Stimulation of single isolated adult ventricular myocytes within a low volume using a planar microelectrode array

Heart-on-Chip for Combined Cellular Dynamics Measurements and Computational Modeling Towards Clinical Applications

Organ-on-chip or micro-engineered three-dimensional cellular or tissue models are increasingly implemented in the study of cardiovascular pathophysiology as alternatives to traditional in vitro cell culture. Drug induced cardiotoxicity is a key issue in drug development pipelines, but the current in vitro and in vivo studies suffer from inter-species differences, high costs, and lack of reliability and accuracy in predicting cardiotoxicity. Microfluidic heart-on-chip devices can impose a paradigm shift to the current tools. They can not only recapitulate cardiac tissue level functionality and the communication between cells and extracellular matrices but also allow higher throughput studies conducive to drug screening especially with their added functionalities or sensors that extract disease-specific phenotypic, genotypic, and electrophysiological information in real-time. Such electrical and mechanical components can tailor the electrophysiology and mechanobiology of the experiment to better mimic the in vivo condition as well. Recent advancements and challenges are reviewed in the fabrication, functionalization and sensor assisted mechanical and electrophysiological measurements, numerical and computational modeling of cardiomyocytes' behavior, and the clinical applications in drug screening and disease modeling. This review concludes with the current challenges and perspectives on the future of such organ-on-chip platforms.

Cardiac Cell Patterning on Customized Microelectrode Arrays for Electrophysiological Recordings

Cardiomyocytes (CMs) and fibroblast cells are two essential elements for cardiac tissue structure and function. The interactions between them can alter cardiac electrophysiology and thus contribute to cardiac diseases, such as arrhythmogenesis. One possible explanation is that fibroblasts can directly affect cardiac electrophysiology through electrical coupling with CMs. Therefore, detecting the electrical activities in the CM-fibroblast network is vital for understanding the coupling dynamics among them. Current commercialized platforms for studying cardiac electrophysiology utilize planar microelectrode arrays (MEAs) to record the extracellular field potential (FP) in real-time, but the prearranged electrode configuration highly limits the measurement capabilities at specific locations. Here, we report a custom-designed MEA device with a novel micropatterning method to construct a controlled network of neonatal rat CMs (rCMs) and fibroblast connections for monitoring the electrical activity of rCM-fibroblast co-cultures in a spatially controlled fashion. For the micropatterning of the co-culture, surface topographical features and mobile blockers were used to control the initial attachment locations of a mixture of rCMs and fibroblasts, to form separate beating rCM-fibroblast clusters while leaving empty space for fibroblast growth to connect these clusters. Once the blockers are removed, the proliferating fibroblasts connect and couple the separate beating clusters. Using this method, electrical activity of both rCMs and human-induced-pluripotent-stem-cell-derived cardiomyocytes (iCMs) was examined. The coupling dynamics were studied through the extracellular FP and impedance profile recorded from the MEA device, indicating that the fibroblast bridge provided an RC-type coupling of physically separate rCM-containing clusters and enabled synchronization of these clusters.

Enabling single cell electrical stimulation and response recording via a microfluidic platform

Electrical stimulation (ES) has been recognized to play important roles in regulating cell behaviors. A microfluidic device was developed for the electrical stimulation of single cells and simultaneous recording of extracellular field potential (EFP). Each single cell was trapped onto an electrode surface by a constriction channel for ES testing and was then driven to the outlet by the pressure afterward. This design allows the application of ES on and detection of EFP of single cells continuously in a microfluidic channel. Human cardiomyocytes and primary rat cortex neurons were tested with specific ES with the device. Each cell's EFP signal was detected and analyzed during the ES process. Results have shown that after applying specific ES on the excitable single cells, the cells evoked electrical responses. In addition, increased secretion of glutamic acid was detected from the stimulated neurons. Altogether, these results indicated that the developed device can be used to continuously apply ES on and accurately determine cell responses of single cells with shorter probing time. The throughput of the measurement can achieve 1 cell per minute, which is higher than the traditional ES methods that need culturing cells or manually positioning the cells onto the electrode surface. Before and after the application of ES, the cell viability had no significant change. Such a device can be used to study the biological process of various types of cells under electrical stimulation.

