Development of Microfluidic Platform with Through-polydimethylsiloxane Microtip Electrode Array for Photosynthetic Microbial Fuel Cell

Abstract

This dissertation proposes and realizes microfluidic platform with through-polydimethylsiloxane microtip electrode array applicable to array level microbial fuel cell. Conventional single cell based insertion methods have drawbacks of low efficiency power extraction. In order to overcome this disadvantage, we aimed to extract power from a stacked cell array using microtip electrode insertion method and microfluidic device fabrication technology. The ultramicroelectrode (UME) on the microtips and an electrode inside the microfluidic chamber were used as an anode and a cathode for the microbial fuel cell, respectively. After the UME was inserted into an array of photosynthetic cells immobilized in the microfluidic chamber, electrons generated during photosynthesis were extracted via the UMEs on the invasive microtips, and were reduced on the cathode in the microfluidic chamber. Integration of UMEs on silicon microtip is proposed with the detailed analysis and verified with the fabrication results of a conductive microtip array having localized UMEs at the tip end. In order to use a microtip electrode in penetrating applications, a study on the variation of electrode design is necessary. A silicon microtip is fabricated using a combination of DRIE and sulfur hexafluoride (SF6) RIE steps. The shape of the microtip is transformed from pillar type to tip type by the variation of the sidewall etching rate of the pillar in the SF6 RIE step. The aspect ratio and the apex radius of the silicon microtip are controlled by mask design with varying diameter and gap of etching mask patterns. A localized UME is formed at the end of the microtip by depositing and patterning the insulating layer without an additional photolithography step. After the fabrication, electrochemical characterization of the UME was performed using cyclic voltammetry (CV), and compared with an estimation based on conical UME theory. The photosynthetic algae cells are immobilized in microfluidic device by inserting the silicon-based electrode into the PDMS fluidic channel. This method is realized by penetrating the bottom thin film of the PDMS fluidic channel with sharp end of the electrode. For easy penetration, the electrode was fabricated in the form of sharp microtip and designed to be positioned in the channel by fabricating arrangement layer around the microtip electrode. Short circuit current and open circuit voltage of the inserted electrode array were measured at 207 pA and 26 mV. Loss analysis of measured current values was done in two ways. First, the decrease of the measured current due to the distance between the working electrode and the counter electrode was verified by electrochemical simulation. As the distance between the working electrode and the counter electrode increases, the magnitude of the current due to the resistance of the electrolyte decreases. Second, it is the deformation of the PDMS thin film by the micro probe electrode moving in the vertical direction during the inserting process. In order to analyze the effect of deformation on the magnitude of the measurement current, the deformation due to the low strength of the thin PDMS thin film was measured and the effective width in the insertion process was calculated by deformation. Simulation results show that the current value is about 53% of the measured value when the distance between electrodes is 5 μm (ideal value). As a result of the effective area calculation, the number of effective electrodes among the total number of 17689 electrodes is 946, and the measured current per electrode is 0.22 pA. If the effective electrode is assumed to have no current loss due to the distance between the electrodes, the current value per electrode is calculated to be 0.40 pA. In conclusion, this dissertation demonstrated the possibility of intracellular insertion of the electrode using the developed microtip electrode and the proposed through-polydimethylsiloxane assembly method. In addition, a short circuit current and an open circuit voltage measurement showed the possibility of being used as a microbial fuel cell. Based on the analysis of the loss of the measured results, a method of reducing the current loss by improving the electrode arrangement and the microfluidic structure was suggested.