David Daomin Zhou

Poly (3,4-Ethylenedioxythiophene) for Chronic Neural Stimulation

Chronic neural stimulation using microelectrode arrays requires highly stable and biocompatible electrode materials with high charge injection capability. Conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT) was electrochemically deposited on thin film Pt electrodes of stimulation electrode arrays to evaluate its properties for chronic stimulation. The coated electrodes demonstrated much lower impedance than thin film Pt due to the high surface area and high ion conductivity across the film. The PEDOT film also presents intrinsic redox activity which contributes to the low impedance as well as a much higher charge storage capacity. The charge injection limit of PEDOT electrode was found to be 2.3 mC/cm2 , comparable to IrOx and much higher than thin film Pt. Under biphasic stimulation, the coated electrodes exhibited lower voltage and linear voltage excursion. Well-coated PEDOT electrodes were stable under chronic stimulation conditions, suggesting that PEDOT is a promising electrode material to be further developed for chronic neural stimulation applications.

Highly stable carbon nanotube doped poly(3,4-ethylenedioxythiophene) for chronic neural stimulation

The function and longevity of implantable microelectrodes for chronic neural stimulation depends heavily on the electrode materials, which need to present high charge injection capability and high stability. While conducting polymers have been coated on neural microelectrodes and shown promising properties for chronic stimulation, their practical applications have been limited due to unsatisfying stability. Here, poly(3,4-ethylenedioxythiophene) (PEDOT) doped with pure carbon nanotubes (CNTs) was electrochemically deposited on Pt microelectrodes to evaluate its properties for chronic stimulation. The PEDOT/CNT coated microelectrodes demonstrated much lower impedance than the bare Pt, and the PEDOT/CNT film exhibited excellent stability. For both acute and chronic stimulation tests, there is no significant increase in the impedance of the PEDOT/CNT coated microelectrodes, and none of the PEDOT/CNT films show any cracks or delamination, which have been the limitation for many conducting polymer coatings on neural electrodes. The charge injection limit of the Pt microelectrode was significantly increased to 2.5 mC/cm(2) with the PEDOT/CNT coating. Further in vitro experiments also showed that the PEDOT/CNT coatings are non-toxic and support the growth of neurons. It is expected that this highly stable PEDOT/CNT composite may serve as excellent new material for neural electrodes.

Electrical properties of retinal–electrode interface

A critical element of a retinal prosthesis is the stimulating electrode array, which is placed in close proximity to the retina. It is via this retinal-electrode interface that a retinal prosthesis electrically stimulates nerve cells to produce the perception of light. The impedance load seen by the current driver consists of the tissue resistance and the complex electrode impedance. The results in this paper show that the tissue resistance of the retina is significantly greater than that of the vitreous humor in the eye. Circuit models of the electrode-retina interface are used to parameterize the different contributors to the overall impedance.

Microsensors and microbiosensors for retinal implants.

This paper concentrates on recent developments in microsensors and microbiosensors for the possible applications in visual prostheses, especially retinal prosthetic devices. A brief introduction on the developments of visual prosthesis will be presented. The importance for in-vivo pH measurements as well as the need for an implantable pH sensor will be demonstrated. Electrochemical biosensors developed for sensitive measurements of glucose and L-glutamate, a known neurotransmitter in the retina and brain will be reviewed. Novel electrode materials such as chemically modified thin-film diamond in applications for implantable biosensors will be shown. The challenges in the development of chronic implantable sensor systems, especially using MEMS technology for medical implants, will be discussed.

Micromachined electrodes for high density neural stimulation systems

High-resolution retinal prosthetic systems require high-density stimulation electrodes to restore the vision of blind patients suffering from retinitis pigmentosa and macular degeneration. Micromachining the surface of electrodes to obtain high aspect ratio features can tremendously increase the effective area of the electrode. A unique process has been developed to obtain high aspect ratio structures by plating on the sidewall of recently developed and insufficiently rinsed resist. A quadrupling of the effective area of the electrode surface has been achieved with modest micromachining and should translate into a similar improvement in visual resolution.

