High-Density Flexible Parylene-Based Multielectrode Arrays for Retinal and Spinal Cord Stimulation

Abstract

Novel flexible parylene-based high-density electrode arrays have been developed for functional electrical stimulation in retinal and spinal cord applications. These electrode arrays are microfabricated according to single-metal-layer and, most recently, dual-metal-layer processes. A new heat-molding process has been implemented to conform electrode arrays to approximate the curvature of canine retinas, and chronic implantation studies have been undertaken to study the mechanical effects of parylene-based prostheses on the retina, with excellent results to date. Electrode arrays have also been implanted and tested on the spinal cords of murine models, with the ultimate goal of facilitation of locomotion after spinal cord injury; these arrays provide a higher density and better spatial control of stimulation and recording than is typically possible using traditional fine-wire electrodes. Spinal cord stimulation typically elicited three muscle responses, an early (direct), a middle (monosynaptic), and a late (polysynaptic) response, classified based on latency after stimulation. Stimulation at different rostrocaudal levels of the cord yielded markedly different muscle responses, highlighting the need for such high-density arrays.

Mechanical Properties of Thin-Film Parylene–Metal–Parylene Devices

“…The effect of bending stress on thin-film devices can be assessed by wrapping films around cylinders of specific diameters ranging from hundreds of microns to several millimeters and then either measuring the resistance across the traces or the testing the performance (Ai et al, 2008;Chen et al, 2010;Kim et al, 2010;Lacour et al, 2010;Ochoa et al, 2013). Alternatively, films can be wrapped around spheres (Ochoa et al, 2013;Rodger et al, 2007) and inflatable balloons (Kim et al, 2011). Some devices are self-curling and were shown to maintain function while being bent 90° (Takeuchi et al, 2004) or down to a radius of 2 mm or less (Johnson and Wise, 2012;Yamagiwa et al, 2013).…”

“…Many different thin-film devices have been demonstrated to be used to stimulate and record from many electrically active tissues such as the spinal cord (Rodger et al, 2007), heart Zhao et al, 2013), muscle (Spence et al, 2007), and cortex Tolstosheeva et al, 2011). Although these studies have demonstrated the performance of the devices for short periods of time for their given application, there have not been comprehensive methods to test the mechanical properties of such thin-film architectures until failure.…”

“…These devices are thin enough to be rolled up and delivered via catheter (Fonseca et al, 2006) or incorporated into the catheter itself (Li et al, 2008a). Thin-film polymeric devices for cochlear stimulation (Johnson and Wise, 2010), spinal cord stimulation (Rodger et al, 2007) as well as electrocardiography (ECG) Yu et al, 2012), electrocorticography (ECOG) (Toda et al, 2011), and intracortical neural recordings have been demonstrated.…”

An Active Thin-Film Cochlear Electrode Array With Monolithic Backing and Curl

“…With the introduction of multielectrodes, investigations began on the effectiveness of multipolar stimulation from one channel [22]. Both bipolar and tripolar configurations have been shown to reduce current spreading, with tripolar achieving the highest degree of selectivity [23][24][25] at the cost of higher power consumption.…”

Implantable Biomedical Devices

“…In 1992, Boppart presented a first flexible electrode and used it to measure evoked potentials in brain slices [66]. The research in this field then followed in more advanced designs by Humayun and Weiland's visual prosthesis team in dual-metallayer processes [67,68]. Recently, another approach for flexible implantable electrode has been presented using ITO/PET (Indium Tin Oxide/PolyEthylene Terephthalate) by ICAS visual prosthesis group [69].…”