Lipid Bilayer Microarray for Parallel Recording of Transmembrane Ion Currents

Label-free quantification of passive membrane permeability of cyclic peptides across lipid bilayers: penetration speed of cyclosporin A across lipid bilayers

Cyclic peptides that passively penetrate cell membranes are under active investigation in drug discovery research. PAMPA (Parallel Artificial Membrane Permeability Assay) and Caco-2 assay are mainly used for permeability measurements in these studies. However, permeability rates across the artificial membrane and the cell monolayer used for these assays are intrinsically different from the ones across pure lipid bilayers. There are also membrane permeability assays for peptides using reconstructed lipid bilayers, but they require labeling for detection, and the absolute membrane permeability of the natural peptides themselves could not be determined. Here, we constructed a lipid bilayer permeability assay and realized the first label-free measurements of the lipid bilayer permeability of cyclic peptides. Quantitative permeability values across lipid bilayers were determined for model cyclic hexapeptides and an important natural product, cyclosporin A (CsA). The obtained quantitative permeability values will provide new and advanced knowledge about the passive permeability of cyclic peptides.

Encapsulated droplet interface bilayers as a platform for high-throughput membrane studies

Whilst it is highly desirable to produce artificial lipid bilayer arrays allowing for systematic high-content screening of membrane conditions, it remains a challenge due to the combined requirements of scaled membrane production, simple measurement access, and independent control over individual bilayer experimental conditions. Here, droplet bilayers encapsulated within a hydrogel shell are output individually into multi-well plates for simple, arrayed quantitative measurements. The afforded experimental throughput is used to conduct a 2D concentration screen characterising the synergistic pore-forming peptides Magainin2 and PGLa. Maximal enhanced activity is revealed at equimolar peptide concentrations via a membrane dye leakage assay, a finding consistent with models proposed from NMR data. The versatility of the platform is demonstrated by performing in situ electrophysiology, revealing low conductance pore activity (∼15 to 20 pA with 4.5 pA sub-states). In conclusion, this array platform addresses the aforementioned challenges and provides new and flexible opportunities for high-throughput membrane studies. Furthermore, the ability to engineer droplet networks within each construct paves the way for "lab-in-a-capsule" approaches accommodating multiple assays per construct and allowing for communicative reaction pathways.

Probing Thermotropic Phase Behavior of Dipalmitoylphosphatidylcholine Bilayers from Electrical and Topographic Data in a Horizontal Black Lipid Membrane Model

Here, a homemade device allowed preparing horizontal lipid bilayer membranes (hBLMs) for recording electrical and topographical data simultaneously and in real-time, under temperature (T)-controlled conditions along a cooling process of dipalmitoylphosphatidylcholine (DPPC) bilayers. Electrical parameters (ionic current intensity, I, and transmembrane resistance, R = ΔV/I) plotted against T exhibited discontinuities at the main transition (T_Pβ'→Lα) and pretransition (T_Lβ→Pβ') temperatures of DPPC. The T-dependent sensitivity to ΔV-induced electrostriction was revealed by capacitance measurements. The patterns of I fluctuation described long-range correlations reflected by 1/f-type noise in the ripple phase (P_β') and Brownian-type fluctuations in the liquid-crystalline (L_α) phase at voltage intensities lower than a voltage threshold (ΔV_th = ±160 mV), indicating that autocorrelations arise from an underlying structural connectivity that takes place within ordered phases. At |V| ≥ V_th, the fluctuation dynamics exhibited a 1/f behavior over the whole temperature range analyzed, suggesting that upon a certain intensity of external electrical perturbation, the membrane system evolves toward a voltage-induced percolated-pore state. At T > T_Pβ'→Lα, differential interference contrast micrographs showed droplet-like structures, probably containing solvent traces of the lipid solution, which were reverted upon cooling. However, droplets did not interfere with the thermotropic equilibrium of the bilayer phase. This suggested that the temperature-induced changes in the electrical properties of the bilayer, as well as in the complexity of the fluctuation patterns (emergency of long- and short-range correlations), were strongly associated with the characteristic thermotropic behavior of DPPC, without significant deviations induced by the presence of residual n-decane in the bilayer. Our hBLM model membrane proved useful for correlating thermotropic phase changes with electro-biophysical and topographical information.

