Asymmetric droplet interface bilayers

In cell membranes, the lipid compositions of the inner and outer leaflets differ. Therefore, a robust model system that enables single-channel electrical recording with asymmetric bilayers would be very useful. We and others recently developed the droplet interface bilayer (DIB), which is formed by connecting lipid monolayer-encased aqueous droplets submerged in an oil-lipid mixture. Here, we incorporate lipid vesicles of different compositions into aqueous droplets and immerse them in an oil bath to form asymmetric DIBs (a-DIBs). Both alpha-helical and beta-barrel membrane proteins insert readily into a-DIBs, and their activity can be measured by single-channel electrical recording. We show that the gating behavior of outer membrane protein G (OmpG) from Escherichia coli differs depending on the side of insertion in an asymmetric DIB with a positively charged leaflet opposing a negatively charged leaflet. The a-DIB system provides a general platform for studying the effects of bilayer leaflet composition on the behavior of ion channels and pores.     

Electrical behavior of droplet interface bilayer networks: experimental analysis and modeling

Aqueous droplets submerged in an oil-lipid mixture become enclosed by a lipid monolayer. The droplets can be connected to form robust networks of droplet interface bilayers (DIBs) with functions such as a biobattery and a light sensor. Such DIB networks might be used as model systems for the study of membrane-based biological phenomena. In this study, we develop and experimentally validate an electrical modeling approach for DIB networks by applying it to describe the current flow through a simple network containing protein pores and blocking molecules. We demonstrate the use of SPICE (Simulation Program with Integrated Circuit Emphasis) for simulating the electrical behavior of DIB networks. The modular and scalable nature of DIB networks should enable a straightforward extension of the analysis presented in this paper to large, complex networks.     

Bilayer Networks within a Hydrogel Shell: A Robust Chassis for Artificial Cells and a Platform for Membrane Studies

The ability to make artificial lipid bilayers compatible with a wide range of environments, and with sufficient structural rigidity for manual handling, would open up a wealth of opportunities for their more routine use in real‐world applications. Although droplet interface bilayers (DIBs) have been demonstrated in a host of laboratory applications, from chemical logic to biosynthesis reaction vessels, their wider use is hampered by a lack of mechanical stability and the largely manual methods employed in their production. Multiphase microfluidics has enabled us to construct hierarchical triple emulsions with a semipermeable shell, in order to form robust, bilayer‐bound, droplet networks capable of communication with their external surroundings. These constructs are stable in air, water, and oil environments and overcome a critical obstacle of achieving structural rigidity without compromising environmental interaction. This paves the way for practical application of artificial membranes or droplet networks in diverse areas such as medical applications, drug testing, biophysical studies and their use as synthetic cells.     

Stepwise synthesis of giant unilamellar vesicles on a microfluidic assembly line

Among the molecular milieu of the cell, the membrane bilayer stands out as a complex and elusive synthetic target. We report a microfluidic assembly line that produces uniform cellular compartments from droplet, lipid, and oil/water interface starting materials. Droplets form in a lipid-containing oil flow and travel to a junction where the confluence of oil and extracellular aqueous media establishes a flow-patterned interface that is both stable and reproducible. A triangular post mediates phase transfer bilayer assembly by deflecting droplets from oil, through the interface, and into the extracellular aqueous phase to yield a continuous stream of unilamellar phospholipid vesicles with uniform and tunable size. The size of the droplet precursor dictates vesicle size, encapsulation of small-molecule cargo is highly efficient, and the single bilayer promotes functional insertion of a bacterial transmembrane pore.     

Droplet shape analysis and permeability studies in droplet lipid bilayers

We apply optical manipulation to prepare lipid bilayers between pairs of water droplets immersed in an oil matrix. These droplet pairs have a well-defined geometry allowing the use of droplet shape analysis to perform quantitative studies of the dynamics during bilayer formation and to determine time-dependent values for the droplet volumes, bilayer radius, bilayer contact angle, and droplet center-line approach velocity. During bilayer formation, the contact angle rises steadily to an equilibrium value determined by the bilayer adhesion energy. When there is a salt concentration imbalance between droplets, there is a measurable change in the droplet volume. We present an analytical expression for this volume change and use this expression to calculate the bilayer permeability to water.     

