A crystal plasticity model for metal matrix composites considering thermal mismatch stress induced dislocations and twins.

Originated at heterogeneous interfaces with distinct coefficient of thermal expansion (CTE), thermal mismatch stress is one of the critical influential factors to mechanical properties of metal matrix composites (MMCs). This stress is normally accommodated plastically by various defects, for example, high-density dislocations and twins in Al near heterogeneous interfaces in SiC/Al composites. Basic knowledge on the influence of defect characteristics is important but difficult to extrapolate from experimental results. However, existed theoretical models more focus on the influence of dislocation density, but less focus on defects variety, volume and distribution. In this paper, we propose a physics-based crystal plasticity model that has the capability of dealing with thermal mismatch stress induced dislocations and twins (denoted as TMDT model). The proposed TMDT model that is implemented in the Visco-Plastic Self-Consistent (VPSC) method considers defect heterogeneous distribution (gradient range), defect type (dislocations vs. twins) and defect volume fraction (twin spacing vs. twin volume). We demonstrate the validity and the capability of the VPSC-TMDT model in SiC/Al composites with thermal mismatch induced dislocations or twins. Furthermore, this model predicts the ultra-high strength of Graphene/Copper composites with high-density nanoscale twins, which is in turn the future aim for such nanocomposites.

A new method to reliably determine elastic strain of various crystal structures from atomic-resolution images.

Elastic strain engineering is an important strategy to design material properties in semiconductor and emerging advanced manufacturing industries. Recently, peak-pair method has drawn great attention compared to geometric phase analysis, owing to its precise determination of atom position at real space. Most current strain characterization methods estimate the local strain by comparing it with the related information from unstrained areas as reference. However, peak-pair method generated large errors in some cases because of the complexity of lower symmetric crystal structures, such as hexagonal structure. In this study, we introduce a new algorithm to overcome this limitation by directly comparing the atom positions with multiple references with different lattice symmetries. Furthermore, this new method is validated through several complicated crystal systems such as hexagonal, orthorhombic, monoclinic, and tetragonal structure, and returns expected values. This finding is essential to reliably determine the localized elastic strain with various crystal structures.

Composite materials with enhanced dimensionless Young's modulus and desired Poisson's ratio.

We have designed a new type of composite materials which not only has a Young's modulus much larger than the Voigt limit, but also is always nearly isotropic. Moreover, its Poisson's ratio can be designed at a desired value, e.g. positive, or negative, or zero. We have also demonstrated that structural hierarchy can help to enhance the stiffness of this type of composite materials. The results obtained in this paper provide a very useful insight into the development of new functional materials and structures.

Identification of a novel allele HLA-DQB1*0622.

A novel human leukocyte antigen-DQ allele, DQB1*0622, was identified in a bone marrow transplantation candidate. DQB1*0622 differs from DQB1*0609 by two nucleotide differences in exon 2, position 122 (A-->T) and 177 (A-->G). The difference at position 122 resulted in an amino acid change (Tyrosine to Phenylalanine). However, the difference at position 177 did not cause any amino acid change.

Characterization of a novel allele HLA-B*3549.

A novel HLA-B allele, B*3549, was identified in a bone marrow transplantation candidate. B*3549 differs from B*3525 by two nucleotides at exon 2, position 142 (T to G) and 165 (G to C). The difference at position 142 resulted in an amino acid difference (serine to alanine). However, the difference at position 165 did not cause any amino acid change. This novel allele was found on a haplotype with A*3101, B*3549, Cw*0401, DRB1*0407, and DQB1*0302.

Optimal protocol for total body irradiation for allogeneic bone marrow transplantation in mice.

