Patrick T. McCarthy

Robust penetrating microelectrodes for neural interfaces realized by titanium micromachining

Neural prosthetic interfaces based upon penetrating microelectrode devices have broadened our understanding of the brain and have shown promise for restoring neurological functions lost to disease, stroke, or injury. However, the eventual viability of such devices for use in the treatment of neurological dysfunction may be ultimately constrained by the intrinsic brittleness of silicon, the material most commonly used for manufacture of penetrating microelectrodes. This brittleness creates predisposition for catastrophic fracture, which may adversely affect the reliability and safety of such devices, due to potential for fragmentation within the brain. Herein, we report the development of titanium-based penetrating microelectrodes that seek to address this potential future limitation. Titanium provides advantage relative to silicon due to its superior fracture toughness, which affords potential for creation of robust devices that are resistant to catastrophic failure. Realization of these devices is enabled by recently developed techniques which provide opportunity for fabrication of high-aspect-ratio micromechanical structures in bulk titanium substrates. Details are presented regarding the design, fabrication, mechanical testing, in vitro functional characterization, and preliminary in vivo testing of devices intended for acute recording in rat auditory cortex and thalamus, both independently and simultaneously.

Simultaneous recording of rat auditory cortex and thalamus via a titanium-based, microfabricated, microelectrode device

Direct recording from sequential processing stations within the brain has provided opportunity for enhancing understanding of important neural circuits, such as the corticothalamic loops underlying auditory, visual, and somatosensory processing. However, the common reliance upon microwire-based electrodes to perform such recordings often necessitates complex surgeries and increases trauma to neural tissues. This paper reports the development of titanium-based, microfabricated, microelectrode devices designed to address these limitations by allowing acute recording from the thalamic nuclei and associated cortical sites simultaneously in a minimally invasive manner. In particular, devices were designed to simultaneously probe rat auditory cortex and auditory thalamus, with the intent of recording auditory response latencies and isolated action potentials within the separate anatomical sites. Details regarding the design, fabrication, and characterization of these devices are presented, as are preliminary results from acute in vivo recording.

Titanium-based multi-channel, micro-electrode array for recording neural signals

Micro-scale brain-machine interface (BMI) devices have provided an opportunity for direct probing of neural function and have also shown significant promise for restoring neurological functions lost to stroke, injury, or disease. However, the eventual clinical translation of such devices may be hampered by limitations associated with the materials commonly used for their fabrication, e.g. brittleness of silicon, insufficient rigidity of polymeric devices, and unproven chronic biocompatibility of both. Herein, we report, for the first time, the development of titanium-based “Michigan” type multi-channel, microelectrode arrays that seek to address these limitations. Titanium provides unique properties of immediate relevance to microelectrode arrays, such as high toughness, moderate modulus, and excellent biocompatibility, which may enhance structural reliability, safety, and chronic recording reliability. Realization of these devices is enabled by recently developed techniques which provide the opportunity for fabrication of high aspect ratio micromechanical structures in bulk titanium substrates. Details regarding the design, fabrication, and characterization of these devices for eventual use in rat auditory cortex and thalamus recordings are presented, as are preliminary results.

Thermionic and Photo-Excited Electron Emission for Energy-Conversion Processes

This article describes advances in thermionic and photoemission materials and applications dating back to the work on thermionic emission by Guthrie in 1873 and the photoelectric effect by Hertz in 1887. Thermionic emission has been employed for electron beam generation from Edison’s work with the light bulb to modern day technologies such as scanning and transmission electron microscopy. The photoelectric effect has been utilized in common devices such as cameras and photocopiers while photovoltaic cells continue to be widely successful and further researched. Limitations in device efficiency and materials have thus far restricted large-scale energy generation sources based on thermionic and photoemission. However, recent advances in the fabrication of nanoscale emitters suggest promising routes for improving both thermionic and photo-enhanced electron emission along with newly developed research concepts, e.g., photonically enhanced thermionic emission. However, the abundance of new emitter materials and reduced dimensions of some nanoscale emitters increases the complexity of electron emission theory and engender new questions related to the dimensionality of the emitter. This work presents derivations of basic two and three-dimensional thermionic and photoemission theory along with comparisons to experimentally acquired data. The resulting theory can be applied to many different material types regardless of composition, bulk and surface structure.