Novel technologies for the formation of 2-D and 3-D droplet interface bilayer networks

Droplet interface bilayer (DIB) networks have vast potential in the field of membrane biophysics, synthetic biology, and functional bio-electronics. However a technological bottleneck exists in network fabrication: existing methods are limited in terms of their automation, throughput, versatility, and ability to form well-defined 3-D networks. We have developed a series of novel and low-cost methodologies which address these limitations. The first involves building DIB networks around the contours of a microfluidic chip. The second uses flow rate and droplet size control to influence droplet packing geometries within a microfluidic chamber. The latter method enables the controlled formation of various 3-D network arrays consisting of thousands of interconnected symmetric and asymmetric lipid bilayers for the first time. Both approaches allow individual droplet position and composition to be controlled, paving the way for complex on-chip functional network synthesis.     

Imaging multiple conductance states in an alamethicin pore

Alamethicin is the archetypal antimicrobial pore-forming peptide. Although the peptide has long been known to form pores of characteristic conductances in lipid membranes, the precise nature of these pores is not known. Simultaneous calcium-flux imaging and single-channel recording in a droplet interface bilayer allowed us to directly attribute multiple conductance states to a single point diffusing in the bilayer.     

Functionalizing Nanoemulsions with Carboxylates: Impact on the Biodistribution and Pharmacokinetics in Mice

Efficiency of drug administration is related to the inhibition of adverse effects, and can be improved by drug targeting through lipid nanocarriers encapsulation. Targeting technology generally goes along with the nanocarrier functionalization that can be surface modification and/or ligand grafting. The great advantage of nanoemulsions is their loading capability and the possibilities to encapsulate several entities in a single droplet, however, the decoration of the lipid droplets with strongly anchored reactive functions is challenging. This study proposes a reliable and innovative method to functionalize lipid droplets, based on the lipophilic polymer poly(maleic anhydride‐alt‐1‐octadecene), solubilized in the droplet core, and able to hydrolyze at the oil/water interface. Interfacial chemistry and physicochemical properties of nanodroplets are characterized. In vitro studies reveal that the presence of carboxylates at interface has a strong impact on the interactions with cells, as the internalization of functionalized droplets is much higher than control ones. This difference is confirmed with longitudinal computed tomography studies in mice after i.v. administration, strongly impacting the pharmacokinetics and biodistributions. This work establishes the proof‐of‐concept of a new method for functionalizing lipid droplets and demonstrates that surface modification can have a significant impact on their interaction with cells, pharmacokinetics, and biodistribution.

     

Voltage control of droplet interface bilayer lipid membrane dimensions

A novel approach to control the area of anchor-free droplet interface bilayer (DIB) lipid membranes is presented. Unsupported DIB lipid membranes are formed at the interface of phospholipid-coated aqueous droplets dispensed in dodecane oil. Using electrodes inserted into the droplets, an external voltage is applied which modulates the effective DIB area. Electrical (capacitance or current) and optical (imaging of DIB lateral length) recordings were simultaneously performed. Alpha-hemolysin (αHL) single channel insertions into the DIB were recorded. Currents across the DIB were measured as a function of voltage and αHL concentration in the droplets. Nonlinear response is observed for current, DIB lateral length and area, and capacitance with respect to voltage. Voltage induced changes in interfacial tension modulated the DIB-oil contact angle and the membrane contact length, which provided control of membrane dimensions. Comparison of these results is made to the electrowetting effect, which is also governed by effect of voltage on the interfacial tension. This approach provides active control of the number of ion channels inserted into the DIB.     

