M. Bak

Parylene as a Chronically Stable, Reproducible Microelectrode Insulator

One of the major problems in the design of neurophysiologiv extracellular microelectrodes is the application and selective removal of the insulation. In addition to the usual problems of achieving pin-hole-free coatings and reproducible tip exposures and impedances, chronic electrode designshave the additional requirement of maintaining megohm levels of electrical isolation for months in vivo. A method of insulating finely tapered microelectrodes with complicated shapes by vapor condensation of Parylene-C is presented, along with a method for exposing controlled, reproducible lengths of their tips. In vivo and in vitro impedance tests and unit records obtained over 100 days in monkey motor cortex are presented. The electric arc process used to expose Parylene-covered iridium and tungsten microelectrodes is found to give cleaner recording surfaces with impedances lower than those obtainable with previously described methods. Chronic iridium microelectrodes so fabricated have recorded unit potentials and maintained constant impedances for over 4 months in vivo.

A floating metal microelectrode array for chronic implantation.

Implantation of multi-electrode arrays is becoming increasingly more prevalent within the neuroscience research community and has become important for clinical applications. Many of these studies have been directed towards the development of sensory and motor prosthesis. Here, we present a multi-electrode system made from biocompatible material that is electrically and mechanically stable, and employs design features allowing flexibility in the geometric layout and length of the individual electrodes within the array. We also employ recent advances in laser machining of thin ceramic substrates, application of ultra-fine line gold conductors to ceramic, fabrication of extremely flexible cables, and fine wire management techniques associated with juxtaposing metal microelectrodes within a few hundred microns of each other in the development of a floating multi-electrode array (FMA). We implanted the FMA in rats and show that the FMA is capable of recording both spikes and local field potentials.

Laser exposure of Parylene-C insulated microelectrodes

The polymer, Parylene-C, has proven to be a biocompatible insulation for microelectrodes. However, due to its inert nature, the removal of the insulation from the tips of microelectrodes is difficult. This paper describes the use of an ultraviolet laser system to micromachine Parylene-C insulation with photoablation to precisely expose an arbitrary shape recording or stimulating surface.

An Improved Time-Amplitude Window Discriminator

Usually extracellular microelectrodes record the activity from several neurons. In order to quantify the activity of an individual neuron in the record some form of spike discrimination is required. This paper describes a window discriminator for the separation of multi-unit neuronal spike trains which will display simultaneously the recorded spike information and the time-amplitude window levels. The window levels are displayed and can be delayed to include any portion of the spike waveform either above or below the initial threshold level. Several instruments can be cascaded for multiple time-amplitude interrogations of the same spike class from a single electrode recording. The instrument utilizes inexpensive commercially available integrated circuits.

Design, Fabrication, and In Vivo Behavior of Chronic Recording Intracortical Microelectrodes

A platinum-glass microelectrode has been developed that permits chronic recording from single cortical neurons in unrestrained animals and without benefit of external manipulation. Details of fabrication and the surgical technique involved are presented. A discussion of the relevant mechanical considerations is also included. A prime advantage of the method is the ease with which it may be realized in the standard neurophysiologic laboratory.

Intracortical Visual Prosthesis Research - Approach and Progress

Following the early work of Brindley in the late 1960s, the NIH began intramural and extramural funding for stimulation of the primary visual cortex using fine-wire electrodes that are inserted into area V1 for the purpose of restoring vision in individuals with blindness. More recently researchers with experience in this project became part of our multi-institutional team with the intention to identify and close technological gaps so that the intracortical approach might be tested in humans on a chronic basis. Our team has formulated an approach for testing a prototype system in a human volunteer. Here, we describe our progress and expectations

Multichannel cortical stimulation for restoration of vision

Development of an intracortical visual prosthesis for restoration of vision, has been, and continues to be an elusive goal of neural prosthesis researchers. Our multi-institutional team has tested the feasibility of implanting and evaluating large numbers of stimulation/recording electrodes in an animal model. Using a combination of 8-electrode arrays and individual electrodes, 152 activated iridium microelectrodes were implanted in area V1 of a macaque. Visual stimuli were used to define a retinotopic map. Spatial coordinates for each electrode were used to train the animal to use electrical stimulation in performing a visual psychophysical task.

Active Floating Micro Electrode Arrays (AFMA)

Neuroscientists have widely used metal microelectrodes inserted into the cortex to record neural signals from, and provide electrical stimulation to, neural tissue for many years. Recently, the demand for implanting electrode arrays within the cortex, for both stimulation and recording, has rapidly increased. We are developing Active-floating-micro-electrode-arrays (AFMA) that are intended for use as a multielectrode cortical interface while minimizing the number of wires leading from the array to extra-dural circuitry or connectors. When combined with a wireless module, these new microelectrode arrays should allow for simulation and recording within free-roaming animals. This paper mainly discusses the design, fabrication, and packing of the first generation AFMA. Our long-term vision is a wireless-transmission electrode system, for stimulation and recording in free-roaming animals, which uses a family of modular active implantable electrode arrays

In-vivo tests of a 16-channel implantable wireless neural stimulator

Wireless stimulation of neural tissue could enable many emerging neural prosthesis designs, and eliminate problems associated with percutaneous wires and connectors. Our laboratory has developed a 16-channel wireless floating microelectrode array (WFMA) for chronic implantation. Here, we report on its first use within in-vivo experiments, using a rat sciatic nerve model. Stimulus currents and associated muscular movements were determined for electrodes of two WFMA devices implanted into four animal subjects.

Characterization and variability of intracortical iridium oxide microelectrodes

Over 200 iridium microelectrodes with activated iridium oxide (AIROF) charge-injection sites were evaluated in vitro by cyclic voltammetry and voltage transient measurements during current pulsing. Variability in the electrochemical behavior of these nominally identical microelectrodes was correlated with variations in the amount of AIROF present and the physical condition of the charge-injection tip. Electrolyte leakage under polymer insulation and delamination of AIROF both gave rise to characteristic shapes in cyclic voltammograms which could be used to identify their presence.