A microfluidic device for noninvasive cell electrical stimulation and extracellular field potential analysis

We developed a device that can quickly apply versatile electrical stimulation (ES) signals to cells suspended in microfluidic channels and measure extracellular field potential simultaneously. The device could trap cells onto the surface of measurement electrodes for ES and push them to the downstream channel after ES by increasing pressure for continuous measurement. Cardiomyocytes, major functional cells in heart, together with human fibroblast cells and human umbilical vein endothelial cells, were tested with the device. Extracellular field potential signals generated from the cells were recorded. We found that under electrical stimulation, cardiomyocytes were triggered to alter their field potential, while non-excitable cells were not triggered. Hence this device can noninvasively distinguish electrically excitable cells from electrically non-excitable cells. Results have also shown that increased cardiomyocyte cell number led to increased magnitude and occurrence of the cell responses. This relationship could be used to detect the viable cells in a cardiac tissue. Application of variable ES signals on different cardiomyocyte clusters has shown that the application of ES clearly boosted cardiomyocytes electrical activities according to the stimulation frequency. In addition, we confirmed that the device can apply ES onto and detect the electrical responses from a mixed cell cluster; the responses from the mixed cluster is dependent on the ratio of cardiomyocytes. These results demonstrated that our device could be used as a tool to optimize ES conditions to facilitate the functional engineered cardiac tissue development.

Biology-inspired microphysiological system approaches to solve the prediction dilemma of substance testing

The recent advent of microphysiological systems - microfluidic biomimetic devices that aspire to emulate the biology of human tissues, organs and circulation in vitro - is envisaged to enable a global paradigm shift in drug development. An extraordinary US governmental initiative and various dedicated research programs in Europe and Asia have led recently to the first cutting-edge achievements of human single-organ and multi-organ engineering based on microphysiological systems. The expectation is that test systems established on this basis would model various disease stages, and predict toxicity, immunogenicity, ADME profiles and treatment efficacy prior to clinical testing. Consequently, this technology could significantly affect the way drug substances are developed in the future. Furthermore, microphysiological system-based assays may revolutionize our current global programs of prioritization of hazard characterization for any new substances to be used, for example, in agriculture, food, ecosystems or cosmetics, thus, replacing laboratory animal models used currently. Thirty-six experts from academia, industry and regulatory bodies present here the results of an intensive workshop (held in June 2015, Berlin, Germany). They review the status quo of microphysiological systems available today against industry needs, and assess the broad variety of approaches with fit-for-purpose potential in the drug development cycle. Feasible technical solutions to reach the next levels of human biology in vitro are proposed. Furthermore, key organ-on-a-chip case studies, as well as various national and international programs are highlighted. Finally, a roadmap into the future is outlined, to allow for more predictive and regulatory-accepted substance testing on a global scale.

Heart-on-a-chip based on stem cell biology

Heart diseases are one of the main causes of death around the world. The great challenge for scientists is to develop new therapeutic methods for these types of ailments. Stem cells (SCs) therapy could be one of a promising technique used for renewal of cardiac cells and treatment of heart diseases. Conventional in vitro techniques utilized for investigation of heart regeneration do not mimic natural cardiac physiology. Lab-on-a-chip systems may be the solution which could allow the creation of a heart muscle model, enabling the growth of cardiac cells in conditions similar to in vivo conditions. Microsystems can be also used for differentiation of stem cells into heart cells, successfully. It will help better understand of proliferation and regeneration ability of these cells. In this review, we present Heart-on-a-chip systems based on cardiac cell culture and stem cell biology. This review begins with the description of the physiological environment and the functions of the heart. Next, we shortly described conventional techniques of stem cells differentiation into the cardiac cells. This review is mostly focused on describing Lab-on-a-chip systems for cardiac tissue engineering. Therefore, in the next part of this article, the microsystems for both cardiac cell culture and SCs differentiation into cardiac cells are described. The section about SCs differentiation into the heart cells is divided in sections describing biochemical, physical and mechanical stimulations. Finally, we outline present challenges and future research concerning Heart-on-a-chip based on stem cell biology.

A microfluidic dual-well device for high-throughput single-cell capture and culture

In vitro culture of single cells facilitates biological studies by deconvoluting complications from cell population heterogeneity. However, there is still a lack of simple yet high-throughput methods to perform single cell culture experiments. In this paper, we report the development and application of a microfluidic device with a dual-well (DW) design concept for high-yield single-cell loading (~77%) in large microwells (285 and 485 μm in diameter) which allowed for cell spreading, proliferation and differentiation. The increased single-cell loading yield is achieved by using sets of small microwells termed "capture-wells" and big microwells termed "culture-wells" according to their utilities for single-cell capture and culture, respectively. This novel device architecture allows the size of the "culture" microwells to be flexibly adjusted without affecting the single-cell loading efficiency making it useful for cell culture applications as demonstrated by our experiments of KT98 mouse neural stem cell differentiation, A549 and MDA-MB-435 cancer cell proliferation, and single-cell colony formation assay with A549 cells in this paper.

