In this review we summarized literature data on the mechanisms of human placental drug transport studied in the isolated perfused placental cotyledon, placental membrane vesicles or trophoblastic cell cultures. Overall human placental drug transport rarely exceeds the transfer of flow-dependent and membrane-limited marker compounds. Interestingly, relatively often placental drug transfer appeared to be much smaller, indicating impaired trans-placental transport, depending on the physico-chemical characteristics of the drug or placental factors such as tissue binding or metabolism. Although in perfusion studies overall human placental drug transport occurs by simple diffusion, at the membrane level several drug transport systems have been found, mainly for drugs structurally related to endogenous compounds.
The effects of single and double pulse resistance spot welding on the microstructures of an advanced high strength automotive steel are presented in this work. The double pulse welding schemes partially remelt the primary weld nugget and anneal the area at the fusion boundary of the nugget. The effects of the annealing treatment on the segregation and the microstructure have been studied by electron probe microanalysis (EPMA) in combination with electron backscatter diffraction (EBSD). Results show that phosphorus has been redistributed at the primary weld nugget edge of the double pulse welds, while the mean block width and ellipticity of the prior austenite grains were smaller in welds subjected to double pulsing compared with single pulse weld. A favourable failure mode was obtained for the double pulse welds although behaviour did not correlate with the measured grain size.
The partitioning behaviour of carbon, phosphorous and boron during the solidification of a resistance spot weld pool was studied using experimental simulations and a phase field model. Steels with varying carbon, phosphorous and boron contents were designed and subjected to a range of resistant spot welding thermal cycles. Mechanical properties were evaluated by hardness and cross tension tests and correlated with the weld microstructure. Phase field modelling results and experimental predictions show that the phosphorus concentration in the last area in the weld pool to solidify can reach about 0.38 wt% for a steel with a bulk concentration of 0.08 wt%. Elemental analysis indicates that in the absence of boron, the grain boundaries of columnar grains in the weld pool are decorated with phosphorous. As a result, a complete interface failure occurs during cross tension testing. With the addition of boron, apart from an increase in weld strength and plug diameter, the failure mode switches to a complete plug mode, resulting from the phosphorous depletion at the grain/inter-phase boundaries.
It has been shown that the introduction of a trailing heat sink following a welding heat
source at a short distance can reduce or even eliminate the buckling deformation of welded thin
plate structures. To increase the insight into the stress development during welding with a trailing
heat sink, residual stress measurements were carried out by means of laboratory X-ray and
synchrotron X-ray diffraction techniques. Both the laboratory X-ray and the synchrotron X-ray
experiments show a difference between the residual stress distributions for the cooled and the
uncooled samples. The longitudinal stresses show a reduction in the compressive stress level
towards the plate edges, to a level below the critical buckling stress. The transverse stresses are
drastically reduced when the active cooling is applied.
It has been proven by several research groups that welding with a trailing heat sink can reduce the buckling distortion of welded thin sheet. However, the transient mechanical behaviour during welding with a trailing heat sink was never fully understood. This paper presents a conceptual model that is capable of explaining both the longitudinal plastic strain and the longitudinal stress development during welding with an active cooling source. The model is based on an existing three bar model for stress and strain development during conventional welding. Comparison with three-dimensional finite element calculations shows that the model gives qualitatively realistic results. The model provides insight into the relevant mechanisms involved in stress development during welding with a trailing heat sink, which is essential for determining the optimum cooling source parameters.
Abstract-The potency and nature of the inhibitory effect of various cationic drugs on the transport of choline across the placental syncytial microvillus membrane was investigated. Tetraethylammonium, a model substrate for organic cation transport, was a poor inhibitor. Enlarging the degree of alkylation of the quaternary ammonium increased the inhibitory effect, in proportion with increasing lipophilicity. Log concentration vs % control uptake curves showed marked differences in inhibitory potency for the different cationic drugs. Hemicholinium-3 inhibited mediated choline uptake in the micromolar range, whereas atropine and mepiperphenidol were less potent. The H2-receptor antagonists cimetidine, ranitidine, and famotidine inhibited choline uptake in the millimolar ranges. Dixon analysis revealed a competitive nature of inhibition for hemicholinium-3 and atropine (.K, = 40 pM and 1.2 mM, respectively). Cimetidine interacted noncompetitively {Kt = 3,4 mM). Since relatively high concentrations were needed to reach half maximal inhibition, impairment of fetal choline supply due to maternal drug use during pregnancy is not to be expected.
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