Geometric Effect for Biological Reactors and Biological Fluids

As expressed "God made the bulk; the surface was invented by the devil" by W. Pauli, the surface has remarkable properties because broken symmetry in surface alters the material properties. In biological systems, the smallest functional and structural unit, which has a functional bulk space enclosed by a thin interface, is a cell. Cells contain inner cytosolic soup in which genetic information stored in DNA can be expressed through transcription (TX) and translation (TL). The exploration of cell-sized confinement has been recently investigated by using micron-scale droplets and microfluidic devices. In the first part of this review article, we describe recent developments of cell-free bioreactors where bacterial TX-TL machinery and DNA are encapsulated in these cell-sized compartments. Since synthetic biology and microfluidics meet toward the bottom-up assembly of cell-free bioreactors, the interplay between cellular geometry and TX-TL advances better control of biological structure and dynamics in vitro system. Furthermore, biological systems that show self-organization in confined space are not limited to a single cell, but are also involved in the collective behavior of motile cells, named active matter. In the second part, we describe recent studies where collectively ordered patterns of active matter, from bacterial suspensions to active cytoskeleton, are self-organized. Since geometry and topology are vital concepts to understand the ordered phase of active matter, a microfluidic device with designed compartments allows one to explore geometric principles behind self-organization across the molecular scale to cellular scale. Finally, we discuss the future perspectives of a microfluidic approach to explore the further understanding of biological systems from geometric and topological aspects.

Tightly Sealed 3D Lipid Structure Monolithically Generated on Transparent SU-8 Microwell Arrays for Biosensor Applications

Artificial lipid membranes are excellent candidates for new biosensing platforms because their structures are similar to cell membranes and it is relatively easy to modify the composition of the membrane. The freestanding structure is preferable for this purpose because of the more manageable reconstitution of the membrane protein. Therefore, most of the lipid membranes for biosensing are based on two-dimensional structures that are fixed on a solid substrate (unlike floating liposomes) even though they have some disadvantages, such as low stability, small surface area, and potential retention of solvent in the membrane. In this paper, three-dimensional freestanding lipid bilayer (3D FLB) arrays were fabricated uniformly on SU-8 microwells without any toxic solvent. The 3D FLBs have better stability and larger surface area due to their cell-like structure. In order to improve the sealing characteristics of the 3D FLBs, the applied frequency of the ac field was controlled during the electroformation. The 3D FLBs were observed through transparent SU-8 microwell arrays using confocal microscopy and demonstrated perfect sealing until 5.5 days after the electroformation at more than 1 kHz. Also, the details of the sealing of a fixed 3D freestanding lipid structure were discussed for the first time. The unilamellarity and biofunctionality of the 3D FLBs were verified by a transport protein (α-hemolysin) assay.

Biomimetic Electronic Devices for Measuring Bacterial Membrane Disruption

Antibiotic discovery has experienced a severe slowdown in terms of discovery of new candidates. In vitro screening methods using phospholipids to model the bacterial membrane provide a route to identify molecules that specifically disrupt bacterial membranes causing cell death. Thanks to the electrically insulating properties of the major component of the cell membrane, phospholipids, electronic devices are highly suitable transducers of membrane disruption. The organic electrochemical transistor (OECT) is a highly sensitive ion-to-electron converter. Here, the OECT is used as a transducer of the permeability of a lipid monolayer (ML) at a liquid:liquid interface, designed to read out changes in ion flux caused by compounds that interact with, and disrupt, lipid assembly. This concept is illustrated using the well-documented antibiotic Polymixin B and the highly sensitive quantitation of permeability of the lipid ML induced by two novel recently described antibacterial amine-based oligothioetheramides is shown, highlighting molecular scale differences in their disruption capabilities. It is anticipated that this platform has the potential to play a role in front-line antimicrobial compound design and characterization thanks to the compatibility of semiconductor microfabrication technology with high-throughput readouts.

Lipid Membrane Nanosensors for Environmental Monitoring: The Art, the Opportunities, and the Challenges

The advent of nanotechnology has brought along new materials, techniques, and concepts, readily adaptable to lipid membrane-based biosensing. The transition from micro-sensors to nano-sensors is neither straightforward nor effortless, yet it leads to devices with superior analytical characteristics: ultra-low detectability, small sample volumes, better capabilities for integration, and more available bioelements and processes. Environmental monitoring remains a complicated field dealing with a large variety of pollutants, several decomposition products, or secondary chemicals produced ad hoc in the short- or medium term, many sub-systems affected variously, and many processes largely unknown. The new generation of lipid membranes, i.e., nanosensors, has the potential for developing monitors with site-specific analytical performance and operational stability, as well as analyte-tailored types of responses. This review presents the state-of-the art, the opportunities for niche applicability, and the challenges that lie ahead.

Functionalized Solid-State Nanopore Integrated in a Reusable Microfluidic Device for a Better Stability and Nanoparticle Detection

Electrical detection based on single nanopores is an efficient tool to detect biomolecules, particles and study their morphology. Nevertheless the surface of the solid-state membrane supporting the nanopore should be better controlled. Moreover, nanopore should be integrated within microfluidic architecture to facilitate control fluid exchanges. We built a reusable microfluidic system integrating a decorated membran, rendering the drain and refill of analytes and buffers easier. This process enhances strongly ionic conductance of the nanopore and its lifetime. We highlight the reliability of this device by detecting gold nanorods and spherical proteins.