Direct detection of membrane channels from gels using water-in-oil droplet bilayers

We form planar lipid bilayers between an aqueous droplet and a hydrogel support immersed in a lipid-oil solution. By scanning the bilayer over the surface of an SDS-PAGE gel, we are able to directly detect membrane proteins from gels using single-channel recording. Using this technique, we are able to examine low levels of endogenous protein from cell extracts without the need for over-expression. We also use droplet bilayers to detect small molecules from hydrogels. The bilayers show enhanced stability compared to conventional planar lipid bilayers, and both bilayer size and position can be controlled during an experiment. Hydrogel scanning with droplet bilayers provides a new method for the discovery and characterization of ion channels with the potential for high-throughput screening.     

Triggered release of molecules across droplet interface bilayer lipid membranes using photopolymerizable lipids

A combination of nonpolymerizable phospholipids (DPPC or DPhPC) and a smaller amount of cross-linking photopolymerizable phospholipids (23:2 DiynePC) is incorporated in an unsupported artificial lipid bilayer formed using the droplet interface bilayer (DIB) approach. The DIB is formed by contacting lipid monolayer-coated aqueous droplets against each other in a dodecane-lipid medium. Cross-linking of the photopolymerizable lipids incorporated in the DIB was obtained by exposure to UV-C radiation (254 nm), resulting in pore formation. The effect of cross-linking on the DIB properties was characterized optically by measuring the diffusion of selectively encapsulated dye molecules (calcein) from one droplet of the DIB to the other droplet. Changes in DIB conductivity due to UV-C exposure were investigated using current-voltage (I-V) measurements. The leakage of dye molecules across the DIB and the increase in DIB conductivity after UV-C exposure indicates the formation of membrane pores. The results indicate that the DIB approach offers a simple and flexible platform for studying phototriggered drug delivery systems in vitro.     

Investigation into the potential ability of Pickering emulsions (food-grade particles) to enhance the oxidative stability of oil-in-water emulsions

In this study the potential ability of food-grade particles (at the droplet interface) to enhance the oxidative stability was investigated. Sunflower oil-in-water emulsions (20%), stabilised solely by food-grade particles (Microcrystalline cellulose (MCC) and modified starch (MS)), were produced under different processing conditions and their physicochemical properties were studied over time. Data on droplet size, surface charge, creaming index and oxidative stability were obtained. Increasing the food-grade particle concentration from 0.1% to 2.5% was found to decrease droplet size, enhance the physical stability of emulsions and reduce the lipid oxidation rate due to the formation of a thicker interfacial layer around the oil droplets. It was further shown that, MCC particles were able to reduce the lipid oxidation rate more effectively than MS particles. This was attributed to their ability to scavenge free radicals, through their negative charge, and form thicker interfacial layers around oil droplets due to the particles size differences. The present study demonstrates that the manipulation of emulsions' interfacial microstructure, based on the formation of a thick interface around the oil droplets by food-grade particles (Pickering emulsions), is an effective approach to slow down lipid oxidation.     

Dynamics of Micelle Formation from Mixed Lipid Droplets

Amphiphilic lipid molecules can form various micelles depending on not only their molecular composition but also their self-assembly pathway. In this work, coarse-grained molecular dynamics simulations have been applied to study the micellization behaviors of mixed di-palmitoylphosphatidylcholine (DPPC)/hexadecylphosphocholine (HPC) droplets. By vary-ing DPPC/HPC composition and the size of lipid droplets, various micelles such as spherical and nonspherical (oblate or prolate) vesicles, disk-like micelles, double or single ring-like and worm-like micelles were observed. It is found that the lipid droplet as an initial state favors forming vesicles and ring-like micelles due to in situ micellization. Our simulation results demonstrate that using special initial conditions combined with various molecular compositions is an effective way to tune lipid micellar structure.     

