Photovoltaic retinal prosthesis is designed to restore sight in patients who lost central vision due to atrophic AMD. Subretinal pixels convert pulsed NIR light projected from augmented-reality glasses into electric current, stimulating the nearby inner retinal neurons. In patients with geographic atrophy, such prosthetic central vision coexists with natural peripheral sight, and its acuity closely matches the 100um pixel pitch of the implant. We present a progress toward 20um pixels based on honeycomb configuration of the stimulating arrays with return electrodes elevated on vertical walls, designed to leverage retinal migration for decoupling the stimulation threshold from pixel size.
Macular degeneration leads to blindness due to loss of the “image capturing” photoreceptors, while neurons in the “image-processing” inner retinal layers are relatively well preserved. Photovoltaic subretinal prosthesis converts light into pulsed electric current, stimulating the nearby inner retinal neurons. Clinical trial with such implants having 100um pixels, as well as preclinical measurements with 75 and 55um pixels, confirm that spatial resolution of prosthetic vision can reach the pixel pitch. For a broad acceptance of this technology, visual acuity should exceed 20/100, which requires pixels smaller than 25um. I will present 3-dimensional electro-neural interface scalable down to cellular-scale pixel size.
To restore vision in patients who lost photoreceptors due to retinal degeneration, we developed a photovoltaic subretinal prosthesis which converts light into pulsed electric current, stimulating the inner retinal neurons. Visual information is projected onto the retina by video goggles using pulsed near-infrared (880nm) light. This design avoids the use of bulky electronics and wiring, thereby greatly reducing the surgical complexity and allows scaling the implants to thousands of electrodes.
We found that similarly to normal vision, retinal response to prosthetic stimulation exhibits flicker fusion at high frequencies (>20Hz), adaptation to static images, antagonistic center-surround receptive fields with non-linear summation of its subunits. In rats, photovoltaic arrays with 55um pixels provided grating visual acuity up to a pixel pitch, which corresponds to about 20/200 acuity in a human eye. In patients with geographic atrophy, implants with 100um pixels provided retinotopically correct pattern percepts with resolution matching the pixel size.
With flat pixels of 40um and smaller, stimulation thresholds are becoming prohibitively high. To reduce the pixel size further, we developed a novel honeycomb configuration of the stimulating electrode array with vertical walls separating the active and return electrodes, designed to leverage retinal migration for reducing the subretinal stimulation threshold and electrical cross-talk between neighboring pixels. Scalability, ease of implantation, and high resolution of these arrays open the door to highly functional restoration of sight in retinal degeneration.
Movements of the cell membrane accompanying action potentials have been detected by various methods, including reflection of a laser beam, atomic force microscopy and even bright-field microscopy. However, imaging of the entire cell dynamics during action potential has not been achieved, and the mechanism behind this phenomenon is still actively debated. Here we report full-field interferometric imaging of cellular movements during action potential by simultaneous quantitative phase microscopy (QPM) and multi-electrode array (MEA) recordings. Using spike-triggered averaging of the movies synchronized to electrical recording, we demonstrate deformations of up to 3 nm (0.9 mrad) during the action potential in spiking HEK-293 cells, with a rise time of 4 ms. The time course of the optically-recorded action potential is very similar to intracellular potential recorded with a whole-cell patch clamp, while the time derivative of the rising edge of the optical spike matches the timing and duration of the extracellular electrical recording on MEA. In some cells, phase increases at the center and decreases along the cell boundaries, while in others it increases on one side and decreases on the other. These findings suggest that optical phase changes during an action potential are due to cellular deformation, likely associated with changes in the membrane tension, rather than refractive index change due to ion influx or cell swelling. High-speed QPM may enable all-optical, label-free, full-field imaging of electrical activity in mammalian cells.
Wide-field interferometric imaging systems can detect mechanical deformations of a cell during an action potential (AP), such as in quantitative phase microscopy, which is highly sensitive to the changing optical path length. This enables non-invasive optophysiology of spiking cells without exogeneous markers, but high-fidelity imaging of such deformations requires averaging of a large number of spikes synchronized by electrical recordings. We have developed new iterative methods for detecting single APs from quantitative phase microscopy of spiking cells, enabling an all-optical detection system with high accuracy and good temporal resolution. We demonstrate performance of the method across multiple preparations of spiking HEK-293 cells and compare the outcomes of the all-optical measurements with the ground truth detected on a multi-electrode array. We initially use a spike-triggered average, synchronized to an electrical recording, to measure deformations during the AP in spiking cells, which reach up to 3 nm (0.9 mrad) with a rise time of 4 ms and fall time of about 120 ms. Based on this knowledge of the AP dynamics, optical data analysis can provide reliable spike detection, within a standard deviation of 11.6 ms (9.7% of the length of the action potential) with an 8.5% false negative detection rate. The method is robust to natural variations between cells and can be modified to function without any prior knowledge of the AP dynamics. Such a system could achieve high-throughput measurements of network activity in culture and help identify the mechanisms linking cell deformations to the changes of transmembrane potential.