Biomedical subjects

P Dimitrakopoulos

Publications and source records attributed to P Dimitrakopoulos.

5 recordsLinked to original sources

PubMed2006

Stress, birefringence, and conformational relaxation of an initially straight stiff bead-rod polymer.

The stress and optical relaxation of an initially straight stiff polymer chain are studied through Brownian dynamics simulations (based on a semiflexible bead-rod model) covering a broad range of time scales and polymer lengths. The strong stress component sigma11 (where "1" is the direction of the original alignment) is shown to be associated with the chain's longitudinal relaxation while the weak stress component sigma22 = sigma33 is shown to depend on the chain's transverse relaxation. The two independent stress components follow a different relaxation; this anisotropy is shown to result from the participation of the different relaxation modes in the transverse direction. The chain's optical relaxation is shown to be affected by the longitudinal dynamics only. The early relaxation of the strong stress component sigma11 and that of the chain's optical properties constitute a universal behavior--i.e., valid for any stiffness of the bead-rod chain, since at the early times the bending forces do not affect the longitudinal dynamics. Based on the knowledge of the physical mechanism and the chain's conformational behavior, we predict and explain the polymer stress and optical relaxation. A nonlinear stress-optic law (valid for any time and chain stiffness) is derived based on the identified relation of the chain configuration with the optical properties and the polymer stress. A coarse-grain model describing extended semiflexible bead road chains is also derived.

Biopolymers↗

PubMed2006

Normal force exerted on vascular endothelial cells.

Hemodynamic forces play an important role in the normal and pathological behavior of vascular endothelial cells as recent studies on the shear stress over the endothelium have shown. Based on computational investigation and scaling analysis, our study shows that the normal force contributes significantly to the total force on the endothelial cells even in large vessels. Therefore, our study suggests that the functions of endothelial cells are also affected by the normal forces exerted on them. The effects of the normal force are more pronounced for smaller vessels and/or less spread cells.

Animals↗

PubMed2004

Longitudinal relaxation of initially straight flexible and stiff polymers.

The relaxation mechanism of an initially straight flexible or stiff polymer chain of length N in a viscous solvent is studied through Brownian dynamics simulations covering a broad range of time scales. After the short-time free diffusion, the chain's longitudinal reduction R2(0)-R2 approximately Nt1/2 at early intermediate times is shown to constitute a universal behavior for any chain stiffness caused by a quasisteady T approximately Nt(-1/2) relaxation of tensions associated with the deforming action of the Brownian forces. Stiff chains with a persistence length E > or = N are shown to exhibit a late intermediate-time longitudinal reduction R2(0)-R2 approximately N2E(-3/4)t1/4 associated with a T approximately N2E(-3/4)t(-3/4) relaxation of tensions affected by the deforming Brownian and the restoring bending forces.

Biopolymers↗

PubMed2001

Short- and intermediate-time behavior of the linear stress relaxation in semiflexible polymers.

The linear viscoelasticity of semiflexible polymers is studied through Brownian Dynamics simulations covering a broad range of chain stiffness and time scales. Our results agree with existing theoretical predictions in the flexible and stiff limits; however, we find that over a wide intermediate-time window spanning several decades, the stress relaxation is described by a single power law t(-alpha), with the exponent alpha apparently varying continuously from 1/2 for flexible chains, to 5/4 for stiff ones. Our study identifies the limits of validity of the t(-3/4) power law at short times predicted by recent theories. An additional regime is identified, the "ultrastiff" chains, where this behavior disappears. In the absence of Brownian motion, the purely mechanical stress relaxation produces a t(-3/4) power law for both short and intermediate times.

Journal Article↗

PubMed2000

PROFILE: Tourism Contribution to Agro-Ecosystems Conservation: The Case of Lesbos Island, Greece.

/ The modernization of agriculture and the development of other economic sectors have prompted the abandonment of cultivated areas, which are marginally productive. Specifically, olive groves in Greece are transformed into pastures due to their location in inaccessible mountainous regions where breeding and raising of sheep and goats are the main economic activities. Overgrazing degrades the environment, exhausts natural resources, and prevents natural regeneration. The Greek islands have limited possibilities of development, except for their coastal areas where the growth of tourism is possible.The objective of this study was to investigate the impact of tourism activities on olive tree cultivation and the human population of the island of Lesbos. The presence or absence of tourism is related with the maintenance or abandonment of olive tree cultivation and population changes for each community. A spatial segregation of the island is evident, related to tourist development, olive tree cultivation, and population change. The results of the study demonstrate that in communities where tourism plays an important role olive tree cultivation is preserved and the population is stable. The preservation of the agro-ecosystem is assured while the olive groves remain productive. Simultaneously, the landscape, which provides specific attractions for tourism, is not altered.

Journal Article↗