Platelet-Rich Blood Clots

Abstract

To determine how the volume fraction of fibrin and platelets affects fluid transport within the interstitial spaces of blood clots, the article in this issue by Wufsus et al. (1xThe hydraulic permeability of blood clots as a function of fibrin and platelet density. Wufsus, A.R., Macera, N.E., and Neeves, K.B. Biophys. J. 2013; 104: 1812–1823Abstract | Full Text | Full Text PDF | PubMed | Scopus (26)See all References1) presents a study of platelet-rich clots (characteristic of the initial stages of hemostatic and thrombotic clots) in which both granular (platelets) and fibrous (fibrin) media are present. The permeability of a clot was estimated by assuming a Darcy’s law approximation at the macroscopic scale and measuring flow through the clots under a maintained constant pressure gradient. Measured permeability values were compared to models of fibrous, granular, and mixed porous media to delineate the relative contributions of each component to the overall hydrodynamic resistance. Confocal microscopy was used to determine the volume fraction of the fibrin gels and platelet-fibrin thrombi, and scanning electron microscopy was used to measure fiber diameters.Permeability measurements are typically used to determine how biochemical conditions affect fibrin polymerization. For example, for a given fibrinogen concentration, fibrin gels formed at higher thrombin concentrations consist of more densely packed thin fibers than in gels formed at low thrombin concentrations. Permeability has also been used as a measure of how a drug or certain disorders affect fibrin polymerization.For fibrin gels, a large (three orders of magnitude) decrease in permeability was found for a modest increase in volume fraction (0.02–0.18), and a much smaller change in permeability for volume fractions in the range 0.18–0.54. These observations were compared with the relationships among fibrin gel permeability, fiber volume fraction, and fiber radius established empirically or derived from simplified analytic (2xFlow through mixed fibrous porous materials. Ethier, C.R. AIChE J. 1991; 37: 1227–1236Crossref | Scopus (31)See all References2) or detailed numerical (lattice-Boltzmann) solutions (3xHydraulic permeability of (un)bounded fibrous media using the lattice Boltzmann method. Clague, D.S., Kandhai, B.D...., and Sloot, P.M. Phys. Rev. E Stat. Phys. Plasmas Fluids Relat. Interdiscip. Topics. 2000; 61: 616–625Crossref | PubMed | Scopus (83)See all References3). The conclusion is that, over a fiber volume fraction of 0.02–0.54, fibrin gels are well described by models of disordered cylinders with uniform diameters.When platelets were added at a volume fraction of 0.19–0.31, the permeability decreased by only ∼50%, compared to a pure fibrin gel. At higher platelet volume fractions (>0.31), however, the contribution of the platelets begins to dominate the overall hydraulic resistance (permeability is an order of magnitude lower at a platelet volume fraction of 0.37). A platelet-rich clot was found to be well described as a Brinkman medium (the Brinkman equation adds a dissipative term to the Darcy equation, arising from the no-slip fluid boundary condition on the surface of large solid inclusions). Viscous losses alone, however, do not fully account for the effect of the addition of platelets into a fibrin gel. Platelets affect fibrin polymerization, and fibers in the presence of platelets are more tortuous and appear much shorter than in platelet-poor plasma.