###### Abstract  
Recent advances in micro-retinal implant technologies have shown promise in vision restoration, yet challenges remain regarding pixel resolution, reliability, power leakage, cost-effectiveness, and device lifespan. Existing devices often face difficulties meeting the long-term safety and reliability requirements set by regulatory bodies such as the FDA and the European Union, making them impractical for widespread use. This research introduces a 256-pixel hexagonal subretinal device with an advanced digital controller designed to overcome these limitations. The proposed system offers solutions to issues of resolution, compactness, economic constraints, and control over biphasic pulse parameters. It was specifically designed to deliver flicker-free vision while increasing resolution and light sensitivity. A prototype was developed using field-programmable gate arrays (FPGAs) and amplitude-shift keying (ASK) modulators for wireless data transmission. The system was fabricated on a microchip and tested against various performance parameters, demonstrating its potential for future commercial viability. field-programmable gated arrays; digital controller; wireless telemetry system; hexagonal stimulation; flicker-free vision Article  
Accademic Editor: [PERSON]  
16 January 202513 March 202513 March 202513 March 202513 March 2025130 April 20  
Rectangular biphasic pulses [11; 12; 13; 14; 15; 18] have evolved as a solution to the most crucial criteria: stimulation efficiency and preventing amplitude mismatch to minimize current leakage. The size, shape, and amplitude of the current pulses must also be controlled by an implanted digital controller or a stimulus driver, as each visually impaired patient has a unique threshold for stimulating retinal neurons. An electric field is created when a charge is injected into retinal tissue through an electrode array, affecting neuron excitation. However, in high-density retinal implants where electrodes are closely spaced, electrical crosstalk between them can occur during stimulation, which may activate unwanted retinal cells. This results in blurred images for patients [19; 20; 21; 22; 23]. Thus, it is essential to design a digital controller that can mitigate the disadvantages of crosstalk. Most retinal implants have employed a single reference electrode, resulting in a greater separation between the active and reference electrodes. Consequently, the electric charge during stimulation flows from the activated electrode to the single reference electrode, which is placed farther away. This phenomenon can trigger undesirable excitation in retinal tissues. Clinically, it has been reported that the density of the electrode array is directly proportional to improvements in visual acuity [24; 25]. Therefore, it is essential to position the reference electrode close to the activated electrode. For high-density retinal implants, the electrical stimulation protocol, adjusted by a digital controller, must also meet the design requirement for flicker-free vision frequency [26]. This means that several electrodes are activated simultaneously, resulting in instantaneous high-power dissipation. Therefore, a digital controller capable of generating stimulation pulse trains should be designed to both reduce power consumption and meet the flicker-free vision frequency of 60 Hz. This work proposes a digital controller for a 256-pixel subretinal implant to reduce the crosstalk effects caused by the increased number of pixels. By employing a hexagonal electrode structure, the reference electrode, typically located far away, can be placed near the activated electrode during stimulation, thereby eliminating undesirable excitation. In addition to the hexagonal electrode structure, an overlapping stimulation method can meet the flicker-free vision frequency and reduce power dissipation. The proposed digital controller provides the overlapping stimulation profile, which will be addressed in the next section.