Memristive effects in oxygenated amorphous carbon nanodevices.

Computing with resistive-switching (memristive) memory devices has shown much recent progress and offers an attractive route to circumvent the von-Neumann bottleneck, i.e. the separation of processing and memory, which limits the performance of conventional computer architectures. Due to their good scalability and nanosecond switching speeds, carbon-based resistive-switching memory devices could play an important role in this respect. However, devices based on elemental carbon, such as tetrahedral amorphous carbon or ta-C, typically suffer from a low cycling endurance. A material that has proven to be capable of combining the advantages of elemental carbon-based memories with simple fabrication methods and good endurance performance for binary memory applications is oxygenated amorphous carbon, or a-CO x . Here, we examine the memristive capabilities of nanoscale a-CO x devices, in particular their ability to provide the multilevel and accumulation properties that underpin computing type applications. We show the successful operation of nanoscale a-CO x memory cells for both the storage of multilevel states (here 3-level) and for the provision of an arithmetic accumulator. We implement a base-16, or hexadecimal, accumulator and show how such a device can carry out hexadecimal arithmetic and simultaneously store the computed result in the self-same a-CO x cell, all using fast (sub-10 ns) and low-energy (sub-pJ) input pulses.

Modeling the formation of ordered nano-assemblies comprised by dendrimers and linear polyelectrolytes: the role of Coulombic interactions.

Models of mixtures of peripherally charged dendrimers with oppositely charged linear polyelectrolytes in the presence of explicit solvent are studied by means of molecular dynamics simulations. Under the influence of varying strength of electrostatic interactions, these systems appear to form dynamically arrested film-like interconnected structures in the polymer-rich phase. Acting like a pseudo-thermodynamic inverse temperature, the increase of the strength of the Coulombic interactions drive the polymeric constituents of the mixture to a gradual dynamic freezing-in. The timescale of the average density fluctuations of the formed complexes initially increases in the weak electrostatic regime reaching a finite limit as the strength of electrostatic interactions grow. Although the models are overall electrically neutral, during this process the dendrimer/linear complexes develop a polar character with an excess charge mainly close to the periphery of the dendrimers. The morphological characteristics of the resulted pattern are found to depend on the size of the polymer chains on account of the distinct conformational features assumed by the complexed linear polyelectrolytes of different length. In addition, the length of the polymer chain appears to affect the dynamics of the counterions, thus affecting the ionic transport properties of the system. It appears, therefore, that the strength of electrostatic interactions together with the length of the linear polyelectrolytes are parameters to which these systems are particularly responsive, offering thus the possibility for a better control of the resulted structure and the electric properties of these soft-colloidal systems.

Tungsten affects the cortical microtubules of Pisum sativum root cells: experiments on tungsten-molybdenum antagonism.

Tungsten (W) is increasingly shown to be toxic to various organisms, including plants. Apart from inactivation of molybdo-enzymes, other potential targets of W toxicity in plants, especially at the cellular level, have not yet been revealed. In the present study, the effect of W on the cortical microtubule array of interphase root tip cells was investigated, in combination with the possible antagonism of W for the pathway of molybdenum (Mo). Pisum sativum seedlings were treated with W, Mo or a combination of the two, and cortical microtubules were examined using tubulin immunofluorescnce and TEM. Treatments with anti-microtubule (oryzalin, colchicine and taxol) or anti-actomyosin (cytochalasin D, BDM or ML-7) drugs and W were also performed. W-affected cortical microtubules were low in number, short, not uniformly arranged and were resistant to anti-microtubule drugs. Cells pre-treated with oryzalin or colchicine and then treated with W displayed W-affected microtubules, while cortical microtubules pre-stabilized with taxol were resistant to W. Treatment with Mo and anti-actomyosin drugs prevented W from affecting cortical microtubules. Cortical microtubule recovery after W treatment was faster in Mo solution than in water. The results indicate that cortical microtubules of plant cells are indirectly affected by W, most probably through a mechanism depending on the in vivo antagonism of W for the Mo-binding site of Cnx1 protein.

