Jakub Kedzierski

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.

Engineering polycrystalline Ni films to improve thickness uniformity of the chemical-vapor-deposition-grown graphene films

It has been shown that few-layer graphene films can be grown by atmospheric chemical vapor deposition using deposited Ni thin films on SiO(2)/Si substrates. In this paper we report the correlation between the thickness variations of the graphene film with the grain size of the Ni film. Further investigations were carried out to increase the grain size of a polycrystalline nickel film. It was found that the minimization of the internal stress not only promotes the growth of the grains with (111) orientation in the Ni film, but it also increases their grain size. Different types of SiO(2) substrates also affect the grain size development. Based upon these observations, an annealing method was used to promote large grain growth while maintaining the continuity of the nickel film. Graphene films grown from Ni films with large versus small grains were compared for confirmation.

Graphene-on-Insulator Transistors Made Using C on Ni Chemical-Vapor Deposition

Graphene transistors are made by transferring a thin graphene film grown on Ni onto an insulating SiO2 substrate. The properties and integration of these graphene-on-insulator transistors are presented and compared to the characteristics of devices made from graphitized SiC and exfoliated graphene flakes.

FDSOI Process Technology for Subthreshold-Operation Ultralow-Power Electronics

Ultralow-power electronics will expand the technological capability of handheld and wireless devices by dramatically improving battery life and portability. In addition to innovative low-power design techniques, a complementary process technology is required to enable the highest performance devices possible while maintaining extremely low power consumption. Transistors optimized for subthreshold operation at 0.3 V may achieve a 97% reduction in switching energy compared to conventional transistors. The process technology described in this article takes advantage of the capacitance and performance benefits of thin-body silicon-on-insulator devices, combined with a workfunction engineered mid-gap metal gate.

Morphology of graphene on SiC(0001¯) surfaces

Graphene is formed on SiC(0001¯) surfaces (the so-called C-face of the crystal) by annealing in vacuum, with the resulting films characterized by atomic force microscopy, Auger electron spectroscopy, scanning Auger microscopy, and Raman spectroscopy. Morphology of these films is compared with the graphene films grown on SiC(0001) surfaces (the Si-face). Graphene forms a terraced morphology on the C-face, whereas it forms with a flatter morphology on the Si-face. It is argued that this difference occurs because of differing interface structures in the two cases. For certain SiC wafers, nanocrystalline graphite is found to form on top of the graphene.

Work-Function-Tuned TiN Metal Gate FDSOI Transistors for Subthreshold Operation

The effective work function of a reactively sputtered TiN metal gate is shown to be tunable from 4.30 to 4.65 eV. The effective work function decreases with nitrogen flow during reactive sputter deposition. Nitrogen annealing increases the effective work function and reduces Dit. Thinner TiN improves the variation in effective work function and reduces gate dielectric charge. Doping of the polysilicon above the TiN metal gate with B or P has negligible effect on the effective work function. The work-function-tuned TiN is integrated into ultralow-power fully depleted silicon-on-insulator CMOS transistors optimized for subthreshold operation at 0.3 V. The following performance metrics are achieved: 64-80-mV/dec subthreshold swing, PMOS/NMOS on-current ratio near 1, 71% reduction in Cgd, and 55% reduction in Vt variation when compared with conventional transistors, although significant short-channel effects are observed.

Low-voltage electrowetting on a lipid bilayer formed on hafnium oxide

We present a class of electrowetting systems in which lipid bilayers on thin hafnium oxide films function as reversibly wettable dielectrics, eliminating the need for solid organic dielectrics (e.g., fluoropolymers) in electrowetting systems. These bilayers self-assemble on hafnium oxide in oil, yielding a dielectric stack that can withstand high electric fields, enabling high contact angle changes at unprecedentedly low voltages. We demonstrate that these electrowetting systems can be virtually free of contact angle saturation with low oil-water surface energies (<1 mJ/m2), allowing for a reversible contact angle change from over 140° to under 10° using less than 1 V actuation.

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.

Microhydraulic Electrowetting Actuators

The conversion of electrical to mechanical power on a sub-centimeter scale is a key technology in many microsystems and energy harvesting devices. In this paper, we present a type of a capacitive energy conversion device that uses capillary pressure and electrowetting to reversibly convert electrical power to hydraulic power. These microhydraulic actuators use a high surface-to-volume ratio to deliver high power at a relatively low voltage with an energy conversion efficiency of over 65%. The capillary pressure generated grows linearly with shrinking capillary diameter, as does the frequency of actuation. We present the pressure, frequency, and power scaling properties of these actuators and demonstrate that power density scales up as the inverse capillary diameter squared, leading to high-efficiency actuators with a strength density exceeding biological muscle. Two potential applications for microhydraulics are also demonstrated: soft-microrobotics and energy harvesting.

Validation of the trapped charge model of electrowetting contact angle saturation on lipid bilayers

The problem of modeling contact angle saturation in electrowetting has resisted a number of concentrated efforts by leading researchers. Several models have been proposed, from charge trapping, to droplet ejection, to thermodynamic instability, but no consensus has been reached as to which model better describes the effect. In this paper, we validate the charge trapping based model of contact angle saturation in electrowetting on lipid bilayers, through careful analysis of charge movement between the liquid charge states and trapped charge states at the solid dielectric interface. We also describe a powerful new methodology for studying electrowetting systems by modeling them with an equivalent circuit and simulating the circuit using the SPICE circuit simulator.