Retinal Implant Pixel Circuit With Dynamic Shunt Discharge
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Solution Overview
Problem
Current retinal implant systems face limitations in achieving high temporal resolution for stimulation due to the interdependence between stimulation pulse repetition rate and efficiency, primarily because of the use of fixed-value shunt resistors, which restrict light pulse rates beyond 30 Hz without compromising efficiency.
Innovation Solution
A pixel cell circuit with a variable shunt resistor that automatically adjusts its resistance based on the stimulation process, utilizing a shunt switch that is in an open state during light input and closed during discharge, allowing for near-infinite resistance at onset and zero resistance during discharge, thereby maximizing charge transfer efficiency and minimizing discharge time.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If a fixed-value shunt resistor is used to speed up discharge, then discharge time is reduced, but stimulation efficiency decreases at high pulse rates
Solution Approach 1:
The patent applies the dynamics principle by replacing the fixed-value shunt resistor with a variable resistor whose resistance changes based on the operational state. The resistor transitions from high resistance during charging phase to low resistance during discharge phase, allowing optimal performance at both stages. This dynamic adaptation resolves the contradiction by having the resistor provide different functions at different times rather than a fixed compromise.
Solution Approach 2:
The patent implements parameter changes by making the shunt resistor's resistance value state-dependent. The resistance parameter is changed from a constant fixed value to a variable value that switches between high and low states based on whether the photodiode is currently receiving light or not. This allows the system to achieve both fast discharge (low resistance) and high stimulation efficiency (high resistance during charging) without compromise.
2Productivity
If light pulse repetition rate is increased beyond 30 Hz, then temporal resolution is improved, but charge accumulation on electrode reduces stimulation efficiency
Solution Approach 1:
The dynamic resistor adapts its value based on the pulse rate and charge accumulation state. At high repetition rates where charge accumulation occurs, the resistor switches to low resistance to accelerate discharge, preventing efficiency degradation. This allows the system to operate at high productivity levels (high pulse rates) while maintaining reliability (stimulation efficiency).
Solution Approach 2:
The resistor's state is influenced by feedback from the operational conditions - specifically the charge accumulation state detected through the photodiode's light reception status. When light is not received (discharge phase), the resistor transitions to low resistance, providing automatic feedback-based control that prevents charge buildup at high pulse rates.
3Loss of time
If shunt resistor value is optimized for discharge speed, then temporal resolution is improved, but charge injection during stimulation pulses is reduced
Solution Approach 1:
The resistor value is dynamically adjusted based on the operational phase. During the charging phase (light pulse), the high resistance minimizes charge loss and maximizes charge injection to the tissue. During the discharge phase, the low resistance rapidly dissipates accumulated charge. This time-dependent optimization resolves the contradiction between discharge speed and charge injection efficiency.
Solution Approach 2:
The resistor undergoes periodic switching between high and low states synchronized with the light pulse sequence. High resistance during charging pulses, low resistance during inter-pulse discharge periods. This periodic action allows the system to optimize for charge injection during stimulation and for discharge speed during recovery, cycling through optimal states for each phase.
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
This approach enables light pulse repetition rates beyond 50 Hz and up to 1000 Hz without decreasing stimulation efficiency, breaking the trade-off between pulse repetition rate and efficiency, thus enhancing the temporal resolution of retinal stimulation.
Implementation Method 1
an array of pixel cells, each pixel cell implementing a microelectronics circuit with an optical receiver pixel array of photodiode elements for converting received pulsed near-IR (～900 nm) light into biphasic pulses of electric current
Implementation Method 2
a shunt switch electrically coupled in parallel across the one or more main photosensitive element... configured for placing the shunt switch in an open state responsive to incident light received at the pixel cell and placing the shunt switch in a closed state if no incident light is received
Data Source
AI summary
A pixel cell circuit comprises an electrode, one or more main photosensitive elements electrically coupled to the electrode for outputting a stimulation signal to the electrode responsive to light illumination, and a shunt arrangement comprising a shunt switch electrically coupled in parallel across the one or more photosensitive elements, and a control arrangement operatively coupled to the shunt switch and configured for placing the shunt switch in an open state responsive to incident light received at the pixel cell and placing the shunt switch in a closed state if no incident light is received at the pixel cell.