Ludwig Galambos

Photovoltaic retinal prosthesis with high pixel density

Retinal degenerative diseases lead to blindness due to loss of the “image capturing” photoreceptors, while neurons in the “image processing” inner retinal layers are relatively well preserved. Electronic retinal prostheses seek to restore sight by electrically stimulating surviving neurons. Most implants are powered through inductive coils, requiring complex surgical methods to implant the coil-decoder-cable-array systems, which deliver energy to stimulating electrodes via intraocular cables. We present a photovoltaic subretinal prosthesis, in which silicon photodiodes in each pixel receive power and data directly through pulsed near-infrared illumination and electrically stimulate neurons. Stimulation was produced in normal and degenerate rat retinas, with pulse durations from 0.5 to 4 ms, and threshold peak irradiances from 0.2 to 10 mW/mm2, two orders of magnitude below the ocular safety limit. Neural responses were elicited by illuminating a single 70 μm bipolar pixel, demonstrating the possibility of a fully-integrated photovoltaic retinal prosthesis with high pixel density.

Photovoltaic retinal prosthesis : implant fabrication and performance

The objective of this work is to develop and test a photovoltaic retinal prosthesis for restoring sight to patients blinded by degenerative retinal diseases. A silicon photodiode array for subretinal stimulation has been fabricated by a silicon-integrated-circuit/MEMS process. Each pixel in the two-dimensional array contains three series-connected photodiodes, which photovoltaically convert pulsed near-infrared light into bi-phasic current to stimulate nearby retinal neurons without wired power connections. The device thickness is chosen to be 30 µm to absorb a significant portion of light while still being thin enough for subretinal implantation. Active and return electrodes confine current near each pixel and are sputter coated with iridium oxide to enhance charge injection levels and provide a stable neural interface. Pixels are separated by 5 µm wide trenches to electrically isolate them and to allow nutrient diffusion through the device. Three sizes of pixels (280, 140 and 70 µm) with active electrodes of 80, 40 and 20 µm diameter were fabricated. The turn-on voltages of the one-diode, two-series-connected diode and three-series-connected diode structures are approximately 0.6, 1.2 and 1.8 V, respectively. The measured photo-responsivity per diode at 880 nm wavelength is ∼0.36 A W(-1), at zero voltage bias and scales with the exposed silicon area. For all three pixel sizes, the reverse-bias dark current is sufficiently low (<100 pA) for our application. Pixels of all three sizes reliably elicit retinal responses at safe near-infrared light irradiances, with good acceptance of the photodiode array in the subretinal space. The fabricated device delivers efficient retinal stimulation at safe near-infrared light irradiances without any wired power connections, which greatly simplifies the implantation procedure. Presence of the return electrodes in each pixel helps to localize the current, and thereby improves resolution.

Cortical responses elicited by photovoltaic subretinal prostheses exhibit similarities to visually evoked potentials

We have previously developed a wireless photovoltaic retinal prosthesis, in which camera-captured images are projected onto the retina using pulsed near-IR light. Each pixel in the subretinal implant directly converts pulsed light into local electric current to stimulate the nearby inner retinal neurons. Here we report that implants having pixel sizes of 280, 140 and 70μm implanted in the subretinal space in rats with normal and degenerate retina elicit robust cortical responses upon stimulation with pulsed near-IR light. Implant-induced eVEP has shorter latency than visible light-induced VEP, its amplitude increases with peak irradiance and pulse duration, and decreases with frequency in the range of 2-20Hz, similar to the visible light response. Modular design of the arrays allows scalability to a large number of pixels, and combined with the ease of implantation, offers a promising approach to restoration of sight in patients blinded by retinal degenerative diseases.

Domain reversal in stoichiometric LiTaO3 prepared by vapor transport equilibration

Domain reversal in stoichiometric lithium tantalate (LiTaO3) single crystals prepared by vapor transport equilibrium (VTE) method was studied. Starting from a virgin VTE crystal and using water electrodes, the coercive fields were found to be 1.39±0.01kV∕cm and 1.23±0.01kV∕cm for the first poling and the second poling, respectively, indicating a built-in internal field of 0.08kV∕cm. The spontaneous polarization, Ps was 55.2±0.5μC∕cm2 and the Curie temperature was Tc=701±2.5°C. The switching time, ts, exhibits an exponential dependence on the external field, E with activation energy of 10.84±0.22kV∕cm. No domain backswitching was observed. These properties are dramatically different from those of congruent and even near-stoichiometric compositions of lithium tantalate grown by Czhochralski method.

One-color one-beam pumping of Er(3+)-doped ZBLAN glasses for a three-dimensional two-step excitation display.

We demonstrate a one-color one-beam pumping method for a three-dimensional two-step excitation fluorescent display with Er(3+) -doped ZBLAN glass. A localized green fluorescent spot is obtained by use of a focused pump-light beam at 979 nm. The quantum efficiency of the two-step excitation fluorescence is investigated in a time-dependent analysis and an experiment with a pulsed pump light.

