Electrical modeling of the cell–electrode interface for recording neural activity from high-density microelectrode arrays

Probing action potentials of single beating cardiomyocytes using atomic force microscopy

This paper presents a method for using atomic force microscopy to probe action potentials of single beating cardiomyocytes at the nanoscale. In this work, the conductive tip of an atomic force microscope (AFM) was used as a nanoelectrode to record the action potentials of self-beating cardiomyocytes in both the non-constant force contact mode and the constant force contact mode. An electrical model of a tip-cell interface was developed and the indentation force effect on the seal of an AFM conductive tip-cell membrane was theoretically analyzed. The force feedback of AFM allowed for the precise control of tip-cell contact, and enabled reliable measurements. The feasibility of simultaneously recording the action potentials and force information during the contraction of the same beating cardiomyocyte was studied. Furthermore, the AFM tip electrode was used to probe the differences of action potentials using different drugs. This method provides a way at the nanoscale for electrophysiological studies on single beating cardiomyocytes, neurons, and ion channels embedded within the cell membrane in relation to disease states, pharmaceutical drug testing and screening.

Electrically conductive scaffolds mimicking the hierarchical structure of cardiac myofibers

Electrically conductive scaffolds, mimicking the unique directional alignment of muscle fibers in the myocardium, are fabricated using the 3D printing micro-stereolithography technique. Polyethylene glycol diacrylate (photo-sensitive polymer), Irgacure 819 (photo-initiator), curcumin (dye) and polyaniline (conductive polymer) are blended to make the conductive ink that is crosslinked using free radical photo-polymerization reaction. Curcumin acts as a liquid filter and prevents light from penetrating deep into the photo-sensitive solution and plays a central role in the 3D printing process. The obtained scaffolds demonstrate well defined morphology with an average pore size of 300 ± 15 μm and semi-conducting properties with a conductivity of ~ 10^-6 S/m. Cyclic voltammetry analyses detect the electroactivity and highlight how the electron transfer also involve an ionic diffusion between the polymer and the electrolyte solution. Scaffolds reach their maximum swelling extent 30 min after immersing in the PBS at 37 °C and after 4 weeks they demonstrate a slow hydrolytic degradation rate typical of polyethylene glycol network. Conductive scaffolds display tunable conductivity and provide an optimal environment to the cultured mouse cardiac progenitor cells.

Multiscale simulation analysis of passive and active micro/nanoelectrodes for CMOS-based in vitro neural sensing devices

Neuron and neural network studies are remarkably fostered by novel stimulation and recording systems, which often make use of biochips fabricated with advanced electronic technologies and, notably, micro- and nanoscale complementary metal-oxide semiconductor (CMOS). Models of the transduction mechanisms involved in the sensor and recording of the neuron activity are useful to optimize the sensing device architecture and its coupling to the readout circuits, as well as to interpret the measured data. Starting with an overview of recently published integrated active and passive micro/nanoelectrode sensing devices for in vitro studies fabricated with modern (CMOS-based) micro-nano technology, this paper presents a mixed-mode device-circuit numerical-analytical multiscale and multiphysics simulation methodology to describe the neuron-sensor coupling, suitable to derive useful design guidelines. A few representative structures and coupling conditions are analysed in more detail in terms of the most relevant electrical figures of merit including signal-to-noise ratio. This article is part of the theme issue 'Advanced neurotechnologies: translating innovation for health and well-being'.

Advances in Cell-Conductive Polymer Biointerfaces and Role of the Plasma Membrane

The plasma membrane (PM) is often described as a wall, a physical barrier separating the cell cytoplasm from the extracellular matrix (ECM). Yet, this wall is a highly dynamic structure that can stretch, bend, and bud, allowing cells to respond and adapt to their surrounding environment. Inspired by shapes and geometries found in the biological world and exploiting the intrinsic properties of conductive polymers (CPs), several biomimetic strategies based on substrate dimensionality have been tailored in order to optimize the cell-chip coupling. Furthermore, device biofunctionalization through the use of ECM proteins or lipid bilayers have proven successful approaches to further maximize interfacial interactions. As the bio-electronic field aims at narrowing the gap between the electronic and the biological world, the possibility of effectively disguising conductive materials to "trick" cells to recognize artificial devices as part of their biological environment is a promising approach on the road to the seamless platform integration with cells.