Microsystems for biomimetic stimulation of cardiac cells

The heart is a complex integrated system that leverages mechanoelectrical signals to synchronize cardiomyocyte contraction and push blood throughout the body. The correct magnitude, timing, and distribution of these signals is critical for proper functioning of the heart; aberrant signals can lead to acute incidents, long-term pathologies, and even death. Due to the heart's limited regenerative capacity and the wide variety of pathologies, heart disease is often studied in vitro. However, it is difficult to accurately replicate the cardiac environment outside of the body. Studying the biophysiology of the heart in vitro typically consists of studying single cells in a tightly controlled static environment or whole tissues in a complex dynamic environment. Micro-electromechanical systems (MEMS) allow us to bridge these two extremes by providing increasing complexity for cell culture without having to use a whole tissue. Here, we carefully describe the electromechanical environment of the heart and discuss MEMS specifically designed to replicate these stimulation modes. Strengths, limitations and future directions of various designs are discussed for a variety of applications.

Electrical Field Stimulation with a Novel Platform: Effect on Cardiomyocyte Gene Expression but not on Orientation

Electrical field stimulation has been shown to improve cardiac cell alignment and functional properties. In this study, neonatal rat cardiomyocytes were exposed to both long-term and short-term stimulation with the goal of investigating whether it is possible to achieve cell orientation and the maturation of cardiomyocytes with a novel, microelectrode array (MEA)-compatible electrical stimulation platform. Cells were viable after electrical stimulation, but no orientation or other morphological changes were observed. However, the electrode wires in MEA dishes affected the cell orientation. Cell contractions synchronized with pacing, but settled back to their original frequency in the absence of stimulation. The expression of genes encoding a gap junction protein connexin-43 (Cx-43), and contractile cardiac protein beta myosin heavy chain 7, was stronger in stimulated cells than in controls (p<0.05). In summary, the surface topography influenced to cardiomyocyte orientation, suggesting that the micro architecture of the biomaterials should be carefully designed for cell applications. However, as electrical stimulation and its duration affected gene expression of some main cardiac proteins, the stimulation system may prove useful to enhance the cardiac differentiation of stem cells.

Single-cell electric lysis on an electroosmotic-driven microfluidic chip with arrays of microwells

Accurate analysis at the single-cell level has become a highly attractive tool for investigating cellular content. An electroosmotic-driven microfluidic chip with arrays of 30-μm-diameter microwells was developed for single-cell electric lysis in the present study. The cellular occupancy in the microwells when the applied voltage was 5 V (82.4%) was slightly higher than that at an applied voltage of 10 V (81.8%). When the applied voltage was increased to 15 V, the cellular occupancy in the microwells dropped to 64.3%. More than 50% of the occupied microwells contain individual cells. The results of electric lysis experiments at the single-cell level indicate that the cells were gradually lysed as the DC voltage of 30 V was applied; the cell was fully lysed after 25 s. Single-cell electric lysis was demonstrated in the proposed microfluidic chip, which is suitable for high-throughput cell lysis.

How do control-based approaches enter into biology?

Control is intrinsic to biological organisms, whose cells are in a constant state of sensing and response to numerous external and self-generated stimuli. Diverse means are used to study the complexity through control-based approaches in these cellular systems, including through chemical and genetic manipulations, input-output methodologies, feedback approaches, and feed-forward approaches. We first discuss what happens in control-based approaches when we are not actively examining or manipulating cells. We then present potential methods to determine what the cell is doing during these times and to reverse-engineer the cellular system. Finally, we discuss how we can control the cell's extracellular and intracellular environments, both to probe the response of the cells using defined experimental engineering-based technologies and to anticipate what might be achieved by applying control-based approaches to affect cellular processes. Much work remains to apply simplified control models and develop new technologies to aid researchers in studying and utilizing cellular and molecular processes.

