Differential pH measurements of metabolic cellular activity in nl culture volumes using microfabricated iridium oxide electrodes

Multifunctional IrOx Neural Probe for In Situ Dynamic Brain Hypoxia Evaluation

Perioperative cerebral hypoxia and neonatal hypoxia-ischemic encephalopathy are the main triggers that lead to temporary or permanent brain dysfunction. The pathogenesis is intimately correlated to neural activities and the pH of the microenvironment, which calls for a high demand for in situ multitype physiological signal acquisition in the brain. However, conventional pH sensing neural interfaces cannot obtain the characteristics of multimodes, multichannels, and high spatial resolution of physiological signals simultaneously. Here, we report a multifunctional implantable iridium oxide (IrOx) neural probe (MIIONP) combined with electrophysiology recording, in situ pH sensing, and neural stimulation for real-time dynamic brain hypoxia evaluation. The neural probe modified with IrOx films exhibits outstanding electrophysiology recording and neural stimulation performance and long-term stable high spatial pH sensing resolution of about 100 μm, and the cytotoxicity of IrOx microelectrodes was investigated as well. In addition, 4 weeks' tracking of the same neuron firing and instantaneous population spike captured during electrical stimulation was achieved by MIIONP. Finally, in a mouse brain hypoxia model, the MIIONP has demonstrated the capability of synchronous in situ recording of the pH and neural firing changes in the brain, which has a valuable application in dynamic brain disease evaluation through real-time acquisition of multiple physiological signals.

SERS microsensors for pH measurements in the lumen and ECM of stem cell derived human airway organoids

Patient derived organoids have the potential to improve the physiological relevance of in vitro disease models. However, the 3D architecture of these self-assembled cellular structures makes probing their biochemistry more complex than in traditional 2D culture. We explore the application of surface enhanced Raman scattering microsensors (SERS-MS) to probe local pH gradients within patient derived airway organoid cultures. SERS-MS consist of solid polymer cores decorated with surface immobilised gold nanoparticles which are functionalised with pH sensitive reporter molecule 4-mercaptobenzoic acid (MBA). We demonstrate that by mixing SERS-MS into the extracellular matrix (ECM) of airway organoid cultures the probes can be engulfed by expanding organoids and report on local pH in the organoid lumen and ECM.

Nanostructured Iridium Oxide: State of the Art

Iridium Oxide (IrO2) is a metal oxide with a rutile crystalline structure, analogous to the TiO2 rutile polymorph. Unlike other oxides of transition metals, IrO2 shows a metallic type conductivity and displays a low surface work function. IrO2 is also characterized by a high chemical stability. These highly desirable properties make IrO2 a rightful candidate for specific applications. Furthermore, IrO2 can be synthesized in the form of a wide variety of nanostructures ranging from nanopowder, nanosheets, nanotubes, nanorods, nanowires, and nanoporous thin films. IrO2 nanostructuration, which allows its attractive intrinsic properties to be enhanced, can therefore be exploited according to the pursued application. Indeed, IrO2 nanostructures have shown utility in fields that span from electrocatalysis, electrochromic devices, sensors, fuel cell and supercapacitors. After a brief description of the IrO2 structure and properties, the present review will describe the main employed synthetic methodologies that are followed to prepare selectively the various types of nanostructures, highlighting in each case the advantages brought by the nanostructuration illustrating their performances and applications.

Iridium and IrOx nanoparticles: an overview and review of syntheses and applications

Precious metals are key in various fields of research and precious metal nanomaterials are directly relevant for optics, catalysis, pollution management, sensing, medicine, and many other applications. Iridium based nanomaterials are less studied than metals like gold, silver or platinum. A specific feature of iridium nanomaterials is the relatively small size nanoparticles and clusters easily obtained, e.g. by colloidal syntheses. Progress over the years overcomes the related challenging characterization and it is expected that the knowledge on iridium chemistry and nanomaterials will be growing. Although Ir nanoparticles have been preferred systems for the development of kinetic-based models of nanomaterial formation, there is surprisingly little knowledge on the actual formation mechanism(s) of iridium nanoparticles. Following the impulse from the high expectations on Ir nanoparticles as catalysts for the oxygen evolution reaction in electrolyzers, new areas of applications of iridium materials have been reported while more established applications are being revisited. This review covers different synthetic strategies of iridium nanoparticles and provides an in breadth overview of applications reported. Comprehensive Tables and more detailed topic-oriented overviews are proposed in Supplementary Material, covering synthesis protocols, the historical role or iridium nanoparticles in the development of nanoscience and applications in catalysis.

