Browsing by Subject "Electrode"

Deep brain stimulation (DBS) electrodes are intended to stimulate specific areas of the brain to treat movement disorders including essential tremor, Parkinson's disease and dystonia. An important goal in the design of next generation DBS electrodes is to minimize the power needed to stimulate specific regions of the brain. A reduction in power consumption will prolong battery life and reduce the size of implanted pulse generator. Electrode geometry is one approach to increase the efficiency of neural stimulation and reduce the power required to produce the level of activation required for clinical efficacy.

We first characterized the impedance of the presently used clinical DBS electrodes in vitro and in vivo. Characterization of the electrode-tissue interface impedance is required to quantify the composition of charge transfer to the brain tissue. The composition of charge transfer was dependent on both the current density and the sinusoidal frequency. The assumption of the DBS electrode being ideally polarizable was not valid under clinical stimulating conditions. This implies that irreversible processes that can cause electrode or tissue damage might occur when high charge injection is required for DBS.

Current density distribution is an important factor in determining patterns of neural excitation, tissue damage and electrode corrosion. We developed a recursive simulation scheme to calculate the current density distribution that incorporates the nonlinear electrode-tissue interface into finite-element based models of electrodes. The current density distributions on the electrode surface were strongly dependent on the sinusoidal frequency. The primary current density distribution without including the electrode-tissue interface can be used to estimate neural excitation, tissue damage and electrode corrosion with rectangular stimulus pulses as most of the signal power is at frequencies where the secondary current density distribution matches closely the primary current density distribution.

We designed and analyzed novel electrode geometries to decrease stimulation thresholds, thus reducing power consumption of implanted stimulators. Our hypothesis was that high-perimeter electrode geometries that increase the variation of current density on the electrode surface will generate larger activating functions for surrounding neurons and thereby increase stimulation efficiency. We investigated three classes of electrodes: segmented cylindrical electrodes, serpentine-perimeter planar electrodes, and serpentine-perimeter cylindrical electrodes. An approach that combined finite element models of potentials and cable models of axonal excitation was used to quantify the stimulation efficiency of electrodes with various geometries. Increasing the electrode perimeter increased the electrode efficiency by decreasing stimulation threshold. Both segmentation and serpentine edges provided means to increase the efficiency of stimulation. Novel cylindrical electrodes that combined segmentation with serpentine edges decreased power consumption by ~20% for axons parallel to the electrode and by ~35% for axons perpendicular to the electrode. These electrode designs could potentially prolong the average battery life of deep brain stimulator by more than one year.

Electrophysiological intracellular recordings from freely behaving animals can provide information and insights, which have been speculated or cannot be reached by traditional recordings from confined animals. Intracellular recordings can reveal a neuron's intrinsic properties and their communication with other neurons. Utilizing this technology in an awake and socially behaving brain can bring brain research one step further.

In this dissertation, a customized miniature electronics and microdrive assembly is introduced for intracellular recording from small behaving animals. This solution has realized in vivo intracellular recording from freely behaving zebra finches and mice. Also, a new carbon nanotube probe is presented as a surface scanning tip and a neural electrode. With the carbon nanotube probe, intracellular and extracellular neural signals were successfully recorded from mouse brains. Previously, carbon nanotubes have only been used as a coating material on a cell-culturing platform or on a metal based neural electrode. This probe is the first pure carbon nanotube neural electrode without an underlying platform or wire, and it is the first one that has achieved intracellular and extracellular recordings from vertebrate cortical neurons.

The electrical interface to neural medical devices is researched from three perspectives, namely, the electronics within the device, the electrodes on the device, and the electromagnetic fields around the device.

A Brain-Machine Interface may allow paralyzed patients to control robotic limbs with neural signals sensed by fine wires inserted into the brain. The neural signals have an amplitude under one millivolt and must be amplified. A totally integrated amplifier is designed, manufactured, and characterized. The amplifier is fabricated in a standard half-micron CMOS process without capacitors or resistors. Two application issues not previously addressed are solved. First, the topology of the amplifier is shown to be less sensitive to long-term drift of transistor parameters than the standard topology. Second, a neural signal corrupted by 10 millivolts of powerline interference can be recovered. The amplifier has a gain of 58 dB, a bandwidth of 750 to 14k Hz, power consumption of 180 uW, and noise of 1.5 uV RMS. The design techniques proven in this amplifier are suitable for clinical Brain Machine Interfaces.

An implanted electroencephalogram (EEG) recorder may aid the diagnosis of infrequent seizure-like events that are currently diagnosed, without proof, as epilepsy. A proof-of-concept study quantifies the electrical characteristics of the electrodes planned for the recorder. The electrodes are implanted in an ovine model for eight weeks. Electrode impedance is less than 800 Ohm throughout the study. A frequency-domain determination of sedation performs similarly for surface versus implanted electrodes throughout the study. The time-domain correlation between an implanted electrode and a surface electrode is almost as high as between two surface electrodes (0.86 versus 0.92). EEG-certified clinicians judge that the implanted electrode quality is at least adequate and that the implanted electrodes provide the same clinical information as surface electrodes except for a noticeable amplitude difference. No significant issues are found that invalidate the concept of an implanted EEG recorder.

Transcranial stimulation may treat a multitude of neural and psychological illnesses. The stimulation may have higher repeatability and lower patient effort if an implanted device provides the stimulation. The shape of the device, 300 mm long by 1 mm in diameter, is unlike any present implanted device. Five techniques that deliver energy to the device are analyzed using computer simulations. The electrode for the techniques that employ an electric field to deliver the energy is a new design that exploits the anatomy of the scalp and skull. The electric field techniques deliver energy that is likely suitable for some stimulation protocols but not for all. The techniques that employ a magnetic field deliver more than the energy required, especially if the shape of the coil that creates the magnetic field is automatically optimized. However, the magnetic-field techniques heat the brain; the electric-field techniques do not heat the brain. This research validates the new delivery concepts and justifies future research.