Bending membranes on demand: fluid phospholipid bilayers on topographically deformable substrates

PDMS as a Substrate for Lipid Bilayers

PDMS (polydimethylsiloxane) is a cheap, optically clear polymer that is elastic and can be easily and quickly fabricated into a wide array of microscale and nanoscale architectures, making it a versatile substrate for biophysical experiments on cell membranes. It is easy to imagine many new experiments will be devised that require a bilayer to be placed upon a substrate that is flexible or easily cast into a desired geometry, such as in lab-on-a-chip, organ-on-chip, and microfluidic applications, or for building accurate membrane models that replicate the surface structure and elasticity of the cytoskeleton. However, PDMS has its limitations, and the extent to which the behavior of membranes is affected on PDMS has not been fully explored. We use AFM and fluorescence optical microscopy to investigate the use of PDMS as a substrate for the formation and study of supported lipid bilayers (SLBs). Lipid bilayers form on plasma-treated PDMS and show free diffusion and normal phase transitions, confirming its suitability as a model bilayer substrate. However, lipid-phase separation on PDMS is severely restricted due to the pinning of domains to surface roughness, resulting in the cessation of lateral hydrodynamic flow. We show the high-resolution porous structure of PDMS and the extreme smoothing effect of oxygen plasma treatment used to hydrophilize the surface, but this is not flat enough to allow domain formation. We also observe bilayer degradation over hour timescales, which correlates with the known hydrophobic recovery of PDMS, and establish a critical water contact angle of 30°, above which bilayers degrade or not form at all. Care must be taken as incomplete surface oxidation and hydrophobic recovery result in optically invisible membrane disruption, which will also be transparent to fluorescence microscopy and lipid diffusion measurements in the early stages.

Structural Characterization of Nanoparticle-Supported Lipid Bilayer Arrays by Grazing Incidence X-ray and Neutron Scattering

Arrays of nanoparticle-supported lipid bilayers (nanoSLB) are lipid-coated nanopatterned interfaces that provide a platform to study curved model biological membranes using surface-sensitive techniques. We combined scattering techniques with direct imaging, to gain access to sub-nanometer scale structural information on stable nanoparticle monolayers assembled on silicon crystals in a noncovalent manner using a Langmuir-Schaefer deposition. The structure of supported lipid bilayers formed on the nanoparticle arrays via vesicle fusion was investigated using a combination of grazing incidence X-ray and neutron scattering techniques complemented by fluorescence microscopy imaging. Ordered nanoparticle assemblies were shown to be suitable and stable substrates for the formation of curved and fluid lipid bilayers that retained lateral mobility, as shown by fluorescence recovery after photobleaching and quartz crystal microbalance measurements. Neutron reflectometry revealed the formation of high-coverage lipid bilayers around the spherical particles together with a flat lipid bilayer on the substrate below the nanoparticles. The presence of coexisting flat and curved supported lipid bilayers on the same substrate, combined with the sub-nanometer accuracy and isotopic sensitivity of grazing incidence neutron scattering, provides a promising novel approach to investigate curvature-dependent membrane phenomena on supported lipid bilayers.

Multiscale compression-induced restructuring of stacked lipid bilayers: From buckling delamination to molecular packing

Lipid membranes in nature adapt and reconfigure to changes in composition, temperature, humidity, and mechanics. For instance, the oscillating mechanical forces on lung cells and alveoli influence membrane synthesis and structure during breathing. However, despite advances in the understanding of lipid membrane phase behavior and mechanics of tissue, there is a critical knowledge gap regarding the response of lipid membranes to micromechanical forces. Most studies of lipid membrane mechanics use supported lipid bilayer systems missing the structural complexity of pulmonary lipids in alveolar membranes comprising multi-bilayer interconnected stacks. Here, we elucidate the collective response of the major component of pulmonary lipids to strain in the form of multi-bilayer stacks supported on flexible elastomer substrates. We utilize X-ray diffraction, scanning probe microscopy, confocal microscopy, and molecular dynamics simulation to show that lipid multilayered films both in gel and fluid states evolve structurally and mechanically in response to compression at multiple length scales. Specifically, compression leads to increased disorder of lipid alkyl chains comparable to the effect of cholesterol on gel phases as a direct result of the formation of nanoscale undulations in the lipid multilayers, also inducing buckling delamination and enhancing multi-bilayer alignment. We propose this cooperative short- and long-range reconfiguration of lipid multilayered films under compression constitutes a mechanism to accommodate stress and substrate topography. Our work raises fundamental insights regarding the adaptability of complex lipid membranes to mechanical stimuli. This is critical to several technologies requiring mechanically reconfigurable surfaces such as the development of electronic devices interfacing biological materials.

