Richard A. Normann

A silicon-based, three-dimensional neural interface: manufacturing processes for an intracortical electrode array

A method is described for the manufacture of a three-dimensional electrode array geometry for chronic intracortical stimulation. This silicon based array consists of a 4.2*4.2*0.12 mm thick monocrystalline substrate, from which project 100 conductive, silicon needles sharpened to facilitate cortical penetration. Each needle is electrically isolated from the other needles, and is about 0.09 mm thick at its base and 1.5 mm long. The sharpened end of each needle is coated with platinum to facilitate charge transfer into neural tissue. The following manufacturing processes were used to create this array: thermomigration of 100 aluminum pads through an n-type silicon block, creating trails of highly conductive p/sup +/ silicon isolated from each other by opposing pn junctions; a combination of mechanical and chemical micromachining which creates individual penetrating needles of the p/sup +/ silicon trails; metal deposition to create active electrode areas and electrical contact pads; and array encapsulation with polyimide.<<ETX>>

Chronic recording capability of the Utah Intracortical Electrode Array in cat sensory cortex

The Utah Intracortical Electrode Array (UIEA) is an array of 100 penetrating silicon microelectrodes designed to focally electrically stimulate or record neurons residing in a single layer up to 1.5 mm beneath the surface of the cerebral cortex. Apart from its use as a unique tool to study parallel processing in the central nervous system, this array could form the platform for a cortical neuroprosthetic system. Although the UIEA has been used extensively in acute neural recording and stimulation experiments, its long-term performance in a chronic application has yet to be demonstrated. As an initial investigation into the feasibility of long-term cortical recording with an array of microelectrodes, we have hard-wired a subset of 11 electrodes of the UIEA to a percutaneous connector. This chronic UIEA assembly was then implanted into the cerebral cortices of ten cats for durations ranging from 2 to 13 months; over which time, both random and stimulus-evoked single and multiple unit action potentials were periodically recorded. On average, after a 6-month implant period, 60% of implanted arrays could still record some type of activity. Post-sacrifice dissections revealed a fibrous encapsulation of the UIEA. Although most implanted cortex was histologically normal, evidence of a chronic astroglial response was seen in a few cases. The results of the reported experiments indicate that the UIEA can be successfully used for limited times in a chronic recording application.

THE UTAH INTRACORTICAL ELECTRODE ARRAY : A RECORDING STRUCTURE FOR POTENTIAL BRAIN-COMPUTER INTERFACES

We investigated the potential of the Utah Intracortical Electrode Array (UIEA) to provide signals for a brain-computer interface (BCI). The UIEA records from small populations of neurons which have an average signal-to-noise ratio (SNR) of 6:1. We provide specific examples that show the activities of these populations of neurons contain sufficient information to perform control tasks. Results from a simple stimulus detection task using these signals as inputs confirm that the number of neurons present in a recording is significant in determining task performance. Increasing the number of units in a recording decreases the sensitivity of the response to the stimulus; decreasing the number of units in the recording, however, increases the variability of the response to the stimulus. We conclude that recordings from small populations of neurons, not single units, provide a reliable source of sufficiently stimulus selective signals which should be suitable for a BCI. In addition, the potential for simultaneous and proportional control of a large number of external devices may be realized through the ability of an array of microelectrodes such as the UIEA to record both spatial and temporal patterns of neuronal activation.

A neural interface for a cortical vision prosthesis

The development of a cortically based vision prosthesis has been hampered by a lack of basic experiments on phosphene psychophysics. This basic research has been hampered by the lack of a means to safely stimulate large numbers of cortical neurons. Recently, a number of laboratories have developed arrays of silicon microelectrodes that could enable such basic studies on phosphene psychophysics. This paper describes one such array, the Utah electrode array, and summarizes neurosurgical, physiological and histological experiments that suggest that such an array could be implanted safely in visual cortex. We also summarize a series of chronic behavioral experiments that show that modest levels of electrical currents passed into cortex via this array can evoke sensory percepts. Pending the successful outcome of biocompatibility studies using such arrays, high count arrays of penetrating microelectrodes similar to this design could provide a useful tool for studies of the psychophysics of phosphene perception in human volunteers. Such studies could provide a proof-of-concept for cortically based artificial vision.

A glass/silicon composite intracortical electrode array

A new manufacturing technique has been developed for creating silicon-based, penetrating electrode arrays intended for implantation into cerebral cortex. The arrays consist of a 4.2 mm×4.2 mm glass/silicon composite base, from which project 100 silicon needle-type electrodes in a 10×10 array. Each needle is approximately 1,500 μm long, 80μm in diameter at the base, and tapers to a sharp point at the metalized tip. The technique used to manufacture these arrays differs from our previous method in that a glass dielectric, rather than ap-n-p junction, provides electrical isolation between the individual electrodes in the array. The new electrode arrays exhibit superior electrical properties to those described previously. We have measured interelectrode impedances of at least 1013 Ω, and interelectrode capacitances of approximately 50 fF for the new arrays. In this paper, we describe the manufacturing techniques used to create the arrays, focusing on the dielectric isolation technique, and discuss the electrical and mechanical characteristics of these arrays.

