Chemical and morphological changes on platinum microelectrode surfaces in AC and DC fields with biological buffer solutions

Deciphering platinum dissolution in neural stimulation electrodes: Electrochemistry or biology?

Platinum (Pt) is the metal of choice for electrodes in implantable neural prostheses like the cochlear implants, deep brain stimulating devices, and brain-computer interfacing technologies. However, it is well known since the 1970s that Pt dissolution occurs with electrical stimulation. More recent clinical and in vivo studies have shown signs of corrosion in explanted electrode arrays and the presence of Pt-containing particulates in tissue samples. The process of degradation and release of metallic ions and particles can significantly impact on device performance. Moreover, the effects of Pt dissolution products on tissue health and function are still largely unknown. This is due to the highly complex chemistry underlying the dissolution process and the difficulty in decoupling electrical and chemical effects on biological responses. Understanding the mechanisms and effects of Pt dissolution proves challenging as the dissolution process can be influenced by electrical, chemical, physical, and biological factors, all of them highly variable between experimental settings. By evaluating comprehensive findings on Pt dissolution mechanisms reported in the fuel cell field, this review presents a critical analysis of the possible mechanisms that drive Pt dissolution in neural stimulation in vitro and in vivo. Stimulation parameters, such as aggregate charge, charge density, and electrochemical potential can all impact the levels of dissolved Pt. However, chemical factors such as electrolyte types, dissolved gases, and pH can all influence dissolution, confounding the findings of in vitro studies with multiple variables. Biological factors, such as proteins, have been documented to exhibit a mitigating effect on the dissolution process. Other biological factors like cells and fibro-proliferative responses, such as fibrosis and gliosis, impact on electrode properties and are suspected to impact on Pt dissolution. However, the relationship between electrical properties of stimulating electrodes and Pt dissolution remains contentious. Host responses to Pt degradation products are also controversial due to the unknown chemistry of Pt compounds formed and the lack of understanding of Pt distribution in clinical scenarios. The cytotoxicity of Pt produced via electrical stimulation appears similar to Pt-based compounds, including hexachloroplatinates and chemotherapeutic agents like cisplatin. While the levels of Pt produced under clinical and acute stimulation regimes were typically an order of magnitude lower than toxic concentrations observed in vitro, further research is needed to accurately assess the mass balance and type of Pt produced during long-term stimulation and its impact on tissue response. Finally, approaches to mitigating the dissolution process are reviewed. A wide variety of approaches, including stimulation strategies, coating electrode materials, and surface modification techniques to avoid excess charge during stimulation and minimise tissue response, may ultimately support long-term and safe operation of neural stimulating devices.

Dielectrophoretic profiling of erythrocytes to study the impacts of metabolic stress, temperature, and storage duration utilizing a point-and-planar microdevice

Dielectrophoresis (DEP) is widely utilized for trapping and sorting various types of cells, including live and dead cells and healthy and infected cells. This article focuses on the dielectric characterization of erythrocytes (red blood cells or RBCs) by quantifying DEP crossover frequency using a novel point-and-planar microwell device platform. Numerical simulations using COMSOL Multiphysics software demonstrate that the distribution of the DEP force is influenced by factors such as the shape of the point electrode, spacing between the point and planar electrodes, and the type of bioparticle being investigated. The dependency on electrode spacing is experimentally evaluated by analyzing the DEP crossover response of erythrocytes. Furthermore, the results are validated against the traditional electrical characterization technique called electrorotation, which typically requires laborious fabrication and operation using quadrupole electrodes. Other significant factors, including erythrocyte storage age and the changes in cell properties over time since collection, osmolarity, and temperature, are also assessed to determine the optimal conditions for erythrocyte characterization. The findings indicate a significant difference between fresh and stored erythrocyte samples (up to 4 days), highlighting the importance of maintaining an isotonic medium for cell storage.

