Open-cell recording of action potentials using active electrode arrays

An organic transistor matrix for multipoint intracellular action potential recording

Electrode arrays are widely used for multipoint recording of electrophysiological activities, and organic electronics have been utilized to achieve both high performance and biocompatibility. However, extracellular electrode arrays record the field potential instead of the membrane potential itself, resulting in the loss of information and signal amplitude. Although much effort has been dedicated to developing intracellular access methods, their three-dimensional structures and advanced protocols prohibited implementation with organic electronics. Here, we show an organic electrochemical transistor (OECT) matrix for the intracellular action potential recording. The driving voltage of sensor matrix simultaneously causes electroporation so that intracellular action potentials are recorded with simple equipment. The amplitude of the recorded peaks was larger than that of an extracellular field potential recording, and it was further enhanced by tuning the driving voltage and geometry of OECTs. The capability of miniaturization and multiplexed recording was demonstrated through a 4 × 4 action potential mapping using a matrix of 5- × 5-μm² OECTs. Those features are realized using a mild fabrication process and a simple circuit without limiting the potential applications of functional organic electronics.

The Design of a CMOS Nanoelectrode Array with 4096 Current-Clamp/Voltage-Clamp Amplifiers for Intracellular Recording/Stimulation of Mammalian Neurons

CMOS microelectrode arrays (MEAs) can record electrophysiological activities of a large number of neurons in parallel but only extracellularly with low signal-to-noise ratio. Patch clamp electrodes can perform intracellular recording with high signal-to-noise ratio but only from a few neurons in parallel. Recently we have developed and reported a neuroelectronic interface that combines the parallelism of the CMOS MEA and the intracellular sensitivity of the patch clamp. Here, we report the design and characterization of the CMOS integrated circuit (IC), a critical component of the neuroelectronic interface. Fabricated in 0.18-μm technology, the IC features an array of 4,096 platinum black (PtB) nanoelectrodes spaced at a 20 μm pitch on its surface and contains 4,096 active pixel circuits. Each active pixel circuit, consisting of a new switched-capacitor current injector--capable of injecting from ±15 pA to ±0.7 μA with a 5 pA resolution--and an operational amplifier, is highly configurable. When configured into current-clamp mode, the pixel intracellularly records membrane potentials including subthreshold activities with ∼23 μV_rms input referred noise while injecting a current for simultaneous stimulation. When configured into voltage-clamp mode, the pixel becomes a switched-capacitor transimpedance amplifier with ∼1 pA_rms input referred noise, and intracellularly records ion channel currents while applying a voltage for simultaneous stimulation. Such voltage/current-clamp intracellular recording/stimulation is a feat only previously possible with the patch clamp method. At the same time, as an array, the IC overcomes the lack of parallelism of the patch clamp method, measuring thousands of mammalian neurons in parallel, with full-frame intracellular recording/stimulation at 9.4 kHz.

High-Aspect-Ratio Nanostructured Surfaces as Biological Metamaterials

Materials patterned with high-aspect-ratio nanostructures have features on similar length scales to cellular components. These surfaces are an extreme topography on the cellular level and have become useful tools for perturbing and sensing the cellular environment. Motivation comes from the ability of high-aspect-ratio nanostructures to deliver cargoes into cells and tissues, access the intracellular environment, and control cell behavior. These structures directly perturb cells' ability to sense and respond to external forces, influencing cell fate, and enabling new mechanistic studies. Through careful design of their nanoscale structure, these systems act as biological metamaterials, eliciting unusual biological responses. While predominantly used to interface eukaryotic cells, there is growing interest in nonanimal and prokaryotic cell interfacing. Both experimental and theoretical studies have attempted to develop a mechanistic understanding for the observed behaviors, predominantly focusing on the cell-nanostructure interface. This review considers how high-aspect-ratio nanostructured surfaces are used to both stimulate and sense biological systems.

