On-Chip Inverted Emulsion Method for Fast Giant Vesicle Production, Handling, and Analysis

Measuring Thousands of Single-Vesicle Leakage Events Reveals the Mode of Action of Antimicrobial Peptides

Host defense or antimicrobial peptides hold promise for providing new pipelines of effective antimicrobial agents. Their activity quantified against model phospholipid membranes is fundamental to a detailed understanding of their structure–activity relationships. However, classical characterization assays often lack the ability to achieve this insight. Leveraging a highly parallelized microfluidic platform for trapping and studying thousands of giant unilamellar vesicles, we conducted quantitative long-term microscopy studies to monitor the membrane-disruptive activity of archetypal antimicrobial peptides with a high spatiotemporal resolution. We described the modes of action of these peptides via measurements of the disruption of the vesicle population under the conditions of continuous peptide dosing using a range of concentrations and related the observed modes to the molecular activity mechanisms of these peptides. The study offers an effective approach for characterizing membrane-targeting antimicrobial agents in a standardized manner and for assigning specific modes of action to the corresponding antimicrobial mechanisms.

Standardizing characterization of membrane active peptides with microfluidics

Antimicrobial peptides (AMPs) are emerging as important players in the fight against antibiotic resistance. In parallel, the field of microfluidics has matured and its benefits are being exploited in applications of biomimetics and standardized testing. Membrane models are essential tools extensively utilized in studying the activity and modes of action of AMPs. Here, we describe how the utilization of microfluidic platforms in characterizing membrane active peptides can develop a reliable colorful image that classical techniques have rendered black and white.

Surfactant-free production of biomimetic giant unilamellar vesicles using PDMS-based microfluidics

Microfluidic production of giant lipid vesicles presents a paradigm-shift in the development of artificial cells. While production is high-throughput and the lipid vesicles are mono-disperse compared to bulk methods, current technologies rely heavily on the addition of additives such as surfactants, glycerol and even ethanol. Here we present a microfluidic method for producing biomimetic surfactant-free and additive-free giant unilamellar vesicles. The versatile design allows for the production of vesicle sizes ranging anywhere from ~10 to 130 µm with either neutral or charged lipids, and in physiological buffer conditions. Purity, functionality, and stability of the membranes are validated by lipid diffusion, protein incorporation, and leakage assays. Usability as artificial cells is demonstrated by increasing their complexity, i.e., by encapsulating plasmids, smaller liposomes, mammalian cells, and microspheres. This robust method capable of creating truly biomimetic artificial cells in high-throughput will prove valuable for bottom-up synthetic biology and the understanding of membrane function.

Scale up of Transmembrane NADH Oxidation in Synthetic Giant Vesicles

The transfer of electrons across and along biological membranes drives the cellular energetics. In the context of artificial cells, it can be mimicked by minimal means, while using synthetic alternatives of the phospholipid bilayer and the electron-transducing proteins. Furthermore, the scaling up to biologically relevant and optically accessible dimensions may provide further insight and allow assessment of individual events but has been rarely attempted so far. Here, we visualized the mediated transmembrane oxidation of encapsulated NADH in giant unilamellar vesicles via confocal laser scanning and time-correlated single photon counting wide-field microscopy. To this end, we first augmented phospholipid membranes with an amphiphilic copolymer in order to check its influence on the oxidation kinetics spectrophotometrically. Then, we scaled up the compartments and followed the process microscopically.

Advances in microfluidic synthesis and coupling with synchrotron SAXS for continuous production and real-time structural characterization of nano-self-assemblies

Microfluidic platforms have become highly attractive tools for synthesis of nanoparticles, including lipid nano-self-assemblies, owing to unique features and at least three important aspects inherent to miniaturized micro-devices. Firstly, the fluids flow under controlled conditions in the microchannels, providing well-defined flow profiles and shorter diffusion lengths that play important roles in enhancing the continuous production of lipid and polymer nanoparticles with relatively narrow size distributions. Secondly, various geometries adapted to microfluidic device designs can be utilized for enhancing the colloidal stability of nanoparticles and improving their drug loading. Thirdly, microfluidic devices are usually compatible with in situ characterization methods for real-time monitoring of processes occurring inside the microchannels. This is unlike conventional nanoparticle synthesis methods, where a final solution or withdrawn aliquots are separately analysed. These features inherent to microfluidic devices provide a tool-set allowing not only precise nanoparticle size control, but also real-time analyses for process optimization. In this review, we focus on recent advances and developments in the use of microfluidic devices for synthesis of lipid nanoparticles. We present different designs based on hydrodynamic flow focusing, droplet-based methods and controlled microvortices, and discuss integration of microfluidic platforms with synchrotron small-angle X ray scattering (SAXS) for in situ structural characterization of lipid nano-self-assemblies under continuous flow conditions, along with major challenges and future directions in this research area.

Forming and loading giant unilamellar vesicles with acoustic jetting

Giant unilamellar vesicles (GUVs) are a useful platform for reconstituting and studying membrane-bound biological systems, offering reduced complexity compared to living cells. Several techniques exist to form GUVs and populate them with biomolecules of interest. However, a persistent challenge is the ability to efficiently and reliably load solutions of biological macromolecules, organelle-like membranes, and/or micrometer-scale particles with controlled stoichiometry in the encapsulated volume of GUVs. Here, we demonstrate the use of acoustic streaming from high-intensity focused ultrasound to make and load GUVs from bulk solutions, without the need for nozzles that can become clogged or otherwise alter the solution composition. In this method, a compact acoustic lens is focused on a planar lipid bilayer formed between two aqueous solutions. The actuation of a planar piezoelectric material coupled to the lens accelerates a small volume of liquid, deforming the bilayer and forming a GUV containing the solution on the transducer side of the bilayer. As demonstrated here, acoustic jetting offers an alternative method for the generation of GUVs for biological and biophysical studies.

Membrane permeability to water measured by microfluidic trapping of giant vesicles

We use a microfluidic method to estimate the water permeability coefficient (p) of membranes. As model lipid membranes we employ giant unilamellar vesicles (GUVs) composed of palmitoyloleoyl phosphatidylcholine and cholesterol (10 mol%). We have developed a microfluidic device with multiple chambers to trap GUVs and allow controlled osmotic exchange. Each chamber has a ring-shaped pressure-controlled valve which upon closure allows isolation of the GUVs in a defined volume. Opening the valves leads to a rapid fluid exchange between the trapping region and the microchannel network outside, thus allowing precise control over solution concentration around the GUVs contrary to other experimental approaches for permeability measurements reported in the literature. The area and volume changes of individual vesicles are monitored with confocal microscopy. The solute concentration in the immediate vicinity of the GUVs, and thus the concentration gradient across the membrane, is independently assessed. The data are well fitted by a simple model for water permeability which assumes that the rate of change in volume of a GUV per unit area is linearly proportional to concentration difference with permeability as the proportionality constant. Experiments of GUV osmotic deflation with hypertonic solutions yield the permeability of POPC/cholesterol 9/1 membranes to be p = 15.7 ± 5.5 μm s-1. For comparison, we also show results using two other approaches, which either do not take into account local concentration changes and/or do not resolve the precise vesicle shape. We point out the errors associated with these limitations. Finally, we also demonstrate the applicability of the microfluidic device for studying the dynamics of vesicles under flow.