A 64-site multiplexed low-profile neural probe for use in neural prostheses.

Abstract

The use of electrical stimulation in the central nervous system (CNS) to restore physiological functions such as limb control, sensation of sound, and perception of light spots has been a research goal for several decades. However, to form a useful and practical prosthesis, thin-film multisite multiplexed probes must be developed to allow the stimulation of many sites within a selected volume of neural tissue while providing precise control over stimulus amplitudes, position, and pulse timing. This thesis research has demonstrated the feasibility of a 64-site 16-shank probe capable of stimulating over a $\pm$127$\mu$A biphasic current range with an amplitude resolution of 1$\mu$A with a clock frequency of 2.5MHz at $\pm$5V supplies. The probe has 8 active parallel channels, operates over five external leads, and is compatible with use alone or in three-dimensional arrays of up to 256 such probes. This probe contains enhanced low-power circuitry, limiting the overall dissipation to less than 50$\mu$W when idle. The probe features an expanded mode set and the ability to record as well as stimulate from any channel on demand. A micromachined CMOS process has been developed for probe fabrication. This process is optimized for operation from $\pm$5V supplies but permits operation at voltages up to $\pm$9.3V in situations where high tissue back voltages are generated due to small stimulating sites or high stimulus currents. Such back voltages have been studied in detail and can reach more than 10V for a 100$\mu$A current driven through a 400$\mu$m$\sp2$ site. A fully complementary BiCMOS process has also been demonstrated for implementations allowing low-voltage operation. The associated circuitry can operate at voltages as low as $\pm$1.3V (with a back voltage of 1V) while providing a full range of probe functions. The research has demonstrated advances in the control of probe shape and the understanding of current flow from such structures. Silicon ribbon cables are used between the penetrating probe shanks and a circuit platform that folds over to rest on the cortical surface. This limits the vertical space of the probe above the cortex to less than 1 mm to prevent tissue overgrowth and anchoring of the probe to the cranium. The use of double-boron-diffused tips has also been introduced to permit probe tip diameters as small as 2$\mu$m. Technology for producing patterned and double-sided sites has been developed, and the electrical profiles and current distributions over such sites have been determined using computer simulations. These probes promise to allow significant advances in the study of neural structures and in facilitating experiments on the multipoint neural stimulation that will be required in next-generation prosthetic devices. The thesis thus sets the stage for the practical realization of neural prostheses for the auditory and perhaps visual systems in the coming decade.