Adhesion Stabilized en Masse Intracellular Electrical Recordings from Multicellular Assemblies

Coordinated collective electrochemical signals in multicellular assemblies, such as ion fluxes, membrane potentials, electrical gradients, and steady electric fields, play an important role in cell and tissue spatial organization during many physiological processes like wound healing, inflammatory responses, and hormone release. This mass of electric actions cumulates in an en masse activity within cell collectives which cannot be deduced from considerations at the individual cell level. However, continuously sampling en masse collective electrochemical actions of the global electrochemical activity of large-scale electrically coupled cellular assemblies with intracellular resolution over long time periods has been impeded by a lack of appropriate recording techniques. Here we present a bioelectrical interface consisting of low impedance vertical gold nanoelectrode interfaces able to penetrate the cellular membrane in the course of cellular adhesion, thereby allowing en masse recordings of intracellular electrochemical potentials that transverse electrically coupled NRK fibroblast, C2C12 myotube assemblies, and SH-SY5Y neuronal networks of more than 200,000 cells. We found that the intracellular electrical access of the nanoelectrodes correlates with substrate adhesion dynamics and that penetration, stabilization, and sealing of the electrode-cell interface involves recruitment of surrounding focal adhesion complexes and the anchoring of actin bundles, which form a caulking at the electrode base. Intracellular recordings were stable for several days, and monitoring of both basal activity as well as pharmacologically altered electric signals with high signal-to-noise ratios and excellent electrode coupling was performed.

Advances in ex vivo models and lab-on-a-chip devices for neural tissue engineering

The technologies related to ex vivo models and lab-on-a-chip devices for studying the regeneration of brain, spinal cord, and peripheral nerve tissues are essential tools for neural tissue engineering and regenerative medicine research. The need for ex vivo systems, lab-on-a-chip technologies and disease models for neural tissue engineering applications are emerging to overcome the shortages and drawbacks of traditional in vitro systems and animal models. Ex vivo models have evolved from traditional 2D cell culture models to 3D tissue-engineered scaffold systems, bioreactors, and recently organoid test beds. In addition to ex vivo model systems, we discuss lab-on-a-chip devices and technologies specifically for neural tissue engineering applications. Finally, we review current commercial products that mimic diseased and normal neural tissues, and discuss the future directions in this field.

Plasmonic meta-electrodes allow intracellular recordings at network level on high-density CMOS-multi-electrode arrays

The ability to monitor electrogenic cells accurately plays a pivotal role in neuroscience, cardiology and cell biology. Despite pioneering research and long-lasting efforts, the existing methods for intracellular recording of action potentials on the large network scale suffer limitations that prevent their widespread use. Here, we introduce the concept of a meta-electrode, a planar porous electrode that mimics the optical and biological behaviour of three-dimensional plasmonic antennas but also preserves the ability to work as an electrode. Its synergistic combination with plasmonic optoacoustic poration allows commercial complementary metal-oxide semiconductor multi-electrode arrays to record intracellular action potentials in large cellular networks. We apply this approach to measure signals from human-induced pluripotent stem cell-derived cardiac cells, rodent primary cardiomyocytes and immortalized cell types and demonstrate the possibility of non-invasively testing a variety of relevant drugs. Due to its robustness and easiness of use, we expect the method will be rapidly adopted by the scientific community and by pharmaceutical companies.

A Multiwell Cardiac μGMEA Platform for Action Potential Recordings from Human iPSC-Derived Cardiomyocyte Constructs

Multielectrode array (MEA) technology has been extensively used for field potential recordings from excitable cells. However, its application for action potential (AP) measurements has not been harnessed. Here, we report a novel platform for high-resolution intracellular AP recordings from induced pluripotent stem cell-cardiomyocyte constructs derived from human cardiac fibroblasts. To gain intracellular access, micro-gold MEAs were used to electroporate multiple constructs simultaneously. High-throughput AP measurements were obtained from 41 multicellular constructs. Repeated electroporations of the same cells did not affect the signal stability. Our model has the capability to distinguish subtle differences in AP morphology to characterize the network profile. Furthermore, we confirm the reliability of the system by recapitulating known drug-induced physiological and arrhythmogenic responses. Overall, the model provides a unique cardio-electronic interface for non-invasive measurements of AP dynamics for drug screening and disease modeling. This technology opens the door for identifying novel cardio-factors to enhance electrophysiological maturation.

