Patch-clamping of primary cardiac cells with micro-openings in polyimide films

From Bernstein's rheotome to Neher-Sakmann's patch electrode. The action potential

The aim of this review was to provide an overview of the most important stages in the development of cellular electrophysiology. The period covered starts with Bernstein's formulation of the membrane hypothesis and the measurement of the nerve and muscle action potential. Technical innovations make discoveries possible. This was the case with the use of the squid giant axon, allowing the insertion of "large" intracellular electrodes and derivation of transmembrane potentials. Application of the newly developed voltage clamp method for measuring ionic currents, resulted in the formulation of the ionic theory. At the same time transmembrane measurements were made possible in smaller cells by the introduction of the microelectrode. An improvement of this electrode was the next major (r)evolution. The patch electrode made it possible to descend to the molecular level and record single ionic channel activity. The patch technique has been proven to be exceptionally versatile. In its whole-cell configuration it was the solution to measure voltage clamp currents in small cells. See also: https://doi.org/10.14814/phy2.13860 & https://doi.org/10.14814/phy2.13862.

Progress in automating patch clamp cellular physiology

Patch clamp electrophysiology has transformed research in the life sciences over the last few decades. Since their inception, automatic patch clamp platforms have evolved considerably, demonstrating the capability to address both voltage- and ligand-gated channels, and showing the potential to play a pivotal role in drug discovery and biomedical research. Unfortunately, the cell suspension assays to which early systems were limited cannot recreate biologically relevant cellular environments, or capture higher order aspects of synaptic physiology and network dynamics. In vivo patch clamp electrophysiology has the potential to yield more biologically complex information and be especially useful in reverse engineering the molecular and cellular mechanisms of single-cell and network neuronal computation, while capturing important aspects of human disease mechanisms and possible therapeutic strategies. Unfortunately, it is a difficult procedure with a steep learning curve, which has restricted dissemination of the technique. Luckily, in vivo patch clamp electrophysiology seems particularly amenable to robotic automation. In this review, we document the development of automated patch clamp technology, from early systems based on multi-well plates through to automated planar-array platforms, and modern robotic platforms capable of performing two-photon targeted whole-cell electrophysiological recordings in vivo.

Priming and testing silicon patch-clamp neurochips

We report on the systematic and automated priming and testing of silicon planar patch-clamp chips after their assembly in Plexiglas packages and sterilization in an air plasma reactor. We find that almost 90% of the chips are successfully primed by our automated setup, and have a shunt capacitance of between 10 pF and 30 pF. Blocked chips are mostly due to glue invasion in the well, and variability in the manual assembly process is responsible for the distribution in shunt capacitance value. Priming and testing time with our automated setup is less than 5 min per chip, which is compatible with the production of large series for use in electrophysiology experiments.

Dual-pore glass chips for cell-attached single-channel recordings

While high-throughput planar patch-clamp instruments are now established to perform whole-cell recordings for drug screening, the conventional micropipette-based approach remains the gold standard for performing cell-attached single-channel recordings. Generally, planar platforms are not well-suited for such studies due to excess noise resulting from low seal resistances and the use of substrates with poor dielectric properties. Since these platforms tend to use the same pore to position a cell by suction and establish a seal, biological debris from the cell suspension can contaminate the pore surface prior to seal formation, reducing the seal resistance. Here, femtosecond laser ablation was used to fabricate dual-pore glass chips optimized for use in cell-attached single-channel recordings that circumvent this problem by using different pores to position a cell and to establish a seal. This dual-pore design also permitted the use of a relatively small patch aperture (D ~ 150 to 300 nm) that is better-suited for establishing high-resistance seals than the micropores used typically in planar patch-clamp setups (D ~ 1 to 2 μm) without compromising the ability of the device to position a cell. Taking advantage of the high seal resistances and low capacitive and dielectric noise realized using glass substrates, patch-clamp experiments with these dual-pore chips consistently achieved high seal resistances (rate of gigaseal formation = 61%, mean seal resistance = 53 GΩ), maintained gigaseals for prolonged durations (up to 6 hours), achieved RMS noise values as low as 0.46 pA at 5 kHz bandwidth, and enabled single-channel recordings in the cell-attached configuration that are comparable to those obtained by conventional patch-clamp.