Insights into Membrane Curvature Sensing and Membrane Remodeling by Intrinsically Disordered Proteins and Protein Regions

Cellular membranes are highly dynamic in shape. They can rapidly and precisely regulate their shape to perform various cellular functions. The protein’s ability to sense membrane curvature is essential in various biological events such as cell signaling and membrane trafficking. As they are bound, these curvature-sensing proteins may also change the local membrane shape by one or more curvature driving mechanisms. Established curvature-sensing/driving mechanisms rely on proteins with specific structural features such as amphipathic helices and intrinsically curved shapes. However, the recent discovery and characterization of many proteins have shattered the protein structure–function paradigm, believing that the protein functions require a unique structural feature. Typically, such structure-independent functions are carried either entirely by intrinsically disordered proteins or hybrid proteins containing disordered regions and structured domains. It is becoming more apparent that disordered proteins and regions can be potent sensors/inducers of membrane curvatures. In this article, we outline the basic features of disordered proteins and regions, the motifs in such proteins that encode the function, membrane remodeling by disordered proteins and regions, and assays that may be employed to investigate curvature sensing and generation by ordered/disordered proteins.

Acetylcholinesterase Activity Influenced by Lipid Membrane Area and Surface Acoustic Waves

According to the current model of nerve propagation, the function of acetylcholinesterase (AChE) is to terminate synaptic transmission of nerve signals by hydrolyzing the neurotransmitter acetylcholine (ACh) in the synaptic cleft to acetic acid (acetate) and choline. However, extra-synaptic roles, which are known as ‘non-classical’ roles, have not been fully elucidated. Here, we measured AChE activity with the enzyme bound to lipid membranes of varying area per enzyme in vitro using the Ellman assay. We found that the activity was not affected by density fluctuations in a supported lipid bilayer (SLB) induced by standing surface acoustic waves. Nevertheless, we found twice as high activity in the presence of small unilamellar vesicles (SUV) compared to lipid-free samples. We also showed that the increase in activity scaled with the available membrane area per enzyme.

A multiplexed phospholipid membrane platform for curvature sensitive protein screening

The curvature of lipid membranes plays a key role in many relevant biological processes such as membrane trafficking, vesicular budding and host-virus interactions. In vitro studies on the membrane curvature of simplified biomimetic models in the nanometer range are challenging, due to their complicated nanofabrication processes. In this work, we propose a simple and low-cost platform for curvature sensitive protein screening, prepared through scanning probe lithography (SPL) methods, where lipid bilayer patches of different compositions can be multiplexed onto substrate areas with tailored local curvature. The curvature is imposed by anchoring nanoparticles of the desired size to the substrate prior to lithography. As a proof of principle, we demonstrate that a positive curvature membrane sensitive protein derived from the BAR domain of Nadrin2 binds selectively to lipid patches patterned on substrate areas coated with 100 nm nanoparticles. The platform opens up a path for screening curvature-dependent protein-membrane interaction studies by providing a flexible and easy to prepare substrate with control over lipid composition and membrane curvature.

Nanoscale Substrate Roughness Hinders Domain Formation in Supported Lipid Bilayers

Supported lipid bilayers are model membranes formed at solid substrate surfaces. This architecture renders the membrane experimentally accessible to surface-sensitive techniques used to study their properties, including atomic force microscopy, optical fluorescence microscopy, quartz crystal microbalance, and X-ray/neutron reflectometry, and allows integration with technology for potential biotechnological applications such as drug screening devices. The experimental technique often dictates substrate choice or treatment, and it is anecdotally recognized that certain substrates are suitable for a particular experiment, but the exact influence of the substrate has not been comprehensively investigated. Here, we study the behavior of a simple model bilayer, phase-separating on a variety of commonly used substrates, including glass, mica, silicon, and quartz, with drastically different results. The distinct micron-scale domains observed on mica, identical to those seen in free-floating giant unilamellar vesicles, are reduced to nanometer-scale domains on glass and quartz. The mechanism for the arrest of domain formation is investigated, and the most likely candidate is nanoscale surface roughness, acting as a drag on the hydrodynamic motion of small domains during phase separation. Evidence was found that the physicochemical properties of the surface have a mediating effect, most likely because of the changes in the lubricating interstitial water layer between the surface and bilayer.

Surface Instability of Bilayer Hydrogel Subjected to Both Compression and Solvent Absorption

The bilayered structure of hard thin film on soft substrate can lose stability and form specific patterns, such as wrinkles or creases, on the surface, induced by external stimuli. For bilayer hydrogels, the surface morphology caused by the instability is usually controlled by the solvent-induced swelling/shrinking and mechanical force. Here, two important issues on the instability of bilayer hydrogels, which were not considered in the previous studies, are focused on in this study. First, the upper layer of a hydrogel is not necessarily too thin. Thus we investigated how the thickness of the upper layer can affect the surface morphology of bilayer hydrogels under compression through both finite element (FE) simulation and theoretical analysis. Second, a hydrogel can absorb water molecules before the mechanical compression. The effect of the pre-absorption of water before the mechanical compression was studied through FE simulations and theoretical analysis. Our results show that when the thickness of the upper layer is very large, surface wrinkles can exist without transforming into period doublings. The pre-absorption of the water can result in folds or unexpected hierarchical wrinkles, which can be realized in experiments through further efforts.