A critical review on the fabrication techniques that can enable higher throughput in dielectrophoresis devices

The sorting of targeted cells in a sample is a cornerstone of healthcare diagnostics and therapeutics. This work focuses on the use of dielectrophoresis for the selective sorting of targeted bioparticles in a sample and how the lack of throughput has been one important practical challenge to its widespread practical implementation. Increasing the cross-sectional area of a channel can lead to higher flow rates and thus the capability to process a larger sample volume per unit of time. However, the required electric field gradient that is generated by polarized electrodes drastically decreases as one moves away from the electrodes. Hence, the scaling up of the channel cross section must be done asymmetrically. One desires a channel aspect ratio AR = height/width that is much smaller or much larger than 1. Since reducing footprint of the DEP device is important to ensure affordability, the use of channels with AR>>1 is desired. This creates the challenge to fabricate electrodes on the sidewalls of multiple channels with AR>>1, or a channel embedding an array of electrodes with a gap in between them with AR >>1. This critical review first details the motivation for using three-dimensional (3D) DEP devices to improve throughput and then describes selected techniques that have been used to fabricate them. Techniques include electrodeposition, deep etching, thick-film photolithography, and co-fabrication. Electrode materials addressed include metals, silicon, carbon, PDMS-based composites as well as conductive polymers and fluids.

Dielectrophoretic Manipulation of Cancer Cells and Their Electrical Characterization

Electromanipulation and electrical characterization of cancerous cells is becoming a topic of high interest as the results reported to date demonstrate a good differentiation among various types of cells from an electrical viewpoint. Dielectrophoresis and broadband dielectric spectroscopy are complementary tools for sorting, identification, and characterization of malignant cells and were successfully used on both primary tumor cells and culture cells as well. However, the literature is presenting a plethora of studies with respect to electrical evaluation of these type of cells, and this review is reporting a collection of information regarding the functioning principles of different types of dielectrophoresis setups, theory of cancer cell polarization, and electrical investigation (including here the polarization mechanisms). The interpretation of electrical characteristics against frequency is discussed with respect to interfacial/Maxwell-Wagner polarization and the parasitic influence of electrode polarization. Moreover, the electrical equivalent circuits specific to biological cells polarizations are discussed for a good understanding of the cells' morphology influence. The review also focuses on advantages of specific low-conductivity buffers employed currently for improving the efficiency of dielectrophoresis and provides a set of synthesized data from the literature highlighting clear differentiation between the crossover frequencies of different cancerous cells.

Dielectric Characterization and Separation Optimization of Infiltrating Ductal Adenocarcinoma via Insulator-Dielectrophoresis

The dielectrophoretic separation of infiltrating ductal adenocarcinoma cells (ADCs) from isolated peripheral blood mononuclear cells (PBMCs) in a ~1.4 mm long Y-shaped microfluidic channel with semi-circular insulating constrictions is numerically investigated. In this work, ADCs (breast cancer cells) and PBMCs' electrophysiological properties were iteratively extracted through the fitting of a single-shell model with the frequency-conductivity data obtained from AC microwell experiments. In the numerical computation, the gradient of the electric field required to generate the necessary dielectrophoretic force within the constriction zone was provided through the application of electric potential across the whole fluidic channel. By adjusting the difference in potentials between the global inlet and outlet of the fluidic device, the minimum (effective) potential difference with the optimum particle transmission probability for ADCs was found. The radius of the semi-circular constrictions at which the effective potential difference was swept to obtain the optimum constriction size was also obtained. Independent particle discretization analysis was also conducted to underscore the accuracy of the numerical solution. The numerical results, which were obtained by the integration of fluid flow, electric current, and particle tracing module in COMSOL v5.3, reveal that PBMCs can be maximally separated from ADCs using a DC power source of 50 V. The article also discusses recirculation or wake formation behavior at high DC voltages (>100 V) even when sorting of cells are achieved. This result is the first step towards the production of a supplementary or confirmatory test device to detect early breast cancer non-invasively.

Chronic intracochlear electrical stimulation at high charge densities results in platinum dissolution but not neural loss or functional changes in vivo

OBJECTIVE

Although there are useful guidelines defining the boundary between damaging and non-damaging electrical stimulation they were derived from acute studies using large surface area electrodes in direct contact with cortical neurons. These parameters are a small subset of the parameters used by neural stimulators. More recently, histological examination of cochleae from patients that were long-term cochlear implant users have shown evidence of particulate platinum (Pt). The pathophysiological effect of Pt within the cochlea is unknown. We examined the response of the cochlea to stimulus levels beyond those regarded as safe, and to evaluate the pathophysiological response of the cochlea following chronic stimulation at charge densities designed to induce Pt corrosion in vivo.