We have previously demonstrated, using chimeric resistant MRL/lpr mice, that a fractionated total body irradiation (FTBI) (5 Gy x 2 with a 4 h interval on the day before allogeneic bone marrow transplantation (BMT)) is the best conditioning regimen for the treatment of autoimmune diseases in radiosensitive MRL/lpr mice. In the present study, using various standard strains of mice (not radiosensitive mice), we explore the best protocol for irradiation (doses and intervals) as the conditioning regimen for allogeneic BMT. Recipient mice were exposed to various irradiation regimens: a single total body irradiation (TBI) of 9.5 or 12 Gy and FTBI of (5+5) Gy to (7+7) Gy with a 1 to 24 h interval. The method generally utilized for humans ((2+2) Gy with a 4 h interval for 3 days (total 12 Gy)) was also used. One day after the last irradiation, donor BMCs from BALB/c, C3H, or C57BL/6 (B6) mice were transplanted into C3H or B6 mice. The irradiation protocol of (2+2) Gy for 3 days was found to be insufficient to enable the complete removal of recipient immunocompetent cells, since donor-reactive T cells were observed in the recipient spleens and many recipient-type NK and CD4(+) cells were also detected in the recipient hematolymphoid tissues. In all the combinations, the highest survival rate was achieved in the recipients irradiated with (6+6) or (6.5+6.5) Gy with a 4 h interval. In the surviving mice, the hematolymphoid tissues had been fully reconstituted with donor cells.

Successful allogeneic bone marrow transplantation (BMT) by injection of bone marrow cells via portal vein: stromal cells as BMT-facilitating cells.

We examined the importance of the coadministration of bone marrow (BM) stromal cells with BM cells via the portal vein. A significant increase in the number of day-14 colony-forming unit-spleen (CFU-S) was observed in the recipient mice injected with hemopoietic stem cells (HSCs) along with donor BM stromal cells obtained after three to four weeks of culture. Histological examination revealed that hematopoietic colonies composed of both donor hemopoietic cells and stromal cells coexist in the liver of these mice. However, when donor HSCs plus BM stromal cells were administered i.v., neither the stimulatory effects on CFU-S formation nor the hemopoietic colonies in the recipient liver were observed. These findings suggest that the interaction of HSCs with stromal cells in the liver is the first crucial step for successful engraftment of allogeneic HSCs. It is likely that donor stromal cells and HSCs trapped in the liver migrate into the recipient BM and spleen, where they form CFU-BM and CFU-S, respectively.

Stimulatory effects of hepatocyte growth factor on hemopoiesis of SCF/c-kit system-deficient mice.

In this study, we report that W/W mutant mice, which have severe macrocytic anemia caused by a deficit of extracellular domain in c-kit molecules and therefore die perinatally, have hemopoietic stem cells (HSCs) and mature hematolymphoid cells in the bone marrow (BM), thymus, and spleen, although there are significant decreases in cell counts. Moreover, the mitogen-induced proliferative response, mixed lymphocyte reaction, and anti-SRBC plaque formation of spleen cells in W/W mice are similar to those in age-matched +/? littermates and normal mice, suggesting that the SCF/c-kit system is necessary for cell proliferation but not essential for HSCs to differentiate. We next examine the stimulatory effects of hepatocyte growth factor (HGF) on hemopoiesis in W/W mice. HGF has a stimulatory effect on the colony formation (CFU-C) of W/W BM cells when cultured using either a methylcellulose assay (containing cytokines) or a long-term culture (LTC) assay. A similar stimulatory effect of HGF is observed in the other W or SI locus-mutant mice (W/Wv and SI/SId mice), which show less severe anemia than W/W. The numbers of nonadherent cells and cobblestone colonies significantly increase in the LTCs using their BM cells. In addition, in vivo administration of HGF shows a transient increase in the CFU-C counts in BM cells and peripheral blood cells. RBC, WBC, and platelet counts also increased. These results suggest that the SCF/c-kit system is not essential to hemopoiesis but that a compensatory system such as the HGF/c-met system functions in the SCF/c-kit system-deficient mice.

Neuronal responses to turtle head rotation in vitro.

Extracellular recordings were made during vestibular stimulation from an in vitro turtle brain stem in which the temporal bones remained attached. Under visual control, microelectrodes were slowly advanced into the vestibular nucleus (VN) while we rotated the brain and searched for a single isolated unit whose spike activity was modulated by the lateral semicircular canals. In some experiments, responses were shown to be due to stimulation of the lateral canals, either by positioning the brains in forward or backward pitch during horizontal rotation or by plugging the vertical canals with wax. VN neurons usually had low spontaneous activity and rectified sinusoidal responses to sinusoidal stimulation. Spike response histograms were averaged from many stimulus cycles and were then fit to a sine function. The fitted phase and amplitude parameters were plotted relative to stimulus frequency and amplitude. The sample of VN cells were quite heterogeneous. Using stimuli at 1 Hz, however, each cell's response phase was weakly correlated with the slope of the plots of response amplitude versus frequency so that a cell could be categorized as sensitive to velocity or acceleration and as sensitive to ipsiversive or contraversive rotation, depending on whether its phase was near -180 degrees, -90 degrees, 0 degrees, or 90 degrees, and whether the gain exceeded 0.4 spikes/s per degrees/s. The properties of these VN cells suggest that there is substantial complexity in the vestibular responses at this first site of central vestibular processing. These data are compared to that of other species where such vestibular signals play an important role in oculomotor and spinal reflexes.

