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The deterioration mechanisms of a Ni–metal hydride (Ni–MH) battery system during operation is investigated. A decrease of the discharge voltage is observed at the early stage of the cycle, which indicates the possible occurrence of an unexpected system shutdown of the battery at low state of charge. Cyclic voltammetry and surface examination are used to investigate the causes of this phenomenon. The elution of elements such as Al and Mn from the metal hydride negative electrodes is shown to affect the oxidation/reduction behavior of Co, the conductive material of the positive electrode. Furthermore, the possible methods to strengthen the conductive network of Co, including pretreatment of the positive electrode, addition of conductive material, and precipitation of the elution elements as insoluble compounds to reduce their effect are also investigated. By combining these strategies, deterioration of the conductive network can be prevented in the early stage.

The pretreatment to modify the valence of cobalt by discharging at 0.2 C rate for 7.5 h before the first initial activation charge process is effective in improving the surface electronic conductivity among fine particles of positive electrode active materials. The discharge curves indicate the same locus within 1800 cycles, and the capacity of the pretreated battery is stable for over 4000 cycles. However, in-situ cell pretreatment with constant current has negative influence on other components. During the constant current pretreatment, the cell voltage rapidly falls to −0.5 V in the first 10 s of in-situ pretreatment. Therefore, we investigate the pretreatment by supplying a constant voltage to the battery instead of a constant current, and find the effective condition to improve the electrochemical performance and not to have any influence on other components of the battery.

To demonstrate the possibility of automatic serial operation in a microanalysis system, components for valve operation, adjustment of mixing time, pH regulation, and sensing functions were integrated. Electrodes for actuation and sensing were formed on a glass substrate and flow channels and compartments were formed using poly(dimethylsiloxane) (PDMS). When a urea standard solution was filled in half of a mixing area, a mixing valve opened automatically and the solution merged with a urease solution initially filled in the other half. The length of time required for the mixing of components was adjusted by using a delay line, which extended from the electrode used for the mixing valve. After a time delay, an injection valve at the end of the mixing chamber was opened automatically. After the solution was injected into the compartment of the pH regulator using a nonstandard three-electrode system, the solution pH was elevated automatically. Gaseous ammonia produced accompanying the change was detected using an air-gap ammonia sensor. The existence of ammonia was clearly indicated by a change in electrode potential. The dependence of the potential on the logarithm of the concentration was linear, confirming the Nernstian response.

A microanalysis system with functions including automatic valve operation for mixing solutions, pH regulation, and detection based on electrochemiluminescence (ECL) was fabricated. Automatic valve operation was achieved by means of direct electrowetting using one of two solutions as a part of an electronic circuit. A valve formed with a gold electrode was opened when the second solution was filled in a compartment, causing the mixing of the two solutions. The pH regulation was achieved by using a nonstandard three-electrode system. The pH of the solution could be changed as desired by changing the working electrode potential, and the negative feedback function maintained the pH at a constant level. The ECL was generated on a platinum working electrode. Amino acids were detected as model analytes. A reagent solution containing tris(2,2′-bipyridyl)ruthenium(II) (Ru(bpy) 3 2+ ) and a sample solution containing an amino acid were mixed, and the pH was adjusted using the pH regulator. When +1.1 V (vs. Ag/AgCl) was applied to the electrode, red luminescence was observed. An increase in the ECL intensity was observed with an increase in the concentration. Amino acids of concentrations ranging from sub nM to 1 mM could be determined on the integrated device.

Electrochemical principles provide key techniques to promote the construction of bio/chemical microsystems of the next generation. There is a wealth of technology for the microfabrication of bio/chemical sensors. In addition, microfluidic transport in a network of flow channels, pH regulation, and automatic switching can be realized by electrochemical principles. Since the basic components of the devices are electrode patterns, the integration of different components is easily achieved. With these techniques, bio/chemical assays that require the exchange of solutions can be conducted on a chip. Furthermore, autonomous microanalysis systems that can carry out necessary procedures are beginning to be realized. In this article, techniques developed in our group will be comprehensively introduced.