APPROACH

19 guinea pigs were systemically deafened and implanted with a cochlear electrode array containing eight Pt electrodes of 0.05, 0.075 or 0.2 mm² area. Animals were electrically stimulated continuously for 28 d using charge balanced current pulses at charge densities of 400, 267 or 100 µC/cm²/phase. Electrically-evoked auditory brainstem responses (EABRs) were recorded to monitor neural function. On completion of stimulation electrodes were examined using scanning electron microscopy (SEM) and cochleae examined histology. Finally, analysis of Pt was measured using energy dispersive x-ray spectroscopy (EDS) and inductively coupled plasma mass spectrometry (ICP-MS).

MAIN RESULTS

Compared with unstimulated control electrodes and electrodes stimulated at 100 µC/cm²/phase, stimulation at 267 or 400 µC/cm²/phase resulted in significant Pt corrosion. Cochleae stimulated at these high charge densities contained particulate Pt. The extent of the foreign body response depended on the level of stimulation; cochleae stimulated at 267 or 400 µC/cm²/phase exhibited an extensive tissue response that included a focal region of necrosis close to the electrode. Despite chronic stimulation at high charge densities there was no loss of auditory neurons (ANs) in stimulated cochleae compared with their contralateral controls. Indeed, we report a statistically significant increase in AN density proximal to electrodes stimulated at 267 or 400 µC/cm²/phase. Finally, there was no evidence of a reduction in AN function associated with chronic stimulation at 100, 267 or 400 µC/cm²/phase as evidenced by stable EABR thresholds over the stimulation program.

SIGNIFICANCE

Chronic electrical stimulation of Pt electrodes at 267 or 400 µC/cm²/phase evoked a vigorous tissue response and produced Pt corrosion products that were located close to the electrode. Despite these changes at the electrode/tissue interface there was no evidence of neural loss or a reduction in neural function.

Platinum corrosion products from electrode contacts of human cochlear implants induce cell death in cell culture models

Despite the technological progress made with cochlear implants (CI), impedances and their diagnosis remain a focus of interest. Increases in impedance have been related to technical defects of the electrode as well as inflammatory and/or fibrosis along the electrode. Recent studies have demonstrated highly increased impedances as the result of corroded platinum (Pt) electrode contacts. This in vitro study examined the effects of Pt ions and compounds generated by corrosion of the electrode contacts of a human CI on cell metabolism. Since traces of solid Pt in surrounding cochlear tissues have been reported, the impact of commercially available Pt nanoparticles (Pt-NP, size 3 nm) on the cell culture model was also determined. For this purpose, the electrode contacts were electrically stimulated in a 0.5% aqueous NaCl solution for four weeks and the mass fraction of the platinum dissolute (Pt-Diss) was determined by mass spectrometry (ICP-MS). Metabolic activity of the murine fibroblasts (NIH 3T3) and the human neuroblastoma (SH-SY5Y) cells was determined using the WST-1 assay following exposure to Pt-Diss and Pt-NP. It was found that 5–50 μg/ml of the Pt-NP did not affect the viability of both cell types. In contrast, 100 μg/ml of the nanoparticles caused significant loss in metabolic activity. Furthermore, transmission electron microscopy (TEM) revealed mitochondrial swelling in both cell types indicating cytotoxicity. Additionally, TEM demonstrated internalized Pt-NP in NIH 3T3 cells in a concentration dependent manner, whereas endocytosis in SH-SY5Y cells was virtually absent. In comparison with the Pt-NP, the corrosion products (Pt-Diss) with concentrations between 1.64 μg/ml and 8.2 μg/ml induced cell death in both cell lines in a concentration dependent manner. TEM imaging revealed both mitochondrial disintegration and swelling of the endoplasmic reticulum, suggesting that Pt ions trigger cytotoxicity in both NIH 3T3 and SH-SY5Y cell lines by interacting with the respiratory chain.