Plant electrical signals: A multidisciplinary challenge

Plant electrical signals, an early event in the plant-stimulus interaction, rapidly transmit information generated by the stimulus to other organs, and even the whole plant, to promote the corresponding response and trigger a regulatory cascade. In recent years, many promising state-of-the-art technologies applicable to study plant electrophysiology have emerged. Research focused on expression of genes associated with electrical signals has also proliferated. We propose that it is appropriate for plant electrical signals to be considered in the form of a "plant electrophysiological phenotype". This review synthesizes research on plant electrical signals from a novel, interdisciplinary perspective, which is needed to improve the efficient aggregation and use of plant electrical signal data and to expedite interpretation of plant electrical signals.

Electrophysiology Read-Out Tools for Brain-on-Chip Biotechnology

Brain-on-Chip (BoC) biotechnology is emerging as a promising tool for biomedical and pharmaceutical research applied to the neurosciences. At the convergence between lab-on-chip and cell biology, BoC couples in vitro three-dimensional brain-like systems to an engineered microfluidics platform designed to provide an in vivo-like extrinsic microenvironment with the aim of replicating tissue- or organ-level physiological functions. BoC therefore offers the advantage of an in vitro reproduction of brain structures that is more faithful to the native correlate than what is obtained with conventional cell culture techniques. As brain function ultimately results in the generation of electrical signals, electrophysiology techniques are paramount for studying brain activity in health and disease. However, as BoC is still in its infancy, the availability of combined BoC-electrophysiology platforms is still limited. Here, we summarize the available biological substrates for BoC, starting with a historical perspective. We then describe the available tools enabling BoC electrophysiology studies, detailing their fabrication process and technical features, along with their advantages and limitations. We discuss the current and future applications of BoC electrophysiology, also expanding to complementary approaches. We conclude with an evaluation of the potential translational applications and prospective technology developments.

Self-repair behaviour of the neuronal cell membrane by conductive atomic force indentation

Conductive atomic force indentation (CAFI) was proposed to study the self-repair behaviour of the neuronal cell membrane here. CAFI was used to detect the changes of membrane potentials by performing the mechanical indentation on neurons with a conductive atomic force microscope. In the experiment, a special insulation treatment was made on the conductive probe, which turned out to be a conductive nanoelectrode, to implement the CAFI function. The mechanical properties of the neuronal cell membrane surface were tested and the membrane potential changes of neurons cultured in vitro were detected. The self-repair behaviour of the neuronal cell membrane after being punctured was investigated. The experiment results show that CAFI provides a new way for the study of self-repair behaviours of neuronal cell membranes and mechanical and electrical properties of living cells.

Electroconductive scaffolds for tissue engineering applications

Materials that conduct electricity are studied in the context of tissue engineering. The mechanisms by which they interact with tissues are unclear and the complexity of the interface between biological and artificial systems is often underestimated.

Opportunities and dilemmas of in vitro nano neural electrodes

Developing electrophysiological platforms to capture electrical activities of neurons and exert modulatory stimuli lays the foundation for many neuroscience-related disciplines, including the neuron–machine interface, neuroprosthesis, and mapping of brain circuitry.

Single-Cell Electrical Stimulation Using CMOS-Based High-Density Microelectrode Arrays

Non-invasive electrical stimulation can be used to study and control neural activity in the brain or to alleviate somatosensory dysfunctions. One intriguing prospect is to precisely stimulate individual targeted neurons. Here, we investigated single-neuron current and voltage stimulation in vitro using high-density microelectrode arrays featuring 26,400 bidirectional electrodes at a pitch of 17.5 μm and an electrode area of 5 × 9 μm2. We determined optimal waveforms, amplitudes and durations for both stimulation modes. Owing to the high spatial resolution of our arrays and the close proximity of the electrodes to the respective neurons, we were able to stimulate the axon initial segments (AIS) with charges of less than 2 pC. This resulted in minimal artifact production and reliable readout of stimulation efficiency directly at the soma of the stimulated cell. Stimulation signals as low as 70 mV or 100 nA, with pulse durations as short as 18 μs, yielded measurable action potential initiation and propagation. We found that the required stimulation signal amplitudes decreased with cell growth and development and that stimulation efficiency did not improve at higher electric fields generated by simultaneous multi-electrode stimulation.

Electrode-Electrolyte Interface Modeling and Impedance Characterizing of Tripolar Concentric Ring Electrode

Electrodes are used to convert ionic currents to electrical currents in biological systems. Modeling the electrode-electrolyte interface and characterizing the impedance of the interface could help to optimize the performance of the electrode interface to achieve higher signal to noise ratios. Previous work has yielded accurate models for single-element biomedical electrodes. This paper introduces a model for a tripolar concentric ring electrode (TCRE) derived from impedance measurements using electrochemical impedance spectroscopy with a Ten20 electrode impedance matching paste. It is shown that the model serves well to predict the performance of the electrode-electrolyte interface for TCREs as well as standard cup electrodes. In this paper, we also discuss the comparison between the TCRE and the standard cup electrode regarding their impedance characterization and demonstrate the benefit of using TCREs in biomedical applications. We have also conducted auditory evoked potential experiments using both TCRE and standard cup electrodes. The results show that electroencephalography (EEG) recorded from tripolar concentric ring electrodes is beneficial, acquiring the auditory brainstem response with less stimuli with respect to recoding EEG using standard cup electrodes.