Transmembrane Signaling with Lipid-Bilayer Assemblies as a Platform for Channel-Based Biosensing

Artificial and natural lipid membranes that elicit transmembrane signaling is are useful as a platform for channel-based biosensing. In this account we summarize our research on the design of transmembrane signaling associated with lipid bilayer membranes containing nanopore-forming compounds. Channel-forming compounds, such as receptor ion-channels, channel-forming peptides and synthetic channels, are embedded in planar and spherical bilayer lipid membranes to develop highly sensitive and selective biosensing methods for a variety of analytes. The membrane-bound receptor approach is useful for introducing receptor sites on both planar and spherical bilayer lipid membranes. Natural receptors in biomembranes are also used for designing of biosensing methods.

Liposomes and lipid bilayers in biosensors

Biosensors for the rapid, specific, and sensitive detection of analytes play a vital role in healthcare, drug discovery, food safety, and environmental monitoring. Although a number of sensing concepts and devices have been developed, many longstanding challenges to obtain inexpensive, easy-to-use, and reliable sensor platforms remain largely unmet. Nanomaterials offer exciting possibilities for enhancing the assay sensitivity and for lowering the detection limits down to single-molecule resolution. In this review, we present an overview of liposomes and lipid bilayers in biosensing applications. Lipid assemblies in the form of spherical liposomes or two-dimensional planar membranes have been widely used in the design of biosensing assays; in particular, we highlight a number of recent promising developments of biosensors based on liposomes in suspension, liposome arrays, and lipid bilayers arrays. Assay sensitivity and specificity are discussed, advantages and drawbacks are reviewed, and possible further developments are outlined.

Formation of droplet interface bilayers in a Teflon tube

Droplet-interface bilayers (DIBs) have applications in disciplines ranging from biology to computing. We present a method for forming them manually using a Teflon tube attached to a syringe pump; this method is simple enough it should be accessible to those without expertise in microfluidics. It exploits the properties of interfaces between three immiscible liquids, and uses fluid flow through the tube to pack together drops coated with lipid monolayers to create bilayers at points of contact. It is used to create functional nanopores in DIBs composed of phosphocholine using the protein α-hemolysin (αHL), to demonstrate osmotically-driven mass transfer of fluid across surfactant-based DIBs, and to create arrays of DIBs. The approach is scalable, and thousands of DIBs can be prepared using a robot in one hour; therefore, it is feasible to use it for high throughput applications.

Hydrodynamic trapping for rapid assembly and in situ electrical characterization of droplet interface bilayer arrays

The droplet interface bilayer (DIB) is a modular technique for assembling planar lipid membranes between water droplets in oil. The DIB method thus provides a unique capability for developing digital, droplet-based membrane platforms for rapid membrane characterization, drug screening and ion channel recordings. This paper demonstrates a new, low-volume microfluidic system that automates droplet generation, sorting, and sequential trapping in designated locations to enable the rapid assembly of arrays of DIBs. The channel layout of the device is guided by an equivalent circuit model, which predicts that a serial arrangement of hydrodynamic DIB traps enables sequential droplet placement and minimizes the hydrodynamic pressure developed across filled traps to prevent squeeze-through of trapped droplets. Furthermore, the incorporation of thin-film electrodes fabricated via evaporation metal deposition onto the glass substrate beneath the channels allows for the first time in situ, simultaneous electrical interrogation of multiple DIBs within a sealed device. Combining electrical measurements with imaging enables measurements of membrane capacitance and resistance and bilayer area, and our data show that DIBs formed in different trap locations within the device exhibit similar sizes and transport properties. Simultaneous, single channel recordings of ion channel gating in multiple membranes are obtained when alamethicin peptides are incorporated into the captured droplets, qualifying the thin-film electrodes as a means for measuring stimuli-responsive functions of membrane-bound biomolecules. This novel microfluidic-electrophysiology platform provides a reproducible, high throughput method for performing electrical measurements to study transmembrane proteins and biomembranes in low-volume, droplet-based membranes.

Membrane platforms for biological nanopore sensing and sequencing

In the past two decades, biological nanopores have been developed and explored for use in sensing applications as a result of their exquisite sensitivity and easily engineered, reproducible, and economically manufactured structures. Nanopore sensing has been shown to differentiate between highly similar analytes, measure polymer size, detect the presence of specific genes, and rapidly sequence nucleic acids translocating through the pore. Devices featuring protein nanopores have been limited in part by the membrane support containing the nanopore, the shortcomings of which have been addressed in recent work developing new materials, approaches, and apparatus resulting in membrane platforms featuring automatability and increased robustness, lifetime, and measurement throughput.

