Implantation and testing of WFMA stimulators in macaque.

Our on-going work to develop an intracortical visual prosthesis has motivated the design, fabrication, and testing of a Wireless Floating Microelectrode Array (WFMA) stimulator. This implantable device can be used for an electrical stimulation interface in the peripheral and the central nervous system. Previously, its use in a sciatic nerve rodent model was described. Here implantation of two WFMAs in motor cortex of two NHP (macaque - two devices/animal) is presented. Preliminary functional tests show the implanted devices to be fully functional with stimulation-induced motor movements obtained. Functional testing is on-going.

Accelerated-stress reliability evaluation for an encapsulated wireless cortical stimulator.

In preparing a wireless cortical stimulator for use in the Intracortical Visual Prosthesis (ICVP) project at the Illinois Institute of Technology (IIT), an accelerated environmental stress test is being performed on prototype stimulator modules. Stimulator devices, containing a custom application specific integrated circuit (ASIC), and encapsulated with PDMS, were soaked in an autoclave chamber at 121°C and 100% relative humidity for more than 2200 hours with and without power supplied to the ASIC. Experimental results showed no physical degradation of the stimulator devices after soaking. Reverse telemetry that measures the stimulator internal power supply, recorded periodically over the entire test time, verified that the devices were electrically functioning, as designed, without deterioration. Taking into consideration other standard reliability test environments, the accelerated moisture resistance-biased autoclave testing duration of 2200 hours, as conducted in this study, overwhelms other less-severe test conditions and demonstrates long term stability of the proposed vision prosthesis device with proven thermo-mechanical and electrical robustness.

Identification and quantification of electrical leakage pathways in floating microelectrode arrays.

The long-term reliability of neural recording and stimulation electrode arrays is becoming the limiting factor for neural interfaces. For effective electrode design, electrical connection to the surrounding neural tissue and fluid should be limited to the electrode tips, with all other leakage currents minimized. It is the goal of this study to identify and quantify electrical leakage within commercially available floating microelectrode arrays (FMAs). Both short term and accelerated stress tests were performed on entire FMAs, as well as on individual electrodes typical of such arrays. Preliminary results of these tests indicate that leakage currents are present due to water penetration of their insulation layer initially, but that prolonged water exposure at high temperature may seal the defects that cause these currents. SEM photos taken of the electrode shafts show extensive defect regions that may correlate with the test data.

An implantable neural stimulator for intraspinal microstimulation.

This paper reports on a wireless stimulator device for use in animal experiments as part of an ongoing investigation into intraspinal stimulation (ISMS) for restoration of walking in humans with spinal cord injury. The principle behind using ISMS is the activation of residual motor-control neural networks within the spinal cord ventral horn below the level of lesion following a spinal cord injury. The attractiveness to this technique is that a small number of electrodes can be used to induce bilateral walking patterns in the lower limbs. In combination with advanced feedback algorithms, ISMS has the potential to restore walking for distances that exceed that produced by other types of functional electrical stimulation. Recent acute animal experiments have demonstrated the feasibility of using ISMS to produce the coordinated walking patterns. Here we described a wireless implantable stimulation system to be used in chronic animal experiments and for providing the basis for a system suitable for use in humans. Electrical operation of the wireless system is described, including a demonstration of reverse telemetry for monitoring the stimulating electrode voltages.

Intrinsic activation of iridium electrodes over a wireless link.

Activated Iridium Oxide Film (AIROF) microelectrodes are regarded as advantage for stimulation of neural tissue owing to their superior charge injection capabilities, as compared to other noble-metal based electrodes. Including AIROF electrodes within an implantable neural stimulator can be challenging since the stimulator fabrication steps often involve elevated temperatures at which the AIROF can be damaged. In this work, a wireless neural stimulator application-specific-integrated-circuit (ASIC) was used to intrinsically activate iridium microelectrodes. This intrinsic activation allows for the growth of the AIROF as the final assembly step after the entire device is assembled, thus avoiding stress on the AIROF. Since a typical neural stimulator is essentially a current-controlled driver with voltage compliance limits, its output waveform can be tuned to match the traditional voltage pulsing/ramp activation waveform. Here the feasibility of the current driven activation of iridium electrodes, over a wireless link, is demonstrated.