Membranes under shear stress: visualization of non-equilibrium domain patterns and domain fusion in a microfluidic device

Parallel on-chip micropipettes enabling quantitative multiplexed characterization of vesicle mechanics and cell aggregates rheology

Micropipette aspiration (MPA) is one of the gold standards for quantifying biological samples' mechanical properties, which are crucial from the cell membrane scale to the multicellular tissue. However, relying on the manipulation of individual home-made glass pipettes, MPA suffers from low throughput and no automation. Here, we introduce the sliding insert micropipette aspiration method, which permits parallelization and automation, thanks to the insertion of tubular pipettes, obtained by photolithography, within microfluidic channels. We show its application both at the lipid bilayer level, by probing vesicles to measure membrane bending and stretching moduli, and at the tissue level by quantifying the viscoelasticity of 3D cell aggregates. This approach opens the way to high-throughput, quantitative mechanical testing of many types of biological samples, from vesicles and individual cells to cell aggregates and explants, under dynamic physico-chemical stimuli.

Shear-Induced Nonequilibrium Patterns in Lipid Bilayer Membranes Exhibiting Phase Separation

The nonequilibrium dynamics of a fluid lipid membrane under external stimuli is an important issue that spans disciplines such as soft matter, biophysical chemistry, and interface science. This study investigated the dynamic response of lipid vesicles with order-disorder phase separation, which mimics a plasma membrane heterogeneity, to shear flow. Lipid vesicles were immobilized in a microfluidic chamber, and shear-induced nonequilibrium patterns on the membrane surface were observed by an optical microscope. We found that phase-separated membranes exhibit a dissipative structure of stripe patterns along the vortex flow on the membrane surface, and the number of stripes increased with the flow rate. At a high flow rate, the membrane exhibited a stripe-to-wave transition, where striped domains often migrated and the replacement of two different phases happened at vortex centers with time. We obtained a dynamic phase diagram of the shear-induced wave pattern by changing the flow rate, membrane components, and temperature. These findings could provide insight into the dissipative structures of lipid membranes out of equilibrium and flow-mediated mechanotransduction of biological membranes.

Mechanoresponsive Drug Delivery Systems for Vascular Diseases

Mechanoresponsive drug delivery systems (DDS) have emerged as promising candidates to improve the current effectiveness and lower the side effects typically associated with direct drug administration in the context of vascular diseases. Despite tremendous research efforts to date, designing drug delivery systems able to respond to mechanical stimuli to potentially treat these diseases is still in its infancy. By understanding relevant biological forces emerging in healthy and pathological vascular endothelium, it is believed that better-informed design strategies can be deduced for the fabrication of simple-to-complex macromolecular assemblies capable of sensing mechanical forces. These responsive systems are discussed through insights into essential parameter design (composition, size, shape, and aggregation state) , as well as their functionalization with (macro)molecules that are intrinsically mechanoresponsive (e.g., mechanosensitive ion channels and mechanophores). Mechanical forces, including the pathological shear stress and exogenous stimuli (e.g., ultrasound, magnetic fields), used for the activation of mechanoresponsive DDS are also introduced, followed by in vitro and in vivo experimental models used to investigate and validate such novel therapies. Overall, this review aims to propose a fresh perspective through identified challenges and proposed solutions that could be of benefit for the further development of this exciting field.

Physical Concept to Explain the Regulation of Lipid Membrane Phase Separation under Isothermal Conditions

Lateral phase separation within lipid bilayer membranes has attracted considerable attention in the fields of biophysics and cell biology. Living cells organize laterally segregated compartments, such as raft domains in an ordered phase, and regulate their dynamic structures under isothermal conditions to promote cellular functions. Model membrane systems with minimum components are powerful tools for investigating the basic phenomena of membrane phase separation. With the use of such model systems, several physicochemical characteristics of phase separation have been revealed. This review focuses on the isothermal triggering of membrane phase separation from a physical point of view. We consider the free energy of the membrane that describes lateral phase separation and explain the experimental results of model membranes to regulate domain formation under isothermal conditions. Three possible regulation factors are discussed: electrostatic interactions, chemical reactions and membrane tension. These findings may contribute to a better understanding of membrane lateral organization within living cells that function under isothermal conditions and could be useful for the development of artificial cell engineering.