Surface characteristics and electrical properties of PMMA chips for incubation-type planar-patch-clamp biosensors

Polymethylmethacrylate (PMMA) plates were successfully applied as sensor chips in an incubation-type planar patch clamp (IPPC). Hot embossing both sides formed the PMMA plates, and a focused ion beam realized micropores. The low seal resistance of the IPPC was investigated by analyzing the surface roughness of the chips. Atomic force microscopy (AFM) showed that the chip surface had a roughness of several nanometers due to the molding process. Coating the molded surface with an anti-adhesive compound further increased the surface roughness of the PMMA chip because the anti-adhesive compound itself had a large roughness and in some case, the compound partially peeled off while detaching the mold. Similarly, coating a chip with extracellular matrix (ECM) poly-l-lysine (PLL) also increased the surface roughness. The measured seal resistance of the PMMA chip for an HEK293 cell was in the range of 4-15 MΩ. The low seal resistance was attributed to the sharp-edge structure of the micropore and the surface roughness of the chip. Nevertheless, the whole cell current was successfully recorded from HEK293 cells expressing channel rhodopsin wide receiver (ChRWR) using salt-bridge-type stable Ag/AgCl electrodes. Another advantage of the PMMA sensor chip was the small parasitic capacitance.

Planar patch clamp for neuronal networks--considerations and future perspectives

The patch-clamp technique is generally accepted as the gold standard for studying ion channel activity allowing investigators to either "clamp" membrane voltage and directly measure transmembrane currents through ion channels, or to passively monitor spontaneously occurring intracellular voltage oscillations. However, this resulting high information content comes at a price. The technique is labor-intensive and requires highly trained personnel and expensive equipment. This seriously limits its application as an interrogation tool for drug development. Patch-clamp chips have been developed in the last decade to overcome the tedious manipulations associated with the use of glass pipettes in conventional patch-clamp experiments. In this chapter, we describe some of the main materials and fabrication protocols that have been developed to date for the production of patch-clamp chips. We also present the concept of a patch-clamp chip array providing high resolution patch-clamp recordings from individual cells at multiple sites in a network of communicating neurons. On this chip, the neurons are aligned with the aperture-probes using chemical patterning. In the discussion we review the potential use of this technology for pharmaceutical assays, neuronal physiology and synaptic plasticity studies.

Ion channel recordings on an injection-molded polymer chip

In this paper, we demonstrate recordings of the ion channel activity across the cell membrane in a biological cell by employing the so-called patch clamping technique on an injection-molded polymer microfluidic device. The findings will allow direct recordings of ion channel activity to be made using the cheapest materials and production platform to date and with the potential for very high throughput. The employment of cornered apertures for cell capture allowed the fabrication of devices without through holes and via a scheme comprising master origination by dry etching in a silicon substrate, electroplating in nickel and injection molding of the final part. The most critical device parameters were identified as the length of the patching capillary and the very low surface roughness on the inside of the capillary. The cross-sectional shape of the orifice was found to be less critical, as both rectangular and semicircular profiles seemed to have almost the same ability to form tight seals with cells with negligible leak currents. The devices were functionally tested using human embryonic kidney cells expressing voltage-gated sodium channels (Nav1.7) and benchmarked against a commercial state-of-the-art system for automated ion channel recordings. These experiments considered current-voltage (IV) relationships for activation and inactivation of the Nav1.7 channels and their sensitivity to a local anesthetic, lidocaine. Both IVs and lidocaine dose-response curves obtained from the injection-molded polymer device were in good agreement with data obtained from the commercial system.

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.