Biocompatible reference electrodes to enhance chronic electrochemical signal fidelity in vivo

In vivo electrochemistry is a vital tool of neuroscience that allows for the detection, identification, and quantification of neurotransmitters, their metabolites, and other important analytes. One important goal of in vivo electrochemistry is a better understanding of progressive neurological disorders (e.g., Parkinson's disease). A complete understanding of such disorders can only be achieved through a combination of acute (i.e., minutes to hours) and chronic (i.e., days or longer) experimentation. Chronic studies are more challenging because they require prolonged implantation of electrodes, which elicits an immune response, leading to glial encapsulation of the electrodes and altered electrode performance (i.e., biofouling). Biofouling leads to increased electrode impedance and reference electrode polarization, both of which diminish the selectivity and sensitivity of in vivo electrochemical measurements. The increased impedance factor has been successfully mitigated previously with the use of a counter electrode, but the challenge of reference electrode polarization remains. The commonly used Ag/AgCl reference electrode lacks the long-term potential stability in vivo required for chronic measurements. In addition, the cytotoxicity of Ag/AgCl adversely affects animal experimentation and prohibits implantation in humans, hindering translational research progress. Thus, a move toward biocompatible reference electrodes with superior chronic potential stability is necessary. Two qualifying materials, iridium oxide and boron-doped diamond, are introduced and discussed in terms of their electrochemical properties, biocompatibilities, fabrication methods, and applications. In vivo electrochemistry continues to advance toward more chronic experimentation in both animal models and humans, necessitating the utilization of biocompatible reference electrodes that should provide superior potential stability and allow for unprecedented chronic signal fidelity when used with a counter electrode for impedance mitigation.

Iridium Oxide Enabled Sensors Applications

There have been numerous studies applying iridium oxides in different applications to explore their proton-change-based reactions since the 1980s. Iridium oxide can be fabricated directly by applying electrodeposition, sputter-coating method, or oxidation of iridium wire. Generally, there have been currently two approaches in applying iridium oxide to enable its sensing applications. One was to improve or create different electrolytes with (non-)electrodeposition method for better performance of Nernst Constant with the temperature-related system. The mechanism behind the scenes were summarized herein. The other was to change the structure of iridium oxide through different kinds of templates such as photolithography patterns, or template-assisted direct growth methods, etc. to improve the sensing performance. The detection targets varied widely from intracellular cell pH, glucose in an artificial sample or actual urine sample, and the hydrogen peroxide, glutamate or organophosphate pesticides, metal-ions, etc. This review paper has focused on the mechanism of electrodeposition of iridium oxide in aqueous conditions and the sensing applications towards different biomolecules compounds. Finally, we summarize future trends on Iridium oxide based sensing and predict future work that could be further explored.

An integrated E-Tube cap for sample preparation, isothermal amplification and label-free electrochemical detection of DNA

A simple, disposable, and integrated electronic-tube cap (E-tube cap) for DNA detection at the point-of-care was designed, fabricated, and tested. The E-tube cap contains a 3D printed electrode substrate for DNA extraction and label-free pH sensing detection. One Flinders Technology Associates (Whatman FTA) membrane was incorporated into the 3D printed electrode substrate for the isolation, concentration, and purification of DNA. The E-tube cap with captured DNA by the membrane was inserted directly into a reaction tube for loop-mediated isothermal amplification (LAMP). The isothermal amplification process was monitored in real-time by a 3D printed electrochemical electrode coated with pH-sensitive material (carbon/iridium oxide layer). The pH sensing electrode showed an excellent linear response within the pH range of 6-9 with a slope of -31.32 ± 0.5 mV/pH at room temperature. The utility of the integrated E-tube cap was demonstrated by detecting the presence of lambda DNA spiked in saliva samples with a sensitivity of 100 copies per mL sample within 30 min. Such a simple, rapid, and affordable diagnostic device is particularly suitable for point-of-care molecular diagnostics of infectious diseases.

