Microelectrode Structure Research Articles

Implantable Neural Microelectrodes: How to Reduce Immune Response.

Implantable neural microelectrodes exhibit the great ability to accurately capture the electrophysiological signals from individual neurons with exceptional submillisecond precision, holding tremendous potential for advancing brain science research, as well as offering promising avenues for neurological disease therapy. Although significant advancements have been made in the channel and density of implantable neural microelectrodes, challenges persist in extending the stable recording duration of these microelectrodes. The enduring stability of implanted electrode signals is primarily influenced by the chronic immune response triggered by the slight movement of the electrode within the neural tissue. The intensity of this immune response increases with a higher bending stiffness of the electrode. This Review thoroughly analyzes the sequential reactions evoked by implanted electrodes in the brain and highlights strategies aimed at mitigating chronic immune responses. Minimizing immune response mainly includes designing the microelectrode structure, selecting flexible materials, surface modification, and controlling drug release. The purpose of this paper is to provide valuable references and ideas for reducing the immune response of implantable neural microelectrodes and stimulate their further exploration in the field of brain science.

Dielectrophoretic force-induced wrinkling of graphene oxide: Enhancing electrical conductivity and expanding biosensing applications

Graphene oxide (GO) has many advantages, making it suitable for various applications. However, it has low electrical conductivity, restricting its applicability to electrochemical biosensors. This study used dielectrophoretic (DEP) force to control the movement and deformation of GO nanosheets to achieve high electrical conductivity without the chemical reduction of oxygen functional groups. Subjecting the DEP force to GO nanosheets induced physical deformation leading to the formation of wrinkled structures. A computational simulation was performed to set an appropriate electrical condition for operating a positive DEP force effect of at least 1019 v2/m3, and the interdigitated microelectrode structure was selected. The resulting wrinkled GO exhibited significantly improved electrical conductivity, reaching 21.721 μS while preserving the essential oxygen functional groups. Furthermore, a biosensor was fabricated using wrinkled GO deposited via DEP force. The biosensor demonstrated superior sensitivity, exhibiting a 9.6-fold enhancement compared with reduced GO (rGO) biosensors, as demonstrated through biological experiments targeting inducible nitric oxide synthase. This study highlights the potential of using DEP force to enhance electrical conductivity in GO-based biosensing applications, opening new avenues for high-performance diagnostics.

A thin film resistive humidity sensor based on polymer and carbon black nanoparticle composites

This paper proposes a resistive humidity sensor that uses a carbon-black and polyvinyl alcohol composites thin film, fabricated with a unique film coating method for thinner thickness and higher sensitivity. Improving the sensitivity of sensing films is still of great importance in the research field of gas sensors. The humidity sensor devices with thin composite film and microelectrode structure are fabricated on the glass substrate for a low cost and a simple fabrication process. The sensor gives a rapid response for humidity levels from 10.9% relative humidity (RH) to 73.7% RH, and the response time is about 5.77 s. Experimental results reveal that the sensor has good sensitivity, reproducibility, fast reaction time, and wide range. In addition to humidity, the sensor also responds well to gases such as ethanol. The proposed gas sensor in this paper can be applied to the other combinations of polymers and nanoparticles to form new gas sensors, which have the potential to be used as a gas sensor array for detecting the composition of complex gases such as volatile organic components.

Demonstration of a roll-to-roll-configurable, all-solution-based progressive assembly of flexible transducer devices consisting of functional nanowires on micropatterned electrodes

We demonstrate continuous fabrication of flexible transducer devices consisting of interdigitated (IDT) Ag microelectrodes interconnected by ZnO nanowires (ZNWs), created via serially connected solution-processable micro- and nanofabrication processes. On an Ag layer obtainable from the mild thermal reduction of an ionic Ag ink coating, the roll-to-roll-driven photolithography process [termed photo roll lithography (PRL)] followed by wet-etching can be applied to continuously define the IDT microelectrode structure. Conformal ZNWs can then be grown selectively on the Ag electrodes to interconnect them via an Ag-mediated hydrothermal ZNW growth that does not require high-temperature seed sintering. Given that all of these constitutive processes are vacuum-free and solution-processable at a low temperature, and are compatible with continuous processing onto flexible substrates, they can be eventually configured into the roll-to-roll-processable progressive assembly. Through parametric optimizations of processes consisting of the roll-to-roll-configurable, solution-based progressive assembly of nanostructures (ROLSPAN), a flexible transducer consisting of ZNW-interconnected, PRL-ed IDT Ag electrodes can be developed. This flexible architecture faithfully performs UV sensing as well as optoelectronic transduction. The ROLSPAN concept along with its specific applicability to flexible devices may inspire many diverse functional systems requiring high-throughput low-temperature fabrication over large-area flexible substrates.

