Saugandhika Minnikanti

Lifetime assessment of atomic-layer-deposited Al2O3-Parylene C bilayer coating for neural interfaces using accelerated age testing and electrochemical characterization

The lifetime and stability of insulation are critical features for the reliable operation of an implantable neural interface device. A critical factor for an implanted insulations performance is its barrier properties that limit access of biological fluids to the underlying device or metal electrode. Parylene C is a material that has been used in FDA-approved implantable devices. Considered a biocompatible polymer with barrier properties, it has been used as a substrate, insulation or an encapsulation for neural implant technology. Recently, it has been suggested that a bilayer coating of Parylene C on top of atomic-layer-deposited Al2O3 would provide enhanced barrier properties. Here we report a comprehensive study to examine the mean time to failure of Parylene C and Al2O3-Parylene C coated devices using accelerated lifetime testing. Samples were tested at 60°C for up to 3 months while performing electrochemical measurements to characterize the integrity of the insulation. The mean time to failure for Al2O3-Parylene C was 4.6 times longer than Parylene C coated samples. In addition, based on modeling of the data using electrical circuit equivalents, we show here that there are two main modes of failure. Our results suggest that failure of the insulating layer is due to pore formation or blistering as well as thinning of the coating over time. The enhanced barrier properties of the bilayer Al2O3-Parylene C over Parylene C makes it a promising candidate as an encapsulating neural interface.

In vivo electrochemical characterization and inflammatory response of multiwalled carbon nanotube-based electrodes in rat hippocampus

Stimulating neural electrodes are required to deliver charge to an environment that presents itself as hostile. The electrodes need to maintain their electrical characteristics (charge and impedance) in vivo for a proper functioning of neural prostheses. Here we design implantable multi-walled carbon nanotubes coating for stainless steel substrate electrodes, targeted at wide frequency stimulation of deep brain structures. In well-controlled, low-frequency stimulation acute experiments, we show that multi-walled carbon nanotube electrodes maintain their charge storage capacity (CSC) and impedance in vivo. The difference in average CSCs (n = 4) between the in vivo (1.111 mC cm(-2)) and in vitro (1.008 mC cm(-2)) model was statistically insignificant (p > 0.05 or P-value = 0.715, two tailed). We also report on the transcription levels of the pro-inflammatory cytokine IL-1beta and TLR2 receptor as an immediate response to low-frequency stimulation using RT-PCR. We show here that the IL-1beta is part of the inflammatory response to low-frequency stimulation, but TLR2 is not significantly increased in stimulated tissue when compared to controls. The early stages of neuroinflammation due to mechanical and electrical trauma induced by implants can be better understood by detection of pro-inflammatory molecules rather than by histological studies. Tracking of such quantitative response profits from better analysis methods over several temporal and spatial scales. Our results concerning the evaluation of such inflammatory molecules revealed that transcripts for the cytokine IL-1beta are upregulated in response to low-frequency stimulation, whereas no modulation was observed for TLR2. This result indicates that the early response of the brain to mechanical trauma and low-frequency stimulation activates the IL-1beta signaling cascade but not that of TLR2.

Charge Storage: Stability measures in implantable electrodes

Here we report on long-term (300 to 600 hours) stability measures for implantable stimulating electrodes. We have considered several measures of stability as they refer to reliability of charge carrying capacity in implantable electrodes. We have designed and manufactured coatings for large area (1 to 2mm2) stainless steel substrates. Materials tested were electrodeposited iridium oxide films, multi-walled carbon nanotube mesh, and PEDOT:PSS. Traditional characterization techniques such as cyclic voltammetry and electrochemical impedance spectroscopy cover a small fraction of the characterization framework needed for ensuring the safety and performance of electrodes designed for long-term implants. The stability measures suggested here rely on continuous low frequency cycling and evaluation of cathodic charge storage capacity during cycling. We experimentally show, in this paper, that the stability may be measured and is relevant for long-term applications of such coatings.

Quasi-static Analysis of Electric Field Distributions by Disc Electrodes in a Rabbit Eye Model

We developed a compartmentalized finite element model (FEM) of the electric fields generated in the rabbit retina due to a biphasic stimulus pulse. The model accounts for the different resistivities and capacitances of the retina, pigment epithelium (PE), and sclera. Axiosymmetric 2-D FEMs were created for monopolar stimulation electrodes using COMSOL. 250 μm diameter electrodes with 10 μm thick insulation were placed at three different locations near the retina: the inner limiting membrane (epiretinal), the subretinal space (PE/retina) (subretinal), and the choroid layer behind the PE/retina (suprachoroidal). A broad return electrode was located at the back of the eye (sclera). The relative dielectric constants of each eyewall layer with linearly varying resistivity for the retina layers were incorporated into the model. Biphasic 1 mA/cm2 current pulses with pulse widths of either 0.5 ms (0.5 μC/cm2), 1ms (1 μC/cm2), and 5 ms (5 μC/cm2) were passed through the tip of the electrode for stimulation. We found that these waveforms, which match waveforms commonly used to activate the retina in retinal implants, show a transient-sustained electric field profile due to charging of the high capacitance and resistivity of the PE. The PE develops high electric fields in all three electrode models. Wider pulses induce greater electric fields in the PE than shorter pulses. This needs to be accounted for when determining safe levels of stimulation. Simulation models that assume constant resistivity (4k Ω-cm) for the retina calculate larger electric fields across the retina than Gaussian resistivity models (3k-7k Ω-cm). Electric field strength is known to be greatly enhanced at the electrode edges. We found that the electric fields at the electrode edge can cause significant damage to the retina even when the nominal current density is below the damage threshold.