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Electroporation-mediated action potential (AP) recordings using MEA technology

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Simultaneous high-throughput AP measurement from multiple cell networks

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Multiple electroporations of the same cells over days with stable signal

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Model validation for developmental, disease, and drug screening studies

Advantageous environment of micro-patterned, high-density complementary metal-oxide-semiconductor electrode array for spiral ganglion neurons cultured in vitro

This study investigated micro-patterned, high-density complementary metal–oxide–semiconductor (CMOS) electrode array to be used as biologically permissive environment for organization, guidance and electrical stimulation of spiral ganglion neurons (SGN). SGNs extracted and isolated from cochleae of P5-P7 rat pups and adult guinea pigs were cultured 1, 4 and 7 days in vitro on glass coverslips (control) and CMOS electrode array. The cultures were analyzed visually and immunohistochemically for SGN presence, outgrowth, neurite alignment, neurite length, neurite asymmetry as well as the contact of a neuronal soma and neurites with the micro-electrodes. Our findings indicate that topographical environment of CMOS chip with micro-patterned pillars enhanced growth, survival, morphology, neural orientation and alignment of SGNs in vitro compared to control. Smaller spacing (0.8–1.6 µm) between protruding pillars on CMOS led SGNs to develop structured and guided neurites oriented along three topographical axes separated by 60°. We found morphological basis for positioning of the micro-electrodes on the chip that was appropriate for direct contact of SGNs with them. This configuration allowed CMOS electrode array to electrically stimulate the SGN whose responses were observed with live Fluo 4 calcium imaging.

A Micropatterned Multielectrode Shell for 3D Spatiotemporal Recording from Live Cells

Microelectrode arrays (MEAs) have proved to be useful tools for characterizing electrically active cells such as cardiomyocytes and neurons. While there exist a number of integrated electronic chips for recording from small populations or even single cells, they rely primarily on the interface between the cells and 2D flat electrodes. Here, an approach that utilizes residual stress-based self-folding to create individually addressable multielectrode interfaces that wrap around the cell in 3D and function as an electrical shell-like recording device is described. These devices are optically transparent, allowing for simultaneous fluorescence imaging. Cell viability is maintained during and after electrode wrapping around the cel and chemicals can diffuse into and out of the self-folding devices. It is further shown that 3D spatiotemporal recordings are possible and that the action potentials recorded from cultured neonatal rat ventricular cardiomyocytes display significantly higher signal-to-noise ratios in comparison with signals recorded with planar extracellular electrodes. It is anticipated that this device can provide the foundation for the development of new-generation MEAs where dynamic electrode-cell interfacing and recording substitutes the traditional method using static electrodes.

Reflective lens-free imaging on high-density silicon microelectrode arrays for monitoring and evaluation of in vitro cardiac contractility

The high rate of drug attrition caused by cardiotoxicity is a major challenge for drug development. Here, we developed a reflective lens-free imaging (RLFI) approach to non-invasively record in vitro cell deformation in cardiac monolayers with high temporal (169 fps) and non-reconstructed spatial resolution (352 µm) over a field-of-view of maximally 57 mm². The method is compatible with opaque surfaces and silicon-based devices. Further, we demonstrated that the system can detect the impairment of both contractility and fast excitation waves in cardiac monolayers. Additionally, the RLFI device was implemented on a CMOS-based microelectrode array to retrieve multi-parametric information of cardiac cells, thereby offering more in-depth analysis of drug-induced (cardiomyopathic) effects for preclinical cardiotoxicity screening applications.