Developing Next-generation Brain Sensing Technologies - A Review

Advances in sensing technology raise the possibility of creating neural interfaces that can more effectively restore or repair neural function and reveal fundamental properties of neural information processing. To realize the potential of these bioelectronic devices, it is necessary to understand the capabilities of emerging technologies and identify the best strategies to translate these technologies into products and therapies that will improve the lives of patients with neurological and other disorders. Here we discuss emerging technologies for sensing brain activity, anticipated challenges for translation, and perspectives for how to best transition these technologies from academic research labs to useful products for neuroscience researchers and human patients.

Optimization of makerspace microfabrication techniques and materials for the realization of planar, 3D printed microelectrode arrays in under four days

Conventional two-dimensional microelectrode arrays (2D MEAs) in the market involve long manufacturing timeframes, have cleanroom requirements, and need to be assembled from multiple parts to obtain the final packaged device. For MEAs to be “used and tossed”, manufacturing has to be moved from the cleanroom to makerspaces. In order to enable makerspace fabricated MEAs comparable to conventional MEAs, the microfabrication processes must be optimized to have similar electrical properties along with biocompatibility and number of recording sites. This work presents a makerspace microfabricated 2D MEA having electrode densities up to a commercially popular 8 × 8 array, all fabricated under four days. Additive manufacturing-based realization of the MEA devices provides immense flexibility in terms of meeting distinct design requirements. A unique non-planar MEA having meso-scale electrodes on the top side of a chip transitioning to traces onto the bottom side through electrical vias is presented in this work. This allows for (a) monolithic integration of a culture well for devices having up to a 6 × 6 MEA array, (b) selective electroplating of the meso-scale electrodes (500 μm diameter) defined by silver ink casting followed by pulsed electroplating of gold or platinum without any masking procedure, (c) casting of a uniform and planar insulation layer via a novel process of confined precision spin coating (CPSC) of SU-8 which acts as a biocompatible insulation atop the meso-scale electrodes; and (d) selective laser micromachining to define the 50 μm × 50 μm microelectrodes. For an 8 × 8 array, the culture well and MEA chip framework are 3D printed as two separate parts and sealed together with a biocompatible epoxy as in commercially available MEAs. The fabricated MEAs have an average 1 kHz impedance of 36.8 kΩ/16 kΩ with a double layer capacitance of 400 nF cm⁻²/520 nF cm⁻² for nano-porous platinum/nano-gold which is comparable to the state-of-art commercially available 2D MEAs. Additionally, it was found out that our 3D printing-based process compares very favorably with traditional glass MEAs in terms of design to device while representing a dramatic reduction in cost, timeline for fabrication, reduction in the number of steps and the need for sophisticated microfabrication and packaging equipment.

“Makerspace microfabrication” with the use of simple tools and materials is used to demonstrate the realization of 2D microelectrode arrays (MEAs) having a density of up to 8 × 8 MEAs in under four days which are comparable to conventional MEAs.

Hydrogel bioelectronics

Hydrogels have emerged as a promising bioelectronic interfacing material. This review discusses the fundamentals and recent advances in hydrogel bioelectronics.

Impedance Spectroscopy and Electrophysiological Imaging of Cells With a High-Density CMOS Microelectrode Array System

A monolithic multi-functional CMOS microelectrode array system was developed that enables label-free electrochemical impedance spectroscopy of cells in vitro at high spatiotemporal resolution. The electrode array includes 59,760 platinum microelectrodes, densely packed within a 4.5 mm × 2.5 mm sensing region at a pitch of 13.5 μm. A total of 32 on-chip lock-in amplifiers can be used to measure the impedance of any arbitrarily chosen subset of electrodes in the array. A sinusoidal voltage, generated by an on-chip waveform generator with a frequency range from 1 Hz to 1 MHz, was applied to the reference electrode. The sensing currents through the selected recording electrodes were amplified, demodulated, filtered, and digitized to obtain the magnitude and phase information of the respective impedances. The circuitry consumes only 412 μW at 3.3 V supply voltage and occupies only 0.1 mm2, for each channel. The system also included 2048 extracellular action-potential recording channels on the same chip. Proof of concept measurements of electrical impedance imaging and electrophysiology recording of cardiac cells and brain slices are demonstrated in this paper. Optical and impedance images showed a strong correlation.