From understanding cellular function to novel drug discovery: the role of planar patch-clamp array chip technology

All excitable cell functions rely upon ion channels that are embedded in their plasma membrane. Perturbations of ion channel structure or function result in pathologies ranging from cardiac dysfunction to neurodegenerative disorders. Consequently, to understand the functions of excitable cells and to remedy their pathophysiology, it is important to understand the ion channel functions under various experimental conditions - including exposure to novel drug targets. Glass pipette patch-clamp is the state of the art technique to monitor the intrinsic and synaptic properties of neurons. However, this technique is labor intensive and has low data throughput. Planar patch-clamp chips, integrated into automated systems, offer high throughputs but are limited to isolated cells from suspensions, thus limiting their use in modeling physiological function. These chips are therefore not most suitable for studies involving neuronal communication. Multielectrode arrays (MEAs), in contrast, have the ability to monitor network activity by measuring local field potentials from multiple extracellular sites, but specific ion channel activity is challenging to extract from these multiplexed signals. Here we describe a novel planar patch-clamp chip technology that enables the simultaneous high-resolution electrophysiological interrogation of individual neurons at multiple sites in synaptically connected neuronal networks, thereby combining the advantages of MEA and patch-clamp techniques. Each neuron can be probed through an aperture that connects to a dedicated subterranean microfluidic channel. Neurons growing in networks are aligned to the apertures by physisorbed or chemisorbed chemical cues. In this review, we describe the design and fabrication process of these chips, approaches to chemical patterning for cell placement, and present physiological data from cultured neuronal cells.

A microfluidic device for simultaneous electrical and mechanical measurements on single cells

This paper presents a microfluidic device for simultaneous mechanical and electrical characterization of single cells. The device performs two types of cellular characterization (impedance spectroscopy and micropipette aspiration) on a single chip to enable cell electrical and mechanical characterization. To investigate the performance of the device design, electrical and mechanical properties of MC-3T3 osteoblast cells were measured. Based on electrical models, membrane capacitance of MC-3T3 cells was determined to be 3.39±1.23 and 2.99±0.82 pF at the aspiration pressure of 50 and 100 Pa, respectively. Cytoplasm resistance values were 110.1±37.7 kΩ (50 Pa) and 145.2±44.3 kΩ (100 Pa). Aspiration length of cells was found to be 0.813±0.351 μm at 50 Pa and 1.771±0.623 μm at 100 Pa. Quantified Young's modulus values were 377±189 Pa at 50 Pa and 344±156 Pa at 100 Pa. Experimental results demonstrate the device's capability for characterizing both electrical and mechanical properties of single cells.

Cell to aperture interaction in patch-clamp chips visualized by fluorescence microscopy and focused-ion beam sections

Patch-clamp is an important method to monitor the electrophysiological activity of cells and the role of pharmacological compounds on specific ion channel proteins. In recent years, planar patch-clamp chips have been developed as a higher throughput approach to the established glass-pipette method. However, proper conditions to optimize the high resistance cell-to-probe seals required to measure the small currents resulting from ion channel activity are still the subject of conjecture. Here, we report on the design of multiple-aperture (sieve) chips to rapidly facilitate assessment of cell-to-aperture interactions in statistically significant numbers. We propose a method to pre-screen the quality of seals based on a dye loading protocol through apertures in the chip and subsequent evaluation with fluorescence confocal microscopy. We also show the first scanning electron micrograph of a focused ion beam section of a cell in a patch-clamp chip aperture.

Whole-Cell Electrical Activity Under Direct Mechanical Stimulus by AFM Cantilever Using Planar Patch Clamp Chip Approach