Strategies for the real-time detection of Ca2+ channel events of single cells: recent advances and new possibilities

Ca(2+) ion channels play key roles in cell physiology and they are important drug targets. The Ca(2+) channel events are mainly measurable by fluorescent and patch clamp methods. This review summarizes the recent advances of these techniques for the detection of Ca(2+) channel events and the prospect of their new directions in the near future. Conventional bulk fluorescent methods are amenable to high-throughput applications, but they are not real-time single-cell measurements, which provide kinetic data on individual cells and offer unparalleled sensitive data for rare cells. Recent advances on real-time single-cell fluorescent measurements are conducted on microfluidic chips with scalable cell-retention sites, integrated with electrical stimulation and fluorescent measuring features. Patch clamp techniques are real-time measurements conducted on single cells, but the measurements are of low throughput. Recent advances are conducted on microfluidic patch clamp chips for high-throughput applications. Future real-time single-cell Ca(2+) channel event measurements will be conducted in a multiparametric manner in an integrated and automated microfluidic chip.

Single-cell chemical lysis on microfluidic chips with arrays of microwells

Many conventional biochemical assays are performed using populations of cells to determine their quantitative biomolecular profiles. However, population averages do not reflect actual physiological processes in individual cells, which occur either on short time scales or nonsynchronously. Therefore, accurate analysis at the single-cell level has become a highly attractive tool for investigating cellular content. Microfluidic chips with arrays of microwells were developed for single-cell chemical lysis in the present study. The cellular occupancy in 30-μm-diameter microwells (91.45%) was higher than that in 20-μm-diameter microwells (83.19%) at an injection flow rate of 2.8 μL/min. However, most of the occupied 20-μm-diameter microwells contained individual cells. The results of chemical lysis experiments at the single-cell level indicate that cell membranes were gradually lysed as the lysis buffer was injected; they were fully lysed after 12 s. Single-cell chemical lysis was demonstrated in the proposed microfluidic chip, which is suitable for high-throughput cell lysis.

A simple and fast microfluidic approach of same-single-cell analysis (SASCA) for the study of multidrug resistance modulation in cancer cells

Due to the cellular heterogeneity in multidrug resistance (MDR) cell populations, positive drug effects on the modulation of MDR can be obscured in conventional methods, especially when only a small number of cells are available. To address cellular variations among different MDR cells, we report a new microfluidic approach to study drug effect on MDR modulation, by investigating drug accumulation of daunorubicin in MDR leukemia cells. We have demonstrated that the new approach of same-single-cell analysis by accumulation (denoted as SASCA-A) is not only superior to different-single-cell analysis, but also has key advantages over our previous approach of same-single-cell analysis. First, SASCA-A is much simpler as it does not require multiple cycles of drug uptake and drug efflux. Second, it is faster, only taking about one fourth of the time used in the previous approach. Third, it provides a more 'identical' and reliable control because it compares the time points just before MDR modulator tests. To help understand the dynamics of drug accumulation in MDR cells, we also developed a mathematical model to describe the kinetics of drug accumulation conducted in individual cells. The SASCA-A method will benefit drug resistance research in minor cell subpopulations (e.g., cancer "stem" cells) because this method requires only a small number of cells in identifying the MDR reversal effect.

Microfluidic perfusion system for maintaining viable heart tissue with real-time electrochemical monitoring of reactive oxygen species

A microfluidic device has been developed to maintain viable heart tissue samples in a biomimetic microenvironment. This device allows rat or human heart tissue to be studied under pseudo in vivo conditions. Effluent levels of lactate dehydrogenase and hydrogen peroxide were used as markers of damaged tissue in combination with in situ electrochemical measurement of the release of reactive oxygen species (ROS). The parameters for perfusion were optimized to maintain biopsies of rat right ventricular or human right atrial tissue viable for up to 5 and 3.5 hours, respectively. Electrochemical assessment of the oxidation current of total ROS, employing cyclic voltammetry, gave results in real-time that were in good agreement to biochemical assessment using conventional, off-chip, commercial assays. This proof-of-principle, integrated microfluidic device, may be exploited in providing a platform technology for future cardiac research, offering an alternative approach for investigating heart pathophysiology and facilitating the development of new therapeutic strategies.

Microfluidic cell arrays for metabolic monitoring of stimulated cardiomyocytes

An array of PDMS microchambers was aligned to an array of sensor electrodes and stimulating microelectrodes, which was used for the electrochemical monitoring of the metabolic activity of single isolated adult ventricular myocytes inside the chamber array, stimulated within a transient electric field. The effect of the accumulation of metabolic byproducts in the limited extracellular volume of the picolitre chambers was demonstrated by measuring single muscle cell contraction optically, while concomitant changes in intracellular calcium transients and pH were recorded independently using fluorescent indicator dyes. Both the amplitude of the cell shortening and the magnitude of the intracellular calcium transients decreased over time and both nearly ceased after 20 min of continuous stimulation in the limited extracellullar volume. The intracellular pH decreased gradually during 20 min of continuous stimulation after which a dramatic pH drop was observed, indicating the breakdown of the intracellular buffering capacity. After continuous stimulation, intracellular lactate was released into the microchamber through cell electroporation and was detected electrochemically at a lactate microbiosensor, within the chamber. A mitochondrial uncoupler was used to mimic ischaemia and thus to enhance the cellular content of lactate. Under these circumstances, intracellular lactate concentrations were found to have risen to approximately 15 mM. This array system has the potential of simultaneous electrochemical and optical monitoring of extracellular and intracellular metabolites from single beating heart cells at a controlled metabolic state.

