Oxygen vacancy dynamics in Pt/TiOx/TaOy/Pt memristors: exchange with the environment and internal electromigration

Memristors are expected to be one of the key building blocks for the development of new bio-inspired nanoelectronics. Memristive effects in transition metal oxides are usually linked to the electromigration at the nanoscale of charged oxygen vacancies (OV). In this paper we address, for Pt/TiO_x/TaO_y/Pt devices, the exchange of OV between the device and the environment upon the application of electrical stress. From a combination of experiments and theoretical simulations we determine that both TiO_xand TaO_ylayers oxidize, via environmental oxygen uptake, during the electroforming process. Once the memristive effect is stabilized (post-forming behavior) our results suggest that oxygen exchange with the environment is suppressed and the OV dynamics that drives the memristive behavior is restricted to an internal electromigration between TiO_xand TaO_ylayers. Our work provides relevant information for the design of reliable binary oxide memristive devices.

Selective activation of memristive interfaces in TaO x -based devices by controlling oxygen vacancies dynamics at the nanoscale

The development of novel devices for neuromorphic computing and non-traditional logic operations largely relies on the fabrication of well controlled memristive systems with functionalities beyond standard bipolar behavior and digital ON-OFF states. In the present work we demonstrate for Ta₂O₅-based devices that it is possible to selectively activate/deactivate two series memristive interfaces in order to obtain clockwise or counter-clockwise multilevel squared remanent resistance loops, just by controlling both the electroforming process and the (a)symmetry of the applied stimuli, and independently of the nature of the used metallic electrodes. Based on our thorough characterization, analysis and modeling, we show that the physical origin of this electrical behavior relies on controlled oxygen vacancies electromigration between three different nanoscopic zones of the active Ta₂O_5-x layer: a central one and two quasi-symmetric interfaces with reduced TaO_2-h(y) layers. Our devices fabrication process is rather simple as it implies the room temperature deposition of only one CMOS compatible oxide-Ta-oxide-and one metal, suggesting that it might be possible to take advantage of these properties at low cost and with easy scability. The tunable opposite remanent resistance loops circulations with multiple-analogic-intermediate stable states allows mimicking the adaptable synaptic weight of biological systems and presents potential for non-standard logic devices.

Effectiveness of a PMTCT programme in rural Western Kenya

We assess the coverage of a Prevention of Mother-to-child Transmission (PMTCT) programme in Busia (Kenya) from 1 January 2006 to 31 December 2008 and estimate the risk of transmission of HIV. We also estimate the odds of HIV transmission according to pharmacological intervention received. Programme coverage was estimated as the proportion of mother-baby pairs receiving any antiretroviral (ARV) regimen among all HIV-positive women attending services. We estimated the mother-to-child transmission (MTCT) rate and their 95% confidence interval (95%CI) using the direct method of calculation (intermediate estimate). A case-control study was established among all children born to HIV-positive mothers with information on outcome (HIV status of the babies) and exposure (data on pharmacological intervention). Cases were all HIV-positive children and controls were the HIV-negative ones. Exposure was defined as: (1) complete protocol: ARV prescribed according World Health Organisation recommendations; (2) partial protocol: does not meet criteria for complete protocol; and (3) no intervention: ARVs were not prescribed to both mother and child. Babies were tested using DNA Polymerase Chain Reaction at six weeks of life and six weeks after breastfeeding ceased. In the study period, 22,566 women accepted testing, 1668 were HIV positive (7.4%; 95%CI 7.05-7.73); 1036 (62%) registered in the programme and 632 were lost. Programme coverage was 40.4% (95%CI 37.9-42.7). Out of the 767 newborns, 28 (3.6%) died, 148 (19.3%) defaulted, 282 (36.7%) were administratively censored and 309 (40.2%) babies completed the follow-up as per protocol; 49 were HIV positive and MTCT risk was 15.86% (95%CI 11.6-20.1). The odds of having an HIV-positive baby was 4.6 times higher among pairs receiving a partial protocol compared to those receiving a complete protocol and 43 times higher among those receiving no intervention. Our data show a good level of enrolment but low global coverage rate. It demonstrates that ARV regimens can be implemented in low resource rural settings with marked decreases of MTCT. Increasing the coverage of PMTCT programmes remains the main challenge.

Antiretroviral therapy for HIV prevention: many concerns and challenges, but are there ways forward in sub-Saharan Africa?

Scientists from the WHO have presented a theoretical mathematical model of the potential impact of universal voluntary HIV testing and counselling followed by immediate antiretroviral therapy (ART). The results of the model suggests that, in a generalised epidemic as severe as that in sub-Saharan Africa (SSA), HIV incidence may be reduced by 95% in 10 years and that this approach may be cost effective in the medium term. This offers a 'ray of hope' to those who have thus far only dreamed of curbing the HIV/AIDS epidemic in SSA, as until now the glaring truth has been pessimistic. When it comes to ART, approximately 7 of 10 people who clinically need ART still do not receive it. From an epidemic point of view, for every person placed on ART an estimated four to six others acquire HIV. The likelihood of achieving the targets of the Millennium Development Goals for 2015 and universal ART access by 2010 are thus extremely low. A new window of opportunity may have now opened, but there are many unanswered feasibility and acceptability issues. In this paper, we highlight four key operational challenges linked to acceptability and feasibility and discuss possible ways forward to address them.

Kinetic fluorimetric determination of the antineoplastic methotrexate (MTX) in human serum

A kinetic study of the oxidation of methotrexate (MTX) in acidic medium and in the presence of potassium permanganate has been made on the basis of the fluorescence-time curves. A kinetic method for the determination of MTX was developed with a range of application between 0.22 and 3.30 microM. The proposed kinetic method permits us to determine MTX in human serum and to avoid the natural fluorescence of the serum. A detection limit of 0.18 microM was calculated in the presence of ascorbic acid as activator. Only 100 s per sample is necessary for the analysis. The interference of pteridin derivatives and the rescue agent folinic acid (leucovorin) was tested.

Analysis of multicomponent formulations containing phenylpropanolamine hydrochloride, caffeine and diazepam by using LC

A reverse phase high performance liquid chromatography assay was carried out for the simultaneous determination of three active principles present in tablets of different origin and wide commercial use in the Province of Córdoba (Argentina). Prescriptions, commercially available as appetite suppressants, very often include the active principles Phenylpropanolamine Hydrochloride (I), Caffeine (II) and Diazepam (III). Simultaneous determination of these three drugs: anorexic, central nervous stimulant and tranquilizer, respectively, in pharmaceutical dosage forms has not been reported. In this study these active principles are quantified. The only sample preparation necessary for the analysis was their dilution with acetonitrile. The resulting solution was filtered and analyzed on a column packed with Supelcosil LC-18 (5 microm) with acetonitrile:water (30:70 v/v) as initial mobile phase (0.4 ml min(-1)) and the detection was performed at 254 nm. Then a linear gradient up to 100% acetonitrile in 18 min (3.0 ml min(-1)) was applied. The procedure was simple and suitable for quality control. The calibration function was established in the ranges of 0.072-0.168 mg ml(-1) for I, 0.036-0.084 mg ml(-1) for II and 0.06-0.196 mg ml(-1) for III. The detection limits of these compounds were 12.8, 4.1 and 11.0 microg ml(-1), respectively with linear response. No chromatographic interference from the tablet excipients was found. The method described in this paper was validated following the analytical performance parameters required by the USP XXIV, and was successfully applied to the commercial tablets.