N. Sabaté

Silicon-based microfabricated microbial fuel cell toxicity sensor

Microbial fuel cells (MFCs) have been used for several years as biosensors for measuring environmental parameters such as biochemical oxygen demand and water toxicity. The present study is focused on the detection of toxic matter using a novel silicon-based MFC. Like other existing toxicity sensors based on MFCs, this device is capable of detecting the variation on the current produced by the cell when toxic compounds are present in the medium. The MFC approach presented in this work aims to obtain a simple, compact and planar device for its further application as a biosensor in the design and fabrication of equipment for toxicity monitoring. It consists on a proton exchange membrane placed between two microfabricated silicon plates that act as current collectors. An array of square 80 μm × 80 μm vertical channels, 300 μm deep, have been defined trough the plates over an area of 6 mm × 6 mm. The final testing assembly incorporates two perspex pieces positioned onto the plates as reservoirs with a working volume of 144 μL per compartment. The operation of the microdevice as a direct electron transfer MFC has been validated by comparing its performance against a larger scale MFC, run under the same conditions. The device has been tested as a toxicity sensor by setting it at a fixed current while monitoring changes in the output power. A drop in the power production is observed when a toxic compound is added to the anode compartment. The compact design of the device makes it suitable for its incorporation into measurement equipment either as an individual device or as an array of sensors for high throughput processing.

Microfluidic fuel cells on paper: meeting the power needs of next generation lateral flow devices

Lateral flow test strips have dominated the rapid diagnostics landscape for decades. Recently, the emergence of paper microfluidics has brought new functionalities to these porous materials, and the search for instrument-free point-of-care devices has driven the development of different types of energy sources to fulfill their power needs. This work presents the development of microfluidic fuel cells as paper-based power sources in a standard lateral flow test format. These fuel cells benefit from the laminar flow occurring in a porous material by capillarity to separately react with two parallel streams, anolyte and catholyte, without an ionic exchange membrane or external pumps. It has been shown that the devices are capable of delivering power densities in the range of 1–5 mW cm−2 using solutions of methanol and KOH. The incorporation of a conjugate pad to store the KOH electrolyte in a solid form and a methanol-rich agar gel on top of the reaction membrane allows the fuel cell to function soaking a single sample pad with just water. The presented microfluidic fuel cell approach would enable a more straightforward integration with typical lateral flow test strips and a cost-effective manufacturing. This work represents the starting point in the development of a power source for capillary-based autonomous sensing systems capable of harvesting the energy needed for the measurement from the biological sample to be analyzed.

Digital image correlation of nanoscale deformation fields for local stress measurement in thin films

In this paper, the application of an in situ stress measurement technique to a silicon nitride thin film deposited onto a thick silicon substrate is presented. The method is based on the measurement of the displacement field originated when a slot is milled into the material under study. Displacements are obtained by digital correlation analysis of scanning electron microscope (SEM) images, whereas milling is performed by ion milling in focused ion-beam equipment. Due to the mechanical constraint introduced by the substrate and the small thickness of the tested layer, the displacements generated by the milling process are in the range 0?5?nm, which is one of the smallest displacement ranges measured up to now in a relaxation-based measurement technique. The local stress value determined with this new method is in good agreement with values obtained by a classical method like the wafer bending test.

Residual Stress Measurement on a MEMS Structure With High-Spatial Resolution

A new approach to the local measurement of residual stress in microstructures is described in this paper. The presented technique takes advantage of the combined milling-imaging features of a focused ion beam (FIB) equipment to scale down the widely known hole drilling method. This method consists of drilling a small hole in a solid with inherent residual stresses and measuring the strains/displacements caused by the local stress release, that takes place around the hole. In the presented case, the displacements caused by the milling are determined by applying digital image correlation (DIC) techniques to high resolution micrographs taken before and after the milling process. The residual stress value is then obtained by fitting the measured displacements to the analytical solution of the displacement fields. The feasibility of this approach has been demonstrated on a micromachined silicon nitride membrane showing that this method has high potential for applications in the field of mechanical characterization of micro/nanoelectromechanical systems

Tolerant Chalcogenide Cathodes of Membraneless Micro Fuel Cells

The most critical issues to overcome in micro direct methanol fuel cells (μDMFCs) are the lack of tolerance of the platinum cathode and fuel crossover through the polymer membrane. Thus, two novel tolerant cathodes of a membraneless microlaminar-flow fuel cell (μLFFC), Pt(x)S(y) and CoSe(2), were developed. The multichannel structure of the system was microfabricated in SU-8 polymer. A commercial platinum cathode served for comparison. When using 5 M CH(3)OH as the fuel, maximum power densities of 6.5, 4, and 0.23 mW cm(-2) were achieved for the μLFFC with Pt, Pt(x)S(y), and CoSe(2) cathodes, respectively. The Pt(x)S(y) cathode outperformed Pt in the same fuel cell when using CH(3)OH at concentrations above 10 M. In a situation where fuel crossover is 100 %, that is, mixing the fuel with the reactant, the maximum power density of the micro fuel cell with Pt decreased by 80 %. However, for Pt(x)S(y) this decrease corresponded to 35 % and for CoSe(2) there was no change in performance. This result is the consequence of the high tolerance of the chalcogenide-based cathodes. When using 10 M HCOOH and a palladium-based anode, the μLFFC with a CoSe(2) cathode achieved a maxiumum power density of 1.04 mW cm(-2). This micro fuel cell does not contain either Nafion membrane or platinum. We report, for the first time, the evaluation of Pt(x)S(y)- and CoSe(2)-based cathodes in membraneless micro fuel cells. The results suggest the development of a novel system that is not size restricted and its operation is mainly based on the selectivity of its electrodes.