Access resistance of stimulation electrodes as a function of electrode proximity to the retina

OBJECTIVE Epiretinal prostheses seek to effectively stimulate the retina by positioning electrode arrays close to its surface so current pulses generate narrow retinal electric fields. Our objective was to evaluate the use of the electrical impedance of insulated platinum electrodes as a measure of the proximity of insulated platinum electrodes to the inner surface of the retina. APPROACH We examined the impedance of platinum disk electrodes, 0.25 mm in diameter, insulated with two widths (0.8 and 1.6 mm outer diameter) of transparent fluoropolymer in a rabbit retinal eyecup preparation. Optical coherence tomography measured the electrodes proximity to the retinal surface which was correlated with changes in the voltage waveform at the electrode. Electrode impedance changes during retinal deformation were also studied. MAIN RESULTS When the 1.6 mm diameter insulated electrodes advanced towards the retinal surface from 1000 μm, their voltage step at current pulse onset increased, reflecting an access resistance increase of 3880 ± 630 Ω, with the 50% midpoint averaging 30 μm, while thin 0.8 mm insulated electrode advancement showed an access resistance increase 50% midpoint averaging 16 μm. Using impedance spectroscopy, electrode-retina proximity differences were seen in the 1.6 mm insulated electrode impedance modulus between 1 and 100 kHz and the waveform phase angle at 0.3-10 kHz, while thin 0.8 mm insulated electrode advancement produced smaller impedance modulus changes with retinal proximity between 3 and 100 kHz. These impedance changes with retinal proximity may reflect different sized zones of eye wall being coupled in series with the insulated platinum electrode. SIGNIFICANCE The proximity of stimulus electrodes to neural tissue in fluid-filled spaces can be estimated from access resistance changes in the stimulus pulse waveform. Because many prosthetic devices allow back telemetry communication of the stimulus electrode waveform, it is possible these series resistance increases observed with retinal proximity could be used as a metric of stimulus electrode placement.

Dielectrophoretic trapping of P19 cells on indium tin oxide based microelectrode arrays

A microfabricated device comprised of a microelectrode array (MEA) and a microfluidic channel is presented here for the purpose of trapping cells using positive dielectrophoresis (DEP). Transparent indium tin oxide (ITO) electrodes are patterned in an array of electrode pairs. A microfluidic channel made up of polydimethylsiloxane (PDMS) is then attached on top of the electrode array. DEP is used to trap P19 cells at specific positions on the ITO electrode array within the PDMS channel. Our method provides exact positioning of cells and better cell access. We show here the design and results on cell trapping with this novel microelectrode array.

Microfluidic based contactless dielectrophoretic device: Modeling and analysis

While there have been many attempts at patterning cells onto substrates, a reliable method for trapping cell clusters and forming cell arrays in a predefined geometry remains to be demonstrated. We intend to develop a multielectrode array platform to initially trap cells via dielectrophoresis (DEP) and to later measure their electrical activity. As a first step toward that objective, here we present an interdigitated microfabricated comb structure. We designed an optimal insulation layer via finite element modeling for maximum dielectrophoretic field strength in solution and minimal cell damage. The microfabricated structure was combined with a microfluidic channel to vertically constrain cell position. With the objective of capturing cells onto the substrate, we here show that there is an optimal thickness of dielectric which limits electrolysis in solution and still allows for sufficient dielectrophoretic force on the cells to pull them onto the surface.

In vitro Models for Measuring Charge Storage Capacity

The interaction of the nervous tissue with electrode surfaces impacts the efficacy of the charge transfer capacity of the electrode. A better understanding of the interface between electrodes and tissue will inform the design of electrically conductive interfaces for implantable electrodes.

Randles Model of Vitreous Humor.

The vitreous is a gel-like structure found in the eyes. It is located above the retina to prevent the passage of fluids. As aging occurs, the vitreous can liquefy and can cause retinal detachment. The literature has little characterization of the vitreous, as it is often a less interesting structure than the retinal tissue. We investigate the impedance properties of the stimulation electrodes such as the constant phase element (Q) and the resistance of the solution (Rsol). We show results on vitreous characterization through electrochemical methods as a first step toward understanding the role of electrical stimulation in retinal prosthetics applications as it pertains to vitreous liquefaction. Our objective is to characterize the vitreous for a wide frequency range and to determine how charge is distributed through its conductive structure. Our electrochemical experiments were performed using insulated stainless steel electrodes (1) in phosphate buffered saline (PBS) and (2) in thimerosal as controls, (3) in vitreous without thimerosal, as well as (4) in vitreous preserved with thimerosal. We also performed cyclic voltammetry to measure the cathodic charge storage capacity for the electrodes for all experimental groups. Our results showed that the resistivity of the vitreous increases as thimerosal is added and that the cathodic charge storage capacity of the vitreous does not show any significant difference in the means as thimerosal is added.

In vitro versus in vivo impedance modeling for electrochemically deposited iridium oxide electrodes

We present here an electric circuit model that fits the behavior of iridium oxide coated electrodes when tested in vitro and in vivo. Our objective is to understand the interface between the active iridium oxide and the biological tissue. In order to do that we have first measured the impedance and the cyclic voltammetry in five electrodes which we later implanted in the hippocampi of rodents. While implanted, we have again measured the charge delivery through cyclic voltammetry and the impedance through spectroscopy. We later explanted the electrodes and performed the exact same tests, demonstrating that the electrodes had not been damaged either by our tests or by the implant. Here we will present the impedance values and the fit of the models we suggest, which include a solution resistand and a constant phase element. A charge transfer resistance is only present in our in vitro model, and this phenomenon is discussed in the current paper.