Visual responses and connectivity in the turtle pretectum.

1. Using an isolated turtle brain preparation, we made extracellular spike recordings in the dorsal midbrain during visual stimulation. Single units were isolated by their response to a slow-moving full-field visual pattern imaged on the contralateral retina. This stimulus elicits responses from the basal optic nucleus (BON) and the cerebellar cortex using a similar preparation. Direction and speed tuning were then analyzed, as well as the size and position of the receptive field. 2. In one brain stem region, anterior to the optic tectum and deep to the dorsal surface, all of the visually responsive neurons were direction sensitive (DS) to contralateral retinal stimulation. The location and properties of these cells indicate that they are in the mesencephalic lentiform nucleus (nLM). Anterograde transport of intravitreally injected horseradish peroxidase revealed that this pretectal nucleus receives direct input from the contralateral eye. 3. All but 2 of the 48 cells of the nLM were strongly DS. The most effective stimulus was a slowly moving complex visual pattern that drifted nasally in the contralateral visual field. Brief flashes of spots, patterns, or diffuse light were much less effective. Receptive fields were large and usually (9 of 13 cells) centered in the superior visual field near the horizon and nasal to the blind spot. 4. The visual responses of nLM cells were compared to those of cells in the superficial layers of the optic tectum. In contrast to nLM, the responses of tectal cells were heterogeneous and frequently not DS. Neither tectum or nLM cells had much spontaneous spike activity during darkness or stationary patterns. On the other hand, visual responses of nLM cells were very similar to those of the BON, where neurons also had low spontaneous activity, preferred slow-moving patterns, and were DS. However, nLM and BON exhibit different distributions of preferred directions. Most nLM cells preferred temporal-to-nasal motion, whereas BON cells preferred almost any direction, although few preferred the nasal direction. nLM cell responses were not affected by removal of the ventral brain stem including the BON. 5. The visual properties of nLM cells recorded in vitro were very similar to those that were recorded in intact turtles.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of prolonged darkness on light responsiveness and spectral sensitivity of cone horizontal cells in carp retina in vivo.

Light responses of cone- and rod-driven horizontal cells were recorded intracellularly from opened eyes of intact, immobilized carp and the effects of prolonged darkness on these cells were examined. When the retina was left in the dark, responses of cone horizontal cells to a moderate test flash presented once every 10 min gradually and steadily decreased in size (dark suppression effect). Following a repetitive presentation of the test flash, the light responsiveness of the cells was enhanced. In contrast, the rod horizontal cells did not show the dark suppression effect and the light responsiveness of these cells increased in the dark. Effects of prolonged darkness on the spectral sensitivity of L-type and R/G-type cone horizontal cells were also studied. The spectral sensitivity of L-type cells, determined just after background illumination was extinguished, agreed reasonably well with that of red-sensitive (R-) cones, but it matched better that of green-sensitive (G-) cones when determined in prolonged darkness. We further show that depolarizing responses of R/G-type cone horizontal cells to long-wavelength flashes, driven by R-cones, were abolished after prolonged darkness. Taken together, these results suggest that input from R-cones converging onto the cone horizontal cells is more significantly suppressed after prolonged dark adaptation than input from G-cones. Due to the suppression of the R-cone input, enhancement of responses of the L-type cone horizontal cells to long-wavelength stimulation in the presence of a green background light was no longer seen after prolonged darkness.(ABSTRACT TRUNCATED AT 250 WORDS)

Visual-response properties of units in the turtle cerebellar granular layer in vitro.