Droplet-interface-bilayer assays in microfluidic passive networks

Basic biophysical studies and pharmacological processes can be investigated by mimicking the intracellular and extracellular environments across an artificial cell membrane construct. The ability to reproduce in vitro simplified scenarios found in live cell membranes in an automated manner has great potential for a variety of synthetic biology and compound screening applications. Here, we present a fully integrated microfluidic system for the production of artificial lipid bilayers based on the miniaturisation of droplet-interface-bilayer (DIB) techniques. The platform uses a microfluidic design that enables the controlled positioning and storage of phospholipid-stabilized water-in-oil droplets, leading successfully to the scalable and automated formation of arrays of DIBs to mimic cell membrane processes. To ensure robustness of operation, we have investigated how lipid concentration, immiscible phase flow velocities and the device geometrical parameters affect the system performance. Finally, we produced proof-of-concept data showing that diffusive transport of molecules and ions across on-chip DIBs can be studied and quantified using fluorescence-based assays.

A droplet microfluidic system for sequential generation of lipid bilayers and transmembrane electrical recordings

This paper demonstrates a microfluidic system that automates i) formation of a lipid bilayer at the interface between a pair of nanoliter-sized aqueous droplets in oil, ii) exchange of one droplet of the pair to form a new bilayer, and iii) current measurements on single proteins. A new microfluidic architecture is introduced - a set of traps designed to localize the droplets with respect to each other and with respect to the recording electrodes. The system allows for automated execution of experimental protocols by active control of the flow on chip with the use of simple external valves. Formation of stable artificial lipid bilayers, incorporation of α-hemolysin into the bilayers and electrical measurements of ionic transport through the protein pore are demonstrated.

Automated formation of lipid membrane microarrays for ionic single-molecule sensing with protein nanopores

Efficient use of membrane protein nanopores in ionic single-molecule sensing requires technology for the reliable formation of suspended molecular membranes densely arrayed in formats that allow high-resolution electrical recording. Here, automated formation of bimolecular lipid layers is shown using a simple process where a poly(tetrafluoroethylene)-coated magnetic bar is remotely actuated to perform a turning motion, thereby spreading phospholipid in organic solvent on a nonpolar surface containing a <1 mm(2) 4 × 4 array of apertures with embedded microelectrodes (microelectrode cavity array). Parallel and high-resolution single-molecule detection by single nanopores is demonstrated on the resulting bilayer arrays, which are shown to form by a classical but very rapid self-assembly process. The technique provides a robust and scalable solution for the problem of reliable, automated formation of multiple independent lipid bilayers in a dense microarray format, while preserving the favorable electrical properties of the microelectrode cavity array.

Screening ion-channel ligand interactions with passive pumping in a microfluidic bilayer lipid membrane chip

We describe a scalable artificial bilayer lipid membrane platform for rapid electrophysiological screening of ion channels and transporters. A passive pumping method is used to flow microliter volumes of ligand solution across a suspended bilayer within a microfluidic chip. Bilayers are stable at flow rates up to ∼0.5 μl/min. Phospholipid bilayers are formed across a photolithographically defined aperture made in a dry film resist within the microfluidic chip. Bilayers are stable for many days and the low shunt capacitance of the thin film support gives low-noise high-quality single ion channel recording. Dose-dependent transient blocking of α-hemolysin with β-cyclodextrin (β-CD) and polyethylene glycol is demonstrated and dose-dependent blocking studies of the KcsA potassium channel with tetraethylammonium show the potential for determining IC50 values. The assays are fast (30 min for a complete IC50 curve) and simple and require very small amounts of compounds (100 μg in 15 μl). The technology can be scaled so that multiple bilayers can be addressed, providing a screening platform for ion channels, transporters, and nanopores.

Lipid bilayer arrays: cyclically formed and measured

Artificial lipid bilayers have many uses. They are well established for scientific studies of reconstituted ion channels, used to host engineered pore proteins for sensing, and can potentially be applied in DNA sequencing. Droplet bilayers have significant technological potential for enabling many of these applications due to their compatibility with automation and array platforms. To further develop this potential, we have simplified the formation and electrical measurement of droplet bilayers using an apparatus that only requires fluid dispensation. We achieved simultaneous bilayer formation and measurement over a 32-element array with ~80% yield and no operator input following fluid addition. Cycling these arrays resulted in the formation and measurement of 96 out of 120 possible bilayers in 80 minutes, a sustainable rate that could significantly increase with automation and greater parallelization. This turn-key, high-yield approach to making artificial lipid bilayers requires no training, making the capability of creating and measuring lipid bilayers and ion channels accessible to a much wider audience. In addition, this approach is low-cost, parallelizable, and automatable, allowing high-throughput studies of ion channels and pore proteins in lipid bilayers for sensing or screening applications.