Dielectrophoresis-based microfluidic platforms for cancer diagnostics

The recent advancement of dielectrophoresis (DEP)-enabled microfluidic platforms is opening new opportunities for potential use in cancer disease diagnostics. DEP is advantageous because of its specificity, low cost, small sample volume requirement, and tuneable property for microfluidic platforms. These intrinsic advantages have made it especially suitable for developing microfluidic cancer diagnostic platforms. This review focuses on a comprehensive analysis of the recent developments of DEP enabled microfluidic platforms sorted according to the target cancer cell. Each study is critically analyzed, and the features of each platform, the performance, added functionality for clinical use, and the types of samples, used are discussed. We address the novelty of the techniques, strategies, and design configuration used in improving on existing technologies or previous studies. A summary of comparing the developmental extent of each study is made, and we conclude with a treatment of future trends and a brief summary.

Separation of nanoparticles by a nano-orifice based DC-dielectrophoresis method in a pressure-driven flow

A novel DC-dielectrophoresis (DEP) method employing a pressure-driven flow for the continuous separation of micro/nano-particles is presented in this paper. To generate the DEP force, a small voltage difference is applied to produce a non-uniformity of the electric field across a microchannel via a larger orifice of several hundred microns on one side of the channel wall and a smaller orifice of several hundred nanometers on the opposite channel wall. The particles experience a DEP force when they move with the flow through the vicinity of the small orifice, where the strongest electrical field gradient exists. Experiments were conducted to demonstrate the separation of 1 μm and 3 μm polystyrene particles by size by adjusting the applied electrical potentials. In order to separate smaller nanoparticles, the electrical conductivity of the suspending solution is adjusted so that the polystyrene nanoparticles of a given size experience positive DEP while the polystyrene nanoparticles of another size experience negative DEP. Using this method, the separation of 51 nm and 140 nm nanoparticles and the separation of 140 nm and 500 nm nanoparticles were demonstrated. In comparison with the microfluidic DC-DEP methods reported in the literature which utilize hurdles or obstacles to induce the non-uniformity of an electric field, a pair of asymmetrical orifices on the channel side walls is used in this method to generate a strong electrical field gradient and has advantages such as capability of separating nanoparticles, and locally applied lower electrical voltages to minimize the Joule heating effect.

Dielectrophoretic applications for disease diagnostics using lab-on-a-chip platforms

Dielectrophoresis is a powerful technique used to distinguish distinct cellular identities in heterogeneous cell populations and to monitor changes in the cell state without the need for biochemical tags, including live and dead cells. Recent studies in the past decade have indicated that dielectrophoresis can be used to discriminate the disease state of cells by exploring the differences in the dielectric polarizabilities of the cells. Factors controlling the dielectric polarizability are dependent on the conductivity and permittivity of the cell and the suspending medium, the cell morphology, the internal structure, and the electric double layer effects associated with the charges on the cell surface. Diseased cells, such as those associated with malaria, cancer, dengue, anthrax and human African trypanosomiasis, could be spatially trapped by positive dielectrophoresis or spatially separated from other healthy cells by negative dielectrophoretic forces. The aim of this review was to provide a better and deeper understanding on how dielectrophoresis can be utilized to manipulate diseased cells. This review compiles and compares the significant findings obtained by researchers in manipulating abnormal or unhealthy cells.

Scanning electron microscopy of chronically implanted intracortical microelectrode arrays in non-human primates

OBJECTIVE

Signal attenuation is a major problem facing intracortical sensors for chronic neuroprosthetic applications. Many studies suggest that failure is due to gliosis around the electrode tips, however, mechanical and material causes of failure are often overlooked. The purpose of this study was to investigate the factors contributing to progressive signal decline by using scanning electron microscopy (SEM) to visualize structural changes in chronically implanted arrays and histology to examine the tissue response at corresponding implant sites.

APPROACH

We examined eight chronically implanted intracortical microelectrode arrays (MEAs) explanted from non-human primates at times ranging from 37 to 1051 days post-implant. We used SEM, in vivo neural recordings, and histology (GFAP, Iba-1, NeuN). Three MEAs that were never implanted were also imaged as controls.

