16-03-2011, 02:49 PM
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Abstract
In this paper, the current version of the artificial retina prosthesis and cortical implant that is being developed will be described. A multi disciplinary team of researchers in Ophthalmology, Neurosurgery, Computer Networking, VLSI, and Sensors has been assembled to develop the novel solutions needed to make artificial vision for the visually impaired a reality. This paper describes the novel approach that has been adopted for providing a complete system for restoring vision to visually
Impaired persons – from the signals generated by an external camera to an array of sensor that electrically stimulate the retina via a wireless interface.
Introduction
In this paper, we describe the current version of the artificial retina prosthesis and cortical implant that is being developed. This research is a multidisciplinary project 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 prostheses may be used to restore visual perception to persons suffering from retinitis pig mentosa, 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 prostheses 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 16 array of sensors and is being fabricated by MOSIS based on the circuit design created in our Smart Sensors and Integrated Devices (SSID) research lab. Our earlier circuit design, which has been fabricated and tested, supports a 10 array of sensors. The package is encapsulated in inert material except for the micro bumps, 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, for example, a pacemaker. Instead, we plan to power the device 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, we have adopted the novel approach of using two frequencies: RF inductance using a frequency of 5 MHz and RF data transmission using a frequency in the range of either 900 MHz or 2.4 GHz. The FCC regulations for low power non ¬licensed transmitters are explained in [1].
Retinal and Cortial 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 the simplified processing that 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 epi¬retinal 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 of 98.6o Fahrenheit. 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, however, that although heat dissipation remains a concern with a cortical implant, the natural heat dissipation within the skull is greater than within the eye.
Given the current state of the research, it is unclear which of these disadvantages will be most difficult to overcome for a chronically implanted device. Therefore, different research groups are investigating different solutions. Here we describe our proposed solution. An implantable version of the current ex-vivo micro sensor array, along with its location within the eye, is shown in Figure 1. The micro bumps 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.
Figure 1: Location of the Smart Sensor within the Eye
As shown in Figure 1, the front side of the retina is in contact with the micro sensor array. This is an example of an epiretinal implant. Transmission into the eye works as follows. The surface of the retina is stimulated electrically, via artificial retina prosthesis, by the sensors on the smart sensor chip. These electrical signals are converted into chemical signals by the ganglia and other underlying tissue structures and the response is carried via the optic nerve to the brain. Signal transmission from the smart sensors implanted in the eye works in a similar manner, only in the reverse direction. The micro sensors pick up the resulting neurological signals from the ganglia and the signal and relative intensity can be transmitted out of the smart sensor. Eventually, the sensor array will be used for both reception and transmission in a feedback system and chronically implanted within the eye. Although the micro sensor array and associated electronics have been developed, they have not yet been tested as a chronic implant. Another challenge at this point is the wireless networking of these micro sensors with an external processing unit in order to process the complex signals to be transmitted to the array.
Smart sensor chip design
Figure 2 shows a close ¬up of the smart sensor shown in figure1. Each micro bump array consists of a cluster of extrusions that will rest on the surface of the