Exploring Molecular-Biomembrane Interactions with Surface Plasmon Resonance and Dual Polarization Interferometry Technology: Expanding the Spotlight onto Biomembrane Structure

The molecular analysis of biomolecular-membrane interactions is central to understanding most cellular systems but has emerged as a complex technical challenge given the complexities of membrane structure and composition across all living cells. We present a review of the application of surface plasmon resonance and dual polarization interferometry-based biosensors to the study of biomembrane-based systems using both planar mono- or bilayers or liposomes. We first describe the optical principals and instrumentation of surface plasmon resonance, including both linear and extraordinary transmission modes and dual polarization interferometry. We then describe the wide range of model membrane systems that have been developed for deposition on the chips surfaces that include planar, polymer cushioned, tethered bilayers, and liposomes. This is followed by a description of the different chemical immobilization or physisorption techniques. The application of this broad range of engineered membrane surfaces to biomolecular-membrane interactions is then overviewed and how the information obtained using these techniques enhance our molecular understanding of membrane-mediated peptide and protein function. We first discuss experiments where SPR alone has been used to characterize membrane binding and describe how these studies yielded novel insight into the molecular events associated with membrane interactions and how they provided a significant impetus to more recent studies that focus on coincident membrane structure changes during binding of peptides and proteins. We then discuss the emerging limitations of not monitoring the effects on membrane structure and how SPR data can be combined with DPI to provide significant new information on how a membrane responds to the binding of peptides and proteins.

Scaling relationships for translational diffusion constants applied to membrane domain dissolution and growth

We compare the way that relationships for diffusion constants scale with the size of diffusing membrane domains and the geometry of their environments. Then, we review our experimental work on the dynamics of dissolution/growth of membrane domains in crowding induced mixing, phase separation, and Ostwald ripening in a highly confined environment. Overall, the scaling relationships applied to diffusion constants obtained by fits to our dynamic data indicate that dissolution and growth is influenced by the diffusion of clusters or small domains of lipids, in addition to kinetic processes and geometrical constraints.

The Detection of Nanoscale Membrane Bending with Polarized Localization Microscopy

The curvature of biological membranes at the nanometer scale is critically important for vesicle trafficking, organelle morphology, and disease propagation. The initiation of membrane bending occurs at a length scale that is irresolvable by most superresolution optical microscopy methods. Here, we report the development of polarized localization microscopy (PLM), a pointillist optical imaging technique for the detection of nanoscale membrane curvature in correlation with single-molecule dynamics and molecular sorting. PLM combines polarized total internal reflection fluorescence microscopy and single-molecule localization microscopy to reveal membrane orientation with subdiffraction-limited resolution without reducing localization precision by point spread function manipulation. Membrane curvature detection with PLM requires fewer localization events to detect curvature than three-dimensional single-molecule localization microscopy (e.g., photoactivated localization microscopy or stochastic optical reconstruction microscopy), which enables curvature detection 10× faster via PLM. With rotationally confined lipophilic fluorophores and the polarized incident fluorescence excitation, membrane-bending events are revealed with superresolution. Engineered hemispherical membrane curvature with a radius ≥24 nm was detected with PLM, and individual fluorophore localization precision was 13 ± 5 nm. Further, deciphering molecular mobility as a function of membrane topology was enabled. The diffusion coefficient of individual DiI molecules was 25 ± 5× higher in planar supported lipid bilayers than within nanoscale membrane curvature. Through the theoretical foundation and experimental demonstration provided here, PLM is poised to become a powerful technique for revealing the underlying biophysical mechanisms of membrane bending at physiological length scales.

Transition of surface-interface creasing in bilayer hydrogels

Controlling the morphologies and properties of the surface and/or interface of bimaterials consisting of soft polymers provides new opportunities in many engineering applications. Crease is a widely observed deformation mode in nature and engineering applications for soft polymers where the smooth surface folds into a region of self-contact with a sharp tip, usually induced by the instability from mechanical compression or swelling. In this work, we explore the competition mechanisms between surface and interface creases through numerical simulations and experimental studies on bilayer hydrogels. The surface or interface crease of the bilayer hydrogels under swelling is governed by both the modulus ratio (M₂/M₁) and the height ratio (H₂/H₁). Through extensive numerical simulations, we find that the interface crease of the bilayer hydrogels can only occur at a moderate modulus ratio (24 < M₂/M₁ < 96) and a large height ratio (H₂/H₁ ≥ 8). Guided by this phase diagram, our experiments confirm that both surface and interface creases can be generated by swelling triggered instability, and the transition of surface to interface creases occurs at the critical value of the height ratio (H₂/H₁) between 5 and 10. Such an observation is in good agreement with our numerical predictions. Fundamental understandings on the switching between the surface and interface creases provide new insights into the design of highly tunable soft materials and devices over a wide range of length scales.