MAIN RESULTS

SEM revealed progressive corrosion of the platinum electrode tips and changes to the underlying silicon. The parylene insulation was prone to cracking and delamination, and in some instances the silicone elastomer also delaminated from the edges of the MEA. Substantial tissue encapsulation was observed and was often seen growing into defects in the platinum and parylene. These material defects became more common as the time in vivo increased. Histology at 37 and 1051 days post-implant showed gliosis, disruption of normal cortical architecture with minimal neuronal loss, and high Iba-1 reactivity, especially within the arachnoid and dura. Electrode tracts were either absent or barely visible in the cortex at 1051 days, but were seen in the fibrotic encapsulation material suggesting that the MEAs were lifted out of the brain. Neural recordings showed a progressive drop in impedance, signal amplitude, and viable channels over time.

SIGNIFICANCE

These results provide evidence that signal loss in MEAs is truly multifactorial. Gliosis occurs in the first few months after implantation but does not prevent useful recordings for several years. Progressive meningeal fibrosis encapsulates and lifts MEAs out of the cortex while ongoing foreign body reactions lead to progressive degradation of the materials. Long-term impedance drops are due to the corrosion of platinum, cracking and delamination of parylene, and delamination of silicone elastomer. Oxygen radicals released by cells of the immune system likely mediate the degradation of these materials. Future MEA designs must address these problems through more durable insulation materials, more inert electrode alloys, and pharmacologic suppression of fibroblasts and leukocytes.

Dielectrophoretic behavior of PEGylated RNase A inside a microchannel with diamond-shaped insulating posts

Ribonuclease A (RNase A) has proven potential as a therapeutic agent, especially in its PEGylated form. Grafting of PEG molecules to this protein yields mono-PEGylated (mono-PEG) and di-PEGylated (di-PEG) RNase A conjugates, and the unreacted protein. Mono-PEG RNase A is of great interest. The use of electrokinetic forces in microdevices represents a novel alternative to chromatographic methods to separate this specie. This work describes the dielectrophoretic behavior of the main protein products of the RNase A PEGylation inside a microchannel with insulators under direct current electric fields. This approach represents the first step in route to design micro-bioprocesses to separate PEGylated RNase A from unreacted native protein. The three proteins exhibited different dielectrophoretic behaviors. All of them experienced a marked streaming pattern at 3000 V consistent with positive dielectrophoresis. Native protein was not captured at any of the conditions tested, while mono-PEG RNase A and di-PEG RNase A were captured presumably due to positive dielectrophoresis at 4000 and 2500 V, respectively. Concentration of mono-PEG RNase A with a maximal enrichment efficiency of ≈9.6 times the feed concentration was achieved in few seconds. These findings open the possibility of designing novel devices for rapid separation, concentration, and recovery of PEGylated RNase A in a one-step operation.

Continuous-flow system and monitoring tools for the dielectrophoretic integration of nanowires in light sensor arrays

Although nanowires (NWs) may improve the performance of many optoelectronic devices such as light emitters and photodetectors, the mass commercialization of these devices is limited by the difficult task of finding reliable and reproducible methods to integrate the NWs on foreign substrates. This work shows the fabrication of zinc oxide NWs photodetectors on conventional glass using transparent conductive electrodes to effectively integrate the NWs at specific locations by dielectrophoresis (DEP). The paper describes the careful preparation of NW dispersions by sedimentation and the dielectrophoretic alignment of NWs in a home-made system. This system includes an impedance technique for the assessment of the alignment quality in real time. Following this procedure, ultraviolet photodetectors based on the electrical contacts formed by the DEP process on the transparent electrodes are fabricated. This cost-effective mean of contacting NWs enables front-and back-illumination operation modes, the latter eliminating shadowing effects caused by the deposition of metals. The electro-optical characterization of the devices shows uniform responsivities in the order of 106 A W(-1) below 390 nm under both modes, as well as, time responses of a few seconds.

Focusing and continuous separation of microparticles by insulator-based dielectrophoresis (iDEP) in stair-shaped microchannel

Focusing and separation of microparticles in a complex mixture have had wide applications in chemistry, biology, medicine, etc. This work presents a numerical and experimental investigation on focusing and continuous separation of microparticles in a geometrically optimized arrangement of steps in the form of a staircase using insulator-based dielectrophoresis (iDEP) mechanism. First, a detailed finite element analysis was performed on important parameters in the focusing and separation of microparticles, such as geometry of stair-shaped microchannel, total voltage, and voltage difference applied to reservoirs. The optimum parameters obtained from numerical analysis were used for experimental work. Theoretically, predicted microparticle trajectories are in good agreement with experimentally observed ones. Experimental and numerical results show that the performance of focusing of microparticles enhances with growth of the total voltage (in a constant voltage difference) and decreases with voltage difference. The fabricated iDEP microchip enhances the performance of focusing and separation of microparticles due to its stair-shaped microchannel and therefore operates at low DC total applied voltages of 90-110 V.

