A multichannel neural probe for selective chemical delivery at thecellular level

A bulk-micromachined multichannel silicon probe capable of selectively delivering chemicals at the cellular level as well as electrically recording from and stimulating neurons in vivo has been developed. The process buries multiple flow channels in the probe substrate, resulting in a hollow-core device, Microchannel formation requires only one mask in addition to those normally used for probe fabrication and is compatible with on-chip signal-processing circuitry. Flow in these microchannels has been studied theoretically and experimentally. For an effective channel diameter of 10 μm, a channel length of 4 mm, and water as the injected fluid, the flow velocity at 11 torr is about 1.3 mm/s, delivering 100 pl in 1 s. Intermixing of chemicals, with the tissue fluid due to natural diffusion through the outlet orifice becomes significant for dwell times in excess of about 30 min, and a shutter is proposed for chronic use. The probe has been used for acute monitoring of the neural responses to various chemical stimuli in guinea pig superior and inferior colliculus     

Silicon-substrate intracortical microelectrode arrays for long-term recording of neuronal spike activity in cerebral cortex

This study investigated the use of planar, silicon-substrate microelectrodes for chronic unit recording in the cerebral cortex. The 16-channel microelectrodes consisted of four penetrating shanks with four recording sites on each shank. The chronic electrode assembly included an integrated silicon ribbon cable and percutaneous connector. In a consecutive series of six rats, 5/6 (83%) of the implanted microelectrodes recorded neuronal spike activity for more than six weeks, with four of the implants (66%) remaining functional for more than 28 weeks. In each animal, more than 80% of the electrode sites recorded spike activity over sequential recording sessions during the postoperative time period. These results provide a performance baseline to support further electrode system development for intracortical neural implant systems for medical applications.     

Strength characterization of silicon microprobes inneurophysiological tissues

Experimentally determined strength characteristics of thin-silicon probes in neural tissues are discussed. It is shown that by proper selection of the substrate length, width, and thickness, silicon substrates can be designed and used to penetrate a variety of biological tissues without breakage or excessive dimpling. Thin-silicon structures have a maximum fracture stress which is a factor of six larger than that of bulk silicon and are very flexible and capable of bending to angles larger than 90°. Silicon substrates 15 μm thick×30 μm wide can easily penetrate guinea pig and rat pia arachnoid layers with minimum dimpling and no breakage, while substrates 30 μm thick×80 μm wide can penetrate guinea pig and rat dura mater repeatedly without breakage. Quantitative comparison on the relative toughness of neurophysiological tissues in rat and guinea pig have also been experimentally obtained     

Chronic neural recording using silicon-substrate microelectrode arrays implanted in cerebral cortex

An important aspect of the development of cortical prostheses is the enhancement of suitable implantable microelectrode arrays for chronic neural recording. The objective of this study was to investigate the recording performance of silicon-substrate micromachined probes in terms of reliability and signal quality. These probes were found to consistently and reliably provide high-quality spike recordings over extended periods of time lasting up to 127 days. In a consecutive series of ten rodents involving 14 implanted probes, 13/14 (93%) of the devices remained functional throughout the assessment period. More than 90% of the probe sites consistently recorded spike activity with signal-to-noise ratios sufficient for amplitudes and waveform-based discrimination. Histological analysis of the tissue surrounding the probes generally indicated the development of a stable interface sufficient for sustained electrical contact. The results of this study demonstrate that these planar silicon probes are suitable for long-term recording in the cerebral cortex and provide an effective platform technology foundation for microscale intracortical neural interfaces for use in humans.     

Realization of Ultra Fine Pitch Traces on LCP Substrates

Liquid crystal polymer (LCP) is being utilized in a variety of applications that could benefit from the realization of ultra fine pitch conductive traces on the surface of the LCP substrate. LCP has unique chemical properties that not only benefit the intended application, but can also be exploited to realize ultra fine pitch traces. Through a series of experiments, it was discovered that a thin film of Ti can form TiC with the surface of an Ar ion ablated LCP substrate. This layer then provides sufficient adhesion for other metals to be deposited so that ultra fine pitch traces can be patterned. Using this process, 10 mum wide traces on a 20 mum pitch with high yield were demonstrated and analyzed. Smaller feature size test structures were also demonstrated.     

Silicon ribbon cables for chronically implantable microelectrodearrays

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 μm thick, 1-5 cm long, and from 60 to 250 μm wide have been fabricated with up to eight leads. The series lead resistance is typically 4 kΩ/cm for polysilicon and 500 Ω/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 gear in the guinea pig CNS, recording driven single-unit activity and maintaining impedance levels in the 1-7 M Ω range     

Batch fabricated thin-film electrodes for stimulation of thecentral auditory system

Silicon micromachining and thin-film technology have been employed to fabricate iridium stimulating arrays which can be used to excite discrete volumes of the central nervous system. Silicon multichannel probes with thicknesses ranging from 1 to 40 microns and arbitrary two-dimensional shapes can be fabricated using a high-yield, circuit-compatible process. Iridium stimulating sites are shown to have similar characteristics to iridium wire electrodes. Accelerated pulse testing with over 8 million 100 microA biphasic current pulses on 8000 microns 2 sites has demonstrated the long-term stability of iridium and activated iridium sites. In vivo tests have been performed in the central auditory pathways to demonstrate neural activation using the devices. These tests show a selective activation both as a function of site separation and site size.