Wataru Satoh

Electrochemical techniques for microfluidic applications

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.

Microfluidic transport based on direct electrowetting

An integrated microfluidic system was fabricated which functions by deliberately manipulating interfacial tension. A distinctive characteristic of our system is the use of an array of adjacent, elongated, working electrodes and protruding polydimethylsiloxane open-flow channels. Microfluidic transport was realized directly on the bare gold electrode surface in the absence of an additional dielectric layer. By changing the potential of the working electrode to a negative potential, a liquid column could be transported from one end of an elongated working electrode to the other end. Transport of the liquid column could be altered without any valves by switching on the adjacent electrode in a given direction. The flow velocity depended on the applied potential, i.e., the velocity could be altered by deliberate manipulation of the electrode potential. In addition, the flow velocity increased as the dimensions of the flow channel decreased. The applied voltage was less than 2 V, and the power consumption was i...

Electrowetting-based valve for the control of the capillary flow

The behavior of a microfluidic device based on transport by capillary action and control by direct electrowetting was examined by changing the device parameters. The device was constructed with a polydimethylsiloxane (PDMS) substrate with a flow channel and a glass substrate with electrodes to control electrowetting. The microfluidic transport and valve action could be explained by Washburn’s model, which suggests that the model can be used for designing such a device. The device could be used for the timely delivery of a solution into many flow channels as well as for the mixing of solutions.

Electrowetting on gold electrodes with microscopic three-dimensional structures for microfluidic devices

To improve the performance of electrowetting-based microfluidic devices, we used micropillar structures to enhance the changes in the wettability of gold electrodes. The changes in the contact angle of a sessile drop were influenced by the diameter of the micropillars and interpillar distances. For a potential change between 0 V and −1.0 V, the change of the contact angle of the KCl sessile drop was 41° on a smooth electrode, but 88° on an electrode with micropillars with a 10 μm diameter. Furthermore, the existence of the micropillars accelerated the change of the contact angle. The gold electrodes with the micropillars were used to generate the capillary force to mobilize a liquid column in a microflow channel. Compared to a device with a smooth electrode, this device showed a fourfold increase in the flow velocity at −0.9 V. The electrodes were also used as a valve. The ability to stop an intruding solution and the switching speed was improved with the micropillar structure.

Micro analysis system with an integrated microfluidic system based on electrowetting

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

Microfluidic Device for On-Chip Manipulation of Liquid Plugs for Biosensing Applications

A microfluidic device that forms a row of liquid plugs in a micro flow channel was fabricated, and its applicability to an on-chip enzyme-linked immunosorbent assay (ELISA) was demonstrated. The nanoliter liquid plugs were formed by six independent pumps and were mobilized in the micro-flow channel using a main pump. The operation of the pumps was based on the volume change caused as a result of the electrochemical production of hydrogen bubbles. When the bubbles were produced in the pumps, a polydimethylsiloxane (PDMS) diaphragm at the bottom of the compartment inflated and closed an inlet hole at the top of the reservoir. The solution in the reservoir was forced to be injected into the flow channel forming a liquid plug. After six plugs were formed, they were mobilized to a reaction chamber one by one to allow an antigen-antibody binding, cleansing, and detection. This simple and reliable liquid handling system could be applied to the detection of a tumor maker, oc-fetoprotein (AFP).

Automatic on-chip sequential processing for bio-microsystems

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.

Microanalysis System Based on Electrochemiluminescence with Automatic Mixing and pH-Regulation Functions

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.

Highly Sophisticated Electrochemical Analysis System with an Integrated Microfluidic System Based on Electrowetting

An integrated device which can conduct timely transport of a sample solution and analyze its components was constructed. The transport of solutions was based on capillary action on hydrophilic glass areas and the control by valves which operate based on electrowetting. Electrochemical sensors including glucose, lactate, GOT, GPT, pH, ammonia, urea, and creatinine were integrated. Sensors for the former four are amperometric whereas the detection of the latter four are based on potentiometry. The ammonia, urea, and creatinine sensors had an air gap structure to realize a fast response. The sensors for GOT and GPT had 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 upon the introduction of a sample solution. Linear relationships were observed between the output signals and the concentration or the logarithm of the concentration of the analytes.

Integrated bio/chemical sensing system with a microfluidic transport module based on electrowetting

An integrated micro analysis system was fabricated using a microfluidic transport system driven by electrowetting and an air-gap ammonia sensor. The basic element in the system was a row of elongated gold working electrodes and a protruding polydimethylsiloxane (PDMS) structure which form an open channel structure. The wettability of the gold electrode was changed by applying a negative potential with respect to a Ag/AgCl electrode, and a solution introduced from an inlet was mobilized through the gap between the working electrode and the protruding structure. Also, a solution could be transported to any desired directions without using any valves. Furthermore, two solutions could be mixed based on the same principle. The open structure of the flow channel facilitated the integration of an air-gap ammonia sensor. Ammonia diffused from the mixing area was detected as the potential change of a pH-indicator electrode. The 90% response time was 45 s for 10 mM ammonia. The relation between the potential of the pH-indicator electrode and the logarithm of ammonia concentration was linear. Furthermore, a biosensing system was constructed by using immobilized urease or creatinine deiminase and the ammonia sensor. The concentration of urea and creatinine could be determined by measuring ammonia produced enzymatically from a sample solution. Linear calibration plot was obtained for urea concentrations down to 100 /spl mu/M and creatinine concentration down to 50 /spl mu/M.