Cylindrical Microelectrodes Research Articles

(Digital Presentation) Fabrication and Characterization of C-MEMS Based 3-Dimensional Microelectrode Arrays for Cardiac Electrophysiological Studies

In-vitro examination of the electrical conduction characteristics and the beating of cardiomyocytes can be done using Microelectrode arrays(MEAs). The microelectrodes continuously monitor the electrical activity of the active cardiac cells on a microsecond timescale and presents a precise data based on that. Since the introduction of MEAs, many efforts have been made by scientists to evolve this technology by means of microfabrication. Previously, planar metallic electrodes were mostly used for this study but they could not penetrate the dead cells and reach the active cell layers. To overcome this, protruding 3-D microelectrodes were introduced that could puncture the dead tissues and reach the active cells for effective recording of cardiac electrical potentials.The characteristics of an electrode are defined by the characteristics of the material used to develop it. In this paper, we focus on the fabrication of cylindrical 3-dimensional Glassy Carbon microelectrode arrays (MEAs) for analyzing cell preparations from cardiac tissues. In order to improve the electrical properties of the commercially available electrodes, we have used a highly conductive material for fabrication. Glassy Carbon microelectrodes were fabricated by the pyrolysis of SU-8 microelectrodes which were developed by a standard lithographic process on Silicon wafer. The ohmic nature and high conductivity of Glassy Carbon makes it a suitable material to be implemented in electrophysiological studies. Further in our work, we have performed the theoretical and structural analysis using Comsol Multiphysics 5.2 considering different geometries of microelectrodes. The skin insertion force is calculated to be 0.012N, which is much lesser than the comprehensive force (15.38N), the maximum bending force (0.885N) and the maximum buckling force (3.28N) per cylindrical microelectrode. The maximum Von Mises stress (as shown in the figure) acting at the base where it is attached to the substrate is found out to be 6.56x106 N/m2 for each cylindrical microneedle. Among all the considered geometries, it can be concluded that the fabricated cylindrical microelectrodes penetrate successfully without breaking or bending and causing minimal damage to the cardiac tissues. The slanted tip microelectrode was also favored as it’s design makes it able to easily pierce the cardiac cell layers. However, the Maximum Von Mises stress distribution showed that the stress at the tip was much higher than the Yield strength of Glassy Carbon (4Gpa) making it evident that the tip might break while penetrating. With the theoretical and simulation analysis performed we were thus able to optimize the geometry and establish that Glassy Carbon proves to be a good material for fabrication of 3D microelectrode arrays. Figure 1

Research on Automatic Detection of Microelectrodes Based on Machine Vision Technology

Background: Microfabrication has gradually become a hot spot in the manufacturing industry in recent years, and in the process of microfabrication, the processing of microelectrodes is very important. The precision and error control of the microelectrodes play a vital role in the final machining results. Based on digital image processing technology and Matlab software, this paper designs a microelectrode automatic measurement system. Through preprocessing and image segmentation of the microelectrode image, the electrode size can be obtained quickly. Compared with the size measured by the microscope, the detection size of this system has an error of less than 4% and coaxiality measured error within 6%. The results show that the system has the characteristics of high measurement accuracy and fast detection speed and can meet the requirements of rapid size measurement of microelectrodes. Objective: The purpose of this research is to improve the efficiency of size detection when manufacturing a large number of microelectrodes by developing an automatic detection system. This system has made relevant optimization on the error control of the size detection to ensure that the error is within the allowable range. Method: Firstly, the original images of the microelectrodes were obtained using an electron microscope. Secondly, the necessary image processing had been done for the image: gray processing, binarization, median filtering, morphological processing, and large background noise removal. These processes are automatically carried out in the system. In addition, three methods of measuring coaxiality error are proposed, and the first measurement algorithm with the minimum deviation is selected and put into the system, which can ensure the accuracy of coaxiality offset. Finally, the diameter of the microelectrode was measured, and the coaxiality offset of the microelectrode was measured by various methods. After the comparison of the measured values manually, the algorithm with the minimum deviation was selected and put into the system. Results: The diameter and coaxiality of the cylindrical microelectrode and the cylindrical microelectrode with a ball head are successfully measured, and the measurement error is within 4% after comparison, showing that the automatic measurement system has great measurement efficiency. Conclusion: The comparison of the electrode measurement results of this system with the manual measurement results shows that the automatic measurement system developed in this paper can meet the accuracy requirements for measuring the diameter of the microelectrodes, and can significantly improve the measurement efficiency compared with manual measurement.