Stochastic detection of Pim protein kinases reveals electrostatically enhanced association of a peptide substrate

In stochastic sensing, the association and dissociation of analyte molecules is observed as the modulation of an ionic current flowing through a single engineered protein pore, enabling the label-free determination of rate and equilibrium constants with respect to a specific binding site. We engineered sensors based on the staphylococcal α-hemolysin pore to allow the single-molecule detection and characterization of protein kinase-peptide interactions. We enhanced this approach by using site-specific proteolysis to generate pores bearing a single peptide sensor element attached by an N-terminal peptide bond to the trans mouth of the pore. Kinetics and affinities for the Pim protein kinases (Pim-1, Pim-2, and Pim-3) and cAMP-dependent protein kinase were measured and found to be independent of membrane potential and in good agreement with previously reported data. Kinase binding exhibited a distinct current noise behavior that forms a basis for analyte discrimination. Finally, we observed unusually high association rate constants for the interaction of Pim kinases with their consensus substrate Pimtide (~10(7) to 10(8) M(-1) · s(-1)), the result of electrostatic enhancement, and propose a cellular role for this phenomenon.

Automated parallel recordings of topologically identified single ion channels

Although ion channels are attractive targets for drug discovery, the systematic screening of ion channel-targeted drugs remains challenging. To facilitate automated single ion-channel recordings for the analysis of drug interactions with the intra- and extracellular domain, we have developed a parallel recording methodology using artificial cell membranes. The use of stable lipid bilayer formation in droplet chamber arrays facilitated automated, parallel, single-channel recording from reconstituted native and mutated ion channels. Using this system, several types of ion channels, including mutated forms, were characterised by determining the protein orientation. In addition, we provide evidence that both intra- and extracellular amyloid-beta fragments directly inhibit the channel open probability of the hBK channel. This automated methodology provides a high-throughput drug screening system for the targeting of ion channels and a data-intensive analysis technique for studying ion channel gating mechanisms.

Wicking: a rapid method for manually inserting ion channels into planar lipid bilayers

The planar lipid bilayer technique has a distinguished history in electrophysiology but is arguably the most technically difficult and time-consuming method in the field. Behind this is a lack of experimental consistency between laboratories, the challenges associated with painting unilamellar bilayers, and the reconstitution of ion channels into them. While there has be a trend towards automation of this technique, there remain many instances where manual bilayer formation and subsequent membrane protein insertion is both required and advantageous. We have developed a comprehensive method, which we have termed “wicking”, that greatly simplifies many experimental aspects of the lipid bilayer system. Wicking allows one to manually insert ion channels into planar lipid bilayers in a matter of seconds, without the use of a magnetic stir bar or the addition of other chemicals to monitor or promote the fusion of proteoliposomes. We used the wicking method in conjunction with a standard membrane capacitance test and a simple method of proteoliposome preparation that generates a heterogeneous mixture of vesicle sizes. To determine the robustness of this technique, we selected two ion channels that have been well characterized in the literature: CLIC1 and α-hemolysin. When reconstituted using the wicking technique, CLIC1 showed biophysical characteristics congruent with published reports from other groups; and α-hemolysin demonstrated Type A and B events when threading single stranded DNA through the pore. We conclude that the wicking method gives the investigator a high degree of control over many aspects of the lipid bilayer system, while greatly reducing the time required for channel reconstitution.

High yield, reproducible and quasi-automated bilayer formation in a microfluidic format

A microfluidic platform is reported for various experimentation schemes on cell membrane models and membrane proteins using a combination of electrical and optical measurements, including confocal microscopy. Bilayer lipid membranes (BLMs) are prepared in the device upon spontaneous and instantaneous thinning of the lipid solution in a 100-μm dry-etched aperture in a 12.5-μm thick Teflon foil. Using this quasi-automated approach, a remarkable 100% membrane formation yield is reached (including reflushing in 4% of the cases), and BLMs are stable for up to 36 h. Furthermore, the potential of this platform is demonstrated for (i) the in-depth characterization of BLMs comprising both synthetic and natural lipids (1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) and L-α-phosphatidylcholine (L-α-PC)/cholesterol, respectively) in terms of seal resistance, capacitance, surface area, specific capacitance, and membrane hydrophobic thickness; (ii) confocal microscopy imaging of phase separation in sphingomyelin/L-α-PC/cholesterol ternary membranes; (iii) electrical measurements of individual nanopores (α-hemolysin, gramicidin); and (iv) indirect assessment of the alteration of membrane properties upon exposure to chemical stimuli using the natural nanopore gramicidin as a sensor.