Lipid Bilayers Are Long-Lived on Solvent Cleaned Plasma-Oxidized poly(dimethyl)siloxane (ox-PDMS)

Although it is well known that phospholipids self-assemble on hydrophilic plasma-oxidized PMDS surfaces (ox-PDMS) to form cell membrane mimetic bilayers, the temporal stability of phospholipid membranes on these surfaces is unknown. Here we report that phospholipid bilayers remain stable on solvent-cleaned ox-PDMS for at least 132 hours after preparation. Absent solvent cleaning, the bilayers were stable for only 36 hours. We characterized the phospholipid bilayers, i) through quantitative comparative analysis of the fluorescence intensity of phospholipid bilayers on ox-PDMS and phospholipid monolayers on native PDMS and, ii) through measurements of the diffusive mobility of the lipids through fluorescence recovery after photobleaching (FRAP). The fluorescence intensity of the phospholipid layer remained consistent with that of a bilayer for 132 hours. The evolution of the diffusive mobility of the phospholipids in the bilayer on ox-PDMS over time was similar to lipids in control bilayers prepared on glass surfaces. Solvent cleaning was essential for the long-term stability of the bilayers on ox-PDMS. Without cleaning in acetone and isopropanol, phospholipid bilayers prepared on ox-PDMS surfaces peeled off in large patches within 36 hours. Importantly, we find that phospholipid bilayers supported on solvent-cleaned ox-PDMS were indistinguishable from phospholipid bilayers supported on glass for at least 36 hours after preparation. Our results provide a link between the two common surfaces used to prepare in vitro biomimetic phospholipid membranes-i) glass surfaces used predominantly in fundamental biophysical experiments, for which there is abundant physicochemical information, with ii) ox-PDMS, the dominant material used in practical, applications-oriented systems to build micro-devices, topographically-patterned surfaces, and biosensors where there is a dearth of information.

Migration of phospholipid vesicles in response to OH(-) stimuli

We demonstrate migration of phospholipid vesicles in response to a pH gradient. Upon simple micro-injection of a NaOH solution, the vesicles linearly moved to the tip of the micro-pipette and the migration velocity was proportional to the gradient of OH(-) concentration. Vesicle migration was characteristic of OH(-) ions and no migration was observed for monovalent salts or nonionic sucrose solutions. The migration of vesicles is quantitatively described by the surface tension gradient model where the hydrolysis of the phospholipids by NaOH solution decreases the surface tension of the vesicle. The vesicles move toward a direction where the surface energy decreases. Thus the chemical modification of lipids produces a mechanical force to drive vesicles.

Reconstituting ring-rafts in bud-mimicking topography of model membranes

During vesicular trafficking and release of enveloped viruses, the budding and fission processes dynamically remodel the donor cell membrane in a protein- or a lipid-mediated manner. In all cases, in addition to the generation or relief of the curvature stress, the buds recruit specific lipids and proteins from the donor membrane through restricted diffusion for the development of a ring-type raft domain of closed topology. Here, by reconstituting the bud topography in a model membrane, we demonstrate the preferential localization of cholesterol- and sphingomyelin-enriched microdomains in the collar band of the bud-neck interfaced with the donor membrane. The geometrical approach to the recapitulation of the dynamic membrane reorganization, resulting from the local radii of curvatures from nanometre-to-micrometre scales, offers important clues for understanding the active roles of the bud topography in the sorting and migration machinery of key signalling proteins involved in membrane budding.

Thermal annealing triggers collapse of biphasic supported lipid bilayers into multilayer islands

The collapse of phase-separating single, supported lipid bilayers, consisting of mixtures of a zwitterionic phospholipid (POPC) and an anionic lipid (DPPA) upon thermal annealing in the presence of ions is examined using a combination of scanning probe, epifluorescence, and ellipsometric microscopies. We find that thermal annealing in the presence of ions in the bathing medium induces an irreversible transition from domain-textured, single supported bilayers to one comprising islands of multibilayer stacks, whose lateral area decays with lamellarity, producing pyramidal staircase "mesa" topography. The higher order lamellae are almost invariably localized above the anionic-lipid rich, gel-phase domains in the parent bilayer and depends on the ions in the bathing medium. The collapse mechanism appears to involve synergistic influences of two independent mechanisms: (1) stabilization of the incipient headgroup-headgroup interface in the emergent multibilayer configuration facilitated by ions in the bath and (2) domain-boundary templated folding. This collapse mechanism is consistent with previous theoretical predictions of topography-induced rippling instability in collapsing lipid monolayers and suggests the role of the mismatch in height and/or spontaneous curvature at domain boundaries in the collapse of phase-separated single supported bilayers.

Fluorescence imaging of MACPF/CDC proteins: new techniques and their application

Structural and biochemical investigations have helped illuminate many of the important details of MACPF/CDC pore formation. However, conventional techniques are limited in their ability to tackle many of the remaining key questions, and new biophysical techniques might provide the means to improve our understanding. Here we attempt to identify the properties of MACPF/CDC proteins that warrant further study, and explore how new developments in fluorescence imaging are able to probe these properties.

Compositional sorting dynamics in coexisting lipid bilayer phases with variations in underlying e-beam formed curvature pattern