Experimental study on fabricating micro-holes in DD5 single-crystal nickel-based superalloy using electrical discharge drilling

The single-crystal superalloy materials have been rapidly developed and widely used in advanced thermal structural components due to their excellent comprehensive physical and chemical properties under high-temperature service conditions. However, conventional micro-hole making results in defects such as edge breakage and burr. In this study, the electrical discharge drilling (EDD) combined with helical microelectrode is first adopted to fabricate DD5 single-crystal nickel-based superalloy. The effects of tool geometry and machining parameters on subsurface damage layer, micro-hole taper, surface morphology, surface roughness and machining time were investigated in detail. Experimental results indicated that helical microelectrode can obtained smoother surface without debris deposition and thinner subsurface damage layer depth lack of micro-cracks compared with cylindrical microelectrodes. Additionally, the computational fluid dynamics model was developed to analyze working fluid movement and reveal effective debris removal mechanism of helical microelectrode. The vortices will be generated in lateral gap fluid between micro-hole and helical microelectrode and have a certain delay time. The surface roughness and dimensional precision of micro-holes fabricated by helical micro-electrodes are greatly improved and machining efficiency is also improved by 30.94% compared to cylindrical microelectrodes. This work could provide theoretical and process guidance to assist in realizing high surface quality and low subsurface damage of micro-holes obtained with EDD process.

Real-Space 3D Simulation of Cyclic Voltammetry in Carbon Felt Electrodes for Application in Vanadium Redox-Flow Batteries Based on a Combination of Micro CT Data, Digital Simulation and Convolutive Modelling

Cyclic voltammetry (CV) is the prevalently used technique for investigating the kinetic performance of carbon felt electrodes for application as porous electrocatalysts in vanadium redox-flow batteries. However, the theoretical treatment of internal diffusion-reaction phenomena of such electrodes, allowing for quantitative data evaluation, is still unexplored and mostly relies on diffusion domain approximations assuming the felt as an array of either planar, spherical or cylindrical microelectrodes in a randomly distributed and finite diffusion domain. With this study, we present a novel three-step strategy for real-space CV simulation of felt electrodes. By using micro x-ray tomography (CT) data of the felts, we obtain a locally resolved 3D template of the diffusion space making any diffusion domain approximations obsolete. CV simulation involving this real diffusion domain template is performed by combining the implicit Crank-Nicolson technique in 3D with convolutive modelling. At first, by utilizing the Crank-Nicolson method, we obtain the chronoamperometric current of the 3D porous network for Cottrell-like Dirichlet boundary conditions. Subsequently, the mass transfer functions, usually generated with the aid of Laplace integral transformation techniques, are calculated from these chronoamperometric currents. Then, CV simulation is performed on the basis of these mass transfer functions involving classical convolution integrals. This strategy offers three significant advantages. First, we avoid the difficulty of implementing non-Dirichlet boundaries in the matrices of the Crank-Nicolson scheme. Second, the difficulty of performing an inverse Laplace transformation is avoided since no direct Laplace transformation step is involved. Third, once the mass transfer functions are obtained the simulation can be done with significant savings in time since the 3D network is converted into a one-dimensional problem that can be solved via classical convolution integrals. Our simulation is supported by experimental data acquired for the VO2+-oxidation reaction underlining the validity of our approach. Figure 1

Universal Algorithm for Simulating and Evaluating Cyclic Voltammetry at Macroporous Electrodes by Considering Random Arrays of Microelectrodes.