Construction of a Ca(2+)-gated artificial channel by fusing alamethicin with a calmodulin-derived extramembrane segment

Using native chemical ligation, we constructed a Ca(2+)-gated fusion channel protein consisting of alamethicin and the C-terminal domain of calmodulin. At pH 5.4 and in the absence of Ca(2+), this fusion protein yielded a burst-like channel current with no discrete channel conductance levels. However, Ca(2+) significantly lengthened the specific channel open state and increased the mean channel current, while Mg(2+) produced no significant changes in the channel current. On the basis of 8-anilinonaphthalene-1-sulfonic acid (ANS) fluorescent measurement, Ca(2+)-stimulated gating may be related to an increased surface hydrophobicity of the extramembrane segment of the fusion protein.

Nano bioresearch approach by microtechnology

To progress in basic science and drug development, convenient methodology for detecting specific biological molecules and their interaction in living organism is in high demand. After more than 20 years of increasing research efforts, micro and nanotechnologies are now mature to propose a new class of miniature devices and principles enabling compartmentalized bioassays. Among them, this review proposes various examples that include array of electro-active microwells for highly parallel single cell analysis, cost-effective nanofluidic for DNA separation, parallel enzymatic reaction in 100pL droplet and high-throughput platform for membrane proteins assays. The micro devices are presented with relevant experiments to foresee their future contribution to translational research and drug discovery.

Formation of lipid bilayers inside microfluidic channel array for monitoring membrane-embedded nanopores of phi29 DNA packaging nanomotor

An efficient method to form lipid bilayers inside an array of microfluidic channels has been developed and applied to monitor the membrane-embedded phi29 DNA packaging motor with an electrochemical characterization on a lab-on-a-chip (LOC) platform. A push-pull junction capturing approach was applied to confine a small amount of the lipid solution inside a microchannel. The selective permeability between solvents and water in PDMS was utilized to extract the solvent from the lipid solution, resulting in a self-formation of the lipid bilayer in the microchannel array. Each microchannel was independently connected to a silver/silver chloride (Ag/AgCl) electrode array, leading to a high-throughput monitoring of the nanopore insertion in the formed lipid bilayers. The formation of multiple lipid bilayers inside an array of microchannels and the simultaneous electrical and optical monitoring of multiple bilayer provides an efficient LOC platform for the further development of single phi29 motor pore sensing and high throughput single pore dsDNA sequencing.

Challenges in the Development of Functional Assays of Membrane Proteins

Lipid bilayers are natural barriers of biological cells and cellular compartments. Membrane proteins integrated in biological membranes enable vital cell functions such as signal transduction and the transport of ions or small molecules. In order to determine the activity of a protein of interest at defined conditions, the membrane protein has to be integrated into artificial lipid bilayers immobilized on a surface. For the fabrication of such biosensors expertise is required in material science, surface and analytical chemistry, molecular biology and biotechnology. Specifically, techniques are needed for structuring surfaces in the micro- and nanometer scale, chemical modification and analysis, lipid bilayer formation, protein expression, purification and solubilization, and most importantly, protein integration into engineered lipid bilayers. Electrochemical and optical methods are suitable to detect membrane activity-related signals. The importance of structural knowledge to understand membrane protein function is obvious. Presently only a few structures of membrane proteins are solved at atomic resolution. Functional assays together with known structures of individual membrane proteins will contribute to a better understanding of vital biological processes occurring at biological membranes. Such assays will be utilized in the discovery of drugs, since membrane proteins are major drug targets.

Miniaturised technologies for the development of artificial lipid bilayer systems

Artificially reproducing cellular environments is a key aim of synthetic biology, which has the potential to greatly enhance our understanding of cellular mechanisms. Microfluidic and lab-on-a-chip (LOC) techniques, which enable the controlled handling of sub-microlitre volumes of fluids in an automated and high-throughput manner, can play a major role in achieving this by offering alternative and powerful methodologies in an on-chip format. Such techniques have been successfully employed over the last twenty years to provide innovative solutions for chemical analysis and cell-, molecular- and synthetic- biology. In the context of the latter, the formation of artificial cell membranes (or artificial lipid bilayers) that incorporate membrane proteins within miniaturised LOC architectures offers huge potential for the development of highly sensitive molecular sensors and drug screening applications. The aim of this review is to give a comprehensive and critical overview of the field of microsystems for creating and exploiting artificial lipid bilayers. Advantages and limitations of three of the most popular approaches, namely suspended, supported and droplet-based lipid bilayers, are discussed. Examples are reported that show how artificial cell membrane microsystems, by combining together biological procedures and engineering techniques, can provide novel methodologies for basic biological and biophysical research and for the development of biotechnology tools.