Multilayer 3D electrodes for neural implants

OBJECTIVE

In many applications, multielectrode arrays employed as neural implants require a high density and a high number of electrodes to precisely record and stimulate the activity of the nervous system while preserving the overall size of the array.

APPROACH

Here we present a multilayer and three-dimensional (3D) electrode array, together with its manufacturing method, enabling a higher electrode density and a more efficient signal transduction with the biological tissue.

MAIN RESULTS

The 3D structure of the electrode array allows for a multilayer placement of the interconnects within a flexible substrate, it narrows the probe size per the same number of electrodes, and it maintains the electrode contacts at the same level within the tissue. In addition, it augments the electrode surface area, leading to a lower electrochemical impedance and a higher charge storage capacity. To characterize the recordings capabilities of the multilayer 3D electrodes, we measured visually evoked cortical potentials in mice and analysed the evolution of the peak prominences and latencies according to different light intensities and recording depths within the brain. The resulting signal-to-noise ratio is improved compared to flat electrodes. Finally, the 3D electrodes have been imaged inside a clarified mouse brain using a light-sheet microscope to visualize their integrity within the tissue.

SIGNIFICANCE

The multilayer 3D electrodes have proved to be a valid technology to ensure tissue proximity and higher recording/stimulating efficiencies while enabling higher electrode density and reducing the probe size.

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.

Lead field theory provides a powerful tool for designing microelectrode array impedance measurements for biological cell detection and observation

Background

Our aim is to introduce a method to enhance the design process of microelectrode array (MEA) based electric bioimpedance measurement systems for improved detection and viability assessment of living cells and tissues. We propose the application of electromagnetic lead field theory and reciprocity for MEA design and measurement result interpretation. Further, we simulated impedance spectroscopy (IS) with two- and four-electrode setups and a biological cell to illustrate the tool in the assessment of the capabilities of given MEA electrode constellations for detecting cells on or in the vicinity of the microelectrodes.

Results

The results show the power of the lead field theory in electromagnetic simulations of cell–microelectrode systems depicting the fundamental differences of two- and four-electrode IS measurement configurations to detect cells. Accordingly, the use in MEA system design is demonstrated by assessing the differences between the two- and four-electrode IS configurations. Further, our results show how cells affect the lead fields in these MEA system, and how we can utilize the differences of the two- and four-electrode setups in cell detection. The COMSOL simulator model is provided freely in public domain as open source.

Conclusions

Lead field theory can be successfully applied in MEA design for the IS based assessment of biological cells providing the necessary visualization and insight for MEA design. The proposed method is expected to enhance the design and usability of automated cell and tissue manipulation systems required for bioreactors, which are intended for the automated production of cell and tissue grafts for medical purposes. MEA systems are also intended for toxicology to assess the effects of chemicals on living cells. Our results demonstrate that lead field concept is expected to enhance also the development of such methods and devices.

Active C4 Electrodes for Local Field Potential Recording Applications

Extracellular neural recording, with multi-electrode arrays (MEAs), is a powerful method used to study neural function at the network level. However, in a high density array, it can be costly and time consuming to integrate the active circuit with the expensive electrodes. In this paper, we present a 4 mm × 4 mm neural recording integrated circuit (IC) chip, utilizing IBM C4 bumps as recording electrodes, which enable a seamless active chip and electrode integration. The IC chip was designed and fabricated in a 0.13 μm BiCMOS process for both in vitro and in vivo applications. It has an input-referred noise of 4.6 μVrms for the bandwidth of 10 Hz to 10 kHz and a power dissipation of 11.25 mW at 2.5 V, or 43.9 μW per input channel. This prototype is scalable for implementing larger number and higher density electrode arrays. To validate the functionality of the chip, electrical testing results and acute in vivo recordings from a rat barrel cortex are presented.

A Low Noise Amplifier for Neural Spike Recording Interfaces

This paper presents a Low Noise Amplifier (LNA) for neural spike recording applications. The proposed topology, based on a capacitive feedback network using a two-stage OTA, efficiently solves the triple trade-off between power, area and noise. Additionally, this work introduces a novel transistor-level synthesis methodology for LNAs tailored for the minimization of their noise efficiency factor under area and noise constraints. The proposed LNA has been implemented in a 130 nm CMOS technology and occupies 0.053 mm-sq. Experimental results show that the LNA offers a noise efficiency factor of 2.16 and an input referred noise of 3.8 μVrms for 1.2 V power supply. It provides a gain of 46 dB over a nominal bandwidth of 192 Hz–7.4 kHz and consumes 1.92 μW. The performance of the proposed LNA has been validated through in vivo experiments with animal models.