An integrated device that carries out the timely transport of solutions and conducts electroanalysis was constructed. The transport of solutions was based on capillary action in overall hydrophilic flow channels and control by valves that operate on the basis of electrowetting. Electrochemical sensors including glucose, lactate, glutamic oxaloacetic transaminase (GOT), glutamic pyruvic transaminase (GPT), pH, ammonia, urea, and creatinine were integrated. An air gap structure was used for the ammonia, urea, and creatinine sensors to realize a rapid response. To enhance the transport of ammonia that existed or was produced by the enzymatic reactions, the pH of the solution was elevated by mixing it with a NaOH solution using a valve based on electrowetting. The sensors for GOT and GPT used a freeze-dried substrate matrix to realize rapid mixing. The sample solution was transported to required sensing sites at desired times. The integrated sensors showed distinct responses when a sample solution reached the respective sensing sites. Linear relationships were observed between the output signals and the concentration or the logarithm of the concentration of the analytes. An interferent, L-ascorbic acid, could be eliminated electrochemically in the sample injection port.

The enzyme activity was determined using a freeze-dried substrate-bovine serum albumin (BSA) matrix packed in a micro flow channel. The device used in the analysis consisted of a glass chip with an amperometric l -glutamate sensor and a polydimethylsiloxane (PDMS) sheet with a micro flow channel. Glutamic oxaloacetic transaminase (GOT), glutamic pyruvic transaminase (GPT), and γ-glutamyl transpeptidase (γ-GTP) were used as the test enzymes, and a reaction product, l -glutamate, was detected using an integrated l -glutamate sensor. The very porous structure of the matrix and very fine substrate precipitates facilitated the smooth introduction of a sample solution and the rapid dissolution of the substrates. The device was demonstrated to be more effective than other devices with different flow channel structures. In one device, the substrates were precipitated on the wall of the flow channel in an atmosphere at room temperature. In another device, the enzyme solution and the substrate solution were mixed in a Y-shaped flow channel. The initial rate of the current increase was much higher with the freeze-dried substrates. The novel device could also be used to determine the activities of the enzymes in human serum samples. The relationship between the slope of the response curve and the enzyme activities was linear up to 100 U l −1 for GOT, 100 U l −1 for GPT, and 300 U l −1 for γ-GTP. The device could also be used for the analysis of enzyme kinetics by changing the amount of the loaded substrates.

A micro analysis system for the electrochemical determination of the activity of protease along with pH sensing was fabricated aiming for its use in telemetric micro analysis systems targeting the testing of the stomach and intestines. The system consisted of a pH-sensing site and two protease assay sites formed in polydimethylsiloxane (PDMS) micro flow channels. To introduce sample solutions, valves were formed with gold electrodes in the inlets, which functioned on the basis of electrowetting. An external sample solution could be introduced into the sensing sites by switching on the valves at appropriate times. In the pH-sensing site, a pH-indicator electrode changed its electrode potential immediately after a sample solution reached an internal liquid-junction reference electrode. The slope of the calibration plot was -74.5 mVpH(-1). Bovine serum albumin (BSA) was used as the substrate for the enzyme and was spotted on the wall of the flow channel that faced the pH-indicator electrode of the protease assay sites. The release of protons accompanying the hydrolysis of BSA by the enzyme was detected using the pH-indicator electrode. When trypsin was contained in the sample solution as a test enzyme, a distinct decrease in pH, which was dependent on the trypsin activity, was observed, indicating that enzymatic hydrolysis was proceeding. The initial rate of potential change varied in proportion to the activity in a range between 1.0 and 51.7 Uml(-1). The integration of the microfluidic and sensing functions provides significant advantages for the use of this system as an isolated telemetric micro system that might operate with small batteries.