Single unit recording capabilities of a 100 microelectrode array

We have developed a three-dimensional silicon electrode array which provides 100 separate channels for neural recording in cortex. The device is manufactured using silicon micromachining techniques, and we have conducted acute recording experiments in cat striate cortex to evaluate the recording capabilities of the array. In a series of five acute experiments, 58.6% of the electrodes in the array were found to be capable of recording visually evoked responses. In the most recent acute study, the average signal-to-noise ratio for recordings obtained from 56 of the electrodes in the array was calculated to be 5.5:1. Using standard window discrimination techniques, an average of 3.4 separable spikes were identified for each of these electrodes. In order to compare the two-dimensional mapping capabilities of the array with those derived from other technologies, orientation preference and ocular dominance maps were generated for each of the evoked responses. Histological evaluation of the implant site indicates some localized tissue insult, but this is likely due to the perfusion procedure since high signal-to-noise ratio neural responses were recorded. The recording capabilities of the Utah Intracortical Electrode Array in combination with the large number of electrodes available for recording make the array a tool well suited for investigations into the parallel processing mechanisms in cortex.

A method for pneumatically inserting an array of penetrating electrodes into cortical tissue

The goal of this research was to find a practical means by which an array of 100 needle-shaped electrodes could be implanted into the cerebral cortex with minimal brain tissue trauma. It was found that insertion of these structures into cortical tissues could only be performed using high insertion speeds. A pneumatically actuated impact insertion system has been developed that is capable of inserting an electrode array into feline brain tissue at speeds from about 1 to 11 m/s. We found that a minimum array insertion speed of 8.3 m/s was necessary for a complete, safe insertion of all 100 electrodes in the array to a depth of 1.5 mm into feline cortex. The performance of the impact insertion system is discussed in terms of a simplified representation of cortical tissue.

Long-term stimulation and recording with a penetrating microelectrode array in cat sciatic nerve

We studied the consequences of long-term implantation of a penetrating microelectrode array in peripheral nerve over the time course of 4-6 mo. Electrode arrays without lead wires were implanted to test the ability of different containment systems to protect the array and nerve during contractions of surrounding muscles. Treadmill walking was monitored and the animals showed no functional deficits as a result of implantation. In a different set of experiments, electrodes with lead wires were implanted for up to 7 mo and the animals were tested at 2-4 week intervals at which time stimulation thresholds and recorded sensory activity were monitored for every electrode. It was shown that surgical technique highly affected the long-term stimulation results. Results between measurement sessions were compared, and in the best case, the stimulation properties stabilized in 80% of the electrodes over the course of the experiment (162 days). The recorded sensory signals, however, were not stable over time. A histological analysis performed on all implanted tissues indicated that the morphology and fiber density of the nerve around the electrodes were normal.

Mobility performance with a pixelized vision system

A visual prosthesis, based on electrical stimulation of the visual cortex, has been suggested as a means for partially restoring functional vision in the blind. The prosthesis would create a pixelized visual sense consisting of punctate spots of light (phosphenes). The present study investigated the feasibility of achieving visually-guided mobility with such a visual sense. Psychophysical experiments were conducted on normally sighted human subjects, who were required to walk through a maze which included a series of obstacles, while their visual input was restricted to information from a pixelized vision simulator. Walking speed and number of body contacts with obstacles and walls were measured as a function of pixel number, pixel spacing, object minification, and field of view. The results indicate that a 25 x 25 array of pixels distributed within the foveal visual area could provide useful visually guided mobility in environments not requiring a high degree of pattern recognition.

A multielectrode array for intrafascicular recording and stimulation in sciatic nerve of cats.

The feasibility of implanting an array of penetrating electrodes into peripheral nerves is studied in acute experiments in the cat sciatic nerve. A novel, silicon-based array of microelectrodes, the Utah Electrode Array, was used, which contains 25 or 100 1-mm long electrodes that project out from a silicon substrate. Electrode arrays of this complexity, when inserted in the peripheral nerve, could cause significant compression of the nerve and block the conduction of action potentials. Using a high velocity insertion technique, the electrode array was implanted into the sciatic nerve. Compound action potentials were evoked by and recorded with cuff electrodes. Compound action potentials recorded 1 h after insertion were only slightly altered from those recorded before insertion. Single units were readily extracted from evoked multiunit neural recordings in response to cutaneous stimulation and limb rotation around joints. Current injections into the nerve through the electrodes evoked muscle twitches with currents in the 10 microA range. Recording and stimulation stability were demonstrated for periods of up to 60 h. We have shown that high density arrays of electrodes can be inserted into the peripheral nerve and can provide a stable recording and stimulating interface to individual peripheral nerve axons. Such an array may be useful in future neuroscience research and potential neuroprosthetic applications.