An algorithm for the simulation and evaluation of cyclic voltammetry (CV) at macroporous electrodes such as felts, foams, and layered structures is presented. By considering 1D, 2D, and 3D arrays of electrode sheets, cylindrical microelectrodes, hollow‐cylindrical microelectrodes, and hollow‐spherical microelectrodes the internal diffusion domains of the macroporous structures are approximated. A universal algorithm providing the time‐dependent surface concentrations of the electrochemically active species, required for simulating cyclic voltammetry responses of the individual planar, cylindrical, and spherical microelectrodes, is presented as well. An essential ingredient of the algorithm, which is based on Laplace integral transformation techniques, is the use of a modified Talbot contour for the inverse Laplace transformation. It is demonstrated that first‐order homogeneous chemical kinetics preceding and/or following the electrochemical reaction and electrochemically active species with non‐equal diffusion coefficients can be included in all diffusion models as well. The proposed theory is supported by experimental data acquired for a reference reaction, the oxidation of [Fe(CN)6]4− at platinum electrodes as well as for a technically relevant reaction, the oxidation of VO2 + at carbon felt electrodes. Based on our calculation strategy, we provide a powerful open source tool for simulating and evaluating CV data implemented into a Python graphical user interface (GUI).

EIS at carbon fiber cylindrical microelectrodes

The theoretical mass-transport impedance at cylindrical microelectrodes has been known for some time, but an experimental verification is given here for the first time, using carbon fiber electrodes taken from a commercial carbon felt. Nonlinear least-squares fitting of the impedance spectrum of a redox couple at the reversible potential enables accurate evaluation of a composite parameter containing the standard rate constant and mean diffusivity, without measuring the fiber diameter, length or area.

Experimental Investigation on Rotating Electrochemical Etching of a Micro Spiral Cylindrical Electrode.

To realize the electrochemical etching of a micro spiral cylindrical electrode, a new method of rotating electrochemical etching is proposed, and its process is further studied. First, according to the electrochemical etching principle, the machining mechanism of rotating electrochemical etching of a micro spiral cylindrical electrode is introduced. Second, based on the spiral vortex theory in the Taylor-Couette system, the effect of the high-speed rotating cylindrical microelectrode on its external flow field is analyzed. Third, the effects of rotation direction, rotation speed, machining voltage, and machining time on the thread structure are analyzed by experiments. Finally, a spiral cylindrical microelectrode with good surface thread shape is fabricated within two minutes by using the optimized machining parameters. It is proved that the rotating electrochemical etching method is an easy way to fabricate a micro spiral cylindrical electrode with high efficiency and low cost.

Molecular dynamics simulation and experimental research on fabrication of micro spherical electrode combined by electrochemical etching with spark discharge

The microstructures with spherical end have been widely applied in micromachining and measurement. In this paper, a microelectrode with spherical end was fabricated combining electrochemical etching with spark discharge machining. Firstly, the formation mechanism of micro spherical electrode was simulated by molecular dynamics LAMMPS, and the energy model of the spherical end was established to guide machining micro spherical electrode. Secondly, on the basis of cylindrical microelectrode, the electrode tip was melted by spark discharge. Simultaneously, the molten electrode formed a spherical end due to surface tension in deionized water. Finally, sets of experiments were carried out to discuss the influence of polarity, applied voltage, and pulse width on the diameter ratio of spherical end to electrode tip. By using optimal machining parameters, the micro spherical electrode with a diameter of 21.25 μm and a coaxial error of 0.9 μm can be obtained, and the diameter ratio is almost 2.

Theory of cyclic voltammetry in random arrays of cylindrical microelectrodes applied to carbon felt electrodes for vanadium redox flow batteries.

In order to quantitatively investigate the kinetic performance and the pore size distribution of carbon felt electrodes for the application in vanadium redox flow batteries, the theory of cyclic voltammetry (CV) is derived for a random network of cylindrical microelectrodes on the base of convolutive modeling. In this context we present an algorithm based on the use of a modified Talbot contour for inverse Laplace transformation, providing the mass transfer functions required for the calculation of the CV responses in external cylindrical finite diffusion space. First-order homogenous chemical kinetics preceding and/or following the electrochemical reactions are implemented in this algorithm as well. The VO2+ oxidation is investigated as model reaction at pristine and electrochemically aged commercial carbon felt electrodes. A fit of simulated data to experimental data clearly shows that an electrochemical aging predominantly affects the kinetics of the electron transfer reaction and that internal electrode surfaces and pore size distributions remain constant. The estimated pore size distributions are in excellent agreement with porosimetry measurements, validating our theory and providing a new strategy to determine electrode porosities and electrode kinetics simultaneously via CV.