Nanometer-scale curvature patterns of an underlying substrate are imposed on lipid multibilayers with each pattern imparting distinctly different sorting dynamics to a metastable pixelation pattern of coexisting liquid ordered (Lo)-liquid disordered (Ld) lipid phases. Therefore, this work provides pathways toward mechanical energy-based separations for analysis of biomembrane-associate species. The central design concept of the patterned sections of the silica substrate is a square lattice pattern of 100 nm projected radius poly(methyl methacrylate) (PMMA) hemispherical features formed by electron beam lithography which pixelates the coexisting phases in order to balance membrane bending and line energy. In one variation, we surround this pattern with three PMMA walls/fences 100 nm in height which substantially slows the loss of the high line energy pixelated Lo phase by altering the balance of two competing mechanism (Ostwald ripening vs. vesiculation). In another walled variation, we form a gradient of the spacing of the 100 nm features which forces partitioning of the Lo phase toward the end of the gradient with the most open (400 nm spacing) lattice pattern where a single vesicle could grow from the Lo phase. We show that two other variations distinctly impact the dynamics, demonstrating locally slowed loss of the high line energy pixelated Lo phase and spontaneous switching of the pixel location on the unit cell, respectively. Moreover, we show that the pixelation patterns can be regenerated and sharpened by a heating and cooling cycle. We argue that localized variations in the underlying curvature pattern have rather complex consequences because of the coupling and/or competition of dynamic processes to optimize mechanical energy such as lipid diffusion, vesiculation and growth, and phase/compositional partitioning.

Nanoscale patterning of membrane-bound proteins formed through curvature-induced partitioning of phase-specific receptor lipids

This work describes a technique for forming high-density arrays and patterns of membrane-bound proteins through binding to a curvature-organized compositional pattern of metal-chelating lipids (Cu(2+)-DOIDA or Cu(2+)-DSIDA). In this bottom-up approach, the underlying support is an e-beam formed, square lattice pattern of hemispheres. This curvature pattern sorts Cu(2+)-DOIDA to the 200 nm hemispherical lattice sites of a 600 nm × 600 nm unit cell in Ld - Lo phase separated lipid multibilayers. Binding of histidine-tagged green fluorescent protein (His-GFP) creates a high density array of His-GFP-bound pixels localized to the square lattice sites. In comparison, the negative pixel pattern is created by sorting Cu(2+)-DSIDA in Ld - Lβ' phase separated lipid multibilayers to the flat grid between the lattice sites followed by binding to His-GFP. Lattice defects in the His-GFP pattern lead to interesting features such as pattern circularity. We also observe defect-free arrays of His-GFP that demonstrate perfect arrays can be formed by this method suggesting the possibility of using this approach for the localization of various active molecules to form protein, DNA, or optically active molecular arrays.

Metastability in pixelation patterns of coexisting fluid lipid bilayer phases imposed by e-beam patterned substrates

We study the dynamic evolution of pixilation patterns of the liquid-ordered (Lo) phase in coexistence with the liquid-disordered phase in lipid multibilayers. The pixilation patterns were formed by imposing lattice patterns of localized high curvature on phase-separating multibilayers using curvature-patterned regions of an underlying support. The projected radius of underlying hemisphere-like features, that provided the local curvature, was varied from 60 nm to 100 nm and the square lattice spacing between the features was varied between 200 nm and 400 nm using standard electron (e) -beam lithography. Over time, the area fraction of the Lo phase on the patterned regions of the substrate decreased toward zero at room temperature. This apparent metastability of the pattern derives from the high line energy of a pixelation pattern where a Boltzmann distribution shows near zero equilibrium partitioning of the Lo phase in the patterned regions. Kinetic rate analysis identifies two pattern-dependent mechanisms that dominate the transition to zero Lo area fraction; diffusion limited dissolution of the Lo phase driven by an Ostwald ripening-type process or the cooperative formation of vesicles containing Lo phase lipids. Interestingly, we observed the spontaneous formation of tubules in the corners of the array due to the high local curvature applied to the membrane. Furthermore we show that it is possible to regenerate pixilation patterns on the curvature-patterned regions by cooling below room temperature. Regenerated area fractions are in agreement with a room-temperature composition of primarily Ld phase and the high degree of overlap with the original patterns is suggestive of fixed nucleation sites.

Substrate Effects on the Formation Process, Structure and Physicochemical Properties of Supported Lipid Bilayers

Supported lipid bilayers are artificial lipid bilayer membranes existing at the interface between solid substrates and aqueous solution. Surface structures and properties of the solid substrates affect the formation process, fluidity, two-dimensional structure and chemical activity of supported lipid bilayers, through the 1–2 nm thick water layer between the substrate and bilayer membrane. Even on SiO₂/Si and mica surfaces, which are flat and biologically inert, and most widely used as the substrates for the supported lipid bilayers, cause differences in the structure and properties of the supported membranes. In this review, I summarize several examples of the effects of substrate structures and properties on an atomic and nanometer scales on the solid-supported lipid bilayers, including our recent reports.

Curvature sorting of peripheral proteins on solid-supported wavy membranes

Cellular membrane deformation and the associated redistribution of membrane-bound proteins are important aspects of membrane function. Current model membrane approaches for studying curvature sensing are limited to positive curvatures and often require complex and delicate experimental setups. To overcome these challenges, we fabricated a wavy substrate by imposing a range of curvatures onto an adhering lipid bilayer membrane. We examined the curvature sorting of several peripheral proteins binding to the wavy membrane and observed them to partition into distinct regions of curvature. Furthermore, single-molecule imaging experiments suggested that the curvature sensing of proteins on low-curvature substrates requires cooperative interactions.