The Argus® II retinal prosthesis system: An overview

The Argus II Retinal Prosthesis System is the only commercially-approved treatment for severe to profound Retinitis Pigmentosa in the world. Over 10 years of research and development have gone into this device; it is now commercially available in the European Economic Area, Saudi Arabia, and in the United States. The Argus II System consists of an implant (including an epiretinal electrode array, an electronics case, and a receiver antenna), as well as a body-worn computer and a pair of glasses with a miniature video camera. It converts video images from the camera into stimulation patterns that are transmitted to the implant in real time, allowing users to perceive patterns of light. All aspects of the System have been extensively bench tested and found to provide long-term reliability. These findings have been borne out in a clinical trial; in over 120 subject-years of testing, there has been only one device failure. Clinical trial results have established the safety and probable benefit of the Argus II System.

Micromachined electrodes for retinal prostheses

High-resolution retinal prosthetic systems need high-density stimulation electrodes to restore the vision of blind patients suffering from retinitis pigmentosa or macular degeneration. Micromachining electrodes to obtain high-aspect-ratio features can increase the area of the electrodes. A process has been developed to obtain high-aspect-ratio microstructures by plating on the sidewall of recently developed photoresist. A quadrupling of the effective area of the electrode surface has been achieved with modest micromachining and could translate into an improvement in visual resolution. A remaining issue is reducing irreversible electrochemical reactions at the electrode.

Microelectronic Visual Prostheses

Research efforts worldwide are developing microelectronic visual prostheses aimed at restoring vision for the blind. Various visual prostheses using neural stimulation techniques targeting different locations along the visual pathway are being pursued. Retinal prostheses have proved to be capable of offering blind subjects in advanced stages of outer retinal diseases the opportunity to regain some visual function. With relatively low-density retinal implants, simple visual tasks that are impossible with the blind subject’s natural light perception vision can be accomplished. Blind subjects can spatially resolve individual electrodes within the array of the implanted retinal prosthesis and can use the system to discriminate and identify oriented patterns. This chapter reviews progress in the development of visual prostheses including visual cortex and optic nerve stimulation devices and retina stimulation devices such as epiretinal, subretinal, and extraocular implants. Second Sight Argus 16 and Argus II 60-electrode Retinal Implants are described. Some engineering challenges for the development of visual prostheses, especially retinal prostheses, are discussed.

Conducting Polymers in Neural Stimulation Applications

With advances in neural prostheses, the demand for high-resolution and site-specific stimulation is driving microelectrode research to develop electrodes that are much smaller in area and longer in lifetime. For such arrays, the choice of electrode material has become increasingly important. Currently, most neural stimulation devices use platinum, iridium oxide, or titanium nitride electrodes. Although those metal electrodes have low electrode impedance, high charge injection capability, and high corrosion resistance, the neural interface between solid metal and soft tissue has undesilable characteristics. Recently, several conducting polymers, also known as inherently conducting polymers (ICPs), have been explored as new electrode materials for neural interfaces. Polypyrrole (PPy), polyaniline (PANi), and poly(3,4-ethylenedioxythiophene) (PEDOT) polymers may offer the organic, improved bionic interface that is necessary to promote biocompatibility in neural stimulation applications. While conducting polymers hold much promise in biomedical applications, more research is needed to further understand the properties of these materials. Factors such as electrode impedance, polymer volume changes under electrical stimulation, charge injection capability, biocompatibility, and long-term stability are of significant importance and may pose as challenges in the future success of conducting polymers in biomedical applications.

Pulse-Clamp Technique for Characterizing Neural-Stimulating Electrodes

A reliable test methsod is required to ensure adequate charge-injection capability for high-resolution neural-stimulation applications that demand both a large amount of charge injection and a small electrode size. A pulse-clamp circuit is designed and employed to characterize the electrode charge-storage capability that will allow different electrodes to be quickly and accurately compared. The custom circuit, which consists of commercial components, has a switching charge noise of 15 pC and a switching time of 1 μs. Pulse-clamp measurements are performed on flat platinum electrodes in 0.15 M saline solution in air with 1 ms long pulses at a charge density of up to 1 mC/cm 2 . Results indicate a safe charge-injection limit of 0.1 mC/cm 2 , and a hydrolysis-dominated regime above 0.5 mC/cm 2 . Comparing different electrode sizes indicates that the charge-storage capability of an electrode is proportional to its surface area. The scalability of the pulse-clamp technique allows it to be used to accurately quantify the roughness of a surface modification. Simulation program with integrated circuits emphasis simulation is used to extract the electrode capacitance and resistance parameters. These parameters, which can vary, depend strongly on the injected charge density.