Activity monitoring of functional OprM using a biomimetic microfluidic device

This paper describes the fabrication and use of a biomimetic microfluidic device for the monitoring of a functional porin reconstituted within a miniaturized suspended artificial bilayer lipid membrane (BLM). Such a microfluidic device allows for (1) fluidic and electrical access to both sides of the BLM and (2) reproducible membrane protein insertion and long-term electrical monitoring of its conductance (G(i)), thanks to the miniaturization of the BLM. We demonstrate here for the first time the feasibility to insert a large trans-membrane protein through its β-barrel, and monitor its functional activity for more than 1 hour (limited by buffer evaporation). In this paper, we specifically used our device for the monitoring of OprM, a bacterial efflux channel involved in the multidrug resistance of the bacteria Pseudomonas aeruginosa. Sub-steps of the OprM channel conductance were detected during the electrical recordings within our device, which might be due to oscillations between several structural conformations (sub-states) adopted by the protein, as part of its opening mechanism. This work is a first step towards the establishment of a genuine platform dedicated to the investigation of bacterial proteins under reconstituted conditions, a very promising tool for the screening of new inhibitors against bacterial channels involved in drug resistance.

Natural and artificial ion channels for biosensing platforms

The single-molecule selectivity and specificity of the binding process together with the expected intrinsic gain factor obtained when utilizing flow through a channel have attracted the attention of analytical chemists for two decades. Sensitive and selective ion channel biosensors for high-throughput screening are having an increasing impact on modern medical care, drug screening, environmental monitoring, food safety, and biowarefare control. Even virus antigens can be detected by ion channel biosensors. The study of ion channels and other transmembrane proteins is expected to lead to the development of new medications and therapies for a wide range of illnesses. From the first attempts to use membrane proteins as the receptive part of a sensor, ion channels have been engineered as chemical sensors. Several other types of peptidic or nonpeptidic channels have been investigated. Various gating mechanisms have been implemented in their pores. Three technical problems had to be solved to achieve practical biosensors based on ion channels: the fabrication of stable lipid bilayer membranes, the incorporation of a receptor into such a structure, and the marriage of the modified membrane to a transducer. The current status of these three areas of research, together with typical applications of ion-channel biosensors, are discussed in this review.

Nanopore-based single-molecule mass spectrometry on a lipid membrane microarray

We report on parallel high-resolution electrical single-molecule analysis on a chip-based nanopore microarray. Lipid bilayers of <20 μm diameter containing single alpha-hemolysin pores were formed on arrays of subpicoliter cavities containing individual microelectrodes (microelectrode cavity array, MECA), and ion conductance-based single molecule mass spectrometry was performed on mixtures of poly(ethylene glycol) molecules of different length. We thereby demonstrate the function of the MECA device as a chip-based platform for array-format nanopore recordings with a resolution at least equal to that of established single microbilayer supports. We conclude that devices based on MECAs may enable more widespread analytical use of nanopores by providing the high throughput and ease of operation of a high-density array format while maintaining or exceeding the precision of state-of-the-art microbilayer recordings.

Microfluidic lipid membrane formation on microchamber arrays

We present a simple method to form free-standing lipid membranes on arrayed microchambers (>100). The formed membranes are perpendicular to an imaging plane with control of solute concentration on each side of the membranes. This platform let us quantitatively detect membrane transport of non-charged fluorescent molecules, induced by membrane proteins.

Automatable lipid bilayer formation and ion channel measurement using sessile droplets

Artificial lipid bilayer membranes have been used to reconstitute ion channels for scientific and technological applications. Membrane formation has traditionally involved slow, labor intensive processes best suited to small scale laboratory experimentation. We have recently demonstrated a high throughput method of membrane formation using automated liquid-handling robotics. We describe here the integration of membrane formation and measurement with two methods compatible with automation and high throughput liquid-handling robotics. Both of these methods create artificial lipid bilayers by joining lipid monolayers self-assembled at the interface of aqueous and organic phases using sessile aqueous droplets in contact with a measurement electrode; one using a pin tool, commonly employed in high throughput fluid handling assays, and the other using a positive displacement pipette. Membranes formed with both methods were high quality and supported measurement of ion channels at the single molecule level. Full automation of bilayer production and measurement with the positive displacement pipette was demonstrated by integrating it with a motion control platform.