Current concepts: diffuse axonal injury-associated traumatic brain injury.

OBJECTIVES: To review the probable physical, physiologic mechanisms that result in the medical and neuropsychologic complications of diffuse axonal injury (DAI)-associated traumatic brain injury (TBI). DATA SOURCES: Various materials were accessed: MEDLINE, textbooks, scientific presentations, and current ongoing research that has been recently reported. STUDY SELECTION: Included were scientific studies involving TBI, particularly direct injury to the axons and glia of the central nervous system (CNS) in both in vitro and in vivo models. These studies include pathologic findings in humans as well as the medical complications and behavioral outcomes of DAI. Studies that addressed animal models of DAI as well as cellular and/or tissue models of neuronal injury were emphasized. The review also covered work on the physical properties of materials involved in the transmission of energy associated with prolonged acceleration-deceleration injuries. DATA EXTRACTION: Studies were selected with regard to those that addressed the mechanism of TBI associated with DAI and direct injury to the axon within the CNS. The material was generally the emphasis of the article and was extracted by multiple observers. Studies that correlate the above findings with the clinical picture of DAI were included. DATA SYNTHESIS: Concepts were developed by the authors based on the current scientific findings and theories of DAI. The synthesis of these concepts involves expertise in physical science, basic science concepts of cellular injury to the CNS, acute medical indicators of DAI, neuropsychologic indicators of DAI, and rehabilitation outcomes from TBI. CONCLUSIONS: The term DAI is a misnomer. It is not a diffuse injury to the whole brain, rather it is predominant in discrete regions of the brain following high-speed, long-duration deceleration injuries. DAI is a consistent feature of TBI from transportation-related injuries as well as some sports injuries. The pathology of DAI in humans is characterized histologically by widespread damage to the axons of the brainstem, parasagittal white matter of the cerebral cortex, corpus callosum, and the gray-white matter junctions of the cerebral cortex. Computed tomography and magnetic resonance imaging scans taken initially after injury are often normal. The deformation of the brain due to plastic flow of the neural structures associated with DAI explains the micropathologic findings, radiologic findings, and medical and neuropsychologic complications from this type of injury mechanism. There is evidence that the types of cellular injury in TBI (DAI, anoxic, contusion, hemorrhagic, perfusion-reperfusion) should be differentiated, as all may involve different receptors and biochemical pathways that impact recovery. These differing mechanisms of cellular injury involving specific biochemical pathways and locations of injury may, in part, explain the lack of success in drug trials to ameliorate TBI.

Physiological and ultrastructural effects of cadmium on wheat (Triticum aestivum L.) leaves.

The effects of a 7-day exposure of 3-day-old wheat plantsto increasing Cd concentrations are described, with special attention beinggiven to chloroplast ultrastructural changes, chlorophyll fluorescenceresponses, chlorophyll and nutrient concentration changes as well as growthchanges of the whole plant. The plants treated with 1 mM Cd showed symptomsof heavy metal toxicity. The root, shoot-leaf length and the root, shoot-leafbiomass progressively decreased with increasing Cd in nutrient solution andin 1 mM of Cd an almost complete inhibition of growth was found. Shoot-leafCd accumulation increased under Cd-treatments, while a Fe, Mg, Ca, and Kdecline in the above ground parts was observed. The growth reduction and theinhibition of chlorophyll content and photosynthesis observed in the upperplant parts seemed principally due to indirect Cd effects on the content ofessential nutrients. Cadmium treatment was shown to damage the structure ofchloroplasts, as manifested by the disturbed shape and the dilation of thethylakoid membranes. These ultrastructural changes suggest that Cd probablyinduced premature senescence.

Differentiation of abnormal sieve elements in roots of wheat (Triticum aestivum L.) affected by colchicine.