Patch clamp is a powerful tool for studying the properties of ion-channels and cellular membrane. In recent years, planar patch clamp chips have been fabricated from various materials including glass, quartz, silicon, silicon nitride, polydimethyl-siloxane (PDMS), and silicon dioxide. Planar patch clamps have made automation of patch clamp recordings possible. However, most planar patch clamp chips have limitations when used in combination with other techniques. Furthermore, the fabrication methods used are often expensive and require specialized equipments. An improved design as well as fabrication and characterization of a silicon-based planar patch clamp chip are described in this report. Fabrication involves true batch fabrication processes that can be performed in most common microfabrication facilities using well established MEMS techniques. Our planar patch clamp chips can form giga-ohm seals with the cell plasma membrane with success rate comparable to existing patch clamp techniques. The chip permits whole-cell voltage clamp recordings on variety of cell types including Chinese Hamster Ovary (CHO) cells and pheochromocytoma (PC12) cells, for times longer than most available patch clamp chips. When combined with a custom microfluidics chamber, we demonstrate that it is possible to perfuse the extra-cellular as well as intra-cellular buffers. The chamber design allows integration of planar patch clamp with atomic force microscope (AFM). Using our planar patch clamp chip and microfluidics chamber, we have recorded whole-cell mechanosensitive (MS) currents produced by directly stimulating human keratinocyte (HaCaT) cells using an AFM cantilever. Our results reveal the spatial distribution of MS ion channels and temporal details of the responses from MS channels. The results show that planar patch clamp chips have great potential for multi-parametric high throughput studies of ion channel proteins.

Lateral patch-clamping in a standard 1536-well microplate format

Lateral patch-clamping has emerged as a chip-based platform for automation of the conventional patch-clamp technique, the 'gold' standard for studying cellular ion channels. The conventional technique, as it relies on skilled-maneuver of glass micropipettes to patch cells, is extremely delicate, low in throughput, and thus cannot be used for primary screening of compounds against ion channels. Direct integration of glass capillaries on silicon provides lateral junctions for automated trapping and patching of cells. We demonstrate here a method of scaling up the lateral junctions to a standard 1536-well microtiter plate format. A single unit of 1536-well plate has been formed here on a 9 mm by 9 mm microstructured silicon with the inclusive of 16 wells molded in a capping layer made of polydimethylsiloxane (PDMS). The silicon substrate provides integrated glass capillaries (total 12) and their associated microfluidic network. Each glass capillary has an independent access through a dedicated well in PDMS and leads to a centralized channel in which cell suspension can be delivered through one of the remaining 4 wells. The unit has been tested on RBL-1 cells by recording whole-cell activity from inwardly rectifying endogenous potassium channels. A revised test protocol has been prescribed to avoid inaccurate readings due to altered ionic composition of the recording buffer when a typical suction is applied to capture cells.

Microfluidic integration of substantially round glass capillaries for lateral patch clamping on chip

High-throughput screening of drug candidates for channelopathies can greatly benefit from an automated patch-clamping assay. Automation of the patch clamping through microfluidics ideally requires on-chip integration of glass capillaries with substantially round cross section. Such round capillaries, if they can only be integrated to connect isolated reservoirs on a substrate surface, will lead to a "lateral" configuration which is simple yet powerful for the patch clamping. We demonstrate here "lateral" patch clamping through microfluidic integration of substantially round glass capillaries in a novel process. The process adopts two well-known phenomena from microelectronics: keyhole-void formation and thermal-reflow of phosphosilicate glass in silicon trenches. The process relies on the same physical principle as the preparation of conventional micropipette electrodes by heat-pulling and fire-polishing glass tubes. The optimized process forms capillaries with a diameter approximately 1.5 microm and variation <10%. Functionality of the integrated glass capillaries for the patch-clamp recording has been verified by statistical test results from a sample of one hundred capillaries on mammalian cells (RBL-1) in suspension: 61% formed gigaseals (>1 GOmega) and of those approximately 48% (29% of all) achieved whole-cell recordings. Pharmacological blockade of ion channel activity and longevity of a whole-cell mode on these capillaries have also been presented.

Atomic force microscopy imaging and electrical recording of lipid bilayers supported over microfabricated silicon chip nanopores: lab-on-a-chip system for lipid membranes and ion channels

We describe a silicon chip-based supported bilayer system to detect the presence of ion channels and their electrical conductance in lipid bilayers. Nanopores were produced in microfabricated silicon membranes by electron beam lithography as well as by using a finely focused ion beam. Thermal oxide was used to shrink pore sizes, if necessary, and to create an insulating surface. The chips with well-defined pores were easily mounted on a double-chamber plastic cell recording system, allowing for controlling the buffer conditions both above and below the window. The double-chamber system allowed using an atomic force microscopy (AFM) tip as one electrode and inserting a platinum wire as the second electrode under the membrane window, to measure electrical current across lipid bilayers that are suspended over the pores. Atomic force imaging, stiffness measurement, and electrical capacitance measurement show the feasibility of supporting lipid bilayers over defined nanopores: a key requirement to use any such technique for structure-function study of ion channels. Online addition of gramicidin, an ion-channel-forming peptide, resulted in electrical current flow across the bilayer, and the I-V curve that was measured using the conducting AFM tip indicates the presence of many conducting gramicidin ion channels.