Nanoparticles of varying hydrophobicity at the emulsion droplet-water interface: adsorption and coalescence stability

The coalescence stability of poly(dimethylsiloxane) emulsion droplets in the presence of silica nanoparticles ( approximately 50 nm) of varying contact angles has been investigated. Nanoparticle adsorption isotherms were determined by depletion from solution. The coalescence kinetics (determined under coagulation conditions at high salt concentration) and the physical structure of coalesced droplets were determined from optical microscopy. Fully hydrated silica nanoparticles adsorb with low affinity, reaching a maximum surface coverage that corresponds to a close packed monolayer, based on the effective particle radius and controlled by the salt concentration. Adsorbed layers of hydrophilic nanoparticles introduce a barrier to coalescence of approximately 1 kT, only slightly reduce the coalescence kinetics, and form kinetically unstable networks at high salt concentrations. Chemically hydrophobized silica nanoparticles, over a wide range of contact angles (25 to >90 degrees ), adsorb at the droplet interface with high affinity and to coverages equivalent to close-packed multilayers. Adsorption isotherms are independent of the contact angle, suggesting that hydrophobic attraction overcomes electrostatic repulsion in all cases. The highly structured and rigid adsorbed layers significantly reduce coalescence kinetics: at or above monolayer surface coverage, stable flocculated networks of droplets form and, regardless of their wettability, particles are not detached from the interface during coalescence. At sub-monolayer nanoparticle coverages, limited coalescence is observed and interfacial saturation restricts the droplet size increase. When the nanoparticle interfacial coverage is >0.7 and <1.0, mesophase-like microstructures have been noted, the physical form and stability of which depends on the contact angle. Adsorbed nanoparticle layers at monolayer coverage and composed of a mixture of nanoparticles with different hydrophobisation levels form stable networks of droplets, whereas mixtures of hydrophobized and hydrophilic nanoparticles do not effectively stabilize emulsion droplets.     

Long-term storable and shippable lipid bilayer membrane platform

The fragility and short lifetimes characteristic of conventionally formed lipid bilayer membranes has necessitated their preparation to be at the time and point of use. By using high freezing-point lipid-solvent mixtures, the process of lipid bilayer self-assembly may be reversibly arrested. In solid form, the bilayer precursor can be stored indefinitely and is sufficiently robust to withstand commercial shipping. Upon thawing, bilayer self-assembly resumes, resulting in a biologically functional membrane. Combination of this membrane precursor with an inexpensive chip results in a compact, practical, and disposable platform for ion channel measurements.     

Bilayer lipid membranes from falling droplets

We describe a system that provides a rapid and simple way of forming suspended lipid bilayers within a microfluidic platform from an aqueous droplet. Bilayer lipid membranes are created in a polymeric device by contacting monolayers formed at a two-phase liquid–liquid interface. Microdroplets, containing membrane proteins, are injected onto an electrode positioned above an aperture machined through a conical cavity that is filled with a lipid–alkane solution. The formation of the BLM depends solely on the device geometry and leads to spontaneous formation of lipid bilayers simply by dispensing droplets of buffer. When an aqueous droplet containing transmembrane proteins or proteoliposomes is injected, straightforward electrophysiology measurements are possible. This method is suitable for incorporation into lab-on-a-chip devices and allows for buffer exchange and electrical measurements.

Deformation and breakup of micro- and nanoparticle stabilized droplets in microfluidic extensional flows

Using a microfluidic flow-focusing device, monodisperse water droplets in oil were generated and their interface populated by either 1 μm or 500 nm amine modified silica particles suspended in the water phase. The deformation and breakup of these Pickering droplets were studied in both pure extensional flow and combined extensional and shear flow at various capillary numbers using a microfluidic hyperbolic contraction. The shear resulted from droplet confinement and increased with droplet size and position along the hyperbolic contraction. Droplet deformation was found to increase with increasing confinement and capillary number. At low confinements and low capillary numbers, the droplet deformation followed the predictions of theory. For fully confined droplets, where the interface was populated by 1 μm silica particles, the droplet deformation increased precipitously and two tails were observed to form at the rear of the droplet. These tails were similar to those seen for surfactant covered droplets. At a critical capillary number, daughter droplets were observed to stream from these tails. Due to the elasticity of the particle-laden interface, these drops did not return to a spherical shape, but were observed to buckle. Although increases in droplet deformation were observed, no tail streaming occurred for the 500 nm silica particle covered droplets over the range of capillary numbers studied.     