Direct current dielectrophoretic simulation of proteins using an array of circular insulating posts

This paper presents a mathematical model for the manipulation of proteins using insulator-based dielectrophoresis (iDEP) and direct current (DC) electric fields. Simulations via COMSOL v4.1 Multiphysics software are implemented to study the response of moderately sized proteins on a lab-on-a-chip platform. The geometry of the device is incorporated in a model that solves current and mass conservation equations within an array of circular insulating silicon posts embedded in a channel. Both micro- and nano-scale geometries are utilized to investigate the protein concentration distributions in the iDEP device. Our results indicate that the trapping of proteins is independent of the scale with respect to the geometry of a device as long as the applied electric field remains constant. DC voltage dependency on concentration distributions has also been explored in both micro- and nano-scale device geometries. To achieve DEP trapping of the proteins, nano-scale geometry is a better selection, as the voltage necessary to generate the required electric field (2.5 MV/cm) is 10(5) × lower compared with the voltage required to generate the same field in the micro-scale device.

Microfabrication technologies in dielectrophoresis applications--a review

DEP is an established technique for particle manipulation. Although first demonstrated in the 1950s, it was not until the development of miniaturization techniques in the 1990s that DEP became a popular research field. The 1990s saw an explosion of DEP publications using microfabricated metal electrode arrays to sort a wide variety of cells. The concurrent development of microfluidics enabled devices for flow management and better understanding of the interaction between hydrodynamic and electrokinetic forces. Starting in the 2000s, alternative techniques have arisen to overcome common problems in metal-electrode DEP, such as electrode fouling, and to increase the throughput of the system. Insulator-based DEP and light-induced DEP are the most significant examples. Most recently, new 3D techniques such as carbon-electrode DEP, contactless DEP, and the use of doped PDMS have further simplified the fabrication process. The constant desire of the community to develop practical solutions has led to devices which are more user friendly, less expensive, and are capable of higher throughput. The state-of-the-art of fabricating DEP devices is critically reviewed in this work. The focus is on how different fabrication techniques can boost the development of practical DEP devices to be used in different settings such as clinical cell sorting and infection diagnosis, industrial food safety, and enrichment of particle populations for drug development.

Dielectrophoretic differentiation of mouse ovarian surface epithelial cells, macrophages, and fibroblasts using contactless dielectrophoresis

Ovarian cancer is the leading cause of death from gynecological malignancies in women. The primary challenge is the detection of the cancer at an early stage, since this drastically increases the survival rate. In this study we investigated the dielectrophoretic responses of progressive stages of mouse ovarian surface epithelial (MOSE) cells, as well as mouse fibroblast and macrophage cell lines, utilizing contactless dielectrophoresis (cDEP). cDEP is a relatively new cell manipulation technique that has addressed some of the challenges of conventional dielectrophoretic methods. To evaluate our microfluidic device performance, we computationally studied the effects of altering various geometrical parameters, such as the size and arrangement of insulating structures, on dielectrophoretic and drag forces. We found that the trapping voltage of MOSE cells increases as the cells progress from a non-tumorigenic, benign cell to a tumorigenic, malignant phenotype. Additionally, all MOSE cells display unique behavior compared to fibroblasts and macrophages, representing normal and inflammatory cells found in the peritoneal fluid. Based on these findings, we predict that cDEP can be utilized for isolation of ovarian cancer cells from peritoneal fluid as an early cancer detection tool.

Isolation of prostate tumor initiating cells (TICs) through their dielectrophoretic signature

In this study, the dielectrophoretic response of prostate tumor initiating cells (TICs) was investigated in a microfluidic system utilizing contactless dielectrophoresis (cDEP). The dielectrophoretic response of prostate TICs was observed to be distinctively different than that for non-TICs, enabling them to be sorted using cDEP. Culturing the sorted TICs generated spheroids, indicating that they were indeed initiating cells. This study presents the first marker-free TIC separation from non-TICs utilizing their electrical fingerprints through dielectrophoresis.