Dielectrophoresis Microfluidic Enrichment Platform with Built-In Capacitive Sensor for Rare Tumor Cell Detection

The manipulation and detection of rare cells are important for many applications in early disease diagnosis and medicine. This study presents a dielectrophoresis (DEP) microfluidic enrichment platform combined with a built-in capacitive sensor for circulating tumor cell detection. The microchip is composed of a lollipop-shaped gold microelectrode structure under a polydimethylsiloxane chamber. A prototype of the device was fabricated using standard micromachining technology. With the proposed device, target cells (in this study, A549 non-small human lung carcinoma and S-180 sarcoma cell lines) are firstly guided toward the center of the working chamber via DEP forces. Then, the target cells are captured by an electrode immobilized by anti-EGFR, which has high affinity toward the target cells. After the cell concentration process, the differential capacitance is read to detect the presence of the target cells. Numerical simulations and measurement experiments were performed to demonstrate the high sensitivity of differential capacitive sensing. The obtained results show high sensitivity for S-180 cell detection (3 mV/cell). The proposed platform is suitable for rapid cancer diagnoses and other metabolic disease applications.

Electrical characterization of 2D and 3D microelectrodes for achieving high-resolution sensing in retinal prostheses with in vitro animal experimental results

In order to provide high-quality visual information to patients with implanted retinal prosthetic devices, the number of microelectrode should be large. As the number of microelectrodes is increased, the dimensions of the microelectrode are also decreased, which in turn results in increased interface impedance of the microelectrodes and decreased dynamic range of injection current. In addition, the reduced maximum limit of injection current may not be sufficiently large to stimulate the ganglion cells in the retina. In order to improve the trade-off between the number of microelectrodes and the current injection limit, a 3D microelectrode structure can be used as an alternative. Due to the advancement of microfabrication technology, the fabrication of highly-accurate 3D structures with small dimensions is possible. This paper presents a comprehensive electrical characterization of 2D and 3D microelectrodes. Microelectrodes that differ in shape and diameter are compared to evaluate the feasibility of use in high-resolution retinal prostheses. Their electrode---electrolyte interface impedances and charge injection limits are quantitatively analyzed. Also, in vitro animal experiments using rd1 mice are performed to observe the evoked neural responses by increasing the stimulation current amplitude from 10 to 100 μA. This research can be used to define requirements for further retinal prosthetic device research.

Controllable cell electroporation using microcavity electrodes

Cell electro-fusion includes four steps, cell alignment, cell electroporation, reconstruction of cytomembrane and cytoplasm exchange. The cell alignment and electroporation steps are highly related to the intensity and distribution of the electric field, which depend on the applied voltage as well as the microelectrode structure. The microelectrode structures were first evaluated based on the numerical analysis of the electric field and the transmembrane potential induced on biological cells when the cell electroporation and electro-fusion were performed based on different designs of microelectrodes. Microelectrodes in a micro-cavity geometry can induce electroporation around the contact area of the paired cells for high-yield electro-fusion. Microfluidic chips with co-planar microelectrodes and microelectrodes within micro-cavities have been fabricated and tested for electro-fusion of Myoblast cells, and the experimental results confirmed the numerical analysis.