Domain dynamics of phase-separated lipid membranes under shear flow

The dynamical behaviour of lateral domains on phase-separated lipid vesicles under external flow is reported. A microfluidic chamber was used for the immobilization of vesicles and the application of shear. Microscopic observation revealed that domains tended to be localized at the vortex center and to exhibit a stripe morphology as the flow speed increased. We clarified the dependency of domain behaviors on the flow speed and lipid mixing fraction. The cholesterol ratio in the membrane affected these domain behaviors. Next, we investigated the growth of domains under flow. We discuss the mechanism of these trends by considering the free energy of phase separation, and reproduce the experimental results by numerical simulations. These findings may lead to a better understanding of the dynamical properties of the membrane under nonequilibrium situations and the biophysical mechanism of cellular mechanotransduction.

Glomerular Biomechanical Stress and Lipid Mediators during Cellular Changes Leading to Chronic Kidney Disease

Hyperfiltration is an important underlying cause of glomerular dysfunction associated with several systemic and intrinsic glomerular conditions leading to chronic kidney disease (CKD). These include obesity, diabetes, hypertension, focal segmental glomerulosclerosis (FSGS), congenital abnormalities and reduced renal mass (low nephron number). Hyperfiltration-associated biomechanical forces directly impact the cell membrane, generating tensile and fluid flow shear stresses in multiple segments of the nephron. Ongoing research suggests these biomechanical forces as the initial mediators of hyperfiltration-induced deterioration of podocyte structure and function leading to their detachment and irreplaceable loss from the glomerular filtration barrier. Membrane lipid-derived polyunsaturated fatty acids (PUFA) and their metabolites are potent transducers of biomechanical stress from the cell surface to intracellular compartments. Omega-6 and ω-3 long-chain PUFA from membrane phospholipids generate many versatile and autacoid oxylipins that modulate pro-inflammatory as well as anti-inflammatory autocrine and paracrine signaling. We advance the idea that lipid signaling molecules, related enzymes, metabolites and receptors are not just mediators of cellular stress but also potential targets for developing novel interventions. With the growing emphasis on lifestyle changes for wellness, dietary fatty acids are potential adjunct-therapeutics to minimize/treat hyperfiltration-induced progressive glomerular damage and CKD.

Role of Negatively Charged Lipids Achieving Rapid Accumulation of Water-Soluble Molecules and Macromolecules into Cell-Sized Liposomes against a Concentration Gradient

Liposomes, molecular self-assemblies resembling biological membranes, are a promising scaffold to investigate the physicochemical logic behind the complexity of living cells. Despite elaborate synthetic studies constructing cell-like chemical systems using liposomes, less attention has been paid to the proactive role of the membrane emerging as dynamics of the molecular self-assembly. This study investigated the liposomes containing anionic phospholipids by exposing them to steady flow conditions using a newly constructed automatic microfluidic observation platform. We demonstrated that the liposomes accumulated even macromolecules under the microfluidic condition without pore formation. By investigating the effect of composition of liposomes and visualizing negatively charged phospholipids upon the flow, we presumed that the external flow caused a compositional asymmetry of anionic phospholipids between the inner/outer leaflets, and the asymmetry enabled a rapid accumulation of those molecules against the concentration gradient. The current study opens new research interests regarding the nature of biological membranes under steady flow conditions.