Optimizing Nanoelectrode Arrays for Scalable Intracellular Electrophysiology

Electrode technology for electrophysiology has a long history of innovation, with some decisive steps including the development of the voltage-clamp measurement technique by Hodgkin and Huxley in the 1940s and the invention of the patch clamp electrode by Neher and Sakmann in the 1970s. The high-precision intracellular recording enabled by the patch clamp electrode has since been a gold standard in studying the fundamental cellular processes underlying the electrical activities of neurons and other excitable cells. One logical next step would then be to parallelize these intracellular electrodes, since simultaneous intracellular recording from a large number of cells will benefit the study of complex neuronal networks and will increase the throughput of electrophysiological screening from basic neurobiology laboratories to the pharmaceutical industry. Patch clamp electrodes, however, are not built for parallelization; as for now, only ∼10 patch measurements in parallel are possible. It has long been envisioned that nanoscale electrodes may help meet this challenge. First, nanoscale electrodes were shown to enable intracellular access. Second, because their size scale is within the normal reach of the standard top-down fabrication, the nanoelectrodes can be scaled into a large array for parallelization. Third, such a nanoelectrode array can be monolithically integrated with complementary metal-oxide semiconductor (CMOS) electronics to facilitate the large array operation and the recording of the signals from a massive number of cells. These are some of the central ideas that have motivated the research activity into nanoelectrode electrophysiology, and these past years have seen fruitful developments. This Account aims to synthesize these findings so as to provide a useful reference. Summing up from the recent studies, we will first elucidate the morphology and associated electrical properties of the interface between a nanoelectrode and a cellular membrane, clarifying how the nanoelectrode attains intracellular access. This understanding will be translated into a circuit model for the nanobio interface, which we will then use to lay out the strategies for improving the interface. The intracellular interface of the nanoelectrode is currently inferior to that of the patch clamp electrode; reaching this benchmark will be an exciting challenge that involves optimization of electrode geometries, materials, chemical modifications, electroporation protocols, and recording/stimulation electronics, as we describe in the Account. Another important theme of this Account, beyond the optimization of the individual nanoelectrode-cell interface, is the scalability of the nanoscale electrodes. We will discuss this theme using a recent development from our groups as an example, where an array of ca. 1000 nanoelectrode pixels fabricated on a CMOS integrated circuit chip performs parallel intracellular recording from a few hundreds of cardiomyocytes, which marks a new milestone in electrophysiology.

Microelectrode array-induced neuronal alignment directs neurite outgrowth: analysis using a fast Fourier transform (FFT)

Many studies have shown that the topography of the substrate on which neurons are cultured can promote neuronal adhesion and guide neurite outgrowth in the same direction as the underlying topography. To investigate this effect, isotropic substrate-complementary metal-oxide-semiconductor (CMOS) chips were used as one example of microelectrode arrays (MEAs) for directing neurite growth of spiral ganglion neurons. Neurons were isolated from 5 to 7-day-old rat pups, cultured 1 day in vitro (DIV) and 4 DIV, and then fixed with 4% paraformaldehyde. For analysis of neurite alignment and orientation, fast Fourier transformation (FFT) was used. Results revealed that on the micro-patterned surface of a CMOS chip, neurons orient their neurites along three directional axes at 30, 90, and 150° and that neurites aligned in straight lines between adjacent pillars and mostly followed a single direction while occasionally branching perpendicularly. We conclude that the CMOS substrate guides neurites towards electrodes by means of their structured pillar organization and can produce electrical stimulation of aligned neurons as well as monitoring their neural activities once neurites are in the vicinity of electrodes. These findings are of particular interest for neural tissue engineering with the ultimate goal of developing a new generation of MEA essential for improved electrical stimulation of auditory neurons.

Advances in nanowire bioelectronics

Semiconductor nanowires represent powerful building blocks for next generation bioelectronics given their attractive properties, including nanometer-scale footprint comparable to subcellular structures and bio-molecules, configurable in nonstandard device geometries readily interfaced with biological systems, high surface-to-volume ratios, fast signal responses, and minimum consumption of energy. In this review article, we summarize recent progress in the field of nanowire bioelectronics with a focus primarily on silicon nanowire field-effect transistor biosensors. First, the synthesis and assembly of semiconductor nanowires will be described, including the basics of nanowire FETs crucial to their configuration as biosensors. Second, we will introduce and review recent results in nanowire bioelectronics for biomedical applications ranging from label-free sensing of biomolecules, to extracellular and intracellular electrophysiological recording.