Thinning and shaping solid films into functional and integrative nanomembranes

Conventional solid films on certain substrates play a crucial role in various applications, for example in flat panel displays, silicon technology, and protective coatings. Recently, tremendous attention has been directed toward the thinning and shaping of solids into so-called nanomembranes, offering a unique and fantastic platform for research in nanoscience and nanotechnology. In this Review, a conceptual description of nanomembranes is introduced and a series of examples demonstrate their great potential for future applications. The thinning of nanomembranes indeed offers another strategy to fabricate nanomaterials, which can be integrated onto a chip and exhibit valuable properties (e.g. giant persistent photoconductivity and thermoelectric property). Furthermore, the stretching of nanomembranes enables a macroscale route for tuning the physical properties of the membranes at the nanoscale. The process by which nanomembranes release from a substrate presents several approaches to shaping nanomembranes into three-dimensional architectures, such as rolled-up tubes, wrinkles, and the resulting channels, which can provide fascinating applications in electronics, mechanics, fluidics, and photonics. Nanomembranes as a new type of nanomaterial promise to be an attractive direction for nanoresearch.

Lipid domain pixelation patterns imposed by e-beam fabricated substrates

This work describes a technique for forming nanometer-scale pixilated lipid domains that are self-organized into geometric patterns residing on a square lattice. In this process, a lipid multibilayer stack is deposited onto a silica substrate patterned with a square lattice array of bumps, hemispherical on their sides, formed by electron beam lithography. Domain patterns are shown to be confined to the flat grid between the bumps and composed of connected and individual domain pixels. Analysis of lattices of varying sizes shows that domain pattern formation is driven by mechanical energy minimization and packing constraints. We demonstrate single lattice sizes and a gradient in lattice size varying from the micrometer to the 100 nm scale applicable to precise arraying, patterning, and transport of biomolecules that partition to lipid domains.

Nonintercalating nanosubstrates create asymmetry between bilayer leaflets

The physical properties of lipid bilayers can be remodeled by a variety of environmental factors. Here we investigate using molecular dynamics simulations the specific effects of nanoscopic substrates or external contact points on lipid membranes. We expose palmitoyl-oleoyl phosphatidylcholine bilayers unilaterally and separately to various model nanosized substrates differing in surface hydroxyl densities. We find that a surface hydroxyl density as low as 10% is sufficient to keep the bilayer juxtaposed to the substrate. The bilayer interacts with the substrate indirectly through multiple layers of water molecules; however, despite such buffered interaction, the bilayers exhibit certain properties different from unsupported bilayers. The substrates modify transverse lipid fluctuations, charge density profiles, and lipid diffusion rates, although differently in the two leaflets, which creates an asymmetry between bilayer leaflets. Other properties that include lipid cross-sectional areas, component volumes, and order parameters are minimally affected. The extent of asymmetry that we observe between bilayer leaflets is well beyond what has been reported for bilayers adsorbed on infinite solid supports. This is perhaps because the bilayers are much closer to our nanosized finite supports than to infinite solid supports, resulting in a stronger support-bilayer electrostatic coupling. The exposure of membranes to nanoscopic contact points, therefore, cannot be considered as a simple linear interpolation between unsupported membranes and membranes supported on infinite supports. In the biological context, this suggests that the exposure of membranes to nonintercalating proteins, such as those belonging to the cytoskeleton, should not always be considered as passive nonconsequential interactions.

Programmed bending reveals dynamic mechanochemical coupling in supported lipid bilayers

In living cells, mechanochemical coupling represents a dynamic means by which membrane components are spatially organized. An extra-ordinary example of such coupling involves curvature-dependent polar localization of chemically-distinct lipid domains at bacterial poles, which also undergo dramatic reequilibration upon subtle changes in their interfacial environment such as during sporulation. Here, we demonstrate that such interfacially-triggered mechanochemical coupling can be recapitulated in vitro by simultaneous, real-time introduction of mechanically-generated periodic curvatures and attendant strain-induced lateral forces in lipid bilayers supported on elastomeric substrates. In particular, we show that real-time wrinkling of the elastomeric substrate prompts a dynamic domain reorganization within the adhering bilayer, producing large, oriented liquid-ordered domains in regions of low curvature. Our results suggest a mechanism in which interfacial forces generated during surface wrinkling and the topographical deformation of the bilayer combine to facilitate dynamic reequilibration prompting the observed domain reorganization. We anticipate this curvature-generating model system will prove to be a simple and versatile tool for a broad range of studies of curvature-dependent dynamic reorganizations in membranes that are constrained by the interfacial elastic and dynamic frameworks such as the cell wall, glycocalyx, and cytoskeleton.

DNA concentration modulation on supported lipid bilayers switched by surface acoustic waves

Spatially addressable arrays of molecules embedded in or anchored to supported lipid bilayers are important for on-chip screening and binding assays; however, methods to sort or accumulate components in a fluid membrane on demand are still limited. Here we apply in-plane surface acoustic shear waves (SAWs) to laterally accumulate double-stranded DNA segments electrostatically bound to a cationic supported lipid bilayer. The fluorescently labeled DNA segments are found to segregate into stripe patterns with a spatial frequency corresponding to the periodicity of the standing SAW wave (~10 μm). The DNA molecules are accumulated 10-fold in the regions of SAW antinodes. The superposition of two orthogonal sets of SAW sources creates checkerboard like arrays of DNA demonstrating the potential to generate arrayed fields dynamically. The pattern relaxation time of 0.58 s, which is independent of the segment length, indicates a sorting and relaxation mechanism dominated by lipid diffusion rather than DNA self-diffusion.

