Asymmetric charge balanced waveforms direct retinal ganglion cell axon growth.

Failure to direct axon regeneration to appropriate targets is a major barrier to restoring function after nerve injury. Development of strategies that can direct targeted regeneration of neurons such as retinal ganglion cells (RGCs) are needed to delay or reverse blindness in diseases like glaucoma. Here, we demonstrate that a new class of asymmetric, charge balanced (ACB) waveforms are effective at directing RGC axon growth, in vitro, without compromising cell viability. Unlike previously proposed direct current (DC) stimulation approaches, charge neutrality of ACB waveforms ensures the safety of stimulation while asymmetry ensures its efficacy. Furthermore, we demonstrate the relative influence of pulse amplitude and pulse width on the overall effectiveness of stimulation. This work can serve as a practical guideline for the potential deployment of electrical stimulation as a treatment strategy for nerve injury.

On the design of efficient multi-coil telemetry system for biomedical implants.

Two-coil based inductive coupling is a commonly used technique for wireless power and data transfer for biomedical implants. Because the source and load resistances are finite, two-coil systems generally achieve a relatively low power transfer efficiency. A novel multi-coil technique (using more than two coils) for wireless power and data transfer is considered to help overcoming this limitation. The proposed multi-coil system is formulated using both network theory and a two-port model. Using three or four coils for the wireless link allows for the source and load resistances to be decoupled from the Q-factor of the coils, resulting in a higher Q -factor and a corresponding improved power transfer efficiency (PTE). Moreover, due to the strong coupling between the driver and the transmitter coil (and/or between the receiver and the load coil), the multi-coil system achieves higher tunable frequency bandwidth as compared to its same sized two-coil equivalent. Because of the wider range of reflected impedance in the multi-coil system case, it is easier to tune the output power to the load and achieve the maximum power transfer condition for given source voltage than in a configuration with two coils. Experimental results showing a three-coil system achieving twice the efficiency and higher gain-bandwidth product compared to its two-coil counterpart are presented. In addition, a figure of merit for telemetry systems is defined to quantify the overall telemetry system performance.

Spatial characterization of electric potentials generated by pulsed microelectrode arrays.

This presentation is a report on the in situ characterization of stimulating microelectrodes in the context of multielectrode retinal prosthetic implants. The experimental system approximately replicates the geometric and electrical parameters of Second Sight Medical Products' Argus II Retinal Implant. Topographic maps of electric potentials have been prepared for a 60 electrode structure in which selected electrodes were stimulated with biphasic repetitively pulsed charge densities at 100 microC·cm(-2). Surface contour maps were prepared using a 10 microm diameter recording electrode.

On the thermal elevation of a 60-electrode epiretinal prosthesis for the blind.

In this paper, the thermal elevation in the human body due to the operation of a dual-unit epiretinal prosthesis to restore partial vision to the blind affected by irreversible retinal degeneration is presented. An accurate computational model of a 60-electrode device dissipating 97 mW power, currently under clinical trials is developed and positioned in a 0.25 mm resolution, heterogeneous model of the human head to resemble actual conditions of operation of the prosthesis. A novel simple finite difference scheme combining the explicit and the alternating-direction implicit (ADI) method has been developed and validated with existing methods. Simulation speed improvement up to 11 times was obtained for the the head model considered in this work with very good accuracy. Using this method, solutions of the bioheat equation were obtained for different placements of the implant. Comparison with in-vivo experimental measurements showed good agreement.

Currents induced in anatomic models of the human for uniform and nonuniform power frequency magnetic fields.

We have used the quasi-static impedance method to calculate the currents induced in the nominal 2 x 2 x 3 and 6 mm resolution anatomically based models of the human body for exposure to magnetic fields at 60 Hz. Uniform magnetic fields of various orientations and magnitudes 1 or 0.417 mT suggested in the ACGIH and ICNIRP safety guidelines are used to calculate induced electric fields or current densities for the various glands and organs of the body including the pineal gland. The maximum 1 cm(2) area-averaged induced current densities for the central nervous system tissues, such as the brain and the spinal cord, were within the reference level of 10 mA/m(2) as suggested in the ICNIRP guidelines for magnetic fields (0.417 mT at 60 Hz). Tissue conductivities were found to play an important role and higher assumed tissue conductivities gave higher induced current densities. We have also determined the induced current density distributions for nonuniform magnetic fields associated with two commonly used electrical appliances, namely a hair dryer and a hair clipper. Because of considerably higher magnetic fields for the latter device, higher induced electric fields and current densities were calculated.

Comparison of numerical and experimental methods for determination of SAR and radiation patterns of handheld wireless telephones.

Some recent developments in both the numerical and experimental methods for determination of SARs and radiation patterns of handheld wireless telephones are described, with emphasis on comparison of results using the two methods. For numerical calculations, it was possible to use the Pro-Engineer CAD Files of cellular telephones for a realistic description of the device. Also, we used the expanding grid formulation of the finite-difference time-domain (FDTD) method for finer-resolution representation of the coupled region, including the antenna, and an increasingly coarser representation of the more-distant, less-coupled region. Together with the truncation of the model of the head, this procedure led to a saving of computer memory needed for SAR calculations by a factor of over 20. Automated SAR and radiation pattern measurement systems were used to validate both the calculated 1-g SARs and radiation patterns for several telephones, including some research test samples, using a variety of antennas. Even though widely different peak 1-g SARs were obtained, ranging from 0.13 to 5.41 W/kg, agreement between the calculated and the measured data for these telephones, five each at 835 and 1900 MHz, was excellent and generally within +/-20% (+/-1 dB). An important observation was that for a maximum radiated power of 600 mW at 800/900 MHz, which may be used for telephones using AMPS technology, the peak 1-g SARs can be higher than 1.6 W/kg unless antennas are carefully designed and placed further away from the head.