Regional electroporation of single cardiac myocytes in a focused electric field

There is now a significant interest in being able to locate single cells within geometrically defined regions of a microfluidic chip and to gain intracellular access through the local electroporation of the cell membrane. This paper describes the microfabrication of electroporation devices which can enable the regional electroporation of adult ventricular myocytes, in order to lower the local electrical resistance of the cell membrane. Initially three different devices, designed to suit the characteristic geometry of the cardiomyocyte, were investigated (all three designs serve to focus the electric field to selected regions of the cell). We demonstrate that one of these three devices revealed the sequence of cellular responses to field strengths of increasing magnitudes, namely, cell contraction, hypercontraction, and lysis. This same device required a reduced threshold voltage for each of these events, including in particular membrane breakdown. We were not only able to show the gradual regional increase in the electric conductivity of the cell membrane but were also able to avoid changes in the local intra- and extracellular pH (by preventing the local generation of protons at the electrode surface, as a consequence of the reduced threshold voltage). The paper provides evidence for new strategies for achieving robust and reproducible regional electroporation, a technique which, in future, may be used for the insertion of large molecular weight molecules (including genes) as well as for on-chip voltage clamping of the primary adult cardiomyocyte.

Real-time detection of the early event of cytotoxicity of herbal ingredients on single leukemia cells studied in a microfluidic biochip

A microfluidic approach has been developed for the real-time detection of drug effects, based on the quantitative measurement of calibrated cytosolic calcium ([Ca(2+)](i)) on single cancer cells. This microfluidic method is rapid by detecting the early event of cytotoxicity of drug candidates on cancer cells, without waiting for a couple of days needed for cell seeding and drug treatment by conventional assays. The miniaturized biochip consists of a V-shaped structure for the single-cell selection and retention. Various test reagents such as the chemotherapy drug (daunorubicin), an ionophore (ionomycin), and herbal ingredients from licorice (isoliquiritigenin or IQ) were investigated for their abilities to stimulate sustained cellular [Ca(2+)](i) elevations. The microfluidic results obtained in hours have been confirmed by conventional cytotoxicity assays which take days to complete. Moreover, any color or chemical interference problems found in the conventional assays of herbal compounds could be resolved. Using the microfluidic approach, IQ (50 microM) has been found to cause a sustained [Ca(2+)](i) elevation and cytotoxic effects on leukemia cells. The microfluidic single-cell analysis not only reduces reagent cost, and demands less cells, but also reveals some phenomena due to cellular heterogeneity that cannot be observed in bulk analysis.

Same-single-cell analysis for the study of drug efflux modulation of multidrug resistant cells using a microfluidic chip

Since multidrug resistance (MDR) is a major cause of failure in cancer chemotherapy, we report a microfluidic approach combined with the same-single-cell analysis to investigate the modulation of MDR, manifested as the inhibition of drug efflux. A microfluidic chip that was capable of selecting and retaining a single multidrug-resistant cancer cell was used to investigate drug efflux inhibition in leukemia cell lines. Three advantages of the microfluidic-based same-single-cell analysis (dubbed as SASCA) method have been revealed. First, it readily detects the modulation of drug efflux of anticancer compounds (e.g., daunorubicin) by MDR modulators (e.g., verapamil) among cellular variations. Second, SASCA is able to compare the different cellular abilities in response to drug efflux modulation based on the drug transport kinetics of single cells. Third, SASCA requires only a small number of cells, which may be beneficial for investigating drug resistance in minor cell subpopulations (e.g., cancer "stem" cells).

