A BIOMEDICAL SMART SENSOR FOR THE VISUALLY IMPAIRED
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21-02-2011, 03:19 PM
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In this paper, we describe the current version of the artificial retina prosthesis and cortical implant that we are developing. This research project and implimentation will require significant advances in a variety of disciplines. A multidisciplinary team of researchers in Ophthalmology, Neurosurgery, Computer Networking, VLSI, and Sensors were assembled to develop the novel solutions needed to make artificial vision for the visually-impaired a reality. This paper describes the novel approach that we have adopted to provide a complete system for restoring vision to visually-impaired persons - from the signals generated by an external camera to an array of sensors that electrically stimulate the retina via a wireless interface.
In this paper, we describe the current version of the artificial retina prosthesis and cortical implant that we are developing. This research is a multidisciplinary project and implimentation involving researchers in Ophthalmology, Neurosurgery, Computer Networking, Sensors, and VLSI. Restoring vision to the blind and visually impaired is possible only through significant progress in all these research areas. In the future, artificial retina prosthesis may be used to restore visual perception to persons suffering from retinitis pigmentosa, macula degeneration, or other diseases of the retina. In patients with these diseases, most of the rods and cones are destroyed, but the other cells of the retina are largely intact. It is well known that the application of electrical charges to the retina can elicit the perception of spots of light. By coupling novel sensing materials with the recent advances in VLSI technology and wireless communication, it is now feasible to develop biomedical smart sensors that can support chronic implantation of a significant number of stimulation points. Although the development and use of artificial retina prosthesis is still in the early stages, the potential benefits of such technology are immense. Similarly, the use of cortical implants has promise for the visually impaired. Unlike the retina prosthesis, a cortical implant bypasses most of the visual system, including the eye and the optic nerve, and directly stimulates the visual cortex, where information from the eyes is processed. Therefore, in addition to overcoming the effects of diseased or damaged retina tissue, a cortical implant could circumvent many other problems in the visual system, including the loss of an eye. The smart sensor package is created through the backside bonding of an array of sensing elements, each of which is a set of microbumps that operate at an extremely low voltage, to a integrated circuit for a corresponding multiplexed grid of transistors that allows individual voltage control of each micro bump sensor. The next generation design supports a 16x 16 array of sensors and is being fabricated and tested, supports a l0x10 array of sensors. The package is encapsulated in inert material except for the microbumps, which must be in contact with the retina. The long-term operation of the device, as well as the difficulty of physically accessing a biomedical device implanted in the eye, precludes the use of a battery-powered smart sensor. Because of the high volume of data that must be transmitted, the power consumption of an implanted retinal chip is much greater than, a pacemaker. Initially the device was planned to power using RF inductance. Because of the difficulties of aligning the two coils, one being within the body and the other one outside the body. For RF power transmission, a low frequency is required to tolerate misalignment of the coils. On the other hand, a relatively high frequency is required to operate in the unlicensed ISM band. For this reason, the novel approach was adopted using two frequencies: RF inductance using a frequency of 5 MHz and RF data two frequencies: RF inductance using a frequency of 5 MHz and RF data.
RETINAL AND CORTICAL IMPLANTS
Proposed retina implants fall into two general categories
• Epiretinal, which are placed on the surface of the retina.
• Subretinal, which are placed under the surface of the retina.
Both approaches have advantages and disadvantages. The main advantages of the sub-retinal implant are that the implant is easily fixed in place, and only the simplified processing is involved, since the signals that are generated replace only the rods and cones with other layers of the retina processing the data from the implant.
The main advantage of the epiretinal implant is the greater ability to dissipate heat because it is not embedded under tissue. This is a significant consideration in the retina. The normal temperature inside the eye is less than the normal body temperature.. Besides the possibility that heat build-up from the sensor electronics could jeopardize the chronic implantation of the sensor, there is also the concern that the elevated temperature produced by the sensor could lead to infection, especially since the implanted device could become a haven for bacteria.
There are also two options for a cortical implant. One option is to place the sensors on the surface of the visual cortex. At this time, it is unknown whether the signals produced by this type of sensor can produce stimuli that are sufficiently localized to generate the desired visual perception. The other option is to use electrodes that extend into the visual cortex. This allows more localized control of the stimulation, but also presents the possibility of long-term damage to the brain cells during chronic use. It should be noted that although heat dissipation remains a concern with a cortical implant, the natural heat dissipation within the skull is greater than within the eye.
An implantable version of the current ex-vivo microsensor array, along with its location within the eye, is shown in Figure 1. The microbumps rest on the surface of the retina rather than embedding themselves into the retina. Unlike some other systems that have been proposed, these smart sensors are placed upon the retina and are small enough and light enough to be held in place with relatively little force. These sensors produce electrical signals that are converted by the underlying tissue into a chemical response, mimicking the normal operating behavior of the retina from light stimulation. The chemical response IS digital (binary), essentially producing chemical serial communication. A similar design is being used for a cortical implant, although the spacing between the micro bumps is larger to match the increased spacing between ganglia in the visual cortex
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