Growth of Sr0.61Ba0.39Nb2O6 fibers: new results regarding orientation

Abstract Single-crystal Sr 0.61 Ba 0.39 Nb 2 O 6 (SBN:61) fibers of high optical quality have been grown by the laser-heated pedestal growth (LHPG) method. Strontium barium niobate, based on its photorefractive properties, is a promising medium for holographic data storage. The use of a fiber bundle in this application is particularly attractive because each individual fiber can store many (10–30) independent holograms with little cross-talk between adjacent fibers. For this purpose, fibers that are oriented with the c -axis perpendicular to the pulling direction are preffered because they provide a significant increase in photorefractive performance when compared to fibers grown along the c -axis. In this paper we report, to our knowledge, the first stable growth of SBN fibers along the [100] and [110] crystallographic axes. Previous attempts to grow bulk crystals with these orientations using the Czochralski technique have been generally unsuccessful because of diameter instability. We do not encounter this problem with the LHPG method and instead find that [100] and [110] fibers can be readily pulled. Growth conditions, morphology and crystal quality are discussed.

Doubly doped stoichiometric and congruent lithium niobate for holographic data storage

Abstract Codoped lithium niobate of congruent and stoichiometric compositions had been successfully grown specifically for holographic data storage applications. The dopants we have selected are Mn and Ce and their role in the recording process is described. Growth methods, sample preparation, doping level measurements, holographic recording data as well as the results of optical characterization and photorefractive properties of the two-photon sensitivity are discussed. Compared to the previously grown crystals, the codoped LiNbO 3 shows a higher sensitivity and dynamic range for recording in red ( λ=632 nm ).

Photovoltaic Pixels for Neural Stimulation: Circuit Models and Performance

Photovoltaic conversion of pulsed light into pulsed electric current enables optically-activated neural stimulation with miniature wireless implants. In photovoltaic retinal prostheses, patterns of near-infrared light projected from video goggles onto subretinal arrays of photovoltaic pixels are converted into patterns of current to stimulate the inner retinal neurons. We describe a model of these devices and evaluate the performance of photovoltaic circuits, including the electrode-electrolyte interface. Characteristics of the electrodes measured in saline with various voltages, pulse durations, and polarities were modeled as voltage-dependent capacitances and Faradaic resistances. The resulting mathematical model of the circuit yielded dynamics of the electric current generated by the photovoltaic pixels illuminated by pulsed light. Voltages measured in saline with a pipette electrode above the pixel closely matched results of the model. Using the circuit model, our pixel design was optimized for maximum charge injection under various lighting conditions and for different stimulation thresholds. To speed discharge of the electrodes between the pulses of light, a shunt resistor was introduced and optimized for high frequency stimulation.

Kinetics of the Local Polarization Switching in Stoichiometric LiTaO3 Under Electric Field Applied Using the Tip of Scanning Probe Microscope

The investigation of the local polarization switching including direct domain writing, local hysteresis measurements, and domain stability testing was carried out in single-crystalline stoichiometric lithium tantalate using the Scanning Probe Microscope. Piezoresponse Force Microscopy was utilized for domain visualization and recording of local switching characteristics. Two variants of domain patterning were shown: “direct writing” and “self-assembled formation.” The polarization reversal in direction opposite to applied field (“anti-parallel switching”) was studied in details and attributed to screening of external field.

Contrast Sensitivity With a Subretinal Prosthesis and Implications for Efficient Delivery of Visual Information.

PURPOSE To evaluate the contrast sensitivity of a degenerate retina stimulated by a photovoltaic subretinal prosthesis, and assess the impact of low contrast sensitivity on transmission of visual information. METHODS We measure ex vivo the full-field contrast sensitivity of healthy rat retina stimulated with white light, and the contrast sensitivity of degenerate rat retina stimulated with a subretinal prosthesis at frequencies exceeding flicker fusion (>20 Hz). Effects of eye movements on retinal ganglion cell (RGC) activity are simulated using a linear-nonlinear model of the retina. RESULTS Retinal ganglion cells adapt to high frequency stimulation of constant intensity, and respond transiently to changes in illumination of the implant, exhibiting responses to ON-sets, OFF-sets, and both ON- and OFF-sets of light. The percentage of cells with an OFF response decreases with progression of the degeneration, indicating that OFF responses are likely mediated by photoreceptors. Prosthetic vision exhibits reduced contrast sensitivity and dynamic range, with 65% contrast changes required to elicit responses, as compared to the 3% (OFF) to 7% (ON) changes with visible light. The maximum number of action potentials elicited with prosthetic stimulation is at most half of its natural counterpart for the ON pathway. Our model predicts that for most visual scenes, contrast sensitivity of prosthetic vision is insufficient for triggering RGC activity by fixational eye movements. CONCLUSIONS Contrast sensitivity of prosthetic vision is 10 times lower than normal, and dynamic range is two times below natural. Low contrast sensitivity and lack of OFF responses hamper delivery of visual information via a subretinal prosthesis.