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Abstract
Restoring sight to blind patients has been an ongoing goal for many research groups worldwide. Many prototypes were developed and to this day, only the Argus II, developed by the Second Sight group, is commercialized. However, research to improve vision is still needed to improve the prostheses performances.
All retinal prostheses rely on the fact that nerve cells that were not harmed by retinal diseases can be electrically stimulated to make the patient sense a visual pattern. This thesis focuses on the use of organic transistors to improve retinal prostheses performances. To increase the resolution of the retinal prosthesis developed at UNSW, an addressable transistor matrix is investigated. This architecture has demonstrated an increase in the number of electrodes being able to be controlled while keeping the wiring on the implant low.
Two types of flexible low-voltage organic field-effect transistors were developed based on ion-gel and self-assembled monolayer (SAM) high capacitance dielectrics. SAM devices showed high mobilities of ~0.9 cm^2.V^-1.s^-1 and Ion/off ratios of 10^6, as well as cut-off frequencies at 1 kHz. Conversely, ion-gel devices showed higher mobilities (~4 cm^2.V^-1s^-1) and Ion/off ratios of 10^4, but slow response times (~100 ms) due to semiconductor doping.
A rabbit retina cell was stimulated on demand using a SAM organic transistor showing that matrix addressing is viable. Parameters such as gate voltage, source drain amplitude and stimulation pulse width were studied to analyze their impact on the threshold stimulation currents of retinal ganglion cells.
In addition, these transistors were associated with organic photodetectors to create a flexible passive pixel sensor and the first all organic flexible active pixel sensor. The photodetectors had an active area of 5 × 4 mm^2, EQE of 28% and a responsivity of 3.3 mA.W^-1. The passive pixel architecture had better performances than flexible organic devices described in the literature with SAM transistors, having an output signal on/off ratio of 450 under an illumination of 41 mW.cm^-2. The active pixel architecture could be used in retinal prostheses but requires improvements in both photodiode and transistor performances. The pixel created can be used for other applications like X-ray detection, scanners or emotional lighting due to a light amplification with a gain of 40.