Alfredo Alexander-Katz

Shear-induced unfolding triggers adhesion of von Willebrand factor fibers

von Willebrand factor (VWF), a protein present in our circulatory system, is necessary to stop bleeding under high shear-stress conditions as found in small blood vessels. The results presented here help unravel how an increase in hydrodynamic shear stress activates VWFs adhesion potential, leading to the counterintuitive phenomena of enhanced adsorption rate under strong shear conditions. Using a microfluidic device, we were able to mimic a wide range of bloodflow conditions and directly visualize the conformational dynamics of this protein under shear flow. In particular, we find that VWF displays a reversible globule-stretch transition at a critical shear rate γ̇crit in the absence of any adsorbing surface. Computer simulations reproduce this sharp transition and identify the large size of VWFs repeating units as one of the keys for this unique hydrodynamic activation. In the presence of an adsorbing collagen substrate, we find a large increase in the protein adsorption at the same critical shear rate, suggesting that the globule unfolding in bulk triggers the surface adsorption in the case of a collagen substrate, which provides a sufficient density of binding sites. Monitoring the adsorption process of multiple VWF fibers, we were able to follow the formation of an immobilized network that constitutes a “sticky” grid necessary for blood platelet adhesion under high shear flow. Because areas of high shear stress coincide with a higher chance for vessel wall damage by mechanical forces, we identified the shear-induced increase in the binding probability of VWF as an effective self-regulating repair mechanism of our microvascular system.

Complex self-assembled patterns using sparse commensurate templates with locally varying motifs

Templated self-assembly of block copolymer thin films can generate periodic arrays of microdomains within a sparse template, or complex patterns using 1:1 templates. However, arbitrary pattern generation directed by sparse templates remains elusive. Here, we show that an array of carefully spaced and shaped posts, prepared by electron-beam patterning of an inorganic resist, can be used to template complex patterns in a cylindrical-morphology block copolymer. We use two distinct methods: making the post spacing commensurate with the equilibrium periodicity of the polymer, which controls the orientation of the linear features, and making local changes to the shape or distribution of the posts, which direct the formation of bends, junctions and other aperiodic features in specific locations. The first of these methods permits linear patterns to be directed by a sparse template that occupies only a few percent of the area of the final self-assembled pattern, while the second method can be used to selectively and locally template complex linear patterns.

Templating three-dimensional self-assembled structures in bilayer block copolymer films

To the Next Level Block copolymers will spontaneously separate into a range of microstructures that depend on the polymer block lengths and chemical compositions, and have been used as a templating material because one can selectively etch or functionalize one of the blocks. However, creating a template that is more than one layer thick is challenging. Tavakkoli K. G. et al. (p. 1294) used an array of posts to provide independent and simultaneous control of the morphology and orientation of two layers of block copolymers and were able to create local variations in the curvature and spacing of the domains. An array of posts guides the bilayer assembly of block copolymers with independent control of morphology and orientation. The registration and alignment of a monolayer of microdomains in a self-assembled block copolymer thin film can be controlled by chemical or physical templating methods. Although planar patterns are useful for nanoscale device fabrication, three-dimensional multilevel structures are required for some applications. We found that a bilayer film of a cylindrical-morphology block copolymer, templated by an array of posts functionalized with a brush attractive to the majority block, can form a rich variety of three-dimensional structures consisting of cylinder arrays with controllable angles, bends, and junctions whose geometry is controlled by the template periodicity and arrangement. This technique allows control of microdomain patterns and the ability to route and connect microdomains in specific directions.

Effect of Particle Diameter and Surface Composition on the Spontaneous Fusion of Monolayer-Protected Gold Nanoparticles with Lipid Bilayers

Anionic, monolayer-protected gold nanoparticles (AuNPs) have been shown to nondisruptively penetrate cellular membranes. Here, we show that a critical first step in the penetration process is potentially the fusion of such AuNPs with lipid bilayers. Free energy calculations, experiments on unilamellar and multilamellar vesicles, and cell studies all support this hypothesis. Furthermore, we show that fusion is only favorable for AuNPs with core diameters below a critical size that depends on the monolayer composition.