Electrodeposition of gold templated by patterned thiol monolayers

The electrochemical deposition of Au onto Au substrates modified by self-assembled monolayers (SAMs) was studied by linear sweep voltammetry (LSV), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Patterned SAMs exhibiting electrochemical contrast were prepared by two different methods. One used microcontact printing (μCP) to generate a binary SAM of ω-(4′-methyl-biphenyl-4-yl)-propane thiol (CH3-C6H4-C6H4-(CH2)3-SH, MBP3) and octadecane thiol (CH3(CH2)17SH, ODT). Templated by the SAM, a gold microelectrode structure was electrodeposited featuring a line 15μm wide and 3mm long. After transfer to an epoxy substrate the structure proved to be electrically conductive across the full length. The other patterning method applied electron beam lithography (EBL) where electrochemical contrast was achieved by crosslinking molecules in a single component SAM of MBP3. An electron dose above 250mC/cm2 results in a high deposition contrast. The choice of parameters for the deposition/lift-off process is found to be more critical for Au compared to Cu studied previously. The origin of the differences and implications for nanoscale patterning are discussed.

A Cell Electrofusion Chip for Somatic Cells Reprogramming.

Cell fusion is a potent approach to explore the mechanisms of somatic cells reprogramming. However, previous fusion methods, such as polyethylene glycol (PEG) mediated cell fusion, are often limited by poor fusion yields. In this study, we developed a simplified cell electrofusion chip, which was based on a micro-cavity/ discrete microelectrode structure to improve the fusion efficiency and to reduce multi-cell electrofusion. Using this chip, we could efficiently fuse NIH3T3 cells and mouse embryonic stem cells (mESCs) to induce somatic cells reprogramming. We also found that fused cells demethylated gradually and 5-hydroxymethylcytosine (5hmC) was involved in the demethylation during the reprogramming. Thus, the cell electrofusion chip would facilitate reprogramming mechanisms research by improving efficiency of cell fusion and reducing workloads.

Electrical Characterization of 3D Au Microelectrodes for Use in Retinal Prostheses.

In order to provide high-quality visual information to patients who have implanted retinal prosthetic devices, the number of microelectrodes should be large. As the number of microelectrodes is increased, the dimensions of each microelectrode must be decreased, which in turn results in an increased microelectrode interface impedance and decreased injection current dynamic range. In order to improve the trade-off envelope between the number of microelectrodes and the current injection characteristics, a 3D microelectrode structure can be used as an alternative. In this paper, the electrical characteristics of 2D and 3D Au microelectrodes were investigated. In order to examine the effects of the structural difference, 2D and 3D Au microelectrodes with different base areas but similar effective surface areas were fabricated and evaluated. Interface impedances were measured and similar dynamic ranges were obtained for both 2D and 3D Au microelectrodes. These results indicate that more electrodes can be implemented in the same area if 3D designs are used. Furthermore, the 3D Au microelectrodes showed substantially enhanced electrical durability characteristics against over-injected stimulation currents, withstanding electrical currents that are much larger than the limit measured for 2D microelectrodes of similar area. This enhanced electrical durability property of 3D Au microelectrodes is a new finding in microelectrode research, and makes 3D microelectrodes very desirable devices.

Tunable liquid crystal cylindrical micro-optical array for aberration compensation

A tunable aberration compensation device for rectangular micro-optical systems is proposed and demonstrated. This device, which is based in nematic liquid crystal and a micro-electrode structure, forms gradients in the index of refraction as a function of voltage. We have developed a fringe skeletonizing application in order to extract the 3D wavefront from an interference pattern. This software tool obtains the optical aberrations using Chebyshev polynomials. By using phase shifted electrical signals the aberrations can be controlled independently. A complete independent control over the spherical and coma aberration has been demonstrated. Also, an independent control over the astigmatism aberration has been demonstrated in a broad range. This device has promising applications where aberration compensation is required. The independent compensation achieved for some coefficients, such as astigmatism for example, is more than 2.4 waves.

Analysis and Simulation of a Planar Microelectrode Structure for Dielectrophoretic Manipulation of a Single Cell and Cell Cluster

In this paper, we proposed a four-electrode microdevice for precise isolating and trapping of a single cell using negative dielectrophoresis (nDEP) forces. To generate appropriate nDEP forces, sinusoidal alternating currents (AC) signals with various phase shifting were applied to the microelectrodes, and the finite element analysis (FEA) techniques were used to analyze the resulted electric field distribution. The simulation results implied that effective trapping and rotation forces can be realized by the proposed device structure under specific excitation condition. The geometry effect on the electric field distributions of electrodes was further studied in details. For the electrodes with 50 μm width, the maximum value of the gradient of the squared field strength could reach 106V2/m3, which is higher than that for electrodes with 20 μm width. The influences of applied voltage to electric field gradient were also simulated and the result shows that the dielectrophoresis (DEP) force increased significantly with the magnitude of applied voltage. These preliminary results may provide useful insight and design guidelines for the future DEP microstructure design and fabrication.