Real-time monitoring of intracellular calcium dynamic mobilization of a single cardiomyocyte in a microfluidic chip pertaining to drug discovery

A microfluidic method for real-time quantitative measurement of cellular response pertaining to drug discovery is reported. This method is capable of multiple-step liquid delivery for measuring the drug response of a single cardiomyocyte, due to the improved cell retention by a newly designed chip. The chip, which consists of a cell-retention chamber with a weir structure, was fabricated just by a one-photomask microfabrication procedure followed by on-chip etching. This method differs from the conventional method, which uses two-mask photolithography to fabricate the microchannel (deep etch) and the weir structure (shallow etch). The dimensions of the weir structure have been predicted by a mathematical model, and confirmed by confocal microscopy. Using this microfluidic method, the dynamic [Ca2+]i mobilization in a single cardiomyocyte during its spontaneous contraction was quantified. Furthermore, we measured the cellular response of a cardiomyocyte on (i) a known cardiotonic agent (caffeine), (ii) a cardiotoxic chemotherapeutic drug (daunorubicin), and (iii) an herbal anticancer drug candidate - isoliquiritigenin (IQ) based on the fluorescent calcium measurement. It was found that IQ had produced a less pronounced effect on calcium mobilization( )of the cardiomyocytes whereas caffeine and daunorubicin had much stronger effects on the cells. These three experiments on cardiomyocytes pertaining to drug discovery were only possible after the improved cell retention provided by the new chip design (MV2) required for multiple-step real-time cellular analysis on a microchip, as compared with our old chip design (MV1).

Anisotropic growth of conducting polymers along heparin-modified surfaces

We have studied the possibility of making biocompatible, conductive patterns on a substrate by controlling the lateral growth rate of conducting polymers upon electropolymerization. Surface modification with heparin was found to enhance the lateral growth of polypyrrole, especially in the presence of dodecylbenzenesulfonate, and thus the micropatterning of heparin around electrodes leads to the formation of polypyrrole patterns.

Microfluidic systems to examine intercellular coupling of pairs of cardiac myocytes

In this paper we describe a microfluidic environment that enables us to explore cell-to-cell signalling between longitudinally linked primary heart cells. We have chosen to use pairs (or doublets) of cardiac myocyte as a model system, not only because of the importance of cell-cell signalling in the study of heart disease but also because the single cardiomyocytes are both mechanically and electrically active and their synchronous activation due to the intercellular coupling within the doublet can be readily monitored on optical and electrical recordings. Such doublets have specialised intercellular contact structures in the form of the intercalated discs, comprising the adhesive junction (fascia adherens and macula adherens or desmosome) and the connecting junction (known as gap junction). The latter structure enables adjacent heart cells to share ions, second messengers and small metabolites (<1 kDa) between them and thus provides the structural basis for the synchronous (syncytical) behaviour of connected cardiomyocytes. Using the unique environment provided by the microfluidic system, described in this paper, we explore the local ionic conditions that enable the propagation of Ca(2+) waves between two heart cells. We observe that the ability of intracellular Ca(2+) waves to traverse the intercalated discs is dependent on the relative concentrations of diastolic Ca(2+) in the two adjacent cells. These experiments rely upon our ability to independently control both the electrical stimulation of each of the cells (using integrated microelectrodes) and to rapidly change (or switch) the local concentrations of ions and drugs in the extracellular buffer within the microfluidic channel (using a nanopipetting system). Using this platform, it is also possible to make simultaneous optical recordings (including fluorescence and cell contraction) to explore the effect of drugs on one or both cells, within the doublet.

Spectroscopic Probing of Dynamic Changes during Stimulation and Cell Remodeling in the Single Cardiac Myocyte

Optical microscopy, involving both fluorescence imaging and confocal Raman microspectroscopy, was used to visualize single, isolated, electrically active heart muscle cells. For example, short-term, dynamic changes in Raman bands during the contraction cycle, as well as persistent band changes during structural remodeling (microscopic rearrangements of cellular structures) in culture over longer periods of time, were obtained from the cellular content (sarcoplasm) of the heart cells. The results of the short-term studies, collected during electrical stimulation, showed dynamic changes in the Raman amide I band intensity, which occurred in phase with changes in cell length during cardiomyocyte contraction. The longer term studies of quiescent cardiomyocytes in culture over 3 days revealed a progressive and sustained increase in the intensity of the amide I band. Over the same period of culture, a decrease in the number of t-tubules (invaginations of the cell membrane, sarcolemma, which ensure the spreading of the action potential into the bulk of the sarcoplasm) was observed using confocal z-sections of the fluorescently labeled sarcolemma. The ability to measure both short-term dynamic changes associated with stimulated contraction and longer term persistent remodeling in the structure of intracellular macromolecules is valuable for assessing the physiological state of the cell, in real time.