PHAIR: a biosensor for pH measurement in air-liquid interface cell culture

In many biological systems, pH can be used as a parameter to understand and study cell dynamics. However, measuring pH in live cell culture is limited by the sensor ion specificity, proximity to the cell surface, and scalability. Commercially available pH sensors are difficult to integrate into a small-scale cell culture system due to their size and are not cost-effective for disposable use. We made PHAIR-a new pH sensor that uses a micro-wire format to measure pH in vitro human airway cell culture. Tungsten micro-wires were used as the working electrodes, and silver micro-wires with a silver/silver chloride coating were used as a pseudo reference electrode. pH sensitivity, in a wide and narrow range, and stability of these sensors were tested in common standard buffer solutions as well as in culture media of human airway epithelial cells grown at the air-liquid interface in a 24 well cell culture plate. When measuring the pH of cells grown under basal and challenge conditions using PHAIR, cell viability and cytokine responses were not affected. Our results confirm that micro-wire-based sensors have the capacity for miniaturization and detection of diverse ions while maintaining sensitivity. This suggests the broad application of PHAIR in various biological experimental settings.

High-fidelity profiling and modeling of heterogeneity in wastewater systems using milli-electrode array (MEA): Toward high-efficiency and energy-saving operation

High energy consumption is a critical problem for wastewater treatment systems currently monitored using conventional "single point" probes and operated with manual or automatic open-loop control strategies, exhibiting significant time lag. This challenge is addressed in this study by profiling the variation of three critical water quality parameters (conductivity, temperature and pH) along the depth of a reactor at high spatiotemporal resolution in a real-time mode using flat thin milli-electrode array (MEA) sensors. The profiling accurately captured the heterogeneous status of the reactor under transient shocks (conductivity and pH) and slow lingering shock (temperature), providing an effective dataset to optimize the chemical dosage and energy requirement of wastewater treatment systems. Transient shock models were developed to validate the MEA profiles and calculate mass transfer coefficients. Monte Carlo simulation revealed high-resolution MEA profiling combined with fast closed-loop control strategies can save 59.50% of energy consumption (Temperature and oxygen consumption controls) and 45.29% of chemical dosage, and reach 16.28% performance improvement over the benchmark (defined with ideal conditions), compared with traditional "single-point" sensors that could only monitor the entire system through a single process state. This study demonstrated the capability of MEA sensors to profile reactor heterogeneity, visualize the variation of water quality at high resolution, provide complete datasets for accurate control, and ultimately lead to energy-saving operation with high resilience.

An organic neurophysiological tool for neuronal metabolic activity monitoring

Monitoring cell metabolism in vitro is considered a relevant methodology in several scientific fields ranging from fundamental biology research to neuro-toxicology. In the last 20 years, several in vitro neuro-pharmacological and neuro-toxicological approaches have been developed, with the intent of addressing the increasing demand for real-time, non-invasive in vitro systems capable of continuously and reliably monitoring cellular activity. In this paper, an Organic Charge Modulated Field Effect Transistor-based device is proposed as a promising tool for neuro-pharmacological applications, thanks to its ultra-high pH sensitivity and a simple fabrication technology. The preliminary characterization of this versatile organic device with primary neuronal cultures shows how these remarkable properties can be exploited for the realization of ultra-sensitive metabolic probes, which are both reference-less and low cost. These features, together with the already assessed capability of this sensor to also monitor the electrical activity of electrogenic cells, could provide important advances in the fabrication of multi-sensing lab-on-chip devices, thus opening up interesting perspectives in the neuro-pharmacological field.

Oxygen Evolution Catalysts Based on Ir-Ti Mixed Oxides with Templated Mesopore Structure: Impact of Ir on Activity and Conductivity

The efficient generation of hydrogen via water electrolysis requires highly active oxygen evolution catalysts. Among the active metals, iridium oxide provides the best compromise in terms of activity and stability. The limited availability and usage in other applications demands an efficient utilization of this precious metal. Forming mixed oxides with titania promises improved Ir utilization, but often at the cost of a low catalyst surface area. Moreover, the role of Ir in establishing a sufficiently conductive mixed oxide has not been elucidated so far. We report a new approach for the synthesis of Ir/TiO_x mixed-oxide catalysts with defined template-controlled mesoporous structure, low crystallinity, and superior oxygen evolution reaction (OER) activity. The highly accessible pore system provides excellent Ir dispersion and avoids transport limitations. A controlled variation of the oxides Ir content reveals the importance of the catalysts electrical conductivity: at least 0.1 S m^-1 are required to avoid limitations owing to slow electron transport. For sufficiently conductive oxides a clear linear correlation between Ir surface sites and OER currents can be established, where all accessible Ir sites equally contribute to the reaction. The optimized catalysts outperform Ir/TiO_x materials reported in literature by about a factor of at least four.