Photonically excited electron emission from modified graphitic nanopetal arrays

Efficient electron emission for energy conversion requires a low work function and a stable emitter material. The work function of graphene-based carbon materials can decrease significantly by intercalation with alkali metals, thus increasing their emission current. In this work, electron emission from potassium-intercalated carbon nanosheet extensions grown on electrode graphite is investigated. These petal-like structures, composed of 5–25 layers of graphene, are synthesized using microwave plasma chemical vapor deposition. Samples are intercalated with potassium, and a hemispherical energy analyzer is used to measure the emission intensity caused by both thermal and photonic excitation. The emission from the potassium-intercalated structures is found to consistently decrease the work function by 2.4 to 2.8 eV relative to non-intercalated samples. High emission intensity induced by photonic excitation from a solar simulator, with a narrow electron energy distribution relative to established theory, sugg...

Work Function Characterization of Potassium-Intercalated, Boron Nitride Doped Graphitic Petals

This paper reports on characterization techniques for electron emission from potassium-intercalated boron nitride modified graphitic petals. Carbon-based materials offer potentially good performance in electron emission applications owing to high thermal stability and a wide range of nanostructures that increase emission current via field enhancement. Furthermore, potassium adsorption and intercalation of carbon-based nanoscale emitters decreases work functions from approximately 4.6 eV to as low as 2.0 eV. In this study, boron nitride modifications of graphitic petals were performed. Hexagonal boron nitride is a planar structure akin to graphene and has demonstrated useful chemical and electrical properties when embedded in graphitic layers. Photoemission induced by simulated solar excitation was employed to characterize the emitter electron energy distributions, and changes in the electron emission characteristics with respect to temperature identified annealing temperature limits. After several heating cycles, a single stable emission peak with work function of 2.8 eV was present for the intercalated graphitic petal sample up to 1000 K. Up to 600 K, the potassium-intercalated boron nitride modified sample exhibited improved retention of potassium in the form of multiple emission peaks (1.8 eV, 2.5 eV, and 3.3 eV) resulting in a large net electron emission relative to the unmodified graphitic sample. However, upon further heating to 1000 K, the unmodified graphitic petal sample demonstrated better stability and higher emission current than the boron nitride modified sample. Both samples deintercalated above 1000 K.

Carbon Nanotube Interfaces for Magneto Thermoelectric Actuation

Magneto thermoelectric generator cell technology uses the ferromagnetic phase transition of gadolinium to drive the movement of a diaphragm ‘shuttle’ whose mechanical energy can be converted to electrical form and which enhances heat transfer through both conduction and convection. This paper describes the thermal behavior of gadolinium foils used in magneto thermoelectric generator cells that, in conjunction with a planar array of similar devices, would form a thermal backplane to a solar photovoltaic panel. In this scenario, the backplane operates as a self-powered cooling device that can simultaneously convert thermal energy to electrical energy as well as improve photovoltaic efficiency through active cooling. This form of energy harvesting and enhancement shows the potential of increasing the energy density of silicon photovoltaic panels. The synthesis and characterization of thermal interfaces applied to the gadolinium shuttles and hot/cold substrates are described. Carbon nanotube arrays are implemented as the thermal interfaces, and their performance under static conditions is assessed. Optimization of the carbon nanotube interfaces on the gadolinium shuttles is achieved using photoacoustic experiments for measuring the thermal interface resistances above and below the gadolinium foil. Carbon nanotube growth studies on gadolinium demonstrated a reduction in thermal interface resistances from 28.8 ± 2.1 mm2 K/W to as low as 17.9 ± 0.8 mm2 K/W. Initial design, fabrication, and experimental techniques and results are presented in this paper.Copyright