Microfluidic selection and retention of a single cardiac myocyte, on-chip dye loading, cell contraction by chemical stimulation, and quantitative fluorescent analysis of intracellular calcium

A microfluidic method to study the contraction of a single cardiac myocyte (heart muscle cell) has been developed. This method integrates various single-cell operations as well as on-chip dye loading, and quantitative analysis of intracellular calcium concentration, [Ca2+]i. After the channel enlargement by on-chip etching to accommodate large-sized cardiac myocytes, a single cell is selected and retained at a V-shaped cell retention structure within the microchip. Owing to the fragile property of the cardiac myocytes that could easily be damaged by centrifugation, the calcium-sensitive fluorescent dye was loaded in the cell by on-chip dye loading. This on-chip method minimized the damage to the cells from the use of a centrifuge in the conventional method and provided a way of cellular analysis of fragile cells. Subsequently, quantitative analysis of [Ca2+]i of a single cardiac myocyte by fluorescence measurement was achieved for the first time in a microfluidic chip, thanks to the intracellular calcium stimulant of ionomycin. The resting [Ca2+]i of the cardiomyocyte determined was consistent with the literature value. From the spontaneous contraction study, it was found that fluorescence intensity cannot represent the [Ca2+]i variation accurately, which implied the importance of the quantitative analysis of [Ca2+]i.

Large-scale single-cell trapping and imaging using microwell arrays

High-throughput single-cell measurements of cellular responses are of great importance for a variety of applications including drug testing, toxicology, and basic cell biology. We present an optimization study for trapping single cells with high efficiency in large arrays of microwells. The method is compatible with standard fluorescence microscopy equipment and is not dependent on, but is compatible with, cell adherence. We have characterized microwell occupancy by cells for a range of microwell dimensions and seeding parameters and optimized it for fibroblasts (as a model of adherent cell) and rat basophilic leukemia cells (as a model of nonadherent cell). We have been able to obtain phase-contrast and fluorescence micrographs with more than 18,000 single cells per image using a 4x objective.

Electrodeposition of anchored polypyrrole film on microelectrodes and stimulation of cultured cardiac myocytes

The electrically conducting polymer polypyrrole (PPy) was electrochemically deposited onto Pt microelectrodes on a polyimide (PI) substrate. Pre-modification of the PI surface with a self-assembled monolayer of octadecyltrichlorosilane-induced anisotropic lateral growth of PPy along the PI surface and enhanced adhesive strength of the PPy film. The lateral growth of PPy film around the electrode anchored the whole film to the substrate. External stimulation of cultured cardiac myocytes was carried out using the PPy-coated microelectrode. The myocytes on the microelectrode substrate were electrically conjugated to form a sheet, and showed synchronized beating upon stimulation. The threshold charge for effective stimulation of a 0.8 cm(2) sheet of myocytes was around 0.2 microC, roughly corresponding to a membrane depolarization of 250 mV.

Microfluidic Partitioning of the Extracellular Space around Single Cardiac Myocytes

This paper describes the partitioning of the extracellular space around an electrically activated single cardiac myocyte, constrained within a microfluidic device. Central to this new method is the production of a hydrophobic gap-structure, which divides the extracellular space into two distinct microfluidic pools. The content of these pools was controlled using a pair of concentric automated pipets (subsequently called "dual superfusion pipet"), each providing the ability to dispense (i.e., the source, inner pipet) and aspirate (the sink, outer pipet) a buffer solution (perfusate) into each of the two pools. For rapid solution switching around the cell, additional dual superfusion pipets were inserted into the microchannel for defined time periods using a piezostepper, enabling us to add a test solution, such as a drug. Three distinct areas of the cell were manipulated, namely, the microfluidic environment, the cellular membrane, and the intracellular space. Planar integrated microelectrodes enabled the electrical stimulation of the cardiomyocyte and the recording of the evoked action potential. The device was mounted on an inverted microscope to allow simultaneous sarcomere length and epifluorescence measurements during evoked electrical activity, including, for example, the response of the stimulated end of the cardiac myocyte in comparison with the untreated cell end.

Multisite Recording of Extracellular Potentials Produced by Microchannel-Confined Neurons In-Vitro

Towards establishing electrical interfaces with patterned in vitro neurons, we have previously described the fabrication of hybrid elastomer-glass devices polymer-on-multielectrode array technology and obtained single-electrode recordings of extracellular potentials from confined neurons (Claverol-Tinturé et al., 2005). Here, we demonstrate the feasibility of spatially localized multisite recordings from individual microchannel-guided neurites extending from microwell-confined somas with good signal-to-noise ratios (20 dB) and spike magnitudes of up to 300 microV. Single-cell current source density (scCSD) analysis of the spatio-temporal patterns of membrane currents along individual processes is illustrated.

