Lipid bilayer arrays: cyclically formed and measured

An ultra-low noise amplifier array system for high throughput single entity analysis

Electrochemical measurements at the single entity level provide ultra-sensitive tools for the precise diagnosis and understanding of basic biological and chemical processes. By decoding current signatures, single-entity electrochemistry provides abundant information on charges, sizes, shapes, catalytic performances and compositions. The accuracy of single-entity electrochemistry highly relies on advanced instrumentation to achieve the amperometric resolution at the sub-picoampere level and the temporal resolution at the sub-microsecond level. Currently, it is still a challenge for paralleling amplifiers to allow low-noise and high bandwidth single-entity electrochemical measurements. Herein, we developed a low-noise four-channel electrochemical instrumentation that integrates an Au electrode array with amplifiers in the circuit board. With this amplifier array, we achieved a high bandwidth (>100 kHz) electrochemical measurement. The further practical experiments proved the capability of this amplifier array system in acquiring transient signals from both single-molecule detection with an aerolysin nanopore and single Pt nanoparticle catalysis during the dynamic collision process. Paired with appropriate microfluidic array systems, our instrumentation will enable an extraordinarily high-throughput feature for single-entity sensing.

Instrumentational implementation for parallelized nanopore electrochemical measurements

Nanopore electrochemistry, as one of the promising tools for single molecule sensing, has proved its capability in DNA sequencing and protein analysis. To achieve a high resolution for obtaining molecular information, the nanopore electrochemical technique not only urgently requires an appropriate nanopore sensing interface with atomic resolution but also requires advanced instrumentation and its related data processing methods. In order to reveal the fundamental biological process and process the point-of-care diagnosis, it is necessary to use a nanopore sensing instrument with a high amperometric and temporal resolution as well as high throughput. The development of the instrumentation requires multi-disciplinary collaboration involving preparing a sensitive nanopore interface, low-noise circuit design, and intelligent data analysis. In this review, we have summarized the recent improvements in the nanopore sensing interface as well as discussed the higher throughput achieved by nanopore arrays and intelligent nanopore data analysis methods. The parallelized nanopore instrumentation could be popularized to all ranges of single-molecule applications.

Microfluidic Systems Applied in Solid-State Nanopore Sensors

Microfluidic system, as a kind of miniature integrated operating platform, has been applied to solid-state nanopore sensors after many years of experimental study. In the process of introducing microfluidic into solid-state nanopore sensors, many novel device structures are designed due to the abundance of analytes and the diversity of detection methods. Here we review the fundamental setup of nanopore-based microfluidic systems and the developments and advancements that have been taking place in the field. The microfluidic systems with a multichannel strategy to elevate the throughput and efficiency of nanopore sensors are then presented. Multifunctional detection represented by optical-electrical detection, which is realized by microfluidic integration, is also described. A high integration microfluidic system with nanopore is further discussed, which shows the prototype of commercialization.

Functional aqueous droplet networks

Droplet interface bilayers (DIBs), comprising individual lipid bilayers between pairs of aqueous droplets in an oil, are proving to be a useful tool for studying membrane proteins. Recently, attention has turned to the elaboration of networks of aqueous droplets, connected through functionalized interface bilayers, with collective properties unachievable in droplet pairs. Small 2D collections of droplets have been formed into soft biodevices, which can act as electronic components, light-sensors and batteries. A substantial breakthrough has been the development of a droplet printer, which can create patterned 3D droplet networks of hundreds to thousands of connected droplets. The 3D networks can change shape, or carry electrical signals through defined pathways, or express proteins in response to patterned illumination. We envisage using functional 3D droplet networks as autonomous synthetic tissues or coupling them with cells to repair or enhance the properties of living tissues.