Sequential storage and release of microdroplets

Droplet microfluidic methods have opened up the possibility of studying a plethora of phenomena ranging from biological to physical or chemical processes at ultra low volumes and high throughput. A key component of such approaches is the ability to trap droplets for observation, and many device architectures for achieving this objective have been developed. A challenge with such approaches is, however, recovering the droplets following their confinement for applications involving further analysis. Here, we present a device capable of generating, confining and releasing microdroplets in a sequential manner. Through a combination of experimental and computational simulations, we shed light on the key features required for successful droplet storage and retrieval. Moreover, we explore the effect of the flow rate of the continuous phase on droplet release, determining that a critical rate is needed to ensure complete droplet deformation through constrictions holding the droplets in place prior to release. Finally, we find that once released, droplets can be retrieved and collected off chip. The ability to generate, store and sequentially release droplets renders such a device particularly promising for future applications where reactions may not only be monitored on-chip, but droplets can also be retrieved for further analysis, facilitating new exploratory avenues in the fields of analytical chemistry and biology.

Core-shell hydrogel microcapsules enable formation of human pluripotent stem cell spheroids and their cultivation in a stirred bioreactor

Cellular therapies based on human pluripotent stem cells (hPSCs) offer considerable promise for treating numerous diseases including diabetes and end stage liver failure. Stem cell spheroids may be cultured in stirred bioreactors to scale up cell production to cell numbers relevant for use in humans. Despite significant progress in bioreactor culture of stem cells, areas for improvement remain. In this study, we demonstrate that microfluidic encapsulation of hPSCs and formation of spheroids. A co-axial droplet microfluidic device was used to fabricate 400 μm diameter capsules with a poly(ethylene glycol) hydrogel shell and an aqueous core. Spheroid formation was demonstrated for three hPSC lines to highlight broad utility of this encapsulation technology. In-capsule differentiation of stem cell spheroids into pancreatic β-cells in suspension culture was also demonstrated.

Imaging non-classical mechanical responses of lipid membranes using molecular rotors

Lipid packing in cellular membranes has a direct effect on membrane tension and microviscosity, and plays a central role in cellular adaptation, homeostasis and disease. According to conventional mechanical descriptions, viscosity and tension are directly interconnected, with increased tension leading to decreased membrane microviscosity. However, the intricate molecular interactions that combine to build the structure and function of a cell membrane suggest a more complex relationship between these parameters. In this work, a viscosity-sensitive fluorophore (‘molecular rotor’) is used to map changes in microviscosity in model membranes under conditions of osmotic stress. Our results suggest that the relationship between membrane tension and microviscosity is strongly influenced by the bilayer's lipid composition. In particular, we show that the effects of increasing tension are minimised for membranes that exhibit liquid disordered (L_d) – liquid ordered (L_o) phase coexistence; while, surprisingly, membranes in pure gel and L_o phases exhibit a negative compressibility behaviour, i.e. they soften upon compression.

Viscosity-sensitive molecular rotors demonstrate that the non-classical mechanical behaviour of model lipid membranes is able to buffer external stress.

Viscosity Landscape of Phase-Separated Lipid Membrane Estimated from Fluid Velocity Field

In cell membranes, the functional constituents such as peptides, proteins, and polysaccharides diffuse in a sea of lipids as single molecules and molecular aggregates. Thus, the fluidity of the heterogeneous multicomponent membrane is important for understanding the roles of the membrane in cell functionality. Recently, Henle and Levine described the hydrodynamics of molecular diffusion in a spherical membrane. A tangential point force at the north pole induces a pair of vortices whose centers lie on a line perpendicular to the point force and are symmetrical with respect to the point force. The position of the vortex center depends on ηm/Rηw, where R is the radius of the spherical membrane, and ηm and ηw are the viscosities of the membrane and the surrounding medium, respectively. Based on this theoretical prediction, we applied a point force to a phase-separated spherical vesicle composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1,2-dioleoyl-sn-glycero-3-phosphocholine/cholesterol by means of a microinjection technique. The pathlines were visualized by trajectories of microdomains. We determined the position of the vortex center and estimated the membrane viscosity using the dependence of the position of the vortex center on ηm/Rηw. The obtained apparent membrane viscosities for various compositions are mapped on the phase diagram. The membrane viscosity is almost constant in the range of 0 <ϕLo ≤ 0.5 (ϕLo: area fraction of the liquid ordered phase), whereas that in the range of 0.5 ≤ ϕLo < 1.0 exponentially increases with increase of ϕLo. The obtained viscosity landscape provides a basic understanding of the fluidity of heterogeneous multicomponent membranes.