Open-cell recording of action potentials using active electrode arrays

The investigation of complex communication in cellular networks requires superior measurement tools than those available to date. Electrode arrays integrated onto silicon electronics are increasingly used to measure the electrical activity of cells in an automated and highly parallelized fashion, but they are restricted to recording extracellular potentials. Here, we report on an array of TiN electrodes built using standard silicon electronics for intracellular action potential recording. Intracellular access, possible at each of the 16 384 electrodes on the chip, was accomplished by local membrane electroporation using electrical stimulation with subcellular, micrometer-sized electrodes. Access to the cell interior was transient and could be tuned in duration by adapting the electroporation protocol. Intracellular sensing was found to be minimally invasive in the short and long-term, allowing consecutive intracellular recordings from the same cell over the course of days. Finally, we applied this method to investigate the effect of an ion channel blocker on cardiac electrical activity. This technique opens the door to massively parallel, long-term intracellular recording for fundamental electrophysiology and drug screening.

An optimization-based study of equivalent circuit models for representing recordings at the neuron-electrode interface

Extracellular neuroelectronic interfacing is an emerging field with important applications in the fields of neural prosthetics, biological computation, and biosensors. Traditionally, neuron-electrode interfaces have been modeled as linear point or area contact equivalent circuits but it is now being increasingly realized that such models cannot explain the shapes and magnitudes of the observed extracellular signals. Here, results were compared and contrasted from an unprecedented optimization-based study of the point contact models for an extracellular "on-cell" neuron-patch electrode and a planar neuron-microelectrode interface. Concurrent electrophysiological recordings from a single neuron simultaneously interfaced to three distinct electrodes (intracellular, "on-cell" patch, and planar microelectrode) allowed novel insights into the mechanism of signal transduction at the neuron-electrode interface. After a systematic isolation of the nonlinear neuronal contribution to the extracellular signal, a consistent underestimation of the simulated suprathreshold extracellular signals compared to the experimentally recorded signals was observed. This conclusively demonstrated that the dynamics of the interfacial medium contribute nonlinearly to the process of signal transduction at the neuron-electrode interface. Further, an examination of the optimized model parameters for the experimental extracellular recordings from sub- and suprathreshold stimulations of the neuron-electrode junctions revealed that ionic transport at the "on-cell" neuron-patch electrode is dominated by diffusion whereas at the neuron-microelectrode interface the electric double layer (EDL) effects dominate. Based on this study, the limitations of the equivalent circuit models in their failure to account for the nonlinear EDL and ionic electrodiffusion effects occurring during signal transduction at the neuron-electrode interfaces are discussed.

Frequency-dependent signal transfer at the interface between electrogenic cells and nanocavity electrodes

We present a model to describe the response of chip-based nanocavity sensors during extracellular recording of action potentials. These sensors feature microelectrodes which are embedded in liquid-filled cavities. They can be used for the highly localized detection of electrical signals on a chip. We calculate the sensor's impedance and simulate the propagation of action potentials. Subsequently we apply our findings to analyze cell-chip coupling properties. The results are compared to experimental data obtained from cardiomyocyte-like cells. We show that both the impedance and the modeled action potentials fit the experimental data well. Furthermore, we find evidence for a large seal resistance of cardiomyocytes on nanocavity sensors compared to conventional planar recording systems.

Nanostructured gold microelectrodes for extracellular recording from electrogenic cells

We present a new biocompatible nanostructured microelectrode array for extracellular signal recording from electrogenic cells. Microfabrication techniques were combined with a template-assisted approach using nanoporous aluminum oxide to develop gold nanopillar electrodes. The nanopillars were approximately 300-400 nm high and had a diameter of 60 nm. Thus, they yielded a higher surface area of the electrodes resulting in a decreased impedance compared to planar electrodes. The interaction between the large-scale gold nanopillar arrays and cardiac muscle cells (HL-1) was investigated via focused ion beam milling. In the resulting cross-sections we observed a tight coupling between the HL-1 cells and the gold nanostructures. However, the cell membranes did not bend into the cleft between adjacent nanopillars due to the high pillar density. We performed extracellular potential recordings from HL-1 cells with the nanostructured microelectrode arrays. The maximal amplitudes recorded with the nanopillar electrodes were up to 100% higher than those recorded with planar gold electrodes. Increasing the aspect ratio of the gold nanopillars and changing the geometrical layout can further enhance the signal quality in the future.