Evolution of supported planar lipid bilayers on step-controlled sapphire surfaces

Self-organized step/terrace structures on a sapphire surface were used to investigate interface properties between a solid surface and a supported planar lipid bilayer (SPB). We prepared random-stepped, single-stepped and multistepped sapphire surfaces. Some multistepped surfaces covered with crossing steps exhibit phase-separation into hydrophilic and hydrophobic domains. We studied evolution of self-spreading lipid bilayers that are subject to the atomic structures and chemical states on the surfaces. The growth direction of SPBs in the self-spreading method is regulated by the atomic steps. While the SPBs were apparently uniform after a 1 h self-spreading, a density gradient of the lipid molecules was observed even after 24 h spreading. We found that various patterns of the SPBs that depend on the density of the lipid molecules are self-assembled on the phase-separated surfaces. Although the SPB is supported on the sapphire surface via an about 1 nm water layer, the self-spreading direction and the morphology of the SPBs are affected by the atomic steps, whose height is much smaller than that of the water layer.

Microelectrode arrays: a physiologically based neurotoxicity testing platform for the 21st century

Microelectrode arrays (MEAs) have been in use over the past decade and a half to study multiple aspects of electrically excitable cells. In particular, MEAs have been applied to explore the pharmacological and toxicological effects of numerous compounds on spontaneous activity of neuronal and cardiac cell networks. The MEA system enables simultaneous extracellular recordings from multiple sites in the network in real time, increasing spatial resolution and thereby providing a robust measure of network activity. The simultaneous gathering of action potential and field potential data over long periods of time allows the monitoring of network functions that arise from the interaction of all cellular mechanisms responsible for spatio-temporal pattern generation. In these functional, dynamic systems, physical, chemical, and pharmacological perturbations are holistically reflected by the tissue responses. Such features make MEA technology well suited for the screening of compounds of interest, and also allow scaling to high throughput systems that can record from multiple, separate cell networks simultaneously in multi-well chips or plates. This article is designed to be useful to newcomers to this technology as well as those who are currently using MEAs in their research. It explains how MEA systems operate, summarizes what systems are available, and provides a discussion of emerging mathematical schemes that can be used for a rapid classification of drug or chemical effects. Current efforts that will expand this technology to an influential, high throughput, electrophysiological approach for reliable determinations of compound toxicity are also described and a comprehensive review of toxicological publications using MEAs is provided as an appendix to this publication. Overall, this article highlights the benefits and promise of MEA technology as a high throughput, rapid screening method for toxicity testing.

Surface Engineering and Patterning Using Parylene for Biological Applications

Parylene is a family of chemically vapour deposited polymer with material properties that are attractive for biomedicine and nanobiotechnology. Chemically inert parylene “peel-off” stencils have been demonstrated for micropatterning biomolecular arrays with high uniformity, precise spatial control down to nanoscale resolution. Such micropatterned surfaces are beneficial in engineering biosensors and biological microenvironments. A variety of substituted precursors enables direct coating of functionalised parylenes onto biomedical implants and microfluidics, providing a convenient method for designing biocompatible and bioactive surfaces. This article will review the emerging role and applications of parylene as a biomaterial for surface chemical modification and provide a future outlook.

Multichannel simultaneous measurements of single-molecule translocation in alpha-hemolysin nanopore array

We present a microarray system that enables simultaneous monitoring of multiple ionic currents through transmembrane alpha-hemolysin nanopores arrayed at bilayer lipid membranes. We applied the self-assembling ability of lipid molecules interfaced between an aqueous solution and organic solvent to induce bilayer membrane formation at a microfluidic device; the device consists of a hydrophobic polymer film that serves to suspend the lipid-containing solvent at micrometer-sized apertures as well as to separate the aqueous solution into two chambers. In this study, we confirmed that expeditious and reproducible bilayer formation is realized by control of the composition of the solvent, a mixture of n-decane and 1-hexanol, which permits simultaneous incorporation of the alpha-hemolysin nanopores to the membrane array. Monitoring the eight wells on the array at once, we obtained a maximum of four relevant, synchronous signals of translocating ionic current through the nanopores. The system was also able to detect translocation events of nucleic acid molecules through the pore via the profile of a blocked current, promising its potential for high-throughput applications.

Free-standing lipid bilayers in silicon chips-membrane stabilization based on microfabricated apertures with a nanometer-scale smoothness

In the present study, we propose a method for preparing stable free-standing bilayer lipid membranes (BLMs). The BLMs were prepared in a microfabricated aperture with a smoothly tapered edge, which was prepared in a nanometer-thick Si(3)N(4) septum by the wet etching method. Owing to this structure, the stress on lipid bilayers at the contact with the septum was minimized, leading to remarkable membrane stability. The BLMs were not broken by applying a constant voltage of +/-1 V. The membrane lifetime was 15-45 h with and without an incorporated gramicidin channel. Gramicidin single-channel currents were recorded from the same BLM preparation when the aqueous solutions surrounding the BLM were repeatedly exchanged, demonstrating the tolerance of the present BLM to repetitive solution exchanges. Such stable membranes enable analysis of channel functions under various solution conditions from the same BLM, which will open up a variety of applications including a high throughput drug screening for ion channels.