As a trial to realize an automatic microfluidic system, functions such as automatic valve operation, adjustment of mixing time, pH-regulation, and sensing were integrated on a chip. Microfluidic transport was achieved by capillary action. The transport and merge of solutions was controlled by gold working electrodes that functioned as valves. The mixing valve was also used as a delay line to take enough time for mixing. The pH of the mixed solution could be adjusted using an electrochemical pH-regulator, and gaseous ammonia produced from the solution could be measured using an air-gap ammonia sensor. The function of the entire system was checked in the enzymatic reaction of urea.

An electrowetting-based pH-responsive valve that uses a nonstandard electrochemical three-electrode system is proposed. The system comprises a gold valve electrode and an iridium electrode that act as the working and auxiliary electrodes, depending on the purpose, while an iridium oxide pH-sensitive electrode acts as the reference electrode. To make the valve open at pH higher than a threshold, the gold valve electrode is used as the working electrode and a voltage is applied to it with respect to the pH-sensitive reference electrode. To make the valve open at pH lower than the threshold, the gold valve electrode is used as the auxiliary electrode, while the iridium electrode is used as the working electrode. The wettability of the valve electrode is altered when a voltage is applied to it. When the pH of a solution crosses the threshold, the potential of the gold valve electrode exceeds a threshold potential because of the change in the potential of the pH-sensitive reference electrode. Consequently, the gold valve electrode becomes more hydrophilic, thereby allowing the solution to pass through the valve. Furthermore, by combining two valve electrodes, we realized a pH filter that allows solutions with pH within a limited range to pass through it. Urea- and glucose-responsive valves that opened at concentrations higher than the threshold could also be formed by immobilizing an enzyme on the pH-sensitive reference electrode.

In conducting a bio/chemical analysis on a chip, several different procedures are usually required. In this study, we demonstrate the possibility to conduct necessary procedures on a single chip. Our fabricated device has functions such as automatic mixing of solutions, pH-regulation, and detection based on electrochemiluminescence (ECL). A reagent solution containing Ru(bpy)3 2+ and a sample solution containing an amino acid were mixed and the solution pH was adjusted using the above- mentioned mechanisms. When a potential was applied to the electrode for ECL, red luminescence was observed. The ECL intensity increased with the increase in the concentration of amino acids.

Aiming for the application to an ingestible micro system, a microanalysis system for the determination of the activity of digestive enzymes was realized. The analysis was conducted by using an on-chip micro-pH-stat which worked on the basis of an irregular use of the electrochemical three-electrode system. The operation using a potentiostat caused a negative feedback, maintaining the solution pH at a constant value. The system used a freeze-dried enzyme substrate. The very porous matrix facilitated rapid dissolution and mixing with the enzyme solution. The determination of the activity of pepsin and trypsin was conducted at the optimum pHs for the respective enzymes by measuring the generated charge accompanying the hydrolysis of hemoglobin. In both cases, a distinct increase in the generated charge was observed as the enzymatic reaction proceeded. Linear relationships were observed between the initial rate of charge increase and the activity of the enzymes.

A freeze-dried matrix packed in a micro flow channel was used to analyze an enzymatic reaction rapidly without using external pumps. The very porous and fine precipitated matrix facilitated the permeation of a sample solution into the flow channel and the dissolution of the components. The amount of products produced as a result of the enzymatic reactions was sufficient and a significant increase in the resulting sensor output was observed. The applicability of the device was demonstrated in the determination of the activities of enzymes, the analysis of enzyme kinetics, and the monitoring of a multi-step reaction. This can be a basic technology in conducting microfluidic diagnostic systems with higher functions including the analysis of enzymatic reactions.