Microsensor systems for cell metabolism - from 2D culture to organ-on-chip

Microsensor systems for cell metabolism are essential tools for investigation and standardization in cell culture. Electrochemical and optical read-out schemes dominate, which enable the marker-free, continuous, online recording of transient effects and deliver information beyond microscopy and end-point tests. There has been much progress in microfluidics and microsensors, but the translation of both into standard cell culture procedures is still limited. Within this critical review, we discuss different cell culture formats ranging from standard culture vessels to dedicated microfluidic platforms. Key aspects are the appropriate supply of cells, mass transport of metabolites to the sensors and generation of stimuli. Microfluidics enable the transition from static to dynamic conditions in culture and measurement. We illustrate the parameters oxygen (respiration), pH (acidification), glucose and lactate (energy metabolism) as well as short-lived reactive species (ROS/RNS) from the perspective of microsensor integration in 2D and 3D cell culture. We discuss different sensor principles and types, along with their limitations, microfabrication technologies and materials. The state-of-the-art of microsensor platforms for cell culture is discussed with respect to sensor performance, the number of parameters and timescale of application. That includes the advances from 2D culture to the increasingly important 3D approaches, with specific requirements for organotypic microtissues, spheroids and solid matrix cultures. We conclude on the current progress, potential, benefits and limitations of cell culture monitoring systems from monolayer culture to organ-on-chip systems.

Concept and Development of an Electronic Framework Intended for Electrode and Surrounding Environment Characterization In Vivo

There has been substantial progress over the last decade towards miniaturizing implantable microelectrodes for use in Active Implantable Medical Devices (AIMD). Compared to the rapid development and complexity of electrode miniaturization, methods to monitor and assess functional integrity and electrical functionality of these electrodes, particularly during long term stimulation, have not progressed to the same extent. Evaluation methods that form the gold standard, such as stimulus pulse testing, cyclic voltammetry and electrochemical impedance spectroscopy, are either still bound to laboratory infrastructure (impractical for long term in vivo experiments) or deliver no comprehensive insight into the material’s behaviour. As there is a lack of cost effective and practical predictive measures to understand long term electrode behaviour in vivo, material investigations need to be performed after explantation of the electrodes. We propose the analysis of the electrode and its environment in situ, to better understand and correlate the effects leading to electrode failure. The derived knowledge shall eventually lead to improved electrode designs, increased electrode functionality and safety in clinical applications. In this paper, the concept, design and prototyping of a sensor framework used to analyse the electrode’s behaviour and to monitor diverse electrode failure mechanisms, even during stimulation pulses, is presented. We focused on the electronic circuitry and data acquisition techniques required for a conceptual multi-sensor system. Functionality of single modules and a prototype framework have been demonstrated, but further work is needed to convert the prototype system into an implantable device. In vitro studies will be conducted first to verify sensor performance and reliability.

An efficient and sensitive fluorescent pH sensor based on amino functional metal–organic frameworks in aqueous environment

In this work, an amino group functionalized MOF (Al-MIL-101-NH₂), which shows strong blue luminescence, is used as pH sensor. Due to the protonated amino group, the fluorescence intensity of Al-MIL-101-NH₂almost increases with increasing pH and gives a good linear relationship (R²= 0.99688) with the pH value.

Real-Time Monitoring of Cellular Bioenergetics with a Multianalyte Screen-Printed Electrode

Real-time monitoring of changes to cellular bioenergetics can provide new insights into mechanisms of action for disease and toxicity. This work describes the development of a multianalyte screen-printed electrode for the detection of analytes central to cellular bioenergetics: glucose, lactate, oxygen, and pH. Platinum screen-printed electrodes were designed in-house and printed by Pine Research Instrumentation. Electrochemical plating techniques were used to form quasi-reference and pH electrodes. A Dimatix materials inkjet printer was used to deposit enzyme and polymer films to form sensors for glucose, lactate, and oxygen. These sensors were evaluated in bulk solution and microfluidic environments, and they were found to behave reproducibly and possess a lifetime of up to 6 weeks. Linear ranges and limits of detection for enzyme-based sensors were found to have an inverse relationship with enzyme loading, and iridium oxide pH sensors were found to have super-Nernstian responses. Preliminary measurements where the sensor was enclosed within a microfluidic channel with RAW 264.7 macrophages were performed to demonstrate the sensors' capabilities for performing real-time microphysiometry measurements.