Effect of microelectrode structure on electrocatalysis at nucleic acid-modified sensors.

The electrochemical detection of nucleic acids using an electrocatalytic reporter system and nanostructured microelectrodes is a powerful approach to ultrasensitive biosensing. In this report we systematically study for the first time the behavior of an electrocatalytic reporter system at nucleic acid-modified electrodes with varying structures and sizes. [Ru(NH3)6](3+) is used as a primary electron acceptor that is electrostatically attracted to nucleic acid-modified electrodes, and [Fe(CN)6](3-) is introduced into the redox system as a secondary electron acceptor to regenerate Ru(3+) after electrochemical reduction. We found that the electrode structure has strong impact on mass transport and electron-transfer kinetics, with structures of specific dimensions yielding much higher electrochemical signals and catalytic efficiencies. The electrocatalytic signals obtained when gold sensors were electrodeposited in both circular and linear apertures were studied, and the smallest structures plated in linear apertures were found to exhibit the best performance with high current densities and turnover rates. This study provides important information for optimal assay performance and insights for the future design and fabrication of high performance biomolecular assays.

Chemistry integrated circuit: chemical system on a complementary metal oxide semiconductor integrated circuit

By integrating chemical reactions on a large-scale integration (LSI) chip, new types of device can be created. For biomedical applications, monolithically integrated sensor arrays for potentiometric, amperometric and impedimetric sensing of biomolecules have been developed. The potentiometric sensor array detects pH and redox reaction as a statistical distribution of fluctuations in time and space. For the amperometric sensor array, a microelectrode structure for measuring multiple currents at high speed has been proposed. The impedimetric sensor array is designed to measure impedance up to 10 MHz. The multimodal sensor array will enable synthetic analysis and make it possible to standardize biosensor chips. Another approach is to create new functional devices by integrating molecular systems with LSI chips, for example image sensors that incorporate biological materials with a sensor array. The quantum yield of the photoelectric conversion of photosynthesis is 100%, which is extremely difficult to achieve by artificial means. In a recently developed process, a molecular wire is plugged directly into a biological photosynthetic system to efficiently conduct electrons to a gold electrode. A single photon can be detected at room temperature using such a system combined with a molecular single-electron transistor.

Preparation and characterization of manganese oxide microelectrodes for microelectromechanical system supercapacitor

The capacity of charge storage for supercapacitor depends on the size of the electrode surface area and the active material on the electrodes. In this article, we fabricate the three-dimensional microelectrodes with SU-8 photoresist based on the microelectromechanical system technology to increase the electrode surface area and to carry more active materials on the electrodes. The three-dimensional microelectrode was directly prepared by the method of anodic electrodeposition. Its surface and composition were characterized by scanning electron microscopy and energy dispersive spectroscopy. Moreover, cyclic voltammetry and galvanostatic charge–discharge current are also used to test the electrochemical characteristics of the manganese oxide electrodes. When the discharge density is 0.5 mA cm−2, the areal capacitance for the fabricated three-dimensional microelectrodes with microstructure and the two-dimensional planar microelectrodes without microstructure is 3.68 mF cm−2 and 1.0 mF cm−2, respectively. The experimental results show that the three-dimensional microelectrode structure prepared with microelectromechanical system technology can effectively increase the electrode surface area and carry more electroactive materials, thereby enhancing the charge storage capacity of the electrodes.