Direct writing of metal film via sputtering of micromachined electrodes

This paper presents the first micro-scale process for local deposition and direct writing of metal films through sputtering of a micromachined target electrode. The deposition process is achieved via a highly confined micro DC glow plasma generated between the electrode’s tip and the substrate at atmospheric pressure. Using cylindrical microelectrodes with diameters down to 40 μm, a micro glow plasma is stably established to show local deposition of the material on silicon substrates. The controlled manipulation of microplasma enables direct film writing of arbitrary patterns, for thicknesses ranging from the 100-nm order up to several microns variable with the discharge and scanning conditions. The process and drawn films are characterized for copper, copper-tungsten alloy, and nickel-200 alloy to reveal its potential applicability for a wide range of target materials. The spectroscopic analysis indicates the films’ elemental compositions to match well with those of the target materials. The direct drawing over non-planar substrates is also demonstrated to show the feasibility of 3D film printing using the developed process.

Theoretical modeling and experimental study on electrochemical etching of micro cylindrical electrode with high rotation precision

In order to realize the fabrication of micro cylindrical electrode with high rotation precision, the electrochemical etching process was studied and improved. Firstly, based on the principle of electrochemical etching, the effect of electrode rotation on the machining accuracy of cylindrical electrode was analyzed, which proved that electrode rotation can improve the rotation precision of the microelectrode. Secondly, according to the experimental induction and theoretical derivation, the function relationships among machining voltage, immersion depth, and rotation speed were established, and then the mathematical model for fabricating micro cylindrical electrode with high rotation precision was founded. Finally, the optimized combination of parameters is selected under the guidance of above model, and series of micro cylindrical electrodes with coaxial error below 1 μm are fabricated successfully, such as, the single-stepped cylindrical microelectrode with diameter of about 100 μm, aspect ratio of more than 20:1, and the multi-stepped cylindrical microelectrode with diameter below 20 μm, aspect ratio of more than 70:1.

高回转精度多阶柱状微电极电化学高效制备试验研究

高回转精度多阶柱状微电极电化学高效制备试验研究

Experimental Study on Electrochemical Etching of Multi-Stepped Cylindrical Microelectrode with High Rotary Accuracy

Background: Microelectrodes have been widely used in the fields of microfabrication, measurement and medical applications. Electrochemical etching machining is a better method for fabricating microelectrode, and the multi-stepped electrode is easily fabricated to very small size. However, the rotary accuracy of multi-stepped electrode fabricated by conventional electrochemical etching is not high, because of the installation error of electrode and the uneven machining gap between the electrode and tool cathode. Various relevant papers and patents have studied different methods to improve the machining accuracy and efficiency of electrodes. Objective: In order to achieve the high-efficiency machining of multi-stepped cylindrical microelectrode with high rotary accuracy, a new electrochemical etching process (rotating electrochemical etching) is introduced. Methods: Firstly, the effect of machining current on the change of electrode diameter was deduced based on the principle of electrochemical etching. Secondly, the experiments proved that the rotation can improve the current change rate and the effective initial current and thus improve the machining efficiency. Following this, the influence of rotation on the rotary accuracy of electrode was analyzed, and the efficient method, which uses different rotation speeds combination, was proposed to fabricate multi-stepped cylindrical microelectrodes with high rotary accuracy. Thirdly, the influence of machining parameters such as the rotation speed, the voltage and the cut-off current, on the shape and size of electrode was analyzed. Results: Finally, a set of three-stepped cylindrical microelectrodes, which have the terminal diameter of less than 15µm and coaxial error within 1µm, was successfully fabricated using the optimized machining parameters. Compared with the conventional electrochemical etching process, the machining efficiency improved significantly. Conclusion: It has been proved that rotating electrochemical etching is a better method to improve the machining efficiency and rotary accuracy of microelectrodes. Keywords: Electrode rotation, machining current, machining efficiency, multi-stepped cylindrical microelectrode, rotary accuracy.