Addition of Cleaved Tail Fragments during Lipid Oxidation Stabilizes Membrane Permeability Behavior

Lipid oxidation has been linked to plasma membrane damage leading to cell death. In previous work, we examined the effect of oxidation on bilayer permeability by replacing defined amounts of an unsaturated lipid species with the corresponding phospholipid product that would result from oxidative tail scission of that species. This study adds the cleaved tail fragment, better mimicking the chemical results of oxidation. Permeability of PEG12-NBD, a small, uncharged molecule, was measured for vesicles with oxidation concentration corresponding to between 0 and 18 mol % of total lipid content. Permeability was measured using a microfluidic trap to capture the vesicles and spinning disk confocal microscopy (SDCM) to measure the transport of fluorescent PEG12-NBD at the equatorial plane. The thicknesses of lipid bilayers containing oxidized species were estimated by measuring capacitance of a black lipid membrane while simultaneously measuring bilayer area. We found that relative to chemically modeled oxidized bilayers without tail fragments, bilayers containing cleaved tail groups were less permeable for the same degree of oxidation. Curiously, membrane capacitance measurements indicated that the addition of tail fragments to chemically modeled oxidized bilayers also thinned these bilayers relative to samples with no tail fragments; in other words, the more permeable membranes were thicker. Above 12.5% chemically modeled oxidation, compositions both with and without the cleaved tail groups showed pore formation. This work highlights the complexity of the relationship between chemically modeled lipid bilayer oxidation and cell membrane properties.

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.

Measurement of Ensemble TRPV1 Ion Channel Currents Using Droplet Bilayers

Electrophysiological characterization of ion channels is useful for elucidation of channel function as well as quantitative assessment of pharmaceutical effects on ion channel conductance. We used droplet bilayers to measure ensemble ion channel currents from membrane preparations made from TRPV1-expressing HEK cells. Conductance measurements showed rectification, activation by acid and capsaicin, and inhibition by capsazepine, SB 452533, and JNJ 17293212. We also quantitatively measured concentration-dependent inhibition of channel conductance through determination of capsazepine IC₅₀ in agreement with previously published studies using patch clamp. These results, combined with the reduced apparatus and material requirements of droplet bilayers, indicate that this platform could be used for study of other physiologically relevant ion channels.

Single channel and ensemble hERG conductance measured in droplet bilayers

The human ether-a-go-go related gene (hERG) encodes the potassium channel Kv11.1, which plays a key role in the cardiac action potential and has been implicated in cardiac disorders as well as a number of off-target pharmaceutical interactions. The electrophysiology of this channel has been predominantly studied using patch clamp, but lipid bilayers have the potential to offer some advantages, including apparatus simplicity, ease of use, and the ability to control the membrane and solution compositions. We made membrane preparations from hERG-expressing cells and measured them using droplet bilayers, allowing measurement of channel ensemble currents and 13.5 pS single channel currents. These currents were ion selective and were blockable by E-4031 and dofetilide in a dose-dependent manner, allowing determination of IC50 values of 17 nM and 9.65 μM for E-4031 and dofetilide, respectively. We also observed time- and voltage- dependent currents following step changes in applied potential that were similar to previously reported patch clamp measurements.

Nanoparticle-lipid bilayer interactions studied with lipid bilayer arrays

The widespread environmental presence and commercial use of nanoparticles have raised significant health concerns as a result of many in vitro and in vivo assays indicating toxicity of a wide range of nanoparticle species. Many of these assays have identified the ability of nanoparticles to damage cell membranes. These interactions can be studied in detail using artificial lipid bilayers, which can provide insight into the nature of the particle-membrane interaction through variation of membrane and solution properties not possible with cell-based assays. However, the scope of these studies can be limited because of the low throughput characteristic of lipid bilayer platforms. We have recently described an easy to use, parallel lipid bilayer platform which we have used to electrically investigate the activity of 60 nm diameter amine and carboxyl modified polystyrene nanoparticles (NH2-NP and COOH-NP) with over 1000 lipid bilayers while varying lipid composition, bilayer charge, ionic strength, pH, voltage, serum, particle concentration, and particle charge. Our results confirm recent studies finding activity of NH2-NP but not COOH-NP. Detailed analysis shows that NH2-NP formed pores 0.3-2.3 nm in radius, dependent on bilayer and solution composition. These interactions appear to be electrostatic, as they are regulated by NH2-NP surface charge, solution ionic strength, and bilayer charge. The ability to rapidly measure a large number of nanoparticle and membrane parameters indicates strong potential of this bilayer array platform for additional nanoparticle bilayer studies.

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.

Editorial: Biotechnology Journal's diverse coverage of biotechnology

This issue of Biotechnology Journal is a regular issue edited by Prof. Michael Wink. The issue covers all the major focus areas of the journal, including medical biotechnology, synthetic biology, and novel biotechnological methods.