Supported lipid bilayers on biocompatible polysaccharide multilayers

Formation of supported lipid bilayers on soft polymer cushions is a useful approach to decouple the membrane from the substrate for applications involving membrane proteins. We prepared biocompatible polymer cushions by the layer-by-layer assembly of two polysaccharide polyelectrolytes, chitosan (CHI) and hyaluronic acid, on glass and silicon substrates. (CHI/HA)(5) films were characterized by atomic force microscopy, giving an average thickness of 57 nm and roughness of 25 nm in aqueous solution at pH 6.5. Formation of zwitterionic lipid bilayers by the vesicle fusion method was attempted using DOPC vesicles at pH 4 and 6.5 on (CHI/HA)(5) films. At higher pH adsorbed lipids had low mobility and large immobile lipid fractions; a combination of fluorescence and AFM indicated that this was attributable to formation of poor quality membranes with defects and pinned lipids rather than to a layer of surface-adsorbed vesicles. By contrast, more uniform bilayers with mobile lipids were produced at pH 4. Fluorescence recovery after photobleaching gave diffusion coefficients that were similar to those for bilayers on PEG cushions and considerably higher than those measured on other polyelectrolyte films. The results suggest that the polymer surface charge is more important than the surface roughness in controlling formation of mobile supported bilayers. These results demonstrate that polysaccharides provide a useful alternative to other polymer cushions, particularly for applications where biocompatibility is important.

ALIGNMENT AND DEFORMATION OF LIPID BILAYER DOMAINS IN VESICLES ADHERING TO MICROSTRUCTURED SUBSTRATES

In heterogeneous lipid membranes, the lateral organization is coupled to local curvature as the membrane bending energies depend on composition. We investigate phase-separated vesicles of ternary lipid composition in contact with structured surfaces, where the distinct elastic properties of the Lo and Ld phases come into play. We show that fused silica substrates with microstructured grooves induce sorting, alignment and deformation of Lo domains in adhering vesicles. The same phenomenon is observed on flat, chemically modified substrates with alternating stripes of rough and smooth regions. In both cases it is the Lo phase which accumulates over the smooth substrate and membrane-spanned groove regions respectively. Deformation of Lo domains occurs when domain diameters grow beyond the width of the microstructured stripes. Domain alignment was also observed in binary membranes featuring gel and fluid phase coexistence showing the generic character of domain sorting on microstructured surfaces.

Autonomic self-healing lipid monolayer: a new class of ultrathin dielectric

The electrical performance of stabilized lipid monolayers on H-terminated silicon is reported for the first time. We show that these 2.7 nm thick only ultrathin layers present extremely low current leakage at high electric field and high breakdown voltage that both compare favorably with the best data reported on organic thin film dielectrics. We demonstrate a very unique property of autonomic self-healing of the layer at room temperature with the total recovery of its performance after electrical breakdown. The mechanisms involved in breakdown and self-healing are described.

Mechanics of surface area regulation in cells examined with confined lipid membranes

Cells are wrapped in inelastic membranes, yet they can sustain large mechanical strains by regulating their area. The area regulation in cells is achieved either by membrane folding or by membrane exo- and endocytosis. These processes involve complex morphological transformations of the cell membrane, i.e., invagination, vesicle fusion, and fission, whose precise mechanisms are still under debate. Here we provide mechanistic insights into the area regulation of cell membranes, based on the previously neglected role of membrane confinement, as well as on the strain-induced membrane tension. Commonly, the membranes of mammalian and plant cells are not isolated, but rather they are adhered to an extracellular matrix, the cytoskeleton, and to other cell membranes. Using a lipid bilayer, coupled to an elastic sheet, we are able to demonstrate that, upon straining, the confined membrane is able to regulate passively its area. In particular, by stretching the elastic support, the bilayer laterally expands without rupture by fusing adhered lipid vesicles; upon compression, lipid tubes grow out of the membrane plane, thus reducing its area. These transformations are reversible, as we show using cycles of expansion and compression, and closely reproduce membrane processes found in cells during area regulation. Moreover, we demonstrate a new mechanism for the formation of lipid tubes in cells, which is driven by the membrane lateral compression and may therefore explain the various membrane tubules observed in shrinking cells.

Transport, separation, and accumulation of proteins on supported lipid bilayers

Transport, separation, and accumulation of proteins in their natural environment are central goals in protein biotechnology. Miniaturized assays of supported lipid bilayers (SLBs) have been proposed as promising candidates to realize such technology on a chip, but a modular system for the controlled transport of membrane proteins does not exist. In this letter, we demonstrate that standing surface acoustic waves drive the in-plane redistribution of proteins on planar SLBs over macroscopic distances (3.5 mm). Accumulation of proteins in periodic patterns of about 10-fold protein concentration difference is accomplished and shown to relax into the homogeneous state by diffusion. Different proteins separate in individual fractions from a homogeneous distribution and are transported and accumulated into clusters using beats. The modular planar setup has the potential of integrating other lab-on-a-chip tools, for monitoring the membrane-protein integrity or adding microfluidic features for blood screening or DNA analysis.