Preparation and Electrochemical Characteristics of Three-dimensional Manganese Oxide Micro-supercapacitor Electrode

In order to increase the electrode surface area and enhance the charge storage capacity, we study the micro electro mechanical system technology to fabricate three-dimensional high aspect ratio micro-electrode structure based on glass. The anodic constant potential method is employed to deposit manganese oxide as electroactive substances on the micro-electrode surface. Cyclic voltammetry and constant current charge-discharge method are both used to prepare electrode electrochemical performance testing, with a two-dimensional electrode without structure for comparison. Experimental results show that three-dimensional electrode structure can effectively enhance the charge storage capacity. At 1.0 mA/cm2 charge-discharge density, the three-dimensional electrode shows a capacitance of 17.88 mF/cm2, seven times higher than the two-dimensional electrode.

Hall-effect Measurement of Organic Semiconductor Layers Using Micro-scale Electrode Chips

A new micro-electrode structure for Hall-effect measurements of organic semiconductor layers was proposed. The equations for the Hall-effect measurements based on the van der Pauw method indicate that narrower electrode gaps can decrease the resistance between contact points and enable Hall-effect measurement even for higher resistance materials such as organic semiconductors. According to this “scaling rule”, we fabricated a new electrode chip which consists of four small finger electrodes with a gap of micrometer-scale between diagonally opposite electrodes. In order to demonstrate the validity of this concept, Hall-effect measurements for FeCl3-doped poly(3,4-ethylenedioxythiophene) (PEDOT) was performed using this new electrode structure, revealing that a 10-μm-gap electrode chip is effective for evaluating carrier concentration of down to around 1019cm-3. These Hall electrode chips are quite useful for quantitative analysis of carrier concentration and carrier mobility in organic semiconductor materials.

Electromagnetic Imprint Technique Combined with Electrophoretic Deposition Technique in Forming Microelectrode Structures

In this study, we integrate the electromagnetic soft mold imprint technique with the electrophoretic deposition technique, and apply them to forming microelectrode structures. The compound casting technology is used to produce a magnetic soft mold of a microelectrode structure, which can effectively reduce the time and cost of molding. The use of an electromagnetic imprint device can apply more evenly distributed imprint pressure, thus, the microelectrode structure can be entirely imprinted onto an indium tin oxide (ITO) soft substrate, and then the electrophoretic deposition technique is employed to deposit titanium dioxide (TiO2) nanopowder on the ITO soft substrate of the microelectrode structure. In addition to the key techniques and processes of electromagnetic soft mold imprinting, In this study, we explore the application of electrophoretic deposition and imprinting to prove that combining these techniques to form a microelectrode structure is a simple, low-cost, high duplication, and high-speed process. It is proven a good choice for producing micro-nanocomponents.

Amperometric Electrochemical Sensor Array for On-Chip Simultaneous Imaging: Circuit and Microelectrode Design Considerations

We propose a switching circuit and an electrode design for measuring multiple microelectrode currents at high speed. Our goal is to realize two-dimensional (2D) imaging of analyte distributions by amperometry. In the proposed amperometric sensor circuit, each electrode is connected to two switches. During measurement, the potential of all electrodes is fixed, the steady-state current is maintained, and the switching is carried out while maintaining the steady-state current. A current buffer circuit was fabricated, its the characteristics were evaluated. The time course of the electrochemical current of an electrolyte was measured using the current buffer circuit. We have also proposed a microelectrode structure to suppress the expansion of the diffusion layer over the microelectrode array. Each electrode has a steady-state current, which is amplified by a redox cycle, and the time to reach the steady state is reduced to about 1/10 of that required by a single microelectrode. The relation between the characteristic lengths of the electrode geometry and the current gain is presented.

Experimental Study of Micro-Electrode Array by Micro-WEDM

This paper discusses a processing preparation for micro-electrode array fabrication by micro wire electrical discharge machining. Based on a self-developed micro-WEDM machine tool and adopted thin tungsten wire electrode of 30μm in diameter, a series of experiments on micro-electrode array machining are introduced. The relationship between the discharge gap and working area of wire discharge under given voltage is investigated. The examination for micro-girder structure forming process has gone along and the axle journal's minimized width of least micro girder which could not bring on machining flexural thermal deformation has been obtained. The clamping method of work-piece intended for electrode array fabrication is analyzed and a special fixture is designed. The factors affecting forming accuracy are summarized. An amounted 5×5 squared-disk-type micro-electrode array structure with a high aspect ration, namely 20μm in axle journal width and 500μm in length of a single electrode, has been machined successfully.