The firm arrest of leukocytes to the endothelium during inflammation is known to be mediated by endothelial intercellular adhesion molecules (ICAMs) binding to activated integrins displayed on leukocyte surface. Selectin-ligand interactions, which mediate rolling, are believed to be important for facilitating firm adhesion, either by activating integrins or by facilitating the transition to firm adhesion by making it easier for integrins to bind. Although leukocytes employ two distinct adhesion molecules that mediate different states of adhesion, the fundamental biophysical mechanisms by which two pairs of adhesion molecules facilitate cell adhesion is not well understood. In this work, we attempt to understand the interaction between two molecular systems using a cell-free system in which polystyrene microspheres functionalized with the selectin ligand, sialyl Lewis(X) (sLe(X)), and an antibody against ICAM-1, aICAM-1, are perfused over P-selectin/ICAM-1 coated surfaces in a parallel plate flow chamber. Separately, sLe(X)/P-selectin interactions support rolling and aICAM-1/ICAM-1 interactions mediate firm adhesion. Our results show that sLe(X)/aICAM-1 microspheres will firmly adhere to P-selectin/ICAM-1 coated surfaces, and that the extent of firm adhesion of microspheres is dependent on wall shear stress within the flow chamber, sLe(X)/aICAM-1 microsphere site density, and P-selectin/ICAM-1 surface density ratio. We show that P-selectin's interaction with sLe(X) mechanistically facilitates firm adhesion mediated by antibody binding to ICAM-1: the extent of firm adhesion for the same concentration of aICAM-1/ICAM-1 interaction is greater when sLe(X)/P-selectin interactions are present. aICAM-1/ICAM-1 interactions also stabilize rolling by increasing pause times and decreasing average rolling velocities. Although aICAM-1 is a surrogate for beta(2)-integrin, the kinetics of association between aICAM-1 and ICAM-1 is within a factor of 1.5 of activated integrin binding ICAM-1, suggesting the findings from this model system may be insightful to the mechanism of leukocyte firm adhesion. In particular, these experimental results show how two molecule systems can interact to produce an effect not achievable by either system alone, a fundamental mechanism that may pervade leukocyte adhesion biology.

To understand the reversible gelation and subsequent aging of hydrogels prepared by freeze/thaw processing of poly(vinyl alcohol) (PVOH) solutions, the microstructures of gels prepared by different freeze/thaw protocols and aged to varying extents are studied by cryogenic transmission electron microscopy, solid‐state nuclear magnetic resonance, X‐ray scattering, and differential scanning calorimetry (DSC). As discussed in the literature, gelation by the freeze/thaw process occurs as a homogeneous aqueous poly(vinyl alcohol) solution is cycled, perhaps multiple times, between temperatures above 0 °C and well below 0 °C. The current investigation has determined that a few percent of chain segments crystallize during the first cycle, organizing themselves into 3–8 nm primary crystallite junctions separated on an irregular mesh by an average spacing of ∼ 30 nm. Aging or imposition of additional freeze/thaw cycles augments the level of crystallinity and transforms the as‐formed liquid‐like microstructure, characterized in the electron microscope by rounded ∼ 30 nm pores, into a fibrillar network. Observation that the transformation occurs at fixed mesh spacing and approximately constant average crystallite size suggests the formation of secondary crystallites that do not affect network connectivity. Dendritic ice crystallization and possibly spinodal decomposition superimpose on this nanoscale structure a matrix of much larger pores. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3438–3454, 1999

The rheo-optical response of solutions of silk in aqueous 9 M LiBr of concentrations ranging from 7.5 to 15 wt % was investigated. These samples were found to be birefringent under flow, with the steady state birefringence having a concentration scaling of c 3.9. This scaling behavior suggests a wormlike conformation of the silk molecules in solution. The relaxation was fast, and Maxwellian behavior was observed at low shear rates. No dichroism was detected suggesting that electrostatic screening suppresses structure formation under shear.

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