Retinal degenerative diseases such as retinitis pigmentosa (RP) and age-related macular degeneration (AMD) are leading causes of blindness, affecting millions worldwide. They damage the photoreceptors, which convert light into electrical signals, but leave the rest of retinal cells functional. The aim of retinal prostheses is to restore vision in these patients by bypassing the damaged photoreceptors and directly stimulating the remaining healthy neurons.
The prosthesis consists of an external system and an implant. The external system captures and processes the visual information that is wirelessly transmitted to the implant through inductively coupled coils. In the same way, power is transmitted via another set of coils. The implant integrates the receiver coils, the IC and the electrode array, which is connected to the retina. The IC receives data and power, and generates biphasic current patterns to stimulate bipolar and ganglion retinal cells. To restore functional visual perception, hundreds of stimulation channels are needed. This requires the use of small electrodes, which increases the impedance of the electrode-tissue interface (>10kOhm).Previous designs used high voltage technologies to support the high output voltage compliance at the expense of area and power consumption. Our approach uses highly-scaled technologies to reduce area and the power consumption, and proposes effective techniques to overcome the limitations of the technology for this application.
In order to restore functional visual perception, hundreds of stimulation channels are needed. This requires the use of small electrodes (≈100μm diameter) which increase the impedance of the electrode-retina interface (>10kOhm). Initial designs targeted current levels up to 1mA to ensure stimulation of retinal cells. For such designs, an output compliance of more than 10V was required and high voltage technologies were used at the expense of area and power consumption. Human clinical trials have recently shown that implanted electrodes present a stimulus threshold as low as 20μA. Thus, our approach uses highly scaled technologies to reduce area and power, and to support hundreds of channels for fully intraocular implants.
A fully intraocular high-density self-calibrating epiretinal prosthesis in 65nm CMOS has been developed. It provides charge-balanced stimulation with highly flexible waveforms. It features dual-band telemetry for power and data, on-chip rectifier and clock recovery, a digital calibration technique to match biphasic stimulation currents, and 512 independent channels capable of arbitrary waveform generation. This project is a collaborative effort with the support of NSF and USC's BMES-ERC (Biomimetic Microelectronic Systems - Engineering Research Center).
Origami Retinal Prosthesis
Our proposed origami design is a 3D integration technique that addresses the size and cost constraints of biomedical implants. Large systems can be split into many smaller chips and connected using 3D integration techniques to be folded compactly for implantation, and then deployed inside the body. Electronics can be partitioned into functional blocks for mass-production and customs implants can be assembled from these relatively cheap modules.
Retinal prostheses can particularly benefit from this approach given their challenging requirements. Instead of a single large chip, many micro-size low-cost chips are distributed over a flexible biocompatible thin film substrate along with the electrodes. Electrodes are micro-manufactured on the top surface of the film in a sub-array fashion. Each sub-array is connected to a microchip by parallel micro-manufactured electrical wires on the film. Power and ground are distributed via such wires avoiding sharp folds. The origami design will place chips facing each other across the fold and wireless (proximity) chip-to-chip communication can be used to reduce reliance on electrical wires . As shown in figure below, when inside the eye, the origami implant will take a curved shape to conform to the shape of the retina improving electrode contact for effective stimulation . The location of the chips and electrodes can be optimized through the design of the origami structure. This high-performance system will achieve the following goals: allows minimally invasive surgery, closely apposes electrodes to the retina, places all components within the eye, reduces the interconnect cable density, and enhances the yield and reliability of the system.
-  M. Loh and A. Emami-Neyestanak, “Capacitive proximity communication with distributed alignment sensing for origami biomedical implants,” in IEEE CICC, 2013, pp. 1-4.
-  Y. Liu et al., “Parylene origami structure for intraocular implantation,” in IEEE Transducers Conf. Dig. Tech. Papers, 2013, pp. 1549-1552.