We present a technique for the directed assembly and self-assembly of micrometer-scale structures based on the control of specific DNA linkages between colloidal particles. The use of DNA links combined with polymer brushes provides an effective way to regulate the range and magnitude of addressable forces between pairs (and further combinations) of different particles. We demonstrate that the autoassembly of alternate microbeads as well as their directed assembly, by using laser tweezers, is reversible. The key to reversibility is preventing the particles from falling into their van der Waals well at close distances. This goal is achieved by the use of adsorbed polymers that limit the number of DNA bridges to one to three between adjacent particles.DNA links ͉ reversible aggregation S tudies of reversible and specific adhesion between colloids are an important step toward understanding various phenomena involving molecular recognition. For instance, they are relevant in the study of cell adhesion (1), cell migration (2), or cell sorting during embryonic development (3). The knowledge and the control of the interplay between nonspecific repulsion and molecular recognition is also fundamental for biotechnological device improvements; e.g., a strategy to improve latex agglutination tests is to reduce aggregation due to nonspecific interactions (4). The present work on DNA links is also a contribution to these more general studies.Controlling and tuning interactions between particles has always been a relevant challenge both experimentally (5, 6) and theoretically (7-11). For example, Tkachenko (11) predicted diverse and unusual crystal morphologies assuming a reversible contact between particles in a binary system of colloids, in which identical particles experience repulsive interactions and differing particles experience attractive ones. Particularly, he predicted a selfassembled diamond lattice structure that would be especially relevant for photonic crystal building. His work was inspired by the work by Mirkin et al. (12), who first used DNA chains as linkers between nanoparticles to build a reversible DNA-mediated assembly of gold nanoparticles. The specificity and magnitude of the attraction is determined by the molecular recognition of complementary DNA strands and the sensitivity of hybridization to solution conditions and temperature. Experimental work involving DNA as a linker between particles has up to now mainly focused on nanosized particles (13-18). To our knowledge, only two studies (19,20) have been reported in the literature with microsized particles, but in both of them the assembly process was not reversible, with the DNA acting as a molecular bridge between the entities of a binary mixture. In this study, we focus on the reversibility of the aggregation process between microsized particles. The specificity and reversibility are proof that the interactions between the colloids are controlled by DNA and thus can be tuned. Materials and MethodsSample Preparation. DNA-functionalized polystyrene mi...

We present new results about the stability of 5CB nematic films spun cast onto silicon wafers. We observe experimentally the dewetting of thin films while thick ones remain stable. We interpret this behavior as a competition between elasticity and van der Waals forces. At later stages, the experimentally observed dewetting instability leads to the formation of structures (islands) which grow and finally merge to form a film of uniform thickness. We show that the islands' characteristic size L͑t͒ scales as t 1͞3 as expected from late stage growth theories. [S0031-9007(99)08781-5] 64.70.Md, 68.45.Gd Dewetting may occur in thin liquid films because of the high deformability of the liquid-air interface. This has been extensively studied for mainly two reasons. Dewetting may hinder technological applications such as coatings or lubrication. From a more fundamental perspective, a better knowledge of specific interactions involved during (de-)wetting processes is necessary. Two main mechanisms have been identified as inducing dewetting [1-3]: dry patches nucleation due to material heterogeneity (dust, defects), or amplification of thermal fluctuations at the free surface of the liquid film. The latter dewetting mechanism is called spinodal dewetting. While the stability of thin polymeric liquid films has been largely studied [4,5], little is actually known about the dewetting of liquid crystals [6]. Spinodal dewetting has been only recently observed in "soft solid" liquid crystal films [7]. As far as liquid crystals are concerned, a significant effort has been carried out, for the last two decades, to achieve a precise understanding of the solid/liquid crystal interface: anchoring mechanisms [8], surface-induced orientational order [9], and orientational wetting [10] have been and still are the objects of intensive research.In previous Letters, we have reported on a detailed investigation of the spreading of 5CB nematic droplets on silicon wafers [11,12]. The anchoring conditions are antagonist (strong homeotropic anchoring at the free nematic-air interface and weak planar anchoring close to the solid surface) and thus induce an elastic distortion in the film. Ellipsometric profiles of the drops reveal two characteristic thicknesses. The first, belonging to the molecular scale, corresponds to a trilayer structure (tilted monolayer bearing a smecticlike bilayer). The second belongs to the mesoscopic scale (tens of nanometers) and corresponds to the equilibrium thickness of the drop. That previous study allowed us to identify the basic interactions which account for the structure of wetting nematic droplets on silicon wafers. Now, we focus our attention on the dewetting of thin 5CB nematic films spun cast onto the same surfaces. The inner structure of the film, induced by the boundary conditions and resulting from the strong coupling of both interfaces, significantly affects the dewetting behavior of such films. We report in this Letter on spinodal dewetting occurring in 5CB nematic films and propose a first quantitat...

As they leave the blood stream and travel to lymph nodes or sites of inflammation, T lymphocytes are captured by the endothelium and migrate along the vascular wall to permissive sites of transmigration. These processes take place under the influence of hemodynamic shear stress; therefore, we investigated how migrational speed and directionality are influenced by variations in shear stress. We examined human effector T lymphocytes on intercellular adhesion molecule 1 (ICAM-1)-coated surfaces under the influence of shear stresses from 2 to 60 dyn.cm(-2). T lymphocytes were shown to respond to shear stress application by a rapid (30 s) and fully reversible orientation of their migration against the fluid flow without a change in migration speed. Primary T lymphocytes migrating on ICAM-1 in the presence of uniformly applied SDF-1α were also found to migrate against the direction of shear flow. In sharp contrast, neutrophils migrating in the presence of uniformly applied fMLP and leukemic HSB2 T lymphocytes migrating on ICAM-1 alone oriented their migration downstream, with the direction of fluid flow. Our findings suggest that, in addition to biochemical cues, shear stress is a contributing factor to leukocyte migration directionality.