David J. Edell

Factors influencing the biocompatibility of insertable silicon microshafts in cerebral cortex

Issues that determine the biocompatibility of insertable microelectrode arrays were investigated. Results from a limited number of tests indicated that there was minimal tissue response along the sides of the shafts when shafts were well sharpened, had sufficiently small tip angles, and were clean. Tissue was usually more reactive at the tips of the shafts. It was concluded that silicon microshafts of appropriate shaft and tip design were biocompatible along the sides of the shaft, but that relatively severe reactions could be anticipated at the tips.<<ETX>>

A Peripheral Nerve Information Transducer for Amputees: Long-Term Multichannel Recordings from Rabbit Peripheral Nerves

A micromachined silicon technology is being developed for the purpose of sensing information from the stumps of amputated mammalian peripheral nerves. Information on long-term biocompatibility, anatomy, and physiology related to the structural design of this sensor is presented. Pertinent materials, fabrication, and surgical implantation issues are discussed. Noise, signal amplitude, and receptive field are considered as the prime determinants of the design of an appropriate electrode contact geometry for the structure. The selectivity of the device is also discussed in terms of the fine structure of the regenerated nerve. Examples of waveforms recorded from rabbit peripheral nerves using this sensor are presented and discussed in terms of electrical and physiological parameters. Possible use of the sensor as a source for a binary code suitable for communication of information to a computer is presented along with discussion of the limitations of the current technology and possible future applications.

Growth of fluorocarbon polymer thin films with high CF2 fractions and low dangling bond concentrations by thermal chemical vapor deposition

Thermal chemical vapor deposition was used to deposit fluorocarbon films with chemical resemblance to bulk polytetrafluoroethylene. X‐ray photoelectron spectroscopy revealed that the films deposited from thermal decomposition of hexafluoropropylene oxide had fluorine to carbon ratios of 2.0 and CF2 fractions of 90% along with 10% of CF3 and CF moieties. Electron spin resonance results found the dangling bond density to be 1.2×1018 spins/cm3, low compared to conventional plasma polymerized films. Low dangling bond densities were achieved by using a clean source of CF2 in the absence of plasma source.

FLEXIBLE FLUOROCARBON WIRE COATINGS BY PULSED PLASMA ENHANCED CHEMICAL VAPOR DEPOSITION

A method to coat thin wires, 25 μm in diameter, with fluorocarbon material that is flexible and conformal has been achieved using pulsed plasma enhanced chemical vapor deposition (PECVD). This process enables the chemical composition of the films to be tailored in order to achieve similar stoichiometry and chemical composition to bulk polytetrafluoroethylene [PTFE, (CF2)n, Teflon™]. The deposited film had a F/C ratio of 1.9 and a CF2 fraction of 65%. In comparison, a film deposited under continuously applied power had a F/C ratio of only 1.6 and a CF2 fraction of 32% and was found to form brittle wire coatings. The flexibility of the pulsed PECVD film was consistent with its connectivity number of 2.1 corresponding to an underconstrained, flexible material. In addition, the connectivity number for the brittle film was 2.5, consistent with an overconstrained brittle network.

Pulsed plasma-enhanced chemical vapor deposition from hexafluoropropylene oxide: Film composition study

Films deposited using pulsed plasma-enhanced chemical vapor deposition (PECVD) from hexafluoropropylene oxide (HFPO) were investigated by X-ray photoelectron spectroscopy (XPS). As compared to continuous rf PECVD, pulsed excitation increases the CF2 fraction in the film. Film composition was determined as a function of plasma processing conditions including on-time, off-time, pressure, flow rate, substrate temperature, electrode spacing, substrate potential, and input power. Varying the on–off pulsing cycle resulted in compositional control of the deposited films. At a low duty cycle [ton/(ton + toff)], up to 70% CF2 could be incorporated into the film. The input gas, HFPO, may facilitate greater CF2 incorporation into the films as this gas thermally decomposes into a difluorocarbene. Both absolute on-time and off-time, rather than simply duty cycle, are important parameters for determining film composition. A simple model was developed to describe the experimentally determined variation %CF2 as a function of substrate temperature and off-time. This model accounts for changes in film composition due to plasma-surface modification and differences in gas-phase chemistry. The model suggests that surface modification by the plasma is the dominant factor only for long on-times or for low deposition rates. However, the gas-phase concentration of CF2 relative to other film-forming species is typically the controlling factor under conditions which achieve the high %CF2 in the film. The gas-phase composition will depend on both abslute on-time and off-time, rather than simply on the duty cycle.