Templating membrane assembly, structure, and dynamics using engineered interfaces

The physical and chemical properties of biological membranes are intimately linked to their bounding aqueous interfaces. Supported phospholipid bilayers, obtained by surface-assisted rupture, fusion, and spreading of vesicular microphases, offer a unique opportunity, because engineering the substrate allows manipulation of one of the two bilayer interfaces as well. Here, we review a collection of recent efforts, which illustrates deliberate substrate-membrane coupling using structured surfaces exhibiting chemical and topographic patterns. Vesicle fusion on chemically patterned substrates results in co-existing lipid phases, which reflect the underlying pattern of surface energy and wettability. These co-existing bilayer/monolayer morphologies are useful both for fundamental biophysical studies (e.g., studies of membrane asymmetry) as well as for applied work, such as synthesizing large-scale arrays of bilayers or living cells. The use of patterned, static surfaces provides new models to design complex membrane topographies and curvatures. Dynamic switchable-topography surfaces and sacrificial trehalose based-substrates reveal abilities to dynamically introduce membrane curvature and change the nature of the membrane-substrate interface. Taken together, these studies illustrate the importance of controlling interfaces in devising model membrane platforms for fundamental biophysical studies and bioanalytical devices.

Dynamic patterns in a supported lipid bilayer driven by standing surface acoustic waves

In the past decades supported lipid bilayers (SLBs) have been an important tool in order to study the physical properties of biological membranes and cells. So far, controlled manipulation of SLBs is very limited. Here we present a new technology to create lateral patterns in lipid membranes controllable in both space and time. Surface acoustic waves (SAWs) are used to generate lateral standing waves on a piezoelectric substrate which create local "traps" in the lipid bilayer and lead to a lateral modulation in lipid concentration. We demonstrate that pattern formation is reversible and does not affect the integrity of the lipid bilayer as shown by extracting the diffusion constant of fluid membranes. The described method could possibly be used to design switchable interfaces for the lateral transport and organization of membrane bound macromolecules to create dynamic bioarrays and control biofilm formation.

Nondisruptive micropatterning of fluid membranes through selective vesicular adsorption and rupture by nanotopography

We report on a nondisruptive method of patterning fluid membranes into micrometer-scale arrays through a selective vesicular rupture pathway by nanotopography. The site- and pathway-selective formation of supported lipid bilayers (SLBs) was achieved by different vesicular adsorption and rupture processes between nanocorrugated and nanosmooth topographies. The SLBs were first developed in the nanocorrugated region due to fast vesicular adsorption and then grew into the nanosmooth region through bilayer edge-induced vesicular rupture. Our topographic approach provides a viable scheme, yet unattainable in conventional ways, of actively controlling the position and the coverage of the SLBs on a variety of substrates without disrupting two-dimensional fluidity for highly integrated membrane devices.

Formation and dynamics of supported phospholipid membranes on a periodic nanotextured substrate

We have studied and modeled the morphology and dynamics of fluid planar lipid bilayer membranes supported on a textured silicon substrate. The substrate is fabricated to have channels on its surface that are a few hundred nanometers across, with a channel depth of a few hundred nanometers perpendicular to the plane of observation. Using atomic force microscopy and quantitative fluorescence microscopy, we have shown that the bilayer assemblies conform to the underlying nanostructured substrate. As far as dynamics is concerned, when observed over length scales exceeding the dimensions of the nanostructured features, the macroscopic diffusion is anisotropic. However, the macroscopic anisotropy is well simulated using models of diffusion on the nanostructured surface that consider the lipids to diffuse homogeneously and isotropically on the supporting substrate. Consistent with previous observations on less well characterized or less periodic nanostructures, we find that the nanostructured substrate produces an effective anisotropy in macroscopic diffusion of the conformal membrane. More importantly, we demonstrate how quantitative analysis of dynamics probed by larger-scale fluorescence imaging can yield information on nanoscale thin-film morphology.

Influence of "Flexible" versus "Rigid" Nanoparticles on the Stability of Matrix Metalloproteinase-7

Matrix Metalloproteinase-7 (MMP-7) is invariably expressed in a variety of cancer cells, and exhibits the potentials to interact with differently charged macromolecular surfaces 1. To ascertain whether the nature of the charge carrying surfaces influences the stability as well as catalytic properties of the enzyme, we compared the effects of differently charged lipid (representative of "flexible") and gold ("rigid") nanoparticles. The experimental data revealed that the catalytic activity of MMP-7 is impaired only by the positively charged lipid nanoparticles, and it remains unaffected by their negatively charged or neutral counterparts. On the other hand, both positively and negatively charged gold nanoparticles impair the enzyme activity with nearly equal potency; no significant influence of neutral gold nanoparticles was noted on the enzyme activity. Unlike lipid nanoparticles, the charged gold nanoparticles mediated effects were found to be manifested partially via the inactivation of the enzyme. Arguments are presented that both the "rigidity" as well as the surface curvature of the lipid ("flexible") vis a vis the gold ("rigid") nanoparticles are responsible for eliciting differential influence on the catalytic activity as well as the stability of MMP-7.