A microfluidic platform for measuring electrical activity across cells

In this paper, we present a microfluidic chip that is capable of measuring electrical conductance through gap junction channels in a 2-dimensional cell sheet. The chip utilizes a tri-stream laminar flow to create a non-conductive sucrose gap between the two conducting solutions so that electrical current can pass across the sucrose gap only through the cells. Using the chip, we tested the effect of a gap junction inhibitor, 2-APB, on the electrical coupling of connexin 43 (Cx43) gap junction channels in NRK-49F cells. We found that 2-APB reversibly blocks the conductivity in a dose-dependent manner. The tri-stream chip further allows us to simultaneously follow the conductance changes and dye diffusion in real time. We show that 2-APB affects both conductance and diffusion, supporting the interpretation that both sets of data reflect the same gap junction activity. The chip provides a generic platform to investigate gap junction properties and to screen drugs that may inhibit or potentiate gap junction transmission.

Quantification of pH gradients and implications in insulator-based dielectrophoresis of biomolecules

Direct current (DC) insulator-based dielectrophoretic (iDEP) microdevices have the potential to replace traditional alternating current dielectrophoretic devices for many cellular and biomolecular separation applications. The use of large DC fields suggest that electrode reactions and ion transport mechanisms can become important and impact ion distributions in the nanoliters of fluid in iDEP microchannels. This work tracked natural pH gradient formation in a 100 μm wide, 1 cm-long microchannel under applicable iDEP protein manipulation conditions. Using fluorescence microscopy with the pH-sensitive dye FITC Isomer I and the pH-insensitive dye TRITC as a reference, pH was observed to drop drastically in the microchannels within 1 min in a 3000 V/cm electric field; pH drops were observed in the range of 6-10 min within a 100 V/cm electric field and varied based on the buffer conductivity. To address concerns of dye transport impacting intensity data, electrokinetic mobilities of FITC were carefully examined and found to be (i) toward the anode and (ii) 1 to 2 orders of magnitude smaller than H⁺ transport which is responsible for pH drops from the anode toward the cathode. COMSOL simulations of ion transport showed qualitative agreement with experimental results. The results indicate that pH changes are severe enough and rapid enough to influence the net charge of a protein or cause aggregation during iDEP experiments. The results also elucidate reasonable time periods over which the phosphate buffering capacity can counter increases in H⁺ and OH⁻ for unperturbed iDEP manipulations.

Direct current insulator-based dielectrophoretic characterization of erythrocytes: ABO-Rh human blood typing

A microfluidic platform developed for quantifying the dependence of erythrocyte (red blood cell, RBC) responses by ABO-Rh blood type via direct current insulator dielectrophoresis (DC-iDEP) is presented. The PDMS DC-iDEP device utilized a 400 x 170 μm² rectangular insulating obstacle embedded in a 1.46-cm long, 200-μm wide inlet channel to create spatial non-uniformities in direct current (DC) electric field density realized by separation into four outlet channels. The DC-iDEP flow behaviors were investigated for all eight blood types (A+, A-, B+, B-, AB+, AB-, O+, O-) in the human ABO-Rh blood typing system. Three independent donors of each blood type, same donor reproducibility, different conductivity buffers (0.52-9.1 mS/cm), and DC electric fields (17.1-68.5 V/cm) were tested to investigate separation dependencies. The data analysis was conducted from image intensity profiles across inlet and outlet channels in the device. Individual channel fractions suggest that the dielectrophoretic force experienced by the cells is dependent on erythrocyte antigen expression. Two different statistical analysis methods were conducted to determine how distinguishable a single blood type was from the others. Results indicate that channel fraction distributions differ by ABO-Rh blood types suggesting that antigens present on the erythrocyte membrane polarize differently in DC-iDEP fields. Under optimized conductivity and field conditions, certain blind blood samples could be sorted with low misclassification rates.