Molecular Design of Fluorocarbon Film Architecture by Pulsed Plasma Enhanced and Pyrolytic Chemical Vapor Deposition

Pulsed plasma enhanced chemical vapor deposition (pulsed PECVD) and pyrolytic chemical vapor deposition (pyrolyric CVD) of fluorocarbon films from hexafluoropropylene oxide (HFPO) have demonstrated the ability to molecularly design film architecture. Film structures ranging from highly amorphous crosslinked matrices to linear perfluoroalkyl chain crystallites can be established by reducing the modulation frequency of plasma discharge in plasma activated deposition and by eventually shifting mechanistically from an electrically activated to a thermally activated process. X-ray photoelectron spectroscopy (XPS) showed CF2 content increasing from 39–65 mol%. Fourier transform infrared spectroscopy (FTIR) showed an increasing resolution between the symmetric and asymmetric CF2 stretches, and a reduction in the intensity of the amorphous PTFE and CF3 bands. High-resolution solid-state 19F nuclear magnetic resonance spectroscopy (NMR) revealed an increasing CF2CF2CF2 character, with the pyrolytic CVD film much like bulk poly(tetrafluoroethylene) (PTFE). X-ray diffraction (XRD) patterns evidenced an increase in crystallinity, with the pyrolytic CVD film showing a characteristic peak at 2θ = 18° representing the (100) plane of the hexagonal structure of crystalline PTFE above 19°C.

Cesium hydroxide (CsOH): a useful etchant for micromachining silicon

The CsOH-H/sub 2/O etchant system was studied over a range of concentrations (10%-76% by weight) and temperatures (25-90 degrees C). The etch rates of

Recording of Electric Signal Passing Through a Pylon in Direct Skeletal Attachment of Leg Prostheses With Neuromuscular Control

Direct recordings were made of electrical signals emanating from the muscles in a rabbits residuum. The signals were transmitted via wires attached on one end to the muscles, and on the other to an external recording system. The cable was held in a titanium tube inside a pylon that had been transcutaneously implanted into the residuums bone. The tube was surrounded by porous titanium cladding to enhance its bond with the bone and with the skin of the residuum. This study was the first known attempt to merge the technology of direct skeletal attachment of limb prostheses with the technology of neuromuscular control of prostheses, providing a safe and reliable passage of the electrical signal from the muscles inside the residuum to the outside recording system.

Chronic intraneural electrical stimulation for prosthetic sensory feedback

The functionality of prosthetic limbs is restricted by the limited availability of sensory feedback. The goal of the present research is the development of a multichannel microelectrode array for the presentation of sensory information directly to the sensory afferent neurons of the transected peripheral nerve of an amputee. Intraneural electrode arrays were developed and implanted in the proximal stump of the transected sciatic nerve in 18 New Zealand White rabbits and monitored for chronic functionality. Several novel nerve regeneration implant designs employing axially oriented Platinum/Iridium wire arrays and regeneration tubes were explored. Somatosensory evoked potentials were monitored via chronically implanted epidural electrodes. A behavioral technique was used to monitor stimulus perception; an electrical stimulus was delivered to the implanted nerve followed by an airpuff to the eye with monitoring of the resultant eye-blink reflex. In this classical conditioning technique, the rabbits associate the two stimuli such that electrical stimulation of the sciatic nerve without an airpuff elicits an eye-blink. Neurophysiological and behavioral tests have demonstrated chronic single channel electrode functionality for greater than 2 years in implanted animals. This research supports the possible viability of an implanted neuroelectric interface in providing chronic sensory feedback in humans.

Chronically implantable neural information transducers

Chronically implantable neural information transducers that interface extracellularly with nearby neurons are currently being developed for applications in rehabilitation. These devices must be both bioresistant and biocompatible. In addition to materials biocompatibility, mechanical issues are also highly important for long term applications. Connective tissue eventually invests all physical objects implanted in living tissue. Interconnects between devices or between devices and connectors that have become invested with connective tissue translate relative motion of tissues to the interconnects and devices, damaging the fragile neural structures and the implant itself. Exchanging data and power using micro-telemetry may be a solution to this persistent problem. This presentation provides an overview of significant problems with neuroprosthetic micro-devices and possible solutions to the mechanical issues using optical micro-telemetry of data and power.