Effects of unidirectional flow shear stresses on the formation, fractal microstructure and rigidity of incipient whole blood clots and fibrin gels

Incipient clot formation in whole blood and fibrin gels was studied by the rheometric techniques of controlled stress parallel superposition (CSPS) and small amplitude oscillatory shear (SAOS). The effects of unidirectional shear stress on incipient clot microstructure, formation kinetics and elasticity are reported in terms of the fractal dimension (df ) of the fibrin network, the gel network formation time (TGP ) and the shear elastic modulus, respectively. The results of this first haemorheological application of CSPS reveal the marked sensitivity of incipient clot microstructure to physiologically relevant levels of shear stress, these being an order of magnitude lower than have previously been studied by SAOS. CSPS tests revealed that exposure of forming clots to increasing levels of shear stress produces a corresponding elevation in df , consistent with the formation of tighter, more compact clot microstructures under unidirectional flow. A corresponding increase in shear elasticity was recorded. The scaling relationship established between shear elasticity and df for fibrin clots and whole blood confirms the fibrin network as the dominant microstructural component of the incipient clot in terms of its response to imposed stress. Supplementary studies of fibrin clot formation by rheometry and microscopy revealed the substantial additional network mass required to increase df and provide evidence to support the hypothesis that microstructural changes in blood clotted under unidirectional shear may be attributed to flow enhanced thrombin generation and activation. CSPS also identified a threshold value of unidirectional shear stress above which no incipient clot formation could be detected. CSPS was shown to be a valuable haemorheological tool for the study of the effects of physiological and pathological levels of shear on clot properties

A robust and fast algorithm for computing exact and approximate shortest visiting routes

Given a simple n-sided polygon in the plane with a boundary partitioned into subchains some of which are convex and colored, we consider the following problem: Which is the shortest route (closed path) contained in the polygon that passes through a given point on the boundary and intersects at least one vertex in each of the colored subchains? We present an optimal algorithm that solves this problem in O(n) time. Previously it was known how to solve the problem optimally when each colored subchain contains one vertex only. Moreover, we show that a solution computed by the algorithm is at most a factor times longer than the overall shortest route that intersects the subchains (not just at vertices) if the minimal distance between vertices of different subchains is at least c times the maximal length of an edge of a subchain. Without such a bound its length can be arbitrarily longer. Furthermore, it is known that algorithms for computing such overall shortest routes suffer from numerical problems. Our algorithm is not subject to such problems.Validerad; 2004; 20070110 (ysko