Experimental study on the microelectrodes fabrication using low speed wire electrical discharge turning (LS-WEDT) combined with multiple cutting strategy

This paper aims at giving an insight into the fabrication of ultra-small microelectrodes and micro-cutting tools using the low speed wire electrical discharge turning (LS-WEDT). The multiple cutting strategy divides the microelectrode machining process into rough cut (RC), semi-finishing trim cut (TC) and finishing trim cut (FTC). Experimental results indicated that the breaking and bending microelectrodes are caused primarily by the improper selection of flushing pressure, peak current and open circuit voltage in FTC. The cylindrical microelectrode of 58μm in diameter is successfully fabricated with good surface quality and high machining accuracy. More importantly, the LS-WEDT method can be used to manufacture the microelectrodes and micro-cutting tools with different micro structures by combining with the numerical control technology. The D-shaped cutting tool of 65μm in diameter and the three spiral micro cutting tool are firstly and successfully manufactured by LS-WEDT method.

The Lithium and Oxygen Concentration Dependence in Li-O2 Batteries Studied with Electrochemical Quartz Crystal Microbalance and Cylindrical Microelectrodes

The Li-O2 battery is a promising chemistry suggesting very high energy densities. In the literature it is agreed upon that during the discharge process Li2O2 is the final product formed via the intermediate LiO2. However, the nature of the intermediate is not fully understood, e.g. how long is the lifetime and how does it proceed to form the final product (chemical or electrochemical reaction). This study sets out to shed light on the reaction mechanism of the Li-O2 battery by studying the effects of different concentrations of lithium and oxygen. Experimentally this is done using an electrochemical quartz crystal microbalance (EQCM) and a cylindrical microelectrode. The EQCM results show that the system is unable to return to its initial mass, i.e. there is a buildup of reaction product during cycling. Further, it is not possible to recover all the discharge electrons during recharge. The intermediate is believed to diffuse away from the electrode rendering it unavailable for recharge. This result is further strengthened by the measurements on microelectrodes where intermediate is believed to diffuse away from the electrode more easily. There is also a discrepancy when looking at the change in mass and the expected mass change from the corresponding current suggesting that both electrochemical and chemical processes occurs. In conclusion it can be said that this study provides additional knowledge to the surface confined processes in the Li-O2 battery. The EQCM provides information that otherwise would go unnoticed, i.e. solely measuring the current response will not provide detailed quantitative information of the mass response.

Fabrication of disk microelectrode arrays and their application to micro-hole drilling using electrochemical micromachining

Minimal-taper microholes are widely used in modern industries. Electrochemical micromachining (EMM) has been demonstrated to be a feasible method to fabricate these microholes. In this study, based on its unique processing properties and productivity, a disk microelectrode array was fabricated via electrolysis for producing micro-holes. The dimensions of the cathode for hydrolysis were optimized by applying the finite element method to the constructed physical model. A 3×3 disk microelectrode array and a 5×5 cylindrical microelectrode array with uniform dimensions were then fabricated using the optimized cathode. Micro-holes were drilled on stainless-steel plates using both disk and cylindrical microelectrode arrays. The taper of the resulting micro holes obtained using the new disk microelectrode array was lower than that of the holes formed using the cylindrical microelectrode array. The effects of EMM parameters, including the applied voltage, feeding speed, and pulse-on time, on the hole diameter and taper were also investigated. The results suggest that appropriate machining parameters should be selected in consideration of the effects of these parameters on hole diameter, taper, localization, and material removal rate.

Electrochemical detection in paper-based analytical devices using microwire electrodes

Microwire electrodes are presented as an alternative to screen-printed electrodes for detection in electrochemical paper-based analytical devices (ePADs). Compared to carbon ink electrodes, microwire electrodes offer lower resistance and a significant increase in current density relative to carbon ink electrodes. Various microwire compositions and diameters, including 30 μm Pt, 25 μm Au, 18 μm Pt with 8% W, and 15 μm Pt with 20% Ir, were tested and compared to theoretically predicted behavior. The measured current in static solution was below predicted levels for cylindrical microelectrodes but greater than levels predicted for hemi-cylindrical electrodes most likely as a result of the proximity of the electrode to the paper surface. Furthermore, the current response was indicative of semi-thin layer behavior, likely due to the confined solution volume in the paper. After electrode characterization, a device was developed for the non-enzymatic detection of glucose, fructose, and sucrose using a Cu electrode in alkaline solution. The limits of detection for glucose, fructose, and sucrose were 270 nM, 340 nM, and 430 nM, respectively, which are significantly below sugar concentrations found in sweetened beverages or glucose levels in serum.

