Voltage-gated nanofluidic devices for protein capture, concentration, and release.

A nanofluidic device with spatially, non-uniformly distributed gate electrodes is reported. In this nanofluidic architecture, multiple nanochannels connect microfluidic reservoirs for the formation of a planar, hybrid microfluidic-nanofluidic device. The gate electrodes are individually addressable, fluidically isolated, and enable a non-uniform electric field distribution within the nanochannels permitting the capture of proteins and a local increase in their concentration. The removal of the gate potential allows the model protein, bovine serum albumin, to move away from the electrodes after concentration at the electrodes for the release of the captured protein. A maximum increase in the protein concentration of nearly an order of magnitude was observed as evaluated by fluorescence intensity.

Direct current electric field regulates endothelial permeability under physiologically relevant fluid forces in a microfluidic vessel bifurcation model.

Previous in vitro studies have reported on the use of direct current electric fields (DC-EFs) to regulate vascular endothelial permeability, which is important for tissue regeneration and wound healing. However, these studies have primarily used static 2D culture models that lack the fluid mechanical forces associated with blood flow experienced by endothelial cells (ECs) in vivo. Hence, the effect of DC-EF on ECs under physiologically relevant fluid forces is yet to be systematically evaluated. Using a 3D microfluidic model of a bifurcating vessel, we report the role of DC-EF on regulating endothelial permeability when co-applied with physiologically relevant fluid forces that arise at the vessel bifurcation. The application of a 70 V m-1 DC-EF simultaneously with 1 µL min-1 low perfusion rate (generating 3.8 dyn cm-2 stagnation pressure at the bifurcation point and 0.3 dyn cm-2 laminar shear stress in the branched vessel) increased the endothelial permeability 7-fold compared to the static control condition (i.e., without flow and DC-EF). When the perfusion rate was increased to 10 µL min-1 (generating 38 dyn cm-2 stagnation pressure at the bifurcation point and 3 dyn cm-2 laminar shear stress in the branched vessel) while maintaining the same electrical stimulation, a 4-fold increase in endothelial permeability compared to the static control was observed. The lower increase in endothelial permeability for the higher fluid forces but the same DC-EF suggests a competing role between fluid forces and the applied DC-EF. Moreover, the observed increase in endothelial permeability due to combined DC-EF and flow was transient and dependent on the Akt signalling pathway. Collectively, these findings provide significant new insights into how the endothelium serves as an electro-mechanical interface for regulating vessel permeability.

Endothelial barrier function is co-regulated at vessel bifurcations by fluid forces and sphingosine-1-phosphate.

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid mediator of endothelial barrier function. Prior studies have implicated mechanical stimulation due to intravascular laminar shear stress in co-regulating S1P signaling in endothelial cells (ECs). Yet, vascular networks in vivo consist of vessel bifurcations, and this geometry generates hemodynamic forces at the bifurcation point distinct from laminar shear stress. However, the role of these forces at vessel bifurcations in regulating S1P-dependent endothelial barrier function is not known. In this study, we implemented a microfluidic platform that recapitulates the flow dynamics of vessel bifurcations with in situ quantification of the permeability of microvessel analogues. Co-application of S1P with impinging bifurcated fluid flow, which is characterized by approximately zero shear stress and 38 dyn•cm-2 stagnation pressure at the vessel bifurcation point, promotes vessel stabilization. Similarly, co-treatment of S1P with 3 dyn•cm-2 laminar shear stress is also protective of endothelial barrier function. Moreover, it is shown that vessel stabilization due to bifurcated fluid flow and laminar shear stress is dependent on S1P receptor 1 or 2 signaling. Collectively, these findings demonstrate the endothelium-protective function of fluid forces at vessel bifurcations and their involvement in coordinating S1P-dependent regulation of vessel permeability.

Competing Fluid Forces Control Endothelial Sprouting in a 3-D Microfluidic Vessel Bifurcation Model.

