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Biomedical subjects

N Petkov

Publications and source records attributed to N Petkov.

8 recordsLinked to original sources
PubMed2005

Interlayer stacking disorder in zeolite beta family: a Raman spectroscopic study.

Raman spectroscopy has been applied to series of BEA- and BEC-type samples differing from each other in size, Si/Al ratio and polymorph percentage in order to analyse the effect of interlayer stacking arrangements on the vibrational modes of zeolite beta. The Raman peaks observed in the spectral range 250-550 cm-1 were assigned to the rings building the basic (001)-layer and to those linking the adjacent layers in zeolite beta. It is shown that the intensity ratio rho between the Raman signals at 314 and 343 cm-1 is most sensitive to the degree of periodicity faults along the c direction. A larger value of rho indicates a larger size of polymorph stacking sequence, i.e. improvement of the stacking faultlessness. The interlayer stacking disorder and the degree of connectivity point defects are higher in nanosized zeolite beta than in micron-sized crystals. The Al content influences the concentration of defected SiOH groups, but is less important for the interlayer stacking sequences in colloidal zeolite beta.

Microscopy, Electron, Transmission↗
PubMed2005

Ordered micro/mesoporous composite prepared as thin films.

A new synthesis method for preparation of thin films and powders consisting of zeolite beta nanocrystals embedded in ordered mesoporous silica matrix is described. The final structures possessing bimodal porosity, i.e., high degree of mesophase order and spatially defined microporous zeolite nanocrystals are obtained via simultaneous solvent evaporation of preformed silica/surfactant/ethanol/nanosized zeolite beta assemblies. The films were characterized with grazing-incident diffraction (GID), nitrogen sorption based on gravimetric measurements with quartz crystal microbalance (QCM) devices, and transmission electron microscopy (TEM). It is shown that the incorporation of beta nanocrystals in the mesoporous silica matrix and the mesophase order itself can be controlled through the variation of the fractional amounts of the zeolite nanoparticles and silica/surfactant solutions. The HR-TEM measurements showed that the nanosized Beta microporous crystals are separated and at the same time connected through an ordered mesostructured matrix.

Journal Article↗
PubMed2004

Comparative study of biological and technological characters in three generations of silkworm Bombyx mori L. ameiotic, parthenogenetically cloned lines.

PURPOSE: Detect any deviation in biologic and technologic characters of eight ameiotic-parthenogenetically cloned lines of Bombyx mori L. from different origins from a normal sexually reproduced control line in three generations. METHODS: Comparative study of the three generations was conducted in SES, Vratza, unit of the National Center for Agrarian Sciences of Bulgaria after fixing all environmental rearing conditions. RESULTS: The ameiotic-parthen-clones displayed good parthenogenetic development, although total hatchability was significantly less than the sexually reproducing control populations. Survival rates between clones and control were not significantly different. All clones displayed significantly longer larval periods. Slight decline in second generation, and a steeper one in the third generation were observed for all eight cloned lines in cocoon weight, shell weight, and shell ratio and these differences were statistically significant. Cocoon yield was significantly lower than the control throughout the three generations. CONCLUSIONS: Our parthen-cloning method has a high rate of success in comparison to other cloning methods, the cloned progeny populations although were weaker technologically (cocoon weight, shell weight, and shell ratio), the biological characters (parthenogenetic development and survival rate) were not compromised. Further study is needed to determine the thermal needs of the cloned embryos and metabolic rate of all stages.

Animals↗
PubMed2000

Computational model of dot-pattern selective cells.

A computational model of a dot-pattern selective neuron is proposed. This type of neuron is found in the inferotemporal cortex of monkeys. It responds strongly to groups of dots and spots of light intensity variation but very weakly or not at all to single dots and spots that are not part of a pattern. This non-linear behaviour is quite different from the spatial frequency filtering behaviour exhibited by other neurons that react to spot-shaped stimuli, such as neurons with centre-surround receptive field profiles found in the lateral geniculate nuclei and layer 4Cbeta of V1. It is implemented in the proposed computational model by using an AND-type non-linearity to combine the responses of centre-surround cells. The proposed model is capable of explaining the results of neurophysiological experiments as well as certain psychophysical observations.

Animals↗
PubMed1997

Computational models of visual neurons specialised in the detection of periodic and aperiodic oriented visual stimuli: bar and grating cells.

Computational models of periodic- and aperiodic-pattern selective cells, also called grating and bar cells, respectively, are proposed. Grating cells are found in areas V1 and V2 of the visual cortex of monkeys and respond strongly to bar gratings of a given orientation and periodicity but very weakly or not at all to single bars. This non-linear behaviour, which is quite different from the spatial frequency filtering behaviour exhibited by the other types of orientation-selective neurons such as the simple cells, is incorporated in the proposed computational model by using an AND-type non-linearity to combine the responses of simple cells with symmetric receptive field profiles and opposite polarities. The functional behaviour of bar cells, which are found in the same areas of the visual cortex as grating cells, is less well explored and documented in the literature. In general, these cells respond to single bars and their responses decrease when further bars are added to form a periodic pattern. These properties of bar cells are implemented in a computational model in which the responses of bar cells are computed as thresholded differences of the responses of corresponding complex (or simple) cells and grating cells. Bar and grating cells seem to play complementary roles in resolving the ambiguity with which the responses of simple and complex cells represent oriented visual stimuli, in that bar cells are selective only for form information as present in contours and grating cells only respond to oriented texture information. The proposed model is capable of explaining the results of neurophysiological experiments as well as the psychophysical observation that the perception of texture and the perception of form are complementary processes.

Animals↗
PubMed1985

[Parasite egg development during methane fermentation].

Investigations were carried out on the development of the most frequently encountered parasitic diseases in farm animals raised under industrial conditions. The parasite eggs were obtained from great numbers of animals with fascioliasis, dicrocoeliasis, ascaridiasis, and paramphistomum infection, etc., and were mixed with fecal homogenate in a lagoon of a fermentor in the conditions of methane fermentation. It was found that the parasite eggs in such conditions retained their vitality in a state of anabiosis. After the fermentation procedure was over and the manure compost mass was spread in the field the parasite eggs could resume their biologic development and become infective. This made it necessary to compost the manure liquid and use it after subsidiary thermal treatment.

Animals↗