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A micromachined silicon sieve electrode has been developed and fabricated to record from and stimulate axons/fibers of the peripheral nervous system by utilizing the nerve regeneration principle. The electrode consists of a 15-microns-thick silicon support rim, a 4-microns-thick diaphragm containing different size holes to allow nerve regeneration, thin-film iridium recording/stimulating sites, and an integrated silicon ribbon cable, all fabricated using boron etch-step and silicon micromachining techniques. The thin diaphragm is patterned using reactive ion etching to obtain different size holes with diameters as small as 1 micron and center-center spacings as small as 10 microns. The holes are surrounded by 100-200 microns 2 anodized iridium oxide sites, which can be used for both recording and stimulation. These sites have impedances of less than 100 k omega @ 1 kHz and charge delivery capacities in the 4-6 mC/cm2 range. The fabrication process is single-sided, has high yield, requires only five masks, and is compatible with integrated multilead silicon ribbon cables. The electrodes were implanted between the cut ends of peripheral taste fibers of rats (glossopharyngeal nerve), and axons functionally regenerated through holes, responding to chemical, mechanical, and thermal stimuli.

This paper describes the design, fabrication, and testing of miniature ultraflexible ribbon cables for use with micromachined silicon microprobes capable of chronic recording and/or stimulation in the central nervous system (CNS). These interconnects are of critical importance in reliably linking these microelectrodes to the external world through a percutaneous connector. The silicon cables allow the realization of multilead, multistrand shielded local interconnects that are extremely flexible and yet strong enough to withstand normal handling and surgical manipulation. Cables 5 microns thick, 1-5 cm long, and from 60 to 250 microns wide have been fabricated with up to eight leads. The series lead resistance is typically 4 k omega/cm for polysilicon and 500 omega/cm for tantalum, with shunt capacitance values of 5-10 pF/cm and an interlead capacitance below 10 fF/cm. Soak tests in buffered saline performed under electrical and mechanical stress have been underway for over three years and show subpicoampere leakage levels. Silicon microprobes with built-in ribbon cables have remained functional for up to one year in the guinea pig CNS, recording driven single-unit activity and maintaining impedance levels in the 1-7 M omega range.

This paper is devoted to a detailed characterization of the neuron-to-microtransducer junction, based on the equivalent electric-circuit approach. As a result, recording of action potentials can be simulated with the general-purpose network-analysis program SPICE. Both noble-metal microelectrodes and insulated-gate FET's are considered. The responses of such devices are characterized as functions of several parameters, e.g., sealing impedance, density of ionic currents in the cell membrane, and spatial discontinuities of the adhesion process. It is shown that the various signal shapes reported in the literature can be reproduced and interpreted in terms of time derivatives of the action potential. In this way, the shape of any experimental signal can be interpreted on the basis of a specific sealing condition. Possible future improvements in microtransducer design, based on the proposed approach, are also suggested.

A microelectrode array capable of recording from and stimulating peripheral nerves at prolonged intervals after surgical implantation has been demonstrated. The microelectrode array, fabricated on a silicon substrate perforated by multiple holes (referred to as via holes), is implanted between the ends of a surgically severed nerve. Regenerating tissue fixes the device in place to provide a stable mapping between the microelectrodes and the axons in the nerve. Processes were developed for the fabrication of thin-film iridium microelectrodes, micromachined via holes, and silicon nitride passivation layers. All fabrication methods were designed to be compatible with standard CMOS/BiCMOS processes to allow for on-chip signal processing circuits in future designs. Such arrays, implanted in the peroneal nerves of rats, were used to record from and stimulate the nerves at up to 13 months postoperatively.

A flexible and perforated 32-element planar microelectrode array has been fabricated and used to measure evoked potentials in brain slices. Electrodes are spaced 200 microns apart in a 4 x 8 array and are sandwiched between layers of insulating polyimide. The polyimide sandwich is lifted off its substrate, making it flexible so that it could shape to contoured tissues. Prior to lift off, holes are etched to expose recording sites 15 microns in diameter and to create perforations which allow increased circulation of artificial cerebrospinal fluid to the recording surface of the tissue and, hence, increased viability. Comparisons of evoked potentials measured over time showed an average increase of 10 h to the viability of the slice while using the perforated versus nonperforated arrays.

Using a mix of thermal and anodic bonding together with microlithographic techniques, the safe transference of a Si3N4 film with a pore (diameter down to 1 micron) to a glass tube tip (external diameter 800 microns) was accomplished, yielding a new geometrical form of micropipette. Compared with conventional glass micropipettes the device has shown lower resistance, more stable capacitance (independent of the tip immersion depth), tip potential closer to that of a salt bridge, and a simplified filling process. Using this device as a potassium ion selective electrode (ISE), a faster response time ISE was achieved. These features indicate that the new device can advantageously substitute the conventional glass micropipettes when cell impalement is not required.