indicated hydrogen uptake by the sample. The time for hydride nucleation--as well as the maximum hydriding rate--was then calculated from the measured decreases in pressure. The time to nucleate a hydride was found to increase whereas the maximum hydriding rate was found to decrease with increasing oxide thickness. The density of hydride pits also decreased with increasing oxide thickness. The observed results support the argument that the nucleation of hydride is controlled somewhat by diffusion of hydrogen through the oxide layer. Vacuum outgassing of samples, thereby removing the oxide impurities and keeping the oxide thickness constant, dramatically decreased the nucleation time and increased the maximum hydriding rate. Again, this is consistent with hydrogen diffusion through the oxide controlling the nucleation of hydride. Impurities in the oxide layer can decrease the diffusivity of hydrogen and therefore delay the nucleation of uranium hydride