Carbon nanospikes grown on metal wires as microelectrode sensors for dopamine.

Carbon nanomaterials are advantageous as electrodes for neurotransmitter detection, but the difficulty of nanomaterials deposition on electrode substrates limits the reproducibility and future applications. In this study, we used plasma enhanced chemical vapor deposition (PECVD) to directly grow a thin layer of carbon nanospikes (CNS) on cylindrical metal substrates. No catalyst is required and the CNS surface coverage is uniform over the cylindrical metal substrate. The CNS growth was characterized on several metallic substrates including tantalum, niobium, palladium, and nickel wires. Using fast-scan cyclic voltammetry (FSCV), bare metal wires could not detect 1 μM dopamine while carbon nanospike coated wires could. The highest sensitivity and optimized S/N ratio was recorded from carbon nanospike-tantalum (CNS-Ta) microwires grown for 7.5 minutes, which had a LOD of 8 ± 2 nM for dopamine with FSCV. CNS-Ta microelectrodes were more reversible and had a smaller ΔE(p) for dopamine than carbon-fiber microelectrodes, suggesting faster electron transfer kinetics. The kinetics of dopamine redox were adsorption controlled at CNS-Ta microelectrodes and repeated electrochemical measurements displayed stability for up to ten hours in vitro and over a ten day period as well. The oxidation potential was significantly different for ascorbic acid and uric acid compared to dopamine. Growing carbon nanospikes on metal wires is a promising method to produce uniformly-coated, carbon nanostructured cylindrical microelectrodes for sensitive dopamine detection.

Estimation of electrode ionomer oxygen permeability and ionomer-phase oxygen transport resistance in polymer electrolyte fuel cells

The oxygen permeability of perfluorinated and hydrocarbon polymer electrolyte membranes (PEMs; Nafion®, SPEEK and SPSU), which are used as electrolytes and electrode ionomers in polymer electrolyte fuel cells (PEFCs), was estimated using chronoamperometry using a modified fuel cell set-up. A thin, cylindrical microelectrode was embedded into the PEM and used as the working electrode. The PEM was sandwiched between 2 gas diffusion electrodes, one of which was catalyzed and served as the counter and pseudo-reference electrode. Independently, from fuel cell experiments, the oxygen transport resistance arising due to transport through the ionomer film covering the catalyst active sites was estimated at the limiting current and decoupled from the overall mass transport resistance. The in situ oxygen permeability measured at 80 °C and 75% RH of perfluorinated ionomers such as Nafion® (3.85 × 10(12) mol cm(-1) s(-1)) was observed to be an order of magnitude higher than that of hydrocarbon-based PEMs such as SPEEK (0.27 × 10(12) mol cm(-1) s(-1)) and SPSU (0.15 × 10(12) mol cm(-1) s(-1)). The obtained oxygen transport (through ionomer film) resistance values (Nafion® - 1.6 s cm(-1), SPEEK - 2.2 s cm(-1) and SPSU - 3.0 s cm(-1); at 80 °C and 75% RH) correlated well with the measured oxygen permeabilities in these ion-containing polymers.

Theoretical and Experimental Research on Fabrication of Multiple Stepped Cylindrical Micro Electrode

In order to achieve a high control accuracy of the cylindrical microelectrode processing,the electrochemical etching technology for fabrication of microelectrode is researched in depth.In view of various shortcomings of single-stepped cylindrical microelectrode,the machining principle and the processing flow of the multi-stepped cylindrical microelectrode are analyzed.According to the three distinguishable strands during the etching process,the etching variety for each stage of the microelectrode is studied through mathematical formula,based on which the size and shape control functions of micro electrode are established.The invariability of the current density during the etching process is confirmed by the experiments,and the influence of different current density on the electrodes diameters is discussed.Then,the corresponding functions among the voltage,current density and immersion depth are built by the experimental results reduction,and at the same time,the correctness of electrode diameter changing linearly with machining time is verified.Based on the above two models,the mathematical control model for fabricating the multi-stepped cylindrical microelectrode is set up.It proves that the experimental and theoretical values have a good agreement by the experiment of a third-stepped microelectrode.A set of different multi-stepped cylindrical microelectrodes are fabricated under the guidance of the control model.