Iridium Oxide Coatings with Templated Porosity as Highly Active Oxygen Evolution Catalysts: Structure-Activity Relationships

Iridium oxide is the catalytic material with the highest stability in the oxygen evolution reaction (OER) performed under acidic conditions. However, its high cost and limited availability demand that IrO2 is utilized as efficiently as possible. We report the synthesis and OER performance of highly active mesoporous IrO2 catalysts with optimized surface area, intrinsic activity, and pore accessibility. Catalytic layers with controlled pore size were obtained by soft-templating with micelles formed from amphiphilic block copolymers poly(ethylene oxide)-b-poly(butadiene)-b-poly(ethylene oxide). A systematic study on the influence of the calcination temperature and film thickness on the morphology, phase composition, accessible surface area, and OER activity reveals that the catalytic performance is controlled by at least two independent factors, that is, accessible surface area and intrinsic activity per accessible site. Catalysts with lower crystallinity show higher intrinsic activity. The catalyst surface area increases linearly with film thickness. As a result of the templated mesopores, the pore surface remains fully active and accessible even for thick IrO2 films. Even the most active multilayer catalyst does not show signs of transport limitations at current densities as high as 75 mA cm(-2) .

Sol-gel deposition of iridium oxide for biomedical micro-devices

Flexible iridium oxide (IrOx)-based micro-electrodes were fabricated on flexible polyimide substrates using a sol-gel deposition process for utilization as integrated pseudo-reference electrodes for bio-electrochemical sensing applications. The fabrication method yields reliable miniature on-probe IrOx electrodes with long lifetime, high stability and repeatability. Such sensors can be used for long-term measurements. Various dimensions of sol-gel iridium oxide electrodes including 1 mm × 1 mm, 500 µm × 500 µm, and 100 µm × 100 µm were fabricated. Sensor longevity and pH dependence were investigated by immersing the electrodes in hydrochloric acid, fetal bovine serum (FBS), and sodium hydroxide solutions for 30 days. Less pH dependent responses, compared to IrOx electrodes fabricated by electrochemical deposition processes, were measured at 58.8 ± 0.4 mV/pH, 53.8 ± 1.3 mV/pH and 48 ± 0.6 mV/pH, respectively. The on-probe IrOx pseudo-reference electrodes were utilized for dopamine sensing. The baseline responses of the sensors were higher than the one using an external Ag/AgCl reference electrode. Using IrOx reference electrodes integrated on the same probe with working electrodes eliminated the use of cytotoxic Ag/AgCl reference electrode without loss in sensitivity. This enables employing such sensors in long-term recording of concentrations of neurotransmitters in central nervous systems of animals and humans.

An iridium oxide microelectrode for monitoring acute local pH changes of endothelial cells

A microelectrode on a chip was modified to detect the local pH changes of the attached endothelial cells under the stimulation of thrombin.

Electrochemical detection of catecholamine release using planar iridium oxide electrodes in nanoliter microfluidic cell culture volumes

Release of neurotransmitters and hormones by calcium regulated exocytosis is a fundamental cellular/molecular process that is disrupted in a variety of psychiatric, neurological, and endocrine disorders. Therefore, this area represents a relevant target for drug and therapeutic development, efforts that will be aided by novel analytical tools and devices that provide mechanistically rich data with increased throughput. Toward this goal, we have electrochemically deposited iridium oxide (IrOx) films onto planar thin film platinum electrodes (20 μm×300 μm) and utilized these for quantitative detection of catecholamine release from adrenal chromaffin cells trapped in a microfluidic network. The IrOx electrodes show a linear response to norepinephrine in the range of 0-400 μM, with a sensitivity of 23.1±0.5 mA/M mm(2). The sensitivity of the IrOx electrodes does not change in the presence of ascorbic acid, a substance commonly found in biological samples. A replica molded polydimethylsiloxane (PDMS) microfluidic device with nanoliter sensing volumes was aligned and sealed to a glass substrate with the sensing electrodes. Small populations of chromaffin cells were trapped in the microfluidic device and stimulated by rapid perfusion with high potassium (50mM) containing Tyrode's solution at a flow rate of 1 nL/s. Stimulation of the cells produced a rapid increase in current due to oxidation of the released catecholamines, with an estimated maximum concentration in the cell culture volume of ~52 μM. Thus, we demonstrate the utility of an integrated microfluidic network with IrOx electrodes for real-time quantitative detection of catecholamines released from small populations of chromaffin cells.

