Abstract
In the current scenario, where over millions of people are affected by visual anomalities, it was with a challenge that this project came into being. It aims at restoring vision to the blind. Today, high-tech resources in microelectronics, Optoelectronic, computer science, biomedical engineering and also in vitreoretinal surgery are working together to realize a device for the electrical stimulation of the visual system.
Electrical Signals From The Brain - Vep
A special part of the brain, the visual cortex, is believed to be the entrance structure to visual perception and cognition Activity of nerve cells within the brain's surface (the cortex) produce electrical fields that can be picked up at some distance with electrodes (like ceiling microphones pick up sound from instruments in an orchestra during a concert). In humans these electrodes are simply "glued" on the scalp with a sticky paste on the back of the head. In the pig model special arrays of electrodes fixed on a silicone-carrier (Fig.1 B) are placed under the scull bone above the duration by neurosurgeons (Fig.1 A) and can be left there for several months.
About
The retina is a thin layer of neural tissue that lines the back wall inside the eye. Some of these cells act to receive light, while others interpret the information and send messages to the brain through the optic nerve. This is part of the process that enables us to see. In damaged or dysfunctional retina, the photoreceptors stop working, causing blindness. By some estimates, there are more than 10 million people worldwide affected by retinal diseases that lead to loss of vision. At present, two general strategies have been pursued. The “Epiretinal” approach involves a semiconductor-based device placed above the retina, close to or in contact with the nerve fiber layer retinal ganglion cells. The information in this approach must be captured by a camera system before transmitting data and energy to the implant. The “Sub retinal” approach involves the electrical stimulation of the inner retina from the sub retinal space by implantation of a semiconductor-based micro photodiode array (MPA) into this location. The concept of the sub retinal approach is that electrical charge generated by the MPA in response to a light stimulus may be used to artificially alter the membrane potential of neurons in the remaining retinal layers in a manner to produce formed images.
Cortical Implants
Scientists have created a device that allows them to communicate directly with large numbers of individual nerve cells in the visual part of the brain. The device is a silicon electrode array may provide a means through which a limited but useful visual sense may be restored to profoundly blind individuals. This shows the development of the first visual prosthesis providing useful "artificial vision" to a blind volunteer by connecting a digital video camera, computer, and associated electronics to the visual cortex of his brain. This device has been the objective of a development effort begun by our group in 1968 and represents realization of the prediction of an artificial vision system made by Benjamin Franklin in his report on the "kite and key" experiment.
Implant Design And Fabrication
The current micro photodiode array (MPA) is comprised of a regular array of individual photodiode subunits, each approximately 20×20-µm square and separated by 10-µm channel stops. Across the different generations examined, the implants have decreased in thickness, from ~250 µm for the earlier devices, to approximately 50 µm for the devices that are currently being used. Because implants are designed to be powered solely by incident light, there are no connections to an external power supply or other device. In their final form, devices generate current in response to a wavelength range of 500 to 1100 nm. Implants are comprised of a doped and ion-implanted silicon substrate disk to produce a PiN (positive-intrinsic-negative) junction. Fabrication begins with a 7.6-cm diameter semiconductor grade N-type silicon wafer. For the MPA device, a photo mask is used to ion-implant shallow P+ doped wells into the front surface of the wafer, separated by channel stops in a pattern of individual micro photodiodes. An intrinsic layer automatically forms at the boundary between the P+-doped wells and the N-type substrate of the wafer. The back of the wafer is then ion-implanted to produce a N+ surface.
Stimulation Parameters
Stimulation delivered to each electrode typically consists of a train of six pulses delivered at 30 Hz to produce each frame of the image. Frames have been produced with 1-50 pulses, and frame rates have been varied from 1 to 20 frames per second. As expected, (4) frame rates of 4 per second currently seem best, even with trains containing only a single pulse. Each pulse is symmetric, biphasic (-/+) with a pulse width of 500 µsec per phase (1,000 µsec total). Threshold amplitudes of 10-20 volts (zero-peak) may vary +/-20% from day to day; they are higher than the thresholds of similar electrodes without the ground plane, presumably because current shunts across the surface of the pia-archnoid and encapsulating membrane.
Conclusion
The application of the research work done is directed towards the people who are visually impaired. People suffering from low vision to, people who are completely blind will benefit from this project. The findings regarding biocompatibility of implant materials will aid in other similar attempts for in human machine interface. Congenital defects in the body, which cannot be fully corrected through surgery, can then be corrected.
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