Metabolic monitoring of the electrically stimulated single heart cell within a microfluidic platform

A device based on five individually addressable microelectrodes, fully integrated within a microfluidic system, has been fabricated to enable the real-time measurement of ionic and metabolic fluxes from electrically active, beating single heart cells. The electrode array comprised one pair of pacing microelectrodes, used for field-stimulation of the cell, and three other microelectrodes, configured as an electrochemical lactate microbiosensor, that were used to measure the amounts of lactate produced by the heart cell. The device also allowed simultaneous in-situ microscopy, enabling optical measurements of cell contractility and fluorescence measurements of extracellular pH and cellular Ca2+. Initial experiments aimed to create a metabolic profile of the beating heart cell, and results show well defined excitation-contraction (EC) coupling at different rates. Ca2+ transients and extracellular pH measurements were obtained from continually paced single myocytes, both as a function of the rate of cell contraction. Finally, the relative amounts of intra- and extra-cellular lactate produced during field stimulation were determined, using cell electroporation where necessary.

Extracellular recordings of field potentials from single cardiomyocytes

Open microfluidic channels were used to separate the extracellular space around a cardiomyocyte into three compartments: the cell ends and a central partition (insulating gap). The microchannels were filled with buffer solution and overlaid with paraffin oil, thus forming the cavities for the cell ends. The central part of the cardiomyocyte rested on the partition between two adjacent microchannels and was entirely surrounded by the paraffin oil. This arrangement increased the extracellular electrical resistance to > 20 MOmega and facilitated the recording of the time course of the change in extracellular voltage and current during subthreshold and suprathreshold stimuli. The waveform of the extracellular current and voltage in response to an extracellular depolarizing stimulus comprised an initial monophasic signal followed by a biphasic signal with a delay of 2-15 ms. The latter was associated with a transient contraction and therefore caused by an action potential. The biphasic signal became monophasic after the depolarization of one cell end by raised extracellular [K+]. This form of differential recording revealed the repolarization phase of the action potential. At rest, the sarcomere length within the gap was 12% +/- 4.8% longer than outside the gap, but intracellular Ca2+ transients occurred to the same extent as that observed in the outer pools. This data demonstrate the feasibility of the use of a microfluidic bath design to limit the extracellular resistance between two ends of an isolated cardiomyocyte.

Characterisation of spatial and temporal changes in pH gradients in microfluidic channels using optically trapped fluorescent sensors

This paper demonstrates the use of micron sized beads, modified with fluorescent dyes, as non-invasive sensors to probe the local changes in pH, within a microfluidic channel. To achieve this, amine modified polystyrene spheres (either 3 microm or 6 microm in diameter) were functionalised with the pH sensitive fluorochrome SNARF-1 to produce point sensors. The modified beads were trapped at defined positions close to a pair of integrated planar gold microelectrodes within the channel, using optical tweezers. Both transient and steady-state electrochemical potentials were applied to the microelectrode pair in order to generate changes in the local pH, associated with electrolysis. The functionalised beads indicated the pH changes in the channel, measured as a change in the fluorescence signal, generated by the immobilised pH sensitive dye. Responses were measured with temporal resolutions of between 1 and 200 ms, whilst the spatial resolution of the pH gradients was limited by the size of the beads to 3 microm.

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.

Single-Cell Chemical Lysis in Picoliter-Scale Closed Volumes Using a Microfabricated Device

Investigating the intracellular contents of single cells is essential for understanding physiologic and pathologic processes at the cellular level. While existing protocols for cell lysis and sample preparation work well for larger samples, scaling to a single-cell level is challenging because of unavoidable analyte dilution and losses. Thus, we are proposing a microfabricated device for the controlled handling and mixing of picoliter cell suspension and lysis solution volumes. Cells and fluids are independently isolated in two microchambers of 25-pL volumes using the geometry of the microchannels and the coordinated action of four on-chip thermopneumatic actuators. Virtual walls formed by liquid-air interfaces in the hydrophobic capillary separate the two volumes, which are subsequently allowed to mix after drawing the air out of the capillary connecting the two microchambers. Following cell lysis, a limited and stable dilution of intracellular components is achieved, simplifying the requirements for subsequent analysis. Two assays at single-cell level, one for direct estimation of the intracellular concentration of a soluble dye and the other for indirect evaluation of intracellular quantities of insoluble actin, demonstrate the use of the microfabricated device for single-cell assays.