The open container-used microfluidic chip using IrO(x) ultramicroelectrodes for the in situ measurement of extracellular acidification

The measurement of metabolic activity based on the extracellular acidification rate has attracted wide interests in the field of biochemical detection. In the study, the chip comprising a microfluid-controlled open container and iridium oxide (IrO(x)) pH ultramicroelectrodes (UMEs) was constructed for the purpose of in situ measurement of extracellular acidification rate. The feasible anodic depositing parameters of IrO(x) film were in the range of +0.53 to +0.8 V by means of exploring the electrochemical properties of alkaline Ir(IV) deposition solution. The IrO(x) pH UMEs electrodeposited for 300 cycles between 0 V and +0.6 V exhibited the near-super-Nernstian sensitivity of -68 to -76 mV/pH and the good stability with potential drifting of 11.7 mV within 24h. The design of the open container connected with a position-raised microchannel improved the sensing stability of IrO(x) pH UMEs, with the potential deviation of as low as 0.1 mV under the flow rate of 20 μl/min. The acidification rate of HeLa cells (2160 cells/mm(2)) repeatedly measured 5 times in the microfluidic chip showed the good reproducibility of 0.021±0.002 pH/min. Moreover, the chip can decrease the acidosis occurrence, a decrease of only 0.13-0.17 pH unit in 8 min interval, during the measurement of cellular metabolic activity.

Modeling the measurements of cellular fluxes in microbioreactor devices using thin enzyme electrodes

An analytic approach to the modeling of stop-flow amperometric measurements of cellular metabolism with thin glucose oxidase and lactate oxidase electrodes would provide a mechanistic understanding of the various factors that affect the measured signals. We divide the problem into two parts: (1) analytic formulas that provide the boundary conditions for the substrate and the hydrogen peroxide at the outer surface of the enzyme electrode layers and the electrode current expressed through these boundary conditions, and (2) a simple diffusion problem in the liquid compartment with the provided boundary conditions, which can be solved analytically or numerically, depending on the geometry of the compartment. The current in an amperometric stop-flow measurement of cellular glucose or lactate consumption/excretion is obtained analytically for two geometries, corresponding to devices developed at the Vanderbilt Institute for Integrative Biosystems Research and Education: a multianalyte nanophysiometer with effective one-dimensional diffusion and a multianalyte microphysiometer, for which plentiful data for metabolic changes in cells are available. The data are calibrated and fitted with the obtained time dependences to extract several cellular fluxes. We conclude that the analytical approach is applicable to a wide variety of measurement geometries and flow protocols.

Towards monitoring real-time cellular response using an integrated microfluidics-matrix assisted laser desorption ionisation/nanoelectrospray ionisation-ion mobility-mass spectrometry platform

The combination of microfluidic cell trapping devices with ion mobility-mass spectrometry offers the potential for elucidating in real time the dynamic responses of small populations of cells to paracrine signals, changes in metabolite levels and delivery of drugs and toxins. Preliminary experiments examining peptides in methanol and recording the interactions of yeast and Jurkat cells with their superfusate have identified instrumental set-up and control parameters and online desalting procedures. Numerous initial experiments demonstrate and validate this new instrumental platform. Future outlooks and potential applications are addressed, specifically how this instrumentation may be used for fully automated systems biology studies of the significantly interdependent, dynamic internal workings of cellular metabolic and signalling pathways.

Microfluidic Device with Integrated Glucose Sensor for Cell-Based Assay in Toxicology

We have developed a cell-based assay platform using a microfluidic device integrating a glucose sensor into a cell culture device with closed-loop perfusion. Online measurement of cell kinetic change associated with cell status change was achieved by measuring glucose concentration change in the device with a cell exposed to a toxic material. The cell-based assay platform, which is integrated with a sensor and a perfusion system, was expected to improve measurement accuracy and efficiency, leading to the discovery of new tools in such wide-ranging fields as drug discovery, life sciences, and medical research.

