A Glucose Fuel Cell for Implantable Brain–Machine Interfaces

We have developed an implantable fuel cell that generates power through glucose oxidation, producing steady-state power and up to peak power. The fuel cell is manufactured using a novel approach, employing semiconductor fabrication techniques, and is therefore well suited for manufacture together with integrated circuits on a single silicon wafer. Thus, it can help enable implantable microelectronic systems with long-lifetime power sources that harvest energy from their surrounds. The fuel reactions are mediated by robust, solid state catalysts. Glucose is oxidized at the nanostructured surface of an activated platinum anode. Oxygen is reduced to water at the surface of a self-assembled network of single-walled carbon nanotubes, embedded in a Nafion film that forms the cathode and is exposed to the biological environment. The catalytic electrodes are separated by a Nafion membrane. The availability of fuel cell reactants, oxygen and glucose, only as a mixture in the physiologic environment, has traditionally posed a design challenge: Net current production requires oxidation and reduction to occur separately and selectively at the anode and cathode, respectively, to prevent electrochemical short circuits. Our fuel cell is configured in a half-open geometry that shields the anode while exposing the cathode, resulting in an oxygen gradient that strongly favors oxygen reduction at the cathode. Glucose reaches the shielded anode by diffusing through the nanotube mesh, which does not catalyze glucose oxidation, and the Nafion layers, which are permeable to small neutral and cationic species. We demonstrate computationally that the natural recirculation of cerebrospinal fluid around the human brain theoretically permits glucose energy harvesting at a rate on the order of at least 1 mW with no adverse physiologic effects. Low-power brain–machine interfaces can thus potentially benefit from having their implanted units powered or recharged by glucose fuel cells

New Generation of Digital Microfluidic Devices

This paper reports on the design, fabrication, and performance of micro-sized fluidic devices that use electrowetting to control and transport liquids. Using standard microfabrication techniques, new pumping systems are developed with significantly more capability than open digital microfluidic systems that are often associated with electrowetting. This paper demonstrates that, by integrating closed microchannels with different channel heights and using electrowetting actuation, liquid interfaces can be controlled, and pressure work can be done, resulting in fluid pumping. The operation of two different on-chip pumps and devices that can form water drops is described. In addition, a theory is presented to explain the details of single-electrode actuation in a closed channel.United States. Air Force (Air Force under Contract FA8721-05-C002

Fuel Cell Cathode.

<p>Scanning electron micrograph of the fuel cell cathode, showing the conducting mesh of carbon nanotubes encapsulated in Nafion ionomer. Scale Bar: 1 µm.</p

Atomic Force Microscopic Measurements of Anode Surface Roughness.

<p>Atomic force microscopic measurements of <i>z</i>-direction (plane-normal) surface roughness at the 10 µm and 1 µm scales (upper and lower images, respectively), comparing atomically smooth platinum (left images) with the roughened anodes we describe here (right images).</p

General Operational Scheme for an Implantable Glucose Fuel Cell.

<p>This schematic conceptually illustrates the structure of an abiotically catalyzed glucose fuel cell, including the essential half-cell and overall reactions, the sites at which they occur within the system, and the flows of reactants and products.</p

Anode Roughening.

<p>These optical micrographs illustrate the effect of the roughening technique on the anode surface, showing the atomically smooth platinum traces (narrow metallic strips) leading to the anode in contrast with the anode itself (large rectangular area). The roughness of the anode surface is detectable optically as an abrupt change in color and texture. The image at right is an enlargement of the central region of the image at left, focusing on the boundary between the smooth and rough platinum surfaces (rotated with the wire trace set vertical). Scale: The wire traces (left and bottom left, respectively) are 100 <i>µ</i>m wide.</p

Photolithography Masks and Fabricated Fuel Cells.

<p>The image at left shows a set of superimposed photolithographic masks for glucose fuel cells of various sizes, arranged for fabrication on a silicon wafer 150 mm (6 inches) in diameter. The largest device depicted has an anode that measures 64 mm by 64 mm. The anodes of the other fuel cells shown are scaled-down versions of the large device, with length and width alternately reduced by factors of two. The schematic was constructed by overlaying the four process layers: yellow, platinum; orange, roughened platinum anode (aluminum deposition for annealing); blue, Nafion; green, cathode (single-walled carbon nanotubes in Nafion). The photograph at right shows the corresponding silicon wafer as fabricated. Scale Bar: 2 cm.</p

Anode Micro- and Nanostructure.

<p>This set of scanning electron micrographs, taken of a fuel cell anode at increasing levels of magnification (as indicated by the scale bars in each image), illustrates the effects of the roughening procedure on electrode surface structure over a hierarchy of length scales from nanometers to micrometers.</p

Fuel Cell Polarization Curve.

<p>The performance of the fuel cell is characterized through its output voltage (blue, left axis) and power density (red, right axis) as functions of output current density. A 2 mm² device exhibits an open-cell voltage of 192 mV and achieves maximum power output of more than 180 µW cm⁻² when sourcing 1.5–1.85 mA cm⁻².</p