Open-access microfluidic patch-clamp array with raised lateral cell trapping sites

A novel open-access microfluidic patch-clamp array chip with lateral cell trapping sites raised above the bottom plane of the chip was developed by combining both a microscale soft-lithography and a macroscale polymer fabrication method. This paper demonstrates the capability of using such an open-access fluidic system for patch-clamp measurements. The surface of the open-access patch-clamp sites prepared by the macroscale hole patterning method of soft-state elastic polydimethylsiloxane (PDMS) is examined; the seal resistances are characterized and correlated with the aperture dimensions. Whole cell patch-clamp measurements are carried out with CHO cells expressing Kv2.1 ion channels. Kv2.1 ion channel blocker (TEA) dosage response is characterized and the binding activity is examined. The results demonstrate that the system is capable of performing whole cell measurements and drug profiling in a more efficient manner than the traditional patch-clamp set-up.

Spatially resolved non-invasive chemical stimulation for modulation of signalling in reconstructed neuronal networks

Functional coupling of reconstructed neuronal networks with microelectronic circuits has potential for the development of bioelectronic devices, pharmacological assays and medical engineering. Modulation of the signal processing properties of on-chip reconstructed neuronal networks is an important aspect in such applications. It may be achieved by controlling the biochemical environment, preferably with cellular resolution. In this work, we attempt to design cell-cell and cell-medium interactions in confined geometries with the aim to manipulate non-invasively the activity pattern of an individual neuron in neuronal networks for long-term modulation. Therefore, we have developed a biohybrid system in which neuronal networks are reconstructed on microstructured silicon chips and interfaced to a microfluidic system. A high degree of geometrical control over the network architecture and alignment of the network with the substrate features has been achieved by means of aligned microcontact printing. Localized non-invasive on-chip chemical stimulation of micropatterned rat cortical neurons within a network has been demonstrated with an excitatory neurotransmitter glutamate. Our system will be useful for the investigation of the influence of localized chemical gradients on network formation and long-term modulation.

Hourglass SiO2 coating increases the performance of planar patch-clamp

Obtaining high-throughput electrophysiological recordings is an ongoing challenge in ion channel biophysics and drug discovery. One particular area of development is the replacement of glass pipettes with planar devices in order to increase throughput. However, successful patch-clamp recordings depend on a surface coating which ideally should promote and stabilize giga-seal formation. Here, we present data supporting the use of a structured SiO(2) coating to improve the ability of cells to form a "seal" with a planar patch-clamp substrate. The method is based on a correlation study taking into account structure and size of the pores, surface roughness and chip capacitance. The influence of these parameters on the quality of the seal was assessed. Plasma-enhanced chemical vapour deposition (PECVD) of SiO(2) led to an hourglass structure of the pore and a tighter seal than that offered by a flat, thermal SiO(2) surface. The performance of PECVD chips was validated by recording recombinant potassium channels, BK(Ca), expressed in stable HEK-293 cell lines and in inducible CHO cell lines and low conductance IRK1, and endogenous cationic currents from CHO cells. This multiparametric investigation led to the production of improved chips for planar patch-clamp applications which allow electrophysiological recordings from a wide range of cell lines.