Enzyme-coated microelectrodes to monitor lactate production in a nanoliter microfluidic cell culture device

Monitoring the degree of anaerobic respiration of cells in high density microscale culture systems is an enabling key technology and essential for cell-based biosensors. We have fabricated and incorporated miniature amperometric lactate sensing electrodes with working areas from 3 to 5×10(-2) mm2 into a microfluidic-based microscale cell culture system to measure the lactate production rate of fibroblasts in nanoliter volumes. Planar thin film platinum electrode arrays on glass substrates were spin coated with lactate oxidase and a protective Nafion layer. The lactate electrodes had a high enzymatic activity described by a Michaelis-Menten constant of 2.6±0.1 mM, a linear response in the range 0.01-2.5 mM and a sensitivity of 7.3×10(-2) mA/mM cm2. A replica-molded polydimethylsiloxane (PDMS) microfluidic device with nanoliter sensing volumes was aligned and sealed to a glass substrate with the sensing electrodes. We trapped fibroblasts in the cell culture volume and measured the lactate production rate using a stop-flow protocol. The average lactate production rate was 0.011±0.0049 mM/min. The lactate production was suppressed with the addition of 2-deoxy-D-glucose, which binds to hexokinase. The blocking of hexokinase prevents the generation of pyruvate, the intermittent substrate required for lactate production even in the presence of glucose.

A Clark-type oxygen chip for in situ estimation of the respiratory activity of adhering cells

A Clark-type oxygen chip consisting of a polydimethylsiloxane (PDMS) reservoir containing an amino group-modified PDMS oxygen-permeable membrane (OPM) and a glass substrate containing a three-electrode detector has been constructed by using microfabrication techniques, and it is utilized for in situ measurement of the respiration activity of adhering cells. Use of the alginate sol electrolyte and the electroplating Ag/AgCl pseudo-reference electrode can effectively diminish the crosstalk between the electrochemical electrodes and supply a stable potential for the detection of dissolved oxygen, respectively. The Clark-type oxygen chips possess only 1.00% residual current, response time of 13.4s and good linearity with a correlation coefficient of 0.9933. The modification of amino groups for the OPM obviously facilitates the adhesion of HeLa cells onto the PDMS OPM surface and allows the cells to spread after 2h of incubation. The oxygen consumption of the cells in the cell-adhesion process increases with the adhesion time, and the increment of cellular oxygen consumption per minute reaches a maximum after 30 min of incubation. Moreover, the change in the respiration activity of adhering HeLa cells stimulated by the high concentration of glucose or propofol anaesthetic can be monitored in real time with the Clark-type oxygen chip.

Enzyme electrodes to monitor glucose consumption of single cardiac myocytes in sub-nanoliter volumes

Monitoring the metabolic activity of cells in automated culture systems is one of the key features of micro-total-analysis-systems. We have developed a microfluidic device that allows us to trap single cardiac myocytes (SCMs) in sub-nanoliter volumes and incorporate amperometric glucose-sensing electrodes with working areas of 0.002 mm(2) to measure the glucose consumption of SCM. The miniaturized planar glucose electrodes were fabricated by spin coating platinum electrodes on glass substrates with a glutaraldehyde/enzyme solution and a protective Nafion membrane. The glucose electrodes demonstrate a high enzymatic activity characterized by an apparent Michaelis-Menten constant of 7.52+/-0.18 mM and a sensitivity of approximately 33.8 and approximately 13.2 mA/Mcm(2) at glucose concentration from 0-6 to 6-20 mM in Tyrode's solution, respectively. The response time of the glucose electrodes was between 5 and 15s, and the sensitivity of the electrodes did not degrade over a period of 8 weeks. A replica molded polydimethylsiloxane microfluidic device with a sub-nanoliter sensing volume was sealed to the glass substrate and aligned with the glucose microelectrodes. SCM can be trapped in the sensing volume above the glucose electrodes to measure the glucose consumption over time. The average glucose consumption of SCM was 0.211+/-0.097 mM/min (n=7) in Tyrode's solution with 5 mM of glucose.

Microfluidics: a new cosset for neurobiology

Recently, microfluidic systems have shown great potential in the study of molecular and cellular biology. With its excellent properties, such as miniaturization, integration and automation, to name just a few, microfluidics creates new opportunities for the spatial and temporal control of cell growth and environmental stimuli in vitro. In the field of neuroscience, microfluidic devices offer precise control of the microenvironment surrounding individual cells, and the delivery of biochemical or physical cues to neural networks or single neurons. The intent of this review is to outline recent advances in microfluidic-based applications in neurobiology, with emphasis on neuron culture, neuron manipulation, neural stem cell differentiation, neuropharmacology, neuroelectrophysiology, and neuron biosensors. It also aims to stimulate development of microfluidic-based applications in neurobiology by involving scientists from various disciplines, especially neurobiology and microtechnology.