Electrolysis-reducing electrodes for electrokinetic devices

Direct current electrokinetic systems generally require Faradaic reactions to occur at a pair of electrodes to maintain an electric field in an electrolyte connecting them. The vast majority of such systems, e.g. electrophoretic separations (capillary electrophoresis) or electroosmotic pumps (EOPs), employ electrolysis of the solvent in these reactions. In many cases, the electrolytic products, such as H+ and OH⁻ in the case of water, can negatively influence the chemical or biological species being transported or separated, and gaseous products such as O₂ and H₂ can break the electrochemical circuit in microfluidic devices. This article presents an EOP that employs the oxidation/reduction of the conjugated polymer poly(3,4-ethylenedioxythiophene), rather than electrolysis of a solvent, to drive flow in a capillary. Devices made with poly(3,4-ethylenedioxythiophene) electrodes are compared with devices made with Pt electrodes in terms of flow and local pH change at the electrodes. Furthermore, we demonstrate that flow is driven for applied potentials under 2 V, and the electrodes are stable for potentials of at least 100 V. Electrochemically active electrodes like those presented here minimize the disadvantage of integrated EOP in, e.g. lab-on-a-chip applications, and may open new possibilities, especially for battery-powered disposable point-of-care devices.

A Microfluidic Passive Pumping Coulter Counter

A microfluidic device using on-chip passive pumping was characterized for use as a particle counter. Flow occurred due to a Young-Laplace pressure gradient between two 1.2 mm diameter inlets and a 4 mm diameter reservoir when 0.5μ L fluid droplets were applied to the inlets using a micropipette. Polystyrene particles (10μm diameter) were enumerated using the resistive pulse technique. Particle counts using passive pumping were within 13% of counts from a device using syringe pumping. All pumping methods produced particle counts that were within 16% of those obtained with a hemocytometer. The effect of intermediate wash steps on particle counts within the passive pumping device was determined. Zero, one, or two wash droplets were loaded after the first of two sample droplets. No statistical difference was detected in the mean particle counts among the loading patterns (p > 0.05). Hydrodynamic focusing using passive pumping was also demonstrated.

DNA manipulation by means of insulator-based dielectrophoresis employing direct current electric fields

Electrokinetic techniques offer a great potential for biological particle manipulation. Among these, dielectrophoresis (DEP) has been successfully utilized for the concentration of bioparticles. Traditionally, DEP is performed employing microelectrodes, an approach with attractive characteristics but expensive due to microelectrode fabrication costs. An alternative is insulator-based DEP, a method where non-uniform electric fields are created with arrays of insulating structures. This study presents the concentration of linear DNA particles (pET28b) employing a microchannel, with an array of cylindrical insulating structures and direct current electric fields. Results showed manipulation of DNA particles with a combination of electroosmotic, electrophoretic, and dielectrophoretic forces. Employing suspending media with conductivity of 104 muS/cm and pH of 11.15, under applied fields between 500 and 1500 V/cm, DNA particles were observed to be immobilized due to negative dielectrophoretic trapping. The observation of DNA aggregates that occurred at higher applied fields, and dispersed once the field was removed is also included. Finally, concentration factors varying from 8 to 24 times the feed concentration were measured at 2000 V/cm after concentration time-periods of 20-40 s. The results presented here demonstrate the potential of insulator-based DEP for DNA concentration, and open the possibility for fast DNA manipulation for laboratory and large-scale applications.

Basic principles of electrolyte chemistry for microfluidic electrokinetics. Part II: Coupling between ion mobility, electrolysis, and acid-base equilibria

We present elements of electrolyte dynamics and electrochemistry relevant to microfluidic electrokinetics experiments. In Part I of this two-paper series, we presented a review and introduction to the fundamentals of acid-base chemistry. Here, we first summarize the coupling between acid-base equilibrium chemistry and electrophoretic mobilities of electrolytes, at both infinite and finite dilution. We then discuss the effects of electrode reactions on microfluidic electrokinetic experiments and derive a model for pH changes in microchip reservoirs during typical direct-current electrokinetic experiments. We present a model for the potential drop in typical microchip electrophoresis device. The latter includes finite element simulation to estimate the relative effects of channel and reservoir dimensions. Finally, we summarize effects of electrode and electrolyte characteristics on potential drop in microfluidic devices. As a whole, the discussions highlight the importance of the coupling between electromigration and electrophoresis, acid-base equilibria, and electrochemical reactions.