Electrical and thermal characterization of nanochannels between a cell and a silicon based micro-pore

Micro and nano fabrication techniques have facilitated the production of new devices for manipulation of single cells on a chip, such as the planar micro-pore electroporation technology. To characterize this technology we have studied the seal that forms at the interface between an individual cell and the micro-pore, in which the cell normally resides, as a function of an electrical field applied across the cell and temperature. Mathematical analysis of non-electroporative electrical fields in experiments with Madin-Darby canine kidney (MDCK) cells suggests that nanoscale channels form between the exterior of the cell and the pore wall. The results indicate that the electrical currents through these channels need to be considered when using planar micro-pores in general and performing micro-pore electroporation in particular. Our results show that the size of these channels is strongly temperature dependent and the cell to pore wall distance can increase by as much as 60% when the temperature of the system is lowered from 35 to 0( composite function)C. Temperature appears to be an important factor in the use of devices for cells on a chip and our results suggest that physiological temperatures should yield better seal formation, thus improved feedback sensitivity, than the traditional use of room temperature in planar micro-pore electroporation devices.

Development of a colorimetric method for functional chloride channel assay

Anion channels play significant physiological roles in humans and animals. However, the effort of screening for anion channel modulators was limited by the available assay technologies. This report discusses the development of a cell-based functional chloride channel assay using iodine as the chloride channel functional indicator. Iodine concentrations were measured with modified Sandell-Kolthoff reaction using colorimetric detection. The assay was rapid and quantitative. When WSS-1 cells were activated by gamma-aminobutyric acid (GABA) in the condition that gamma-aminobutyric acid type A receptor (GABAA receptor) conducted outwardly rectifying chloride channel function, the EC50 of GABA was 7.69 microM. IC50s were 0.53 microM for bicuculline and 3.1 microM for picrotoxin, respectively, in the presence of 10 microM GABA. When Capan-1 cells were activated by forskolin, the EC50 was 0.14 microM. The assay can also be applied to inwardly rectifying anion channels as exemplified by GABAA channel with an EC50 of 294 microM. Thus, the assay is universal and reliable and can be used for anion channel high-throughput screening.

Stimulation of Isolated Ventricular Myocytes Within an Open Architecture Microarray

This paper is concerned with the physiological responses of single heart cells within microfluidic chambers, in response to stimulation by integrated microelectrodes. To enable these investigations, which included the measurement of action potential duration, intracellular Ca2+ and cell shortening, a series of microfluidic chambers (50 microm wide, 180 microm long, 400 microm high, 500 microm pitch) and connecting channels (200 microm wide, 5000 microm long, 50 microm high, 500 microm pitch) were replica-moulded into the silicone elastomer, polydimethylsiloxane (PDMS). The structures were formed against a master of posts and lines, photolithograhically patterned into the high aspect ratio photoresist SU-8. The chambers within the slab of PDMS were aligned against pairs of stimulating gold microelectrodes (50 microm long, 20 microm wide, 0.1-10 microm thick, 180 microm apart) patterned on a microscope coverslip base, thus defining cavities of approximately 4 nL volume. The assembly was filled with physiological saline and single isolated rabbit ventricular myocytes were introduced by micropipetting, thus creating limited volumes of saline above individual myocytes that could be varied between 4 nL and > or = 4 microL. The application of transient current pulses to the cells via the electrodes caused transient contractions with constant amplitude (recorded as changes in sarcomere length), confirming that excitation contraction coupling (EC coupling) remained functional in these limited volumes. Continuous monitoring of the intracellular Ca2+ (using calcium sensitive dyes) showed, that in the absence of bath perfusion, the amplitude of the transients remained constant for approximately 3 min in the 4-nL volume and approximately 20 min for the 4 microL volume. Beyond this time, the cells became unexcitable until the bath was renewed. The action potential duration (APD) was recorded at stimulation frequencies of 1 Hz and 0.5 Hz using potential sensitive dyes and was prolonged at the higher pacing rate. These studies show the prolonged electrical stimulation of isolated adult cardiac myocytes in microchambers with unimpaired EC coupling as verified on optical records of the action potential, Ca2+ transients and cell shortening. The open architecture provided free (pipetting) access for drug dispensation without cross talk between neighboring microwells, and multiplexed optical detection can be realized to study EC coupling on arrays of cells under both control and experimental conditions.