22 research outputs found
Memristor-based Synaptic Networks and Logical Operations Using In-Situ Computing
We present new computational building blocks based on memristive devices.
These blocks, can be used to implement either supervised or unsupervised
learning modules. This is achieved using a crosspoint architecture which is an
efficient array implementation for nanoscale two-terminal memristive devices.
Based on these blocks and an experimentally verified SPICE macromodel for the
memristor, we demonstrate that firstly, the Spike-Timing-Dependent Plasticity
(STDP) can be implemented by a single memristor device and secondly, a
memristor-based competitive Hebbian learning through STDP using a synaptic network. This is achieved by adjusting the memristor's
conductance values (weights) as a function of the timing difference between
presynaptic and postsynaptic spikes. These implementations have a number of
shortcomings due to the memristor's characteristics such as memory decay,
highly nonlinear switching behaviour as a function of applied voltage/current,
and functional uniformity. These shortcomings can be addressed by utilising a
mixed gates that can be used in conjunction with the analogue behaviour for
biomimetic computation. The digital implementations in this paper use in-situ
computational capability of the memristor.Comment: 18 pages, 7 figures, 2 table
Memristor MOS Content Addressable Memory (MCAM): Hybrid Architecture for Future High Performance Search Engines
Large-capacity Content Addressable Memory (CAM) is a key element in a wide
variety of applications. The inevitable complexities of scaling MOS transistors
introduce a major challenge in the realization of such systems. Convergence of
disparate technologies, which are compatible with CMOS processing, may allow
extension of Moore's Law for a few more years. This paper provides a new
approach towards the design and modeling of Memristor (Memory resistor) based
Content Addressable Memory (MCAM) using a combination of memristor MOS devices
to form the core of a memory/compare logic cell that forms the building block
of the CAM architecture. The non-volatile characteristic and the nanoscale
geometry together with compatibility of the memristor with CMOS processing
technology increases the packing density, provides for new approaches towards
power management through disabling CAM blocks without loss of stored data,
reduces power dissipation, and has scope for speed improvement as the
technology matures.Comment: 10 pages, 11 figure
An Analytical Approach for Memristive Nanoarchitectures
As conventional memory technologies are challenged by their technological
physical limits, emerging technologies driven by novel materials are becoming
an attractive option for future memory architectures. Among these technologies,
Resistive Memories (ReRAM) created new possibilities because of their
nano-features and unique - characteristics. One particular problem that
limits the maximum array size is interference from neighboring cells due to
sneak-path currents. A possible device level solution to address this issue is
to implement a memory array using complementary resistive switches (CRS).
Although the storage mechanism for a CRS is fundamentally different from what
has been reported for memristors (low and high resistances), a CRS is simply
formed by two series bipolar memristors with opposing polarities. In this paper
our intention is to introduce modeling principles that have been previously
verified through measurements and extend the simulation principles based on
memristors to CRS devices and hence provide an analytical approach to the
design of a CRS array. The presented approach creates the necessary design
methodology platform that will assist designers in implementation of CRS
devices in future systems.Comment: 12 pages, 10 figures, 4 table
Selectively enhanced expression of prophenoloxidase activating enzyme 1 (PPAE1) at a bacteria clearance site in the white shrimp, Litopenaeus vannamei
<p>Abstract</p> <p>Background</p> <p>The prophenoloxidase-activating (PO activating) system plays an important role in the crustacean innate immunity, particularly in wound healing and pathogen defense. A key member of this system is prophenoloxidase-activating enzyme (PPAE), which is the direct activator of prophenoloxidase (proPO). Despite their importance in crustacean PO activating system, the studies on them remain limited.</p> <p>Results</p> <p>Here we report on a PPAE of white shrimp, <it>Litopenaeus vannamei </it>(lvPPAE1), which showed 94% similarity to PPAE1 of <it>Penaeus monodon</it>. We found that lvPPAE1 in fluid hemocytes was down regulated after challenge by <it>Vibrio harveyi </it>but was enhanced when shrimps were exposed to a bacteria-rich environment for long-term. In <it>vivo </it>gene silence of lvPPAE1 by RNAi can significantly reduce the phenoloxidase activity (PO) and increase the susceptibility of shrimps to <it>V. harveyi</it>. Although lvPPAE1 was down-regulated in fluid hemocytes by <it>Vibrio </it>challenge, its expression increased significantly in gill after bacteria injection, which is the primary bacteria-clearance tissue.</p> <p>Conclusion</p> <p>Suppressed expression in fluid hemocytes and enhanced expression in gill indicates selectively enhanced expression at the bacterial clearance site. This is a novel feature for PPAE expression. The results will contribute to our understanding of the PO activating system in crustaceans.</p
Analysis of the dihydrofolate reductase-thymidylate synthase gene sequences in Plasmodium vivax field isolates that failed chloroquine treatment
<p>Abstract</p> <p>Background</p> <p>To use pyrimethamine as an alternative anti-malarial drug for chloroquine-resistant malaria parasites, it was necessary to determine the enzyme's genetic variation in dihydrofolate reductase-thymidylate syntase (DHFR-TS) among Korean strains.</p> <p>Methods</p> <p>Genetic variation of <it>dhfr-ts </it>genes of <it>Plasmodium vivax </it>clinical isolates from patients who did not respond to drug treatment (<it>n </it>= 11) in Korea were analysed. The genes were amplified using the polymerase chain reaction (PCR) with genomic DNA as a template.</p> <p>Results</p> <p>Sequence analysis showed that the open reading frame (ORF) of 1,857 nucleotides encoded a deduced protein of 618 amino acids (aa). Alignment with the DHFR-TS genes of other malaria parasites showed that a 231-residue DHFR domain and a 286-residue TS domain were seperated by a 101-aa linker region. This ORF shows 98.7% homology with the <it>P. vivax </it>Sal I strain (XM001615032) in the DHFR domain, 100% in the linker region and 99% in the TS domain. Comparison of the DHFR sequences from pyrimethamine-sensitive and pyrimethamine-resistant <it>P. vivax </it>isolates revealed that nine isolates belonged to the sensitive strain, whereas two isolates met the criteria for resistance. In these two isolates, the amino acid at position 117 is changed from serine to asparagine (S117N). Additionally, all Korean isolates showed a deletion mutant of THGGDN in short tandem repetitive sequences between 88 and 106 amino acid.</p> <p>Conclusions</p> <p>These results suggest that sequence variations in the DHFR-TS represent the prevalence of antifolate-resistant <it>P. vivax </it>in Korea. Two of 11 isolates have the Ser to Asn mutation in codon 117, which is the major determinant of pyrimethamine resistance in <it>P. vivax</it>. Therefore, the introduction of pyrimethamine for the treatment of chloroquine-resistant vivax malaria as alternative drug in Korea should be seriously considered.</p