Sprouting angiogenesis-the infiltration and extension of endothelial cells from pre-existing blood vessels-helps orchestrate vascular growth and remodeling. It is now agreed that fluid forces, such as laminar shear stress due to unidirectional flow in straight vessel segments, are important regulators of angiogenesis. However, regulation of angiogenesis by the different flow dynamics that arise due to vessel branching, such as impinging flow stagnation at the base of a bifurcating vessel, are not well understood. Here we used a recently developed 3-D microfluidic model to investigate the role of the flow conditions that occur due to vessel bifurcations on endothelial sprouting. We observed that bifurcating fluid flow located at the vessel bifurcation point suppresses the formation of angiogenic sprouts. Similarly, laminar shear stress at a magnitude of ~3 dyn/cm2 applied in the branched vessels downstream of the bifurcation point, inhibited the formation of angiogenic sprouts. In contrast, co-application of ~1 µm/s average transvascular flow across the endothelial monolayer with laminar shear stress induced the formation of angiogenic sprouts. These results suggest that transvascular flow imparts a competing effect against bifurcating fluid flow and laminar shear stress in regulating endothelial sprouting. To our knowledge, these findings are the first report on the stabilizing role of bifurcating fluid flow on endothelial sprouting. These results also demonstrate the importance of local flow dynamics due to branched vessel geometry in determining the location of sprouting angiogenesis.

Surface dependent enhancement in water vapor permeation through nanochannels.

Selective permeation of water vapor over liquid phase water through hydrophobic conduits finds broad use in separation processes, including desalination and membrane distillation. The tangential momentum accommodation coefficient (TMAC), a fundamental parameter that dictates momentum changes to a molecule colliding with a wall remains unknown for water vapor at room temperature and pressure conditions. Here, a nanofluidic platform with tunable hydrophobic regions that selectively barricaded flow of liquid water was patterned within glass nanochannels. The surface functionalization with an alkyltrichlorosilane led to either a fluoride or a methyl terminal group generating partially hydrophobic regions along the length of the nanochannels. Differential osmotic pressure solutions on either side of the hydrophobic region cause an isothermal evaporation-condensation process, which drives net water vapor transport from higher to lower vapor pressure solution, similar to osmotic distillation. Water vapor transport under such conditions for the 80 nm deep nanochannels was in the transitional regime with the Knudsen number ∼O(1). The TMAC was estimated experimentally to be of the order of 10-4-10-3 for both the hydrophobic coatings leading to a near-elastic collision of H2O molecules with the nanochannel walls. Use of the low TMAC surfaces was evaluated in two proof-of-concept technology demonstrations: (1) osmotic distillation using hyper-saline (brine) 3 M Utica shale flowback water as both the feed and draw and (2) separation of trace amounts of toluene and chloroform from water at high flux and selectivity. The results reported here likely provide new insights in designing hydrophilic-hydrophobic junctions for nanoscale liquid/vapor fluid transport with enhanced flux and selectivity.

Flow dynamics control endothelial permeability in a microfluidic vessel bifurcation model.

Endothelial barrier function is known to be regulated by a number of molecular mechanisms; however, the role of biomechanical signals associated with blood flow is comparatively less explored. Biomimetic microfluidic models comprised of vessel analogues that are lined with endothelial cells (ECs) have been developed to help answer several fundamental questions in endothelial mechanobiology. However, previously described microfluidic models have been primarily restricted to single straight or two parallel vessel analogues, which do not model the bifurcating vessel networks typically present in physiology. Therefore, the effects of hemodynamic stresses that arise due to bifurcating vessel geometries on ECs are not well understood. Here, we introduce and characterize a microfluidic model that mimics both the flow conditions and the endothelial/extracellular matrix (ECM) architecture of bifurcating blood vessels to systematically monitor changes in endothelial permeability mediated by the local flow dynamics at specific locations along the bifurcating vessel structure. We show that bifurcated fluid flow (BFF) that arises only at the base of a vessel bifurcation and is characterized by stagnation pressure of ∼38 dyn cm-2 and approximately zero shear stress induces significant decrease in EC permeability compared to the static control condition in a nitric oxide (NO)-dependent manner. Similarly, intravascular laminar shear stress (LSS) (3 dyn cm-2) oriented tangential to ECs located downstream of the vessel bifurcation also causes a significant decrease in permeability compared to the static control condition via the NO pathway. In contrast, co-application of transvascular flow (TVF) (∼1 µm s-1) with BFF and LSS rescues vessel permeability to the level of the static control condition, which suggests that TVF has a competing role against the stabilization effects of BFF and LSS. These findings introduce BFF at the base of vessel bifurcations as an important regulator of vessel permeability and suggest a mechanism by which local flow dynamics control vascular function in vivo.

