Retinal Prosthesis Pixels With Transient Return Electrodes
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Solution Overview
Problem
The spatial resolution of retinal prostheses is limited by pixel size and crosstalk from neighboring electrodes, with electric field penetration depth constrained by pixel width, leading to increased stimulation thresholds and safe charge injection limits, especially with smaller pixels.
Innovation Solution
Preconditioning active electrodes as transient return electrodes by optically controlling photodiodes or phototransistors to manage electric field confinement through spatiotemporal modulation of projected images, using a common return electrode and series-connected photodiodes to optimize pixel conductivity and electric field penetration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If local return electrodes are used in bipolar pixels to reduce crosstalk, then lateral selectivity is improved, but electric field penetration depth is constrained to about a pixel radius, causing stimulation threshold to rapidly increase with decreasing pixel size
Solution Approach 1:
The patent applies preliminary action by pre-charging active electrodes during the display of one image frame so that they are ready to serve as return electrodes for the next frame. This advance preparation allows the electrodes to immediately function as return electrodes when needed, enabling effective field confinement without requiring permanent bipolar structures. The pre-charging process accumulates charge on the active electrode, creating the necessary potential difference to confine the electric field laterally when the electrode subsequently acts as a return electrode.
Solution Approach 2:
The patent implements dynamics by making the return electrode function transient and time-dependent. Instead of fixed bipolar pixels where both electrodes are always present, the system dynamically switches electrodes between active and return roles from frame to frame. The return electrode is created temporarily through pre-charging and lasts only for the duration of the subsequent frame display. This dynamic reconfiguration allows flexible control of field confinement while maintaining simpler electrode geometry.
2Measurement precision
If pixel size is reduced to increase spatial resolution, then acuity is improved, but penetration depth of electric field is constrained and stimulation threshold exceeds safe charge injection limit
Solution Approach 1:
The patent uses preliminary action to pre-charge active electrodes before they are needed as return electrodes. This advance charge accumulation ensures that when small pixels are used, sufficient charge is already stored on the active electrode to create an effective return path, maintaining safe charge injection levels even with sub-40 µm pixels. The pre-charging process occurs during the display of the current frame, preparing electrodes for their return electrode role in the next frame without requiring excessive charge injection during stimulation.
Solution Approach 2:
The patent applies parameter changes by dynamically adjusting the charge state of electrodes through optical control of photodiodes. The photodiodes are charged to different voltage levels (e.g., 0V, 0.3V, 0.6V, or higher) depending on whether they should act as active or return electrodes in the next frame. This parameter modulation allows the system to optimize the electric field configuration for each pixel based on the desired stimulation pattern, enabling safe operation with high-resolution small pixels by controlling the charge injection parameters.
3Reliability
If 3-D honeycomb-shaped array with elevated return electrode is used to decouple field penetration depth from pixel width, then stimulation threshold is reduced, but fabrication complexity increases and functionality of migrated neurons remains unconfirmed
Solution Approach 1:
The patent extracts the return electrode function from the physical electrode structure and implements it through optically controlled charge accumulation on active electrodes. Instead of requiring complex 3-D honeycomb structures with elevated return electrodes, the invention takes out the return electrode functionality and realizes it through temporal control of charge on planar electrodes. This extraction simplifies the fabrication process to standard planar electrode arrays while maintaining the beneficial low stimulation threshold through optical pre-charging of the photodiodes.
Solution Approach 2:
The patent replaces the mechanical/structural solution (3-D honeycomb array with physically elevated return electrodes) with an optical/electrical solution (temporal control of charge on planar electrodes through photodiode pre-charging). Instead of relying on complex 3-D fabrication and physical electrode positioning, the invention uses optical control of photodiodes to dynamically create return electrode functionality. This substitution maintains effective field confinement and low stimulation thresholds while dramatically simplifying the device structure and fabrication process to compatible planar technologies.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables high-resolution prosthetic vision by improving electric field confinement and lateral selectivity, allowing pixel sizes below 40 µm to achieve visual acuity comparable to natural spatial resolution, reducing crosstalk and stimulation thresholds.
Implementation Method 1
Each pixel includes one or more photodiodes connected in series between the common return electrode and the corresponding active electrode
Implementation Method 2
photovoltaic pixels can be turned into transient returns by optically controlling the discharge with photosensitive transistors responding to a different range of wavelengths
Data Source
Figure 1
Figure 2A~2B
Figure 3
AI summary
Photovoltaic retinal prosthesis is provided with optically configurable confinement of electrical field. A video stream is projected onto a retinal implant. An array of photovoltaic pixels is configured to provide retinal stimulus responsive to the video stream. The photovoltaic pixels have a common return electrode. Each pixel has an active electrode that is coupled to retinal tissue via a capacitive interface or a faradaic interface. Each pixel includes photodiode(s) connected in series between the common return electrode and the corresponding active electrode. The projected video stream is configured based on a source video stream such that one or more pixels of the retinal implant that will be dark in a next frame of the projected video stream are optically preconditioned (pre-charged) by the projected video stream during the previous frame to become sufficiently conductive to act as transient local return electrodes during the next frame of the projected video stream.