Characterization of dorsal root ganglion neurons cultured on silicon micro-pillar substrates

Our study focuses on characterization of dorsal root ganglion (DRG) neurons cultured on silicon micro-pillar substrates (MPS) with the ultimate goal of designing micro-electrode arrays (MEAs) for successful electrophysiological recordings of DRG neurons. Adult and neonatal DRG neurons were cultured on MPS and glass coverslips for 7 days in vitro. DRG neuronal distribution and morphometric analysis, including neurite alignment and length, was performed on MPS areas with different pillar width and spacing. We showed that MPS provide an environment for growth of adult and neonatal DRG neurons as permissive as control glass surfaces. Neonatal DRG neurons were present on MPS areas with narrow pillar spacing, while adult neurons preferred wider pillar spacing. Compared to the control glass surfaces the neonatal and adult DRG neurons in regions with narrow pillar spacing range developed a smaller number of longer neurites. In the same area, neurites were preferentially oriented along three directional axes at 30°, 90° and 150°. MPS architecture influenced growth directionality of all main DRG neuronal subtypes. We can conclude that specific micro-pillar substrate topography affects the morphology of DRG neurons. This knowledge can enable development of MEAs with precisely defined physical features for various neuroscience applications.

Multisite electrophysiological recordings by self-assembled loose-patch-like junctions between cultured hippocampal neurons and mushroom-shaped microelectrodes

Substrate integrated planar microelectrode arrays is the “gold standard” method for millisecond-resolution, long-term, large-scale, cell-noninvasive electrophysiological recordings from mammalian neuronal networks. Nevertheless, these devices suffer from drawbacks that are solved by spike-detecting, spike-sorting and signal-averaging techniques which rely on estimated parameters that require user supervision to correct errors, merge clusters and remove outliers. Here we show that primary rat hippocampal neurons grown on micrometer sized gold mushroom-shaped microelectrodes (gMμE) functionalized simply by poly-ethylene-imine/laminin undergo self-assembly processes to form loose patch-like hybrid structures. More than 90% of the hybrids formed in this way record monophasic positive action potentials (APs). Of these, 34.5% record APs with amplitudes above 300 μV and up to 5,085 μV. This self-assembled neuron-gMμE configuration improves the recording quality as compared to planar MEA. This study characterizes and analyzes the electrophysiological signaling repertoire generated by the neurons-gMμE configuration, and discusses prospects to further improve the technology.

An integrated multi-electrode-optrode array for in vitro optogenetics

Modulation of a group of cells or tissue needs to be very precise in order to exercise effective control over the cell population under investigation. Optogenetic tools have already demonstrated to be of great value in the study of neuronal circuits and in neuromodulation. Ideally, they should permit very accurate resolution, preferably down to the single cell level. Further, to address a spatially distributed sample, independently addressable multiple optical outputs should be present. In current techniques, at least one of these requirements is not fulfilled. In addition to this, it is interesting to directly monitor feedback of the modulation by electrical registration of the activity of the stimulated cells. Here, we present the fabrication and characterization of a fully integrated silicon-based multi-electrode-optrode array (MEOA) for in vitro optogenetics. We demonstrate that this device allows for artifact-free electrical recording. Moreover, the MEOA was used to reliably elicit spiking activity from ChR2-transduced neurons. Thanks to the single cell resolution stimulation capability, we could determine spatial and temporal activation patterns and spike latencies of the neuronal network. This integrated approach to multi-site combined optical stimulation and electrical recording significantly advances today's tool set for neuroscientists in their search to unravel neuronal network dynamics.

Recording, labeling, and transfection of single neurons in deep brain structures

Genetic tools that permit functional or connectomic analysis of neuronal circuits are rapidly transforming neuroscience. The key to deployment of such tools is selective transfection of target neurons, but to date this has largely been achieved using transgenic animals or viral vectors that transduce subpopulations of cells chosen according to anatomical rather than functional criteria. Here, we combine single‐cell transfection with conventional electrophysiological recording techniques, resulting in three novel protocols that can be used for reliable delivery of conventional dyes or genetic material in vitro and in vivo. We report that techniques based on single cell electroporation yield reproducible transfection in vitro, and offer a simple, rapid and reliable alternative to established dye‐labeling techniques in vivo, but are incompatible with targeted transfection in deep brain structures. In contrast, we show that intracellular electrophoresis of plasmid DNA transfects brainstem neurons recorded up to 9 mm deep in the anesthetized rat. The protocols presented here require minimal, if any, modification to recording hardware, take seconds to deploy, and yield high recovery rates in vitro (dye labeling: 89%, plasmid transfection: 49%) and in vivo (dye labeling: 66%, plasmid transfection: 27%). They offer improved simplicity compared to the juxtacellular labeling technique and for the first time offer genetic manipulation of functionally characterized neurons in previously inaccessible brain regions.