Oriented reconstitution of a membrane protein in a giant unilamellar vesicle: experimental verification with the potassium channel KcsA

We report a method for the successful reconstitution of the KcsA potassium channel with either an outside-out or inside-out orientation in giant unilamellar vesicles, using the droplet-transfer technique. The procedure is rather simple. First, we prepared water-in-oil droplets lined with a lipid monolayer. When solubilized KcsA was encapsulated in the droplet, it accumulated at monolayers of phosphatidylglycerol (PG) and phosphoethanolamine (PE) but not at a monolayer of phosphatidylcholine (PC). The droplet was then transferred through an oil/water interface having a preformed monolayer. The interface monolayer covered the droplet so as to generate a bilayer vesicle. By creating chemically different lipid monolayers at the droplet and oil/water interface, we obtained vesicles with asymmetric lipid compositions in the outer and inner leaflets. KcsA was spontaneously inserted into vesicles from the inside or outside, and this was accelerated in vesicles that contained PE or PG. Integrated insertion into the vesicle membrane and the KcsA orientation were examined by functional assay, exploiting the pH sensitivity of the opening of the KcsA when the pH-sensitive cytoplasmic domain (CPD) faces toward acidic media. KcsA loaded from the inside of the PG-containing vesicles becomes permeable only when the intravesicular pH is acidic, and the KcsA loaded from the outside becomes permeable when the extravesicular pH is acidic. Therefore, the internal or external insertion of KcsA leads to an outside-out or inside-out configuration so as to retain its hydrophilic CPD in the added aqueous side. The CPD-truncated KcsA exhibited a random orientation, supporting the idea that the CPD determines the orientation. Further application of the droplet-transfer method is promising for the reconstitution of other types of membrane proteins with a desired orientation into cell-sized vesicles with a targeted lipid composition of the outer and inner leaflets.     

Interfacial organic synthesis in a simple droplet-based microfluidic system

A spherical liquid-liquid interface can be obtained by dispersing one liquid phase into another to form droplets, which will facilitate the two-phase reactions between the immiscible participating fluids. The phase transfer catalysts assembled at the droplet "wall" catalyze the reactions between the aqueous and organic phases. The study illustrates an interfacial synthetic approach which is ideal for the biphasic reaction by taking advantage of the droplet-based microdevice. The improved reaction efficiency can be attributed to the high surface-to-volume ratio and internal flow circulation in the droplets.     

Silica-coated quantum dots for optical evaluation of perfluorocarbon droplet interactions with cells

There has been recent interest in developing new, targeted, perfluorocarbon (PFC) droplet-based contrast agents for medical imaging (e.g., magnetic resonance imaging, X-ray/computed tomography, and ultrasound imaging). However, due to the large number of potential PFCs and droplet stabilization strategies available, it is challenging to determine in advance the PFC droplet formulation that will result in the optimal in vivo behavior and imaging performance required for clinical success. We propose that the integration of fluorescent quantum dots (QDs) into new PFC droplet agents can help to rapidly screen new PFC-based candidate agents for biological compatibility early in their development. QD labels can allow the interaction of PFC droplets with single cells to be assessed at high sensitivity and resolution using optical methods in vitro, complementing the deeper depth penetration but lower resolution provided by PFC droplet imaging using in vivo medical imaging systems. In this work, we introduce a simple and robust method to miscibilize silica-coated nanoparticles into hydrophobic and lipophobic PFCs through fluorination of the silica surface via a hydrolysis-condensation reaction with 1H,1H,2H,2H-perfluorodecyltriethoxysilane. Using CdSe/ZnS core/shell QDs, we show that nanoscale, QD-labeled PFC droplets can be easily formed, with similar sizes and surface charges as unlabeled PFC droplets. The QD label can be used to determine the PFC droplet uptake into cells in vitro by fluorescence microscopy and flow cytometry, and can be used to validate the fate of PFC droplets in vivo in small animals via fluorescence microscopy of histological tissue sections. This is demonstrated in macrophage and cancer cells, and in rabbits, respectively. This work reveals the potential of using QD labels for rapid, preclinical, optical assessment of different PFC droplet formulations for their future use in patients.