Osmotic-Tension-Induced Membrane Lateral Organization

Alteration of lipid raft organization manifesting as phase separation is important for cellular processes, such as signaling and trafficking. Such behaviors and dynamics of lipid membranes can be affected by external stimuli including both physical and chemical stimuli. In this study, we focused on osmotic-tension-induced phase separation. The effects of osmotic tension on the phase behaviors of vesicles consisting of dioleoylphosphocholine (DOPC)/dipalmitoylphosphocholine (DPPC)/cholesterol (Chol) were quantitatively studied at different temperatures by fluorescence microscopy. We determined the ternary phase diagrams and found that tension leads to a shift in the miscibility temperature. Cholesterol plays a key role in determining the extent of this shift. In addition, we found that osmotic tension can enhance the line tension. The physicochemical mechanism of osmotic-pressure-induced phase separation is discussed.

Shear-induced microstructures and dynamics processes of phospholipid cylinders in solutions

Shear-induced microstructures and their corresponding dynamic processes are investigated for phospholipid cylinders in aqueous solution by dissipative particle dynamic simulation. Various phospholipid cylinders with cross-sections, which are formed under shear-free flow, are selected to examine the effects of shear flow on their structures and dynamic processes. Shear flow induces the transition from cylinders into vesicles at weak rate and the transition into vesicle–lamella mixtures with increased shear rate and lamella structures at the strong shear rate. Then, the average radius of gyration and shape factors of the polymer chains in the dynamic processes are discussed in detail. Results show that shear flow causes the structure of the polymer chains to be elongated along the shear direction, and the configuration of the polymer chain can be rapidly transformed into an ellipsoid structure under strong shear.

Microfluidic Handling and Analysis of Giant Vesicles for Use as Artificial Cells: A Review

One of the goals of synthetic biology is the bottom-up construction of an artificial cell, the successful realization of which could shed light on how cellular life emerged and could also be a useful tool for studying the function of modern cells. Using liposomes as biomimetic containers is particularly promising because lipid membranes are biocompatible and much of the required machinery can be reconstituted within them. Giant lipid vesicles have been used extensively in other fields such as biophysics and drug discovery, but their use as artificial cells has only recently seen an increase. Despite the prevalence of giant vesicles, many experiments remain challenging or impossible due to their delicate nature compared to biological cells. This review aims to highlight the effectiveness of microfluidic technologies in handling and analyzing giant vesicles. The advantages and disadvantages of different microfluidic approaches and what new insights can be gained from various applications are introduced. Finally, future directions are discussed in which the unique combination of microfluidics and giant lipid vesicles can push forward the bottom-up construction of artificial cells.

Ultra-high capacity microfluidic trapping of giant vesicles for high-throughput membrane studies

Biomimetic systems such as model lipid membranes are vital to many research fields including synthetic biology, drug discovery and membrane biophysics. One of the most commonly used are giant unilamellar vesicles (GUVs) due to their size similarity with biological cells and their ease of production. Typical methods for handling such delicate objects are low-throughput and do not allow solution exchange or long-term observations, all of which limits the experimental options. Herein, we present a new device designed to confine large assemblies of GUVs in microfluidic traps but is still able to perform precise and fast solution exchanges. An optimised design allows efficient filling with as many as 114 GUVs per trap and over 23 000 GUVs per device. This allows high-throughput dataset acquisitions which we demonstrate with two proof-of-concept experiments: (i) end-point measurements of vesicle interior pH and (ii) membrane transport kinetics. Moreover, we show that the design is able to selectively trap sub-populations of specific vesicle sizes and assemble them in different layers. The device can easily be applied to other high-throughput membrane studies and will pave the way for future applications using vesicle assemblies to model cellular tissues or even prototissues.