The ability to label individual neurons after electrophysiological characterization of their functional properties is a foundational technique in neuroscience. A number of approaches that achieve this goal have been described, but all are technically challenging. Here, we describe a simple approach that is rapid, reliable, and compatible with delivery of conventional dyes or large plasmid DNA molecules.

Packaging commercial CMOS chips for lab on a chip integration

Combining integrated circuitry with microfluidics enables lab-on-a-chip (LOC) devices to perform sensing, freeing them from benchtop equipment. However, this integration is challenging with small chips, as is briefly reviewed with reference to key metrics for package comparison. In this paper we present a simple packaging method for including mm-sized, foundry-fabricated dies containing complementary metal oxide semiconductor (CMOS) circuits within LOCs. The chip is embedded in an epoxy handle wafer to yield a level, large-area surface, allowing subsequent photolithographic post-processing and microfluidic integration. Electrical connection off-chip is provided by thin film metal traces passivated with parylene-C. The parylene is patterned to selectively expose the active sensing area of the chip, allowing direct interaction with a fluidic environment. The method accommodates any die size and automatically levels the die and handle wafer surfaces. Functionality was demonstrated by packaging two different types of CMOS sensor ICs, a bioamplifier chip with an array of surface electrodes connected to internal amplifiers for recording extracellular electrical signals and a capacitance sensor chip for monitoring cell adhesion and viability. Cells were cultured on the surface of both types of chips, and data were acquired using a PC. Long term culture (weeks) showed the packaging materials to be biocompatible. Package lifetime was demonstrated by exposure to fluids over a longer duration (months), and the package was robust enough to allow repeated sterilization and re-use. The ease of fabrication and good performance of this packaging method should allow wide adoption, thereby spurring advances in miniaturized sensing systems.

Superimposed topographic and chemical cues synergistically guide neurite outgrowth

Guidance of neuronal extensions is a complex process essential for linking neurons into complex functional networks underlying the workings of the neural system. Decades of research have suggested the ability of neuronal growth cones to integrate multiple types of cues during the extension process, but also have raised numerous still unanswered questions about synergy or antagonism between the superimposed chemical and mechanical signaling inputs. In this study, using a novel microfabricated analysis platform, we investigate the response of primary mouse embryonic hippocampal neurons to superimposed topographic and soluble chemical cues. We find that an optimal spatial frequency of topographic cues exists, maximizing the precision of the neurite extension. This optimal frequency can help the extending neurites navigate a topographically complex environment, providing pronounced directional selectivity. We also demonstrate that this cue can synergistically enhance attractive and suppress repulsive guidance by the bi-functional soluble cue Netrin-1, and eliminate the repulsive guidance by a chemorepellent Semaphorin3A (Sema3A). These results suggest that topographic cues can provide optimal periodic input into the guidance signaling processes involved in growth cone chemoattraction and can synergistically interact with chemical gradients of soluble guidance cues, shedding light on complex events accompanying the development of the functional nervous system.

Multi-electrode array technologies for neuroscience and cardiology

At present, the prime methodology for studying neuronal circuit-connectivity, physiology and pathology under in vitro or in vivo conditions is by using substrate-integrated microelectrode arrays. Although this methodology permits simultaneous, cell-non-invasive, long-term recordings of extracellular field potentials generated by action potentials, it is 'blind' to subthreshold synaptic potentials generated by single cells. On the other hand, intracellular recordings of the full electrophysiological repertoire (subthreshold synaptic potentials, membrane oscillations and action potentials) are, at present, obtained only by sharp or patch microelectrodes. These, however, are limited to single cells at a time and for short durations. Recently a number of laboratories began to merge the advantages of extracellular microelectrode arrays and intracellular microelectrodes. This Review describes the novel approaches, identifying their strengths and limitations from the point of view of the end users--with the intention to help steer the bioengineering efforts towards the needs of brain-circuit research.