Artificial Retina Using Thin-film Transistors Driven By Wireless Power Supply
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Blindness is caused by damage to clear structures in the eye, that allow the light to pass through, the nerves within the eye, the Optic nerve and brain. Various diseases of eye are
1) Retinitis Pigmentosa - Hereditary genetic disease. It is caused by degeneration of the retina. Gradually progress towards center of eye. Spares the foveal region. Tunnel vision results.
2) Macular Degeneration - This is also genetically related. Cones in macula region degenerate. Loss or damage of central vision. Common among aged people. Peripheral retina spared.
Artificial Thin-Film Transistor Retina
Artificial Thin-Film Transistor Retina recovers the sight sense for sight-handicapped people. Electronic Photo devices and circuits substitutes deteriorated photoreceptor cells. Implanted inside the eyes. Implanting classified into two types: Epiretinal implant and Subretinal implant. Thin-Film Transistors, fabricated on transparent and flexible substrates. Implantable microelectronic retinal prostheses. Externally worn digital camera which samples the wearers visual environment. The first application of an implantable stimulator for vision restoration.
Epiretinal implant - They sit in the inner surface of the retina. They bypass a large portion of the retina. Provides visual perception to individuals. The implants receive input from a camera. Electrodes from the implants electrically stimulate the ganglion cells and axons at the start of the optic nerve.
Subretinal implant - Subretinal implants sit on the outer surface of the retina. Directly stimulates the retinal cells. Replace damaged rods and cones by Silicon plate carrying 1000’s of light-sensitive micro photodiodes each with a stimulation electrode. Light from image activates the micro photodiodes, the electrodes inject currents into the neural cells.
The epiretinal implant have high image resolutions. Stimulus signal can be directly conducted to neuron cells. Here living retinas are not seriously damaged. The input to the Epiretinal Implant is more easily controlled.
Fabrication of Thin-Film Transistor
Low temperature poly-Si TFTs have been developed. For integrated drivers, CMOS configurations are used. High speed operation due to parasitic capacitance in Self-Aligned TFT’s. Ion implantation is one of the key factors in fabricating such as TFTs and CMOS configurations. Ions from discharged gas are accelerated by an extraction electrode and an acceleration electrode. Then implanted into the substrate. Impurities can be implanted over the entire 300 mm square substrate. Maximum accelerating voltage of over 110 KeV which is sufficient for implanting impurities through the 150nm SiO2 gate insulator.
Fabrication of Thin-Film Transistor
New Masking Technique - An SiO2 buffer layer is deposited on the glass substrate. Then, pad poly-Si patterns are formed for source and drain regions. A 25 nm channel poly-Si layer is deposited by low pressure chemical vapor deposition (LPCVD) at 600 Degree Celsius. Then a 150 nm SiO2 gate insulator is deposited by electron cyclotron resonance chemical vapor deposition (ECR-CVD) at 100 Degree Celsius in a vacuum. Then, a Cr film is deposited at 180 Degree Celsius. First, only p-channel gate electrodes are formed. The next step is to form source and drain regions of p-channel TFTs by the new I/D technique. Boron ions are implanted through the gate insulator with a dose of of 80 keV. N-channel gate electrodes are also formed and phosphorus ions are implanted with a dose of 110 keV by the new I/D technique. Impurities are activated by a XeCl excimer laser.
White light from a halogen lamp is reflected by a triangular prism and irradiated through the glass substrates to the back surfaces of the TFPT. Although the light from a halogen lamp includes the light from 400 to 750 nm with a peak around 600 nm and is therefore reddish despite a built-in infrared filter.
Wireless Power Supply Using Inductive Coupling
Inductive coupling of magnetic field. Electrical energy can be easily converted to magnetic energy and back using conductive coil. Traditionally, a pair of inductive coils are used. The secondary coil can be located within the eye and the primary coil external to the eye. Problems are first problem is difficulty in placing a large receive coil inside the eye and complicated surgical procedure.
Major challenge in implementing a wireless power are Large separation between the coils, Constant relative motion between the primary and secondary coils, Reduction in power transfer to the device. In order to overcome these problems use of an intermediate link. The secondary coil is located under the sclera (eye wall) and is connected to the implanted device via electrical wires which are embedded under the wall of the eye. The transmit coil is placed at the back of the ear. Intermediate coils are positioned with one end on the sclera over the receive coil and the other end under the skin beneath the transmit coil. Immunity to variation in coupling due to rapid movements of the eye as relative motion between adjacent coils is restricted. To increase the power transfer efficiency compared to a one-pair coil system.