Protophloem sieve elements (PSEs) of seminal roots of wheat (Triticum aestivum L.) treated with 2 MM colchicine solution differentiate into an extraordinary cell type not found in normal roots. Colchicine-treated PSEs stop elongating and increase in diameter considerably. Abnormal ultrastructural features appear gradually with increasing time of exposure to the drug. Microtubules disorganize and disappear first. Cell divisions are blocked and nuclei become polyploid, with multiple lobes and deep imaginations. Profuse paracrystalline material, presumably consisting of tubulin or, more likely, of tubulin-colchicine polymers, accumulates in the cytoplasm of the polyploid cells. Endoplasmic reticulum (ER) cisternae do not aggregate in stacks and mitochondria are not enveloped by ER cisternae, as normal. Golgi bodies appear active and distinct, while unusual vesicles with dense contents occur in the cytoplasm. However, development of plastids and degeneration of nuclei do not seem to be significantly affected by colchicine. The rate of development of affected PSEs is retarded considerably, as judged from ultrastructural features (plastid inclusions, wall thickenings), which appear much later in the vertical files of cells. Due to the retardation and to the overall stoppage of Toot elongation, differentiation of PSEs is caught up by other vascular elements such as protoxylem elements and metaphloem sieve elements. The aberrant PSEs undergo autolysis and reach maturity, but plastids, mitochondria, nuclear remnants and ER configurations characterizing normal PSEs are retained. Transverse walls differentiate into abnormal sieve plates, in which sieve pores are not formed. Lateral walls develop unusual thickenings projecting in the cytoplasm. The observations show that: (a) Differentiation of the affected PSEs proceeds in the absence of divisions; (b) the number and degree of aberrations depend largely on the timing of exposure to the drug, and (c) strands of paracrystalline material and vacuoles are formed in microtubule-free PSEs after long and continuous colchicine treatment.

Microtubules and root protophloem ontogeny in wheat.

Protophloem ontogeny in roots of Triticum aestivum has been investigated ultrastructurally. Each protophloem pole consists of three cells, a protophloem sieve element and two companion cells, all originating from a single precursor cell usually having a pentahedral shape. This protophloem mother cell (PMC) undergoes two successive asymmetrical divisions: the first one gives rise to a smaller cell that will differentiate into a companion cell, and a larger one that divides again asymmetrically yielding another companion cell and a protophloem sieve element. The latter divides once more, but now symmetrically, increasing the number of cells. Both asymmetrical and symmetrical divisions are preceded by preprophase microtubule bands (PMBs), well demarcated by a great number (more than 100 profiles in a single band section) of microtubules (MTs). The plane of a PMB coincides with that of the succeeding cell plate, which fuses with parent walls at sites previously occupied by the PMB. The strict correspondence between PMB and cell plate suggests that a cytokinesis the latter bisects the PMB cortical zone. The possible role of PMB cortical zone in positioning the cell plate and guiding its expanding edges towards predetermined sites is discussed in relation to recent discoveries in other anatomical situations. The plane of PMBs (and hence of divisions) changes from one division to the next, so that the three successive divisions occur in three spatial planes transversely to each other. This change is probably influenced by cell polarity. Prior to each asymmetrical division peri-nuclear MTs were observed besides the MTs of the PMB. They appear before the PMB organization and persist throughout preprophase, but they change their position and orientation in response to the transition from PMB to the spindle organization.

Abundance of microtubules in preprophase bands of some Triticum species.

Root tip procambial cells of Triticum speltoides, T. tauschii, T. turgidum and T. aestivum have been investigated ultrastructurally for the detection of preprophase microtubule bands (PMBs) and to estimate the number of microtubules comprising the bands. The species selected are phylogenetically related but differ in the ploidy level. It was found that all species develop well-defined PMBs prior to mitosis. Estimations of microtubule abundance in the PMBs was carried out in midpreprophase cells, a stage judged by a feature of the nucleus in which electron-transparent canals are formed around the initial condensations of the chromatin material and the nucleoli. Triticum speltoides bears the smaller average number of microtubules per PMB and T. aestivum the greater. The results indicate that the increase follows the upgrade of the number of chromosome sets. It is suggested that the average number of microtubules of PMBs is related to the ploidy level.