Microfluidic platform for real-time signaling analysis of multiple single T cells in parallel

Deciphering the signaling pathways that govern stimulation of naïve CD4+ T helper cells by antigen-presenting cells via formation of the immunological synapse is key to a fundamental understanding of the progression of successful adaptive immune response. The study of T cell-APC interactions in vitro is challenging, however, due to the difficulty of tracking individual, non-adherent cell pairs over time. Studying single cell dynamics over time reveals rare, but critical, signaling events that might be averaged out in bulk experiments, but these less common events are undoubtedly important for an integrated understanding of a cellular response to its microenvironment. We describe a novel application of microfluidic technology that overcomes many limitations of conventional cell culture and enables the study of hundreds of passively sequestered hematopoietic cells for extended periods of time. This microfluidic cell trap device consists of 440 18 micromx18 micromx10 microm PDMS, bucket-like structures opposing the direction of flow which serve as corrals for cells as they pass through the cell trap region. Cell viability analysis revealed that more than 70% of naïve CD4+ T cells (TN), held in place using only hydrodynamic forces, subsequently remain viable for 24 hours. Cytosolic calcium transients were successfully induced in TN cells following introduction of chemical, antibody, or cellular forms of stimulation. Statistical analysis of TN cells from a single stimulation experiment reveals the power of this platform to distinguish different calcium response patterns, an ability that might be utilized to characterize T cell signaling states in a given population. Finally, we investigate in real time contact- and non-contact-based interactions between primary T cells and dendritic cells, two main participants in the formation of the immunological synapse. Utilizing the microfluidic traps in a daisy-chain configuration allowed us to observe calcium transients in TN cells exposed only to media conditioned by secretions of lipopolysaccharide-matured dendritic cells, an event which is easily missed in conventional cell culture where large media-to-cell ratios dilute cellular products. Further investigation into this intercellular signaling event indicated that LPS-matured dendritic cells, in the absence of antigenic stimulation, secrete chemical signals that induce calcium transients in T(N) cells. While the stimulating factor(s) produced by the mature dendritic cells remains to be identified, this report illustrates the utility of these microfluidic cell traps for analyzing arrays of individual suspension cells over time and probing both contact-based and intercellular signaling events between one or more cell populations.

On-chip acidification rate measurements from single cardiac cells confined in sub-nanoliter volumes

The metabolic activity of cells can be monitored by measuring the pH in the extracellular environment. Microfabrication and microfluidic technologies allow the sensor size and the extracellular volumes to be comparable to single cells. A glass substrate with thin film pH sensitive IrO( x ) electrodes was sealed to a replica-molded polydimethylsiloxane (PDMS) microfluidic network with integrated valves. The device, termed NanoPhysiometer, allows the trapping of single cardiac myocytes and the measurement of the pH in a detection volume of 0.36 nL. For wild-type (WT) single cardiac myocytes an acidification rate of 6.45 +/- 0.38 mpH/min was measured in comparison to 19.5 +/- 0.38 mpH/min for very long chain Acyl-CoA dehydrogenase (VLCAD) deficient mice in 0.8 mM of Ca(2+). VLCAD deficiency is a fatty acid oxidation disease leading to cardiomyopathy and arrhythmias. The acidification rate increased to 11.96 +/- 1.33 mpH/min for WT and to 32.0 +/- 4.64 mpH/min for VLCAD -/- in 1.8 mM of Ca(2+). The NanoPhysiometer concept can be extended to study ischemia/reperfusion injury or disorders of other biological systems to identify strategies for treatment and possible pharmacological targets.

An integrated microfluidic system for long-term perfusion culture and on-line monitoring of intestinal tissue models

Conventional cell-based assays in life science and medical applications can be difficult to maintain functionally over long periods. Microfluidics is an emerging technology with potential to provide integrated environments for cell maintenance, continuous perfusion, and monitoring. In this study, we developed an integrated microfluidic device with on-chip pumping and detection functionalities. The microfluidic structure in the device is divided into two independent channels separated by a semipermeable membrane on which cells are inoculated and cultured. Perfusion and fluorescence measurements of culture media for each channel can be conducted by the on-chip pumping system and optical fiber detection system. Performance of the device was examined through long-term culture and monitoring of polarized transport activity of intestinal tissue models (Caco-2 cells). The cells could be cultured for more than two weeks, and monolayer transport of rhodamine 123 was successfully monitored by on-line fluorescent measurement. This device may have applications in toxicity testing and drug screening.