Cation Dependent Surface Charge Regulation in Gated Nanofluidic Devices.

Surface charge governs nanoscale aqueous electrolyte transport, both in engineered analytical systems and in biological entities such as ion channels and ion pumps as a function of ion type and concentration. Embedded electrodes in a nanofluidic channel, isolated from the fluid in the channel by a dielectric layer, act as active, tunable gates to systematically modify local surface charge density at the interface between the nanochannel surface and the aqueous electrolyte solution, causing significant changes in measured nanochannel conductance. A systematic comparison of transport of monovalent electrolytes [potassium chloride (KCl), sodium chloride (NaCl)], 2:1 electrolytes [magnesium chloride (MgCl2), calcium chloride (CaCl2)], and electrolyte mixtures (KCl + CaCl2) through a gated nanofluidic device was performed. Ion-surface interactions between divalent Ca2+ and Mg2+ ions and the nanochannel walls reduced the native surface charge density by up to ∼4-5 times compared to the monovalent cations. In electrolyte mixtures, Ca2+ was the dominating cation with nanochannel conductance independent of KCl concentration. Systematic changes in local electrostatic surface state induced by the gate electrode are impacted by the divalent cation-surface interactions, limiting modulation of the local surface potential by the gate electrode and resulting in cation dependent nanoscale ion transport as seen through conductance measurements and numerical models.

Effect of surface modification on interfacial nanobubble morphology and contact line tension.

Past research has confirmed the existence of surface nanobubbles on various hydrophobic substrates (static contact angle >90°) when imaged in air-equilibrated water. Additionally, the use of solvent exchange techniques (based on the difference in saturation levels of air in various solvents) also introduced surface nanobubbles on hydrophilic substrates (static contact angle <90°). In this work, tapping mode atomic force microscopy was used to image interfacial nanobubbles formed on bulk polycarbonate (static contact angle of 81.1°), bromo-terminated silica (BTS; static contact angle of 85.5°), and fluoro-terminated silica (FTS; static contact angle of 105.3°) surfaces when immersed in air-equilibrated water without solvent exchange. Nanobubbles formed on the above three substrates were characterized on the basis of Laplace pressure, bubble density, and contact line tension. Results reported here show that (1) the Laplace pressures of all nanobubbles formed on both BTS and polycarbonate were an order of magnitude higher than those of FTS, (2) the nanobubble number density per unit area decreased with an increase in substrate contact angle, and (3) the contact line tension of the nanobubbles was calculated to be positive for both BTS and polycarbonate (lateral radius, Rs < 50 nm for all nanobubbles), and negative for FTS (Rs > 50 nm for all nanobubbles). The nanobubble morphology and distribution before and after using the solvent exchange method (ethanol-water), on the bulk polycarbonate substrate was also characterized. Analysis for these polycarbonate surface nanobubbles showed that both the Laplace pressure and nanobubble density reduced by ≈98% after ethanol-water exchange, accompanied by a flip in the magnitude of contact line tension from positive (0.19 nN) to negative (-0.11 nN).

A three-state nanofluidic field effect switch.

We report a three-state nanofluidic field effect switch in an asymmetrically gated device with a forward (positive), off (zero), and a reverse (negative) current state for tunable control of ionic transport by systematically controlling the gate potential. The embedded gate electrode allows for modulation of the ionic current through the 16 nm deep channels as a function of electrolyte concentration and gate electrode location for a fixed streamwise potential.