Controlled surface-induced flows from the motion of self-assembled colloidal walkers

Biological flows at the microscopic scale are important for the transport of nutrients, locomotion, and differentiation. Here, we present a unique approach for creating controlled, surface-induced flows inspired by a ubiquitous biological system, cilia. Our design is based on a collection of self-assembled colloidal rotors that “walk” along surfaces in the presence of a rotating magnetic field. These rotors are held together solely by magnetic forces that allow for reversible assembly and disassembly of the chains. Furthermore, rotation of the magnetic field allows for straightforward manipulation of the shape and motion of these chains. This system offers a simple and versatile approach for designing microfluidic devices as well as for studying fundamental questions in cooperative-driven motion and transport at the microscopic level.

Lipid tail protrusions mediate the insertion of nanoparticles into model cell membranes

Recent work has demonstrated that charged gold nanoparticles (AuNPs) protected by an amphiphilic organic monolayer can spontaneously insert into the core of lipid bilayers to minimize the exposure of hydrophobic surface area to water. However, the kinetic pathway to reach the thermodynamically stable transmembrane configuration is unknown. Here, we use unbiased atomistic simulations to show the pathway by which AuNPs spontaneously insert into bilayers and confirm the results experimentally on supported lipid bilayers. The critical step during this process is hydrophobic-hydrophobic contact between the core of the bilayer and the monolayer of the AuNP that requires the stochastic protrusion of an aliphatic lipid tail into solution. This last phenomenon is enhanced in the presence of high bilayer curvature and closely resembles the putative pre-stalk transition state for vesicle fusion. To the best of our knowledge, this work provides the first demonstration of vesicle fusion-like behaviour in an amphiphilic nanoparticle system.

Hierarchical Nanostructures by Sequential Self-Assembly of Styrene-Dimethylsiloxane Block Copolymers of Different Periods

Poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) block copolymers with a period as low as 13 nm have been self-assembled on a template formed from PS-b-PDMS of a 34–40 nm period, which is itself templated by micron-scale substrate features prepared using conventional lithography. This hierarchical process provides a simple method for directing the self-assembly of sub-10 nm features and registering them on the substrate.

Aligned Sub-10-nm Block Copolymer Patterns Templated by Post Arrays

Self-assembly of block copolymer films can generate useful periodic nanopatterns, but the self-assembly needs to be templated to impose long-range order and to control pattern registration with other substrate features. We demonstrate here the fabrication of aligned sub-10-nm line width patterns with a controlled orientation by using lithographically formed post arrays as templates for a 16 kg/mol poly(styrene-block-dimethylsiloxane) (PS-b-PDMS) diblock copolymer. The in-plane orientation of the block copolymer cylinders was controlled by varying the spacing and geometry of the posts, and the results were modeled using 3D self-consistent field theory. This work illustrates how arrays of narrow lines with specific in-plane orientation can be produced, and how the post height and diameter affect the self-assembly.

Penetration of lipid bilayers by nanoparticles with environmentally-responsive surfaces: simulations and theory

Understanding the interactions between nanoparticles (NPs) and lipid bilayers is critical for the design of drug delivery carriers, biosensors, and biocompatible materials. In particular, it is desirable to understand how to effectively translocate synthetic molecules through the cellular membrane, which acts as a selective barrier to regulate transport into the cell. In this work, we use simulations and theory to explore the role that surface reconstruction may play in non-specific interactions between NPs and lipid bilayers. We show that NPs with a mixed hydrophobic/hydrophilic surface functionalization capable of rearranging their surfaces to maximize hydrophobic matching with the bilayer core are able to spontaneously establish a thermodynamically-favored position at the bilayer midplane. Furthermore, this penetration behavior is most favorable thermodynamically when the surface of the NP is near an order-disorder transition. Our analysis provides design criteria for future synthetic NPs, with the goal of designing particles that can maintain a stable transmembrane orientation.

Elongational flow induces the unfolding of von Willebrand factor at physiological flow rates.

The unfolding of von Willebrand Factor (vWF), one of the largest multimeric proteins in our body, has been shown to be a crucial step in the process of blood clotting. Here we show that elongational flows, which appear during vasoconstriction or stenosis, are the primary activation mechanisms of vWF, and unfold the multimeric protein at flow rates that are two orders-of -magnitude below those corresponding to pure shear. The findings presented here complement the current understanding of blood clotting from the molecular to the physiological level, and provide new physical insights into the connection between clotting anomalies, such as Heydes syndrome and stenosis. These findings also represent a new paradigm in the function and activation of vWF.