Key Points Hemostatic plugs develop a regional architecture defined by the extent of platelet activation and packing density. The regional composition of a hemostatic plug is due to the interaction of local conditions with the platelet-signaling network.

Key Points• Thrombus formation and contraction alters local molecular transport, which regulates agonist distribution and platelet activation.• Semaphorin 4D contactdependent signaling increases platelet activation, but does not affect platelet packing or agonist transport.Hemostatic thrombi develop a characteristic architecture in which a core of highly activated platelets is covered by a shell of less-activated platelets. Here we have used a systems biology approach to examine the interrelationship of this architecture with transport rates and agonist distribution in the gaps between platelets. Studies were performed in mice using probes for platelet accumulation, packing density, and activation plus recently developed transport and thrombin activity probes. The results show that intrathrombus transport within the core is much slower than within the shell. The region of slowest transport coincides with the region of greatest packing density and thrombin activity, and appears prior to full platelet activation. Deleting the contact-dependent signaling molecule, Sema4D, delays platelet activation, but not the emergence of the low transport region. Collectively, these results suggest a timeline in which initial platelet accumulation and the narrowing gaps between platelets create a region of reduced transport that facilitates local thrombin accumulation and greater platelet activation, whereas faster transport rates within the shell help to limit thrombin accumulation and growth of the core. Thus, from a systems perspective, platelet accumulation produces an altered microenvironment that shapes thrombus architecture, which in turn affects agonist distribution and subsequent thrombus growth. (Blood. 2014;124(11):1808-1815 IntroductionThe hemostatic response balances the need to halt bleeding with the need to avoid inappropriate vascular occlusion. Recent reports of hemostatic thrombi formed in vivo have demonstrated that the extent of platelet activation within a growing thrombus is heterogeneous [1][2][3][4][5] and can result in a characteristic core-and-shell architecture. We have shown that the core region develops adjacent to the injury site and consists of fully activated, closely packed platelets that have undergone a-granule exocytosis, which allows them to be recognized by the appearance of the a-granule membrane protein, P-selectin, on their surface. 3 The shell is a less stable region that coats the core and consists of loosely packed, less activated platelets. 3Regional differences in the extent of platelet activation can potentially be driven by multiple factors. Here we have adopted a systems biology perspective, looking beyond the events in any one platelet to test the idea that the emerging architecture of the hemostatic response serves as both a driver and a reflection of differences in intrathrombus molecular transport rates and consequent differences in agonist distribution. Numerous platelet agonists are present during vascular injury, including collagen, thrombin, adenosine 59-diphosphate (ADP)...

A review of the characteristics of hydrophobicity is presented, with the goal of investigating the relationship, if any, between the contact angle (a macroscopically observed property) and the slip length (a microscopic phenomenon). An analysis of simulations, and of their evolution through the years, sheds light on some inherent differences between contact angle and slip length behavior on flat and patterned surfaces. Previous studies lead to the conclusion that epitaxial layering of fluid near the solid is intricately related to the magnitude of fluid slip. Epitaxial layer data help to explain unexpected slip length behavior in relation to the contact angle, and reported inconsistencies between slip length experiments and simulations. Therefore, it seems that solids that can produce favorable epitaxial layering of the fluid will cause larger slip. Dimensional analysis is used to elucidate the contact angle-slip length relationship. Results can be applied to the development of artificial supersolvophobic surfaces that would exhibit predictable fluid slip with important practical applications.

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Correlations between contact angle, a measure of the wetting of surfaces, and slip length are developed using nonequilibrium molecular dynamics for a Lennard-Jones fluid in Couette flow between graphitelike hexagonal-lattice walls. The fluid-wall interaction is varied by modulating the interfacial energy parameter epsilonr=epsilonsfepsilonff and the size parameter sigmar=sigmasfsigmaff, (s=solid, f=fluid) to achieve hydrophobicity (solvophobicity) or hydrophilicity (solvophilicity). The effects of surface chemistry, as well as the effects of temperature and shear rate on the slip length are determined. The contact angle increases from 25 degrees to 147 degrees on highly hydrophobic surfaces (as epsilonr decreases from 0.5 to 0.1), as expected. The slip length is functionally dependent on the affinity strength parameters epsilonr and sigmar: increasing logarithmically with decreasing surface energy epsilonr (i.e., more hydrophobic), while decreasing with power law with decreasing size sigmar. The mechanism for the latter is different from the energetic case. While weak wall forces (small epsilonr) produce hydrophobicity, larger sigmar smoothes out the surface roughness. Both tend to increase the slip. The slip length grows rapidly with a high shear rate, as wall velocity increases three decades from 100 to 10(5) ms. We demonstrate that fluid-solid interfaces with low epsilonr and high sigmar should be chosen to increase slip and are prime candidates for drag reduction.