Full ceramic micro solid oxide fuel cells: towards more reliable MEMS power generators operating at high temperatures

Batteries, with a limited capacity, have dominated the power supply of portable devices for decades. Recently, the emergence of new types of highly efficient miniaturized power generators like micro fuel cells has opened up alternatives for continuous operation on the basis of unlimited fuel feeding. This work addresses for the first time the development of a full ceramic micro solid oxide fuel cell fabricated in silicon technology. This full-ceramic device represents a new generation of miniaturized power generators able to operate at high temperatures, and therefore able to work with a hydrocarbon fuel supply. Dense yttria-stabilized zirconia free-standing large-area membranes on micromachined silicon were used as the electrolyte. Thin-film porous electrodes of La0.6Sr0.4CoO3−δ and gadolinia-doped ceria were employed as cathode and anode materials, respectively. The electrochemical performance of all the components was evaluated by partial characterization using symmetrical cells, yielding excellent performance for the electrolyte (area specific resistance of 0.15 Ω cm2 at temperatures as low as 450 °C) and the electrodes (area specific resistance of the cathode and anode below 0.3 Ω cm2 at 700 °C). A micro solid oxide fuel cell with an active area of 2 mm2 and less than 1 micrometer in thickness was characterized under fuel cell conditions, using hydrogen as a fuel and air as an oxidant. A maximum power density of 100 mW cm−2 and 2 mW per single membrane was generated at 750 °C, having an open circuit voltage of 1.05 V. Impedance spectroscopy of the all-ceramic membrane showed a total area-specific resistance of ∼3.5 Ω cm2.

Replica molding for multilevel micro-/nanostructure replication

The development of micro- and nanofabrication processes with the capability to achieve three-dimensional structures is of great interest for a wide variety of applications. In this paper, a replica molding process for the replication of combined micro- and nanostructures in an epoxy-based photo resin is reported. First, multilevel masters were realized using standard micro- and nanofabrication processes. The structures from these masters were transferred to poly(dimethylsiloxane) (PDMS) stamps by soft lithography. Finally, the PDMS stamps were used for the replication of nano- and microstructures in the epoxy-based resin by UV casting. With this process, micro- and nanostructures of minimal dimensions of 50 nm were successfully replicated. Furthermore, the capabilities of the process were confirmed by the fabrication of a microfluidic device. In this system, the surface of micro channels was structured to modify its wetting properties and create hydrophobic and hydrophilic areas without any additional chemical treatment.

A micro alkaline direct ethanol fuel cell with platinum-free catalysts

This paper presents the fabrication and characterization of a micro alkaline direct ethanol fuel cell. The device has been conceived as a feasibility demonstrator, using microtechnologies for the fabrication of the current collectors and traditional techniques for the membrane electrode assembly production. The fuel cell works in passive mode, as expected for the simplicity required for micro power systems. Non-noble catalysts have been used in order to implement the main advantage of alkaline systems, showing the feasibility of such a device as a potential very-low-cost power device at mini- and micro scales. (Some figures may appear in colour only in the online journal)

Comprehensive characterization and understanding of micro-fuel cells operating at high methanol concentrations.

Summary We report on the analysis of the performance of each electrode of an air-breathing passive micro-direct methanol fuel cell (µDMFC) during polarization, stabilization and discharge, with CH3OH (2–20 M). A reference electrode with a microcapillary was used for separately measuring the anode the cathode potential. Information about the open circuit potential (OCP), the voltage and the mass transport related phenomena are available. Using 2 M CH3OH, the anode showed mass transport problems. With 4 and 6 M CH3OH both electrodes experience this situation, whereas with 10 and 20 M CH3OH the issue is attributed to the cathode. The stabilization and fuel consumption time depends mainly on the cathode performance, which is very sensitive to fuel crossover. The exposure to 20 M CH3OH produced a loss in performance of more than 75% of the highest power density (16.3 mW·cm−2).

A test structure for the design of thermal flow sensors

A test structure for the design and optimisation of thermal gas flow sensors has been developed. This test structure provides information about the temperature distribution created around a heating element as well as its modification due to a gas flow, thus permitting to obtain information about the thermal conductivity of the membrane material. Data obtained from the characterisation of the structure can be used in the optimisation of a flow sensor designed for any specific application.