1. Single units were recorded extracellularly in the turtle's cerebellar cortex from an isolated brain preparation during visual stimulation. Only a small fraction of the isolated units responded to visual stimuli. For these visually responsive units, the most effective visual stimulus was a moving check pattern that covered the entire surface of the retinal eyecup. The visually responsive units had little or no spontaneous spike activity, nor were they driven by flashes of diffuse light or stationary patterns. 2. All the visually responsive units were direction sensitive and were driven exclusively by the contralateral eye. This direction tuning was well fit by a limaçon model (mean correlation coefficient, 0.89). The distribution of the entire sample indicates a slight preponderance of upward preferred directions. 3. The direction tuning of these cerebellar units was independent of stimulus contrast or the pattern's configuration (such as checkerboards or random check or dot patterns). In the preferred direction, a unit's spike frequency increased monotonically as a function of stimulus velocity until approximately 10 degrees/s, but remained direction sensitive (relative to the opposite direction) at speeds as fast as 100 degrees/s. 4. In some experiments the ventrocaudal brain stem was transected in the frontal plane just caudal to the cerebellar peduncles. Although this lesion presumably removes climbing fiber input from the inferior olivary nuclei, the visual-response properties in the cerebellar cortex were unaffected. 5. The response properties of these units indicate that they encode retinal slip information in the cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiological evidence for a bisynaptic retinocerebellar pathway.

1. Electrical microstimulation was applied to an in vitro turtle brain preparation while recording extracellular activity from the cerebellar cortex. A visual input to the cerebellum was investigated by measuring spike responses evoked by stimulation of drifting visual patterns imaged onto the contralateral retinal eyecup. A vestibular input was assessed by extracellular field potentials following brief current pulses through monopolar suction electrodes holding the eighth cranial nerve (nVIII). 2. The cortical topography of visual and vestibular inputs was first examined. Visual units and vestibular fields show considerable topographic overlap in the rostrolateral quadrant of the cerebellum. In addition, granule layer units were isolated that responded to current stimulation of nVIII (60-150 microA monopolar). In some cases, spikes occurred at short and fixed latency after each current pulse for stimulus frequencies of 100 Hz. The responses of these units suggest a direct path between the stimulating and recording electrodes without intervening synapses. Alternatively, extracellular units were also encountered that responded with longer, more variable latencies but only for low stimulation frequencies (< or = 20 Hz). Of the units that responded to nVIII stimulation, three units also responded to visual stimuli, yet those units all failed to follow high-frequency stimulation of nVIII. This cortical area may then be a site for convergence of visual and vestibular signals on postsynaptic cells. 3. The cellular identity of the visual units in the granule layer and the visual pathways leading there were next investigated.(ABSTRACT TRUNCATED AT 250 WORDS)

Dark adaptation of horizontal cells in the teleost fish retina.

The dark adaptation behaviors of rod-driven and cone-driven horizontal cells were examined by analyzing their light responses recorded intracellularly in the intact, immobilized carp, and compared with that of the electroretinographic b-wave recorded simultaneously. Like the b-wave, the light responsiveness of rod horizontal cells increased gradually with time in the dark and attained a steady level at 60 min. On the other hand, cone horizontal cells initially increased in light responsiveness in the first 10 min, but thereafter decreased steadily so that the response amplitudes of these cells to bright light flashes were only 3-5 mV. The results suggest that cone horizontal cells are strongly suppressed in prolonged darkness.

Electroretinographic b-wave merely reflects the activity of the rod system in the dark-adapted carp retina.

The electroretinogram and responses of intermediate horizontal cells (IHCs) were recorded simultaneously and studied under dark-adapted conditions in intact crucian carp, with the hope of providing an answer to the controversy whether the scotopic b-wave contains contribution from both rods and cones. Our observations indicate that the dark-adapted b-wave follows the univariance principle perfectly in that: (1) b-wave forms and shapes of response amplitude vs light intensity relationships did not depend on the stimulus wavelength; (2) response amplitudes to flashes of mixed wavelengths agreed exactly with the values predicted in accordance with the univariance principle. Furthermore, the spectral sensitivity of the b-wave matched well that of the IHC, receiving input exclusively from rods. In consequence, we conclude that the dark-adapted b-wave merely reflects the activity of the rod system, and the discrepancy between the spectral sensitivities of the b-wave and IHC response compared with the absorption spectrum of the rod photopigment may be due to the self-screening effect caused by the high optical density of porphyropsin in situ.