Carlos J. Cela

Computation of Induced Current Densities in the Human Body at Low Frequencies Due to Contact Electrodes Using the ADI-FDTD Method

We report the use of the alternating direction implicit (ADI) finite-difference time-domain (FDTD) method in a D-H formulation to compute induced current densities and recruitment volumes in the human body due to contact electrodes for human electromuscular incapacitation devices at frequencies below 200 kHz. A computational model resolution of 1 mm has been used for most of the human body model, including regions proximal to the electrode contact points, while a progressively coarser resolution up to 5 mm is utilized, according to an expanding grid scheme for body regions distant from the source, such as the lower extremities. Using quasi-static assumptions, discrete Fourier transforms have been used to average the electric field values at the desired frequencies for times much shorter than their time periods. The field values induced in the human body were then obtained as ratios with respect to the source, which can be scaled depending on the magnitude. This study suggests that the ADI-FDTD method can be used for the solution of low-frequency large-scale bioelectromagnetic problems. It is shown that, when used with quasi-static assumptions, Fourier series decomposition, and expanding grid, the D-H ADI-FDTD can be an effective computational bioelectromagnetics tool.

Modeling and percept of transcorneal electrical stimulation in humans

Retinal activation via transcorneal electrical stimulation (TcES) in normal humans was investigated by comparing subject perception, model predictions, and brain activation patterns. The preferential location of retinal stimulation was predicted from 3-D admittance modeling. Visual cortex activation was measured using positron emission tomography (PET) and 18F-fluorodeoxyglucose (FDG). Two different corneal electrodes were investigated: DTL-Plus and ERG-Jet. Modeling results predicted preferential stimulation of the peripheral, inferior, nasal retina during right eye TcES using DTL-Plus, but more extensive activation of peripheral, nasal hemiretina using ERG-Jet. The results from human FDG PET study using both corneal electrodes showed areas of visual cortex activation that consistently corresponded with the reported phosphene percept and modeling predictions. ERG-Jet was able to generate brighter phosphene percept than DTL-Plus and elicited retinotopically mapped primary visual cortex activation. This study demonstrates that admittance modeling and PET imaging consistently predict the perceived location of electrically elicited phosphenes produced during TcES.

Modeling Cellular Lysis in Skeletal Muscle Due to Electric Shock

High-voltage electrical trauma frequently results in injury patterns that cannot be completely attributed to Joule heating. An electrical-injury model describing cellular lysis damage caused by supraphysiological electric fields is introduced, and used to evaluate the effects of high-voltage electric shock on the skeletal muscle of a human upper limb in a configuration that simulates hand-to-hand contact. A novel multiresolution admittance method, capable of efficiently handling large computational models while maintaining excellent accuracy, was used to perform the numerical computations. Values for the computed current through the arm and the upper limb impedance are reported.

Computational Modeling of Electromagnetic and Thermal Effects for a Dual-Unit Retinal Prosthesis: Inductive Telemetry, Temperature Increase, and Current Densities in the Retina

Recent advances in electromagnetic and thermal modeling of a dual-unit retinal prosthesis are presented. The focus is on the latest computational methods to quantify electrical and thermal deposition in the human tissue with the ultimate goal of addressing safety concerns and optimizing the overall performance of the system. A Partial Inductance Method (PIM) is used for the computation of the electrical coupling parameters of the radiating and receiving telemetry coils. Results for the inductive coil coupling are presented and different coil geometries are compared. Further, a finite difference method for the solution of a bio-heat equation is used to compute the temperature increase caused by the implanted electronics and the electromagnetic absorption due to the external power and data telemetry link. Temperature increases due to the implanted microchip, coils, and stimulating electrode array are shown. Finally, our computational approach based on a multi-resolution impedance method is used for the computation current spread in the human tissue. Results are presented showing variations of current spread in the retina and eye due to different electrode array geometries and placement configurations.

In Situ Characterization of Stimulating Microelectrode Arrays: Study of an Idealized Structure Based on Argus II Retinal implants

The development of a retinal prosthesis for artificial sight includes a study of the factors affecting the structural and functional stability of chronically implanted microelectrode arrays. Although neuron depolarization and propagation of electrical signals have been studied for nearly a century, the use of multielectrode stimulation as a proposed therapy to treat blindness is a frontier area of modern ophthalmology research. Mapping and characterizing the topographic information contained in the electric field potentials and understanding how this information is transmitted and interpreted in the visual cortex is still very much a work in progress. In order to characterize the electrical field patterns generated by the device, an in vitro prototype that mimics several of the physical and chemical parameters of the in vivo visual implant device was fabricated. We carried out multiple electrical measurements in a model “eye,” beginning with a single electrode, followed by a 9-electrode array structure, both idealized components based on the Argus II retinal implants. Correlating the information contained in the topographic features of the electric fields with psychophysical testing in patients may help reduce the time required for patients to convert the electrical patterns into graphic signals.

Retinal Cell Excitation Modeling

As the electrode density of implantable retinal prosthesis increases, simulation becomes a valuable tool to characterize excitation performance, evaluate implant electrical safety, determine optimal geometry and placement of implant current return, and understand charge distribution due to stimulation. To gain an insight into the effectiveness of a retina stimulator, quasi-static numerical electromagnetic methods can help estimate current densities, potentials, and their gradients in retinal layers and neural cells. Detailed discrete three-dimensional models of the retina, implant and surrounding tissue can be developed to account for the anatomical complexity of the human eye and appropriate dielectric properties. This chapter will cover the basics of quasi-static methods that can be used for this purpose. Specifically, authors will focus on the admittance method, the output it produces, and possibilities it offers to determine the potential effectiveness of a retinal stimulator, ranging from evaluating the current density magnitude in the ganglion cell layer, to calculating local activation function in the areas targeted by the electrical stimulation.

Numerical prediction of neural activation in electrically stimulated retina

Degenerative retinal diseases such as Retinitis Pigmentosa (RP) and Age Related Macular Degeneration (AMD) result in progressive loss of photoreceptor cells in the retina without completely disrupting the rest of the visual neural path. Implantable retinal prostheses have been shown to be capable of restoring some form of vision to patients affected by these conditions by providing systematic electrical stimulation to the retinal ganglion cells, thus partially replacing the functionality of the diseased photoreceptors cells. In this paper, we determine the areas where retinal ganglion cells are depolarized and hyperpolarized by the artificial electrical stimulation. This is achieved by calculating the activation function from the potential scalar field obtained using the admittance method with three-dimensional models of the retina, implanted electrode, and surrounding tissue.

Bioelectromagnetics for a Retinal Prosthesis to Restore Partial Vision to the Blind

The retinal prosthesis is aimed to restore partial vision to the blind affected by outer retinal degenerative diseases. Almost all research efforts rely on a dual-unit system, with components internal and external to the human body, including a video camera, an inductive wireless link for power and/or data transfer between the external and internal components, a processing microchip, and a stimulating electrode array. In this paper, we discuss modeling and computational methods to calculate the electromagnetic interaction of some of the system components with the head tissues. Specifically, we will focus on two aspects of the system: the safety considerations of an inductive link operating at the carrier frequency of 10 MHz and the current spread in the retina under different conditions.