RESUMEN
Resistive random access memories (RRAMs) can be programmed to discrete resistive levels on demand via voltage pulses with appropriate amplitude and widths. This tuneability enables the design of various emerging concepts, to name a few: neuromorphic applications and reconfigurable circuits. Despite the wide interest in RRAM technologies there is still room for improvement and the key lies with understanding better the underpinning mechanism responsible for resistive switching. This work presents a methodology that aids such efforts, by revealing the nature of the resistive switching through assessing the transport properties in the non-switching operation regimes, before and after switching occurs. Variation in the transport properties obtained by analysing the current-voltage characteristics at distinct temperatures provides experimental evidence for understanding the nature of the responsible mechanism. This study is performed on prototyped device stacks that possess common Au bottom electrodes, identical TiO2 active layers while employing three different top electrodes, Au, Ni and Pt. Our results support in all cases an interface controlled transport due to Schottky emission and suggest that the acquired gradual switching originates by the bias induced modification of the interfacial barrier. Throughout this study, the top electrode material was found to play a role in determining the electroforming requirements and thus indirectly the devices' memristive characteristics whilst both the top and bottom metal/oxide interfaces are found to be modified as result of this process.
RESUMEN
The purpose of this paper was to evaluate the outcome of a double embryo transfer during the same cycle for patients who had had three or more implantation failures in IVF-ET or ICSI-ET programs after the transfer of good quality embryos in all attempts. Forty-five women who had had previous unsuccessful attempts in IVF-ET or ICSI-ET programs after transfer of good quality embryos (Group A) were included in the study. Group A was divided into two subgroups, Group A1 consisted of 34 patients who underwent embryo transfer on day 2 and day 4 after pick-up and Group A2 consisted of ten patients who underwent embryo transfer on day 2 and day 5 after pick-up. Forty-two other women with a similar unsuccessful history in IVF-ET (Group B) were studied as controls. The patients in this group had a day 4 or 5 only transfer without having an additional day 2 transfer. The outcome of the procedure was compared in the two groups. Double embryo transfer had beneficial effects on patients with good embryos but with previous failure attempts. These patients had a 38.2% clinical pregnancy rate and a 50% total pregnancy rate if the additional embryo transfer was done on day 4 and a 60% clinical and 60% total pregnancy rate if the additional embryo transfer was done on day 5. Our data showed that excellent pregnancy rates can be obtained with a commercially available medium and double embryo transfers on days 2 and 4 or 5 after pick-up for patients with good quality embryos that have had previous failure attempts in an IVF-ET program. Due to the fact that endometrial maturation varies considerably in each patient, an adequate endometrial maturation and improved uterine receptivity seem to be the reason for improved pregnancy rates with double embryo transfers. It was also shown that morullae have high viability and high potential for implantation and pregnancy.