An on-chip pH-regulator based on an irregular use of an electrochemical thin-film three-electrode system will be presented. The device is operated using only an ordinary potentiostat. A pH-sensitive electrode was used for the reference electrode, whereas a non-polarizable Ag/AgCl electrode was used for the working electrode. Auxiliary electrode was placed around the working electrode, which worked as an actuator electrode. A pH change of the solution in the reaction chamber increased the overpotential and then a large current was generated. Accompanying the electrolysis of water on the auxiliary electrode, the pH of the reaction chamber recovered to the initial value rapidly. Recovery of pH was rapid and the pH in the reaction chamber could be controlled. The device could be used for the titration of an acid or a base. Linear relationships were observed between the generated charge and the concentration of analytes including acetic acid, ammonia, and urea. The output values were not affected by the buffering capacity.

A microfluidic system for the analysis of the activities of glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) was fabricated. The device consists of a glass chip with a micro-electrochemical L-glutamate sensor and a polydimethylsiloxane (PDMS) sheet with a Y-shaped micro-flow channel. A sample solution and a substrate solution for the enzymes were introduced from two injection ports at the end of the flow channel. When the flows were stopped, substrates in a solution mixed immediately with either of the enzymes by diffusion in a mixing channel. L-glutamate produced by the enzymatic reaction of GOT or GPT in the flow channel was detected by using the L-glutamate sensor. A distinct current increase was observed immediately after mixing, and the initial slope of the response curve varied in proportion to the activity of GOT or GPT. The relation between the slope of the response curve and the enzyme activity was linear between 7 and 228 U l-1 for GOT and 9 and 250 U l-1 for GPT. The quality of the response curve was improved with an increase in the channel height. The measurement based on the rate analysis in the micro-flow channel facilitated the reduction of the influence of interferents. The influence of the viscosity of the sample solution was also checked for the analysis of real samples. The determination of the enzyme activities was also conducted in a system with micropumps fabricated for a sample injection. Two solutions could be mixed in the mixing channel, and the activity of the enzymes could be measured as in the experiments using microsyringe pumps.

Electrowetting was used to fabricate an integrated highly efficient microfluidic transport system. As an option, a row of elongated gold working electrodes were used to generate a driving force to mobilize a solution in a flow channel. Flow velocity could be changed by changing the potential of the working electrode. The solutions could be transported to any desired direction without using any valves. In another option, a working electrode was formed in a hydrophilic flow channel and used as a valve. When the valve was switched on, a solution in the flow channel passed the area and went forward by capillary action. Furthermore, two solutions transported through two flow channels could be mixed based on the same principle. Integrated biosensing systems could easily be constructed using these microfluidic components. In this study, the concentration of L-amino acids was determined by generating electrochemical luminescence (ECL) on an integrated platinum working electrode. Also, the activity of protease could be determined using an integrated potentiometric pH-sensing system

The possibility to develop a microanalysis system for the acquisition of gastrointestinal information is presented here. The system consists of four assay sites for trypsin, pepsin, and other biochemical compounds. The major components in each assay site were a pH-responsive valve, a pH-stat used to maintain the pH of the solution to be analyzed and used for electrochemical pH-titration, and a freeze-dried enzyme substrate stored in the pH-stat. The operation of the valve is based on electrowetting, and the valve is made pH-responsive by means of a nonstandard three-electrode system. The sample solution was automatically injected into the compartment and rapidly dissolved into the substrate layer. The automatic pH-stat, based on another nonstandard use of the electrochemical three-electrode system, maintained the solution pH and, at the same time, conducted pH-titration. The determination of the activity of the proteases was conducted at their optimum pHs. The output current showed a clear dependence on t...

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Three-dimensional surface structures composed of micropillars with different diameters (8–110 µm) and interpillar distances were fabricated using a thick-film photoresist, and the surface was modified with a hydrophobic plasma-polymerized film using hexamethyldisiloxane (HMDS) as a monomer. Surface hydrophobicity was examined by measuring contact angle using a pure water droplet. The effect of adding ethanol was also examined. Water static contact angle increased significantly on the surface coated with the HMDS plasma polymer. Structural parameters, including the diameter and height of the micropillars, were changed, as was the interpillar distance. The dependence of apparent contact angle on these parameters was explained using Cassie's and Wenzel's models.