Self-assembly of phospholipid molecules in solutions under shear flows: Microstructures and phase diagrams

Shear-induced microstructures and their phase diagrams were investigated for phospholipid molecules in aqueous solution by dissipative particle dynamic simulation. Self-assembled microstructures, including spherical and cylindrical micelles, spherical vesicles, lamellae, undulated lamellae, perforated lamellae, and continuous networks, were observed under various shear flows and phospholipid concentrations, where the spatial inhomogeneity and symmetry were analysed. A series of phase diagrams were constructed based on the chain lengths under various phospholipid concentrations. The phase distributions showed that the structures with spherical symmetry could be shear-induced to structures with cylindrical symmetry in the dilute solutions. In the semi-concentrated solutions, the lamellae were located in most spaces under zero shear flows, which could be shear-induced into undulated lamellae and then into cylindrical micelles. For the concentrated solutions, the strong shear flows oriented the directions of multilayer lamellae and phase transitions appeared between several cylindrical network structures. These observations on shear-induced microstructures and their distributions revealed a promising approach that could be used to design bio-microstructures based on phospholipid molecules under shear flows.

Membrane deformation and layer-by-layer peeling of giant vesicles induced by the pore-forming toxin pneumolysin

Membranes under attack by the pore-forming toxin pneumolysin reveal a hitherto unknown layer-by-layer peeling mechanism and disclose the multilamellar structure.

Tunable Membrane Potential Reconstituted in Giant Vesicles Promotes Permeation of Cationic Peptides at Nanomolar Concentrations

We investigate the influence of membrane potential on the permeation of cationic peptides. Therefore, we employ a microfluidic chip capable of capturing giant unilamellar vesicles (GUVs) in physical traps and fast exchange of chemical compounds. Control experiments with calcein proved that the vesicle membranes' integrity is not affected by the physical traps and applied shear forces. Combined with fluorescence correlation spectroscopy, permeation of fluorescently labeled peptides across vesicle membranes can be measured down to the nanomolar level. With the addition of a lipophilic ruthenium(II) complex Ru(C17)22+, GUVs consisting of mixed acyl phospholipids are prepared with a negative membrane potential, resembling the membrane asymmetry in cells. The membrane potential serves as a driving force for the permeation of cationic cell-penetrating peptides (CPPs) nonaarginine (Arg9) and the human immunodeficiency virus trans-activator of transcription (TAT) peptide already at nanomolar doses. Hyperpolarization of the membrane by photo-oxidation of Ru(C17)22+ enhances permeation significantly from 55 to 78% for Arg9. This specific enhancement for Arg9 (cf. TAT) is ascribed to the higher affinity of the arginines to the phosphoserine head groups. On the other hand, permeation is decreased by introducing an additional negative charge in close proximity to the N-terminal arginine residue when changing the fluorophore. In short, with the capability to reconstitute membrane potential as well as shear stress, our system is a suitable platform for modeling the membrane permeability of pharmaceutics candidates. The results also highlight the membrane potential as a major cause of discrepancies between vesicular and cellular studies on CPP permeation.

Observation of molecular self-assembly events in massively parallel microdroplet arrays

The self-assembly of peptide and protein molecules into nanoscale filaments is a process associated with both biological function and malfunction. Microfluidic techniques can provide powerful tools in the study of such aggregation phenomena while providing access to exploring the role of molecular interactions in disease development. Yet, a common challenge encountered in the study of protein aggregation is the difficulty in achieving spatial and temporal control of the underlying processes. Here, we present a planar (2-D) device allowing for both the generation and confinement of 10 000 monodisperse water-in-oil droplets in an array of chambers with a trapping efficiency of 99%. Due to the specific geometry of the device, droplets can be formed and immediately trapped on the same chip, without the need for continuous flow of the oil phase. Furthermore, we demonstrate the capability of this device as a platform to study the aggregation kinetics and determine stochastic molecular nanoscale self-assembly events in a highly parallel manner for the aggregation of the dipeptide, diphenylalanine, the core recognition motif of the Aβ-42 peptide associated with Alzheimer's disease. The ability to reproducibly generate and confine monodisperse water-in-oil droplets with an extremely high trapping efficiency while maintaining entrapment under zero-flow conditions, on timescales compatible with observing molecular self-assembly events, renders it promising for numerous potential further applications in the biological and biophysical fields.

Single-Vesicle Assays Using Liposomes and Cell-Derived Vesicles: From Modeling Complex Membrane Processes to Synthetic Biology and Biomedical Applications

The plasma membrane is of central importance for defining the closed volume of cells in contradistinction to the extracellular environment. The plasma membrane not only serves as a boundary, but it also mediates the exchange of physical and chemical information between the cell and its environment in order to maintain intra- and intercellular functions. Artificial lipid- and cell-derived membrane vesicles have been used as closed-volume containers, representing the simplest cell model systems to study transmembrane processes and intracellular biochemistry. Classical examples are studies of membrane translocation processes in plasma membrane vesicles and proteoliposomes mediated by transport proteins and ion channels. Liposomes and native membrane vesicles are widely used as model membranes for investigating the binding and bilayer insertion of proteins, the structure and function of membrane proteins, the intramembrane composition and distribution of lipids and proteins, and the intermembrane interactions during exo- and endocytosis. In addition, natural cell-released microvesicles have gained importance for early detection of diseases and for their use as nanoreactors and minimal protocells. Yet, in most studies, ensembles of vesicles have been employed. More recently, new micro- and nanotechnological tools as well as novel developments in both optical and electron microscopy have allowed the isolation and investigation of individual (sub)micrometer-sized vesicles. Such single-vesicle experiments have revealed large heterogeneities in the structure and function of membrane components of single vesicles, which were hidden in ensemble studies. These results have opened enormous possibilities for bioanalysis and biotechnological applications involving unprecedented miniaturization at the nanometer and attoliter range. This review will cover important developments toward single-vesicle analysis and the central discoveries made in this exciting field of research.

Out-of-equilibrium microcompartments for the bottom-up integration of metabolic functions

Self-sustained metabolic pathways in microcompartments are the corner-stone for living systems. From a technological viewpoint, such pathways are a mandatory prerequisite for the reliable design of artificial cells functioning out-of-equilibrium. Here we develop a microfluidic platform for the miniaturization and analysis of metabolic pathways in man-made microcompartments formed of water-in-oil droplets. In a modular approach, we integrate in the microcompartments a nicotinamide adenine dinucleotide (NAD)-dependent enzymatic reaction and a NAD-regeneration module as a minimal metabolism. We show that the microcompartments sustain a metabolically active state until the substrate is fully consumed. Reversibly, the external addition of the substrate reboots the metabolic activity of the microcompartments back to an active state. We therefore control the metabolic state of thousands of independent monodisperse microcompartments, a step of relevance for the construction of large populations of metabolically active artificial cells.

Pneumolysin-damaged cells benefit from non-homogeneous toxin binding to cholesterol-rich membrane domains

Nucleated cells eliminate lesions induced by bacterial pore-forming toxins, such as pneumolysin via shedding patches of damaged plasmalemma into the extracellular milieu. Recently, we have shown that the majority of shed pneumolysin is present in the form of inactive pre-pores. This finding is surprising considering that shedding is triggered by Ca²⁺-influx following membrane perforation and therefore is expected to positively discriminate for active pores versus inactive pre-pores. Here we provide evidence for the existence of plasmalemmal domains that are able to attract pneumolysin at high local concentrations. Within such a domain an immediate plasmalemmal perforation induced by a small number of pneumolysin pores would be capable of triggering the elimination of a large number of not yet active pre-pores/monomers and thus pre-empt more frequent and perilous perforation events. Our findings provide further insights into the functioning of the cellular repair machinery which benefits from an inhomogeneous plasmalemmal distribution of pneumolysin.

The Effects of Shear Force Transmission Across Vesicle Membranes

We report a comprehensive study on mechanotransmission of shear forces across lipid bilayer membranes of giant unilamellar vesicles (GUVs). GUVs containing fluorescent tracer particles were immobilized on a microfluidic platform and exposed to shear flows. A method was developed for the visualization of three-dimensional flows at high precision by defocusing microscopy. We quantify the symmetry of external flow around the GUV and show its effects on vortex flows and luminal dynamics. With increasing asymmetry, luminal vortices merged while liquid exchange in between them increased. The effect of membrane composition was studied through addition of cholesterol. Mechanotransmission efficacy, quantified by the ratio of luminal flow to external flow, ranged from ε = 0.094 (0 mol % cholesterol) to ε = 0.043 (16 mol % cholesterol). Our findings give new cues to the mechanisms underlying the sensing of strength and spatial distribution of shear forces by cells and the impact of membrane composition.

Rheological Droplet Interface Bilayers (rheo-DIBs): Probing the Unstirred Water Layer Effect on Membrane Permeability via Spinning Disk Induced Shear Stress

A new rheological droplet interface bilayer (rheo-DIB) device is presented as a tool to apply shear stress on biological lipid membranes. Despite their exciting potential for affecting high-throughput membrane translocation studies, permeability assays conducted using DIBs have neglected the effect of the unstirred water layer (UWL). However as demonstrated in this study, neglecting this phenomenon can cause significant underestimates in membrane permeability measurements which in turn limits their ability to predict key processes such as drug translocation rates across lipid membranes. With the use of the rheo-DIB chip, the effective bilayer permeability can be modulated by applying shear stress to the droplet interfaces, inducing flow parallel to the DIB membranes. By analysing the relation between the effective membrane permeability and the applied stress, both the intrinsic membrane permeability and UWL thickness can be determined for the first time using this model membrane approach, thereby unlocking the potential of DIBs for undertaking diffusion assays. The results are also validated with numerical simulations.

Phase Behavior of Charged Vesicles Under Symmetric and Asymmetric Solution Conditions Monitored with Fluorescence Microscopy

Phase-separated giant unilamellar vesicles (GUVs) exhibiting coexisting liquid-ordered and liquid-disordered domains are a common biophysical tool to investigate the lipid raft hypothesis. Numerous studies, however, neglect the impact of physiological solution conditions. On that account, the current work presents the effect of high-salinity buffer and trans-membrane solution asymmetry on liquid-liquid phase separation in charged GUVs grown from dioleylphosphatidylglycerol, egg sphingomyelin, and cholesterol. The effects were studied under isothermal and varying temperature conditions.

We describe equipment and experimental strategies applicable for monitoring the stability of coexisting liquid domains in charged vesicles under symmetric and asymmetric high-salinity solution conditions. This includes an approach to prepare charged multicomponent GUVs in high-salinity buffer at high temperatures. The protocol entails the option to perform a partial exchange of the external solution by a simple dilution step while minimizing the vesicle dilution. An alternative approach is presented utilizing a microfluidic device that allows for a complete external solution exchange. The solution effects on phase separation were also studied under varying temperatures. To this end, we present the basic design and utility of an in-house built temperature control chamber. Furthermore, we reflect on the assessment of the GUV phase state, pitfalls associated with it and how to circumvent them.

A microfluidic device for the delivery of enzymes into cells by liposome fusion

Liposomes are versatile carriers of drugs or biomolecules and are ideally suited to transport molecules into cells. However, mechanistic studies to understand and improve the fusion of liposomes with cell membranes and endosomes are difficult. Here, we report a method that allows for stable coimmobilization of liposomes and living cells, thereby bringing the membranes into close contact, which is essential for membrane fusion. The small unilamellar liposomes are tethered to the surface by a linker so that no modification of the liposome membrane for cell binding is required. The cells are positioned above the liposomes by posts that are integrated into the microfluidic device, and a pH drop induces the fusion of the cell-liposome membranes. Both membrane fusion and release of molecules into the cytosol are visualized by fluorescence dequenching assays. Furthermore, we proved the efficient delivery of the enzyme β-galactosidase into the cells when a fusogenic liposome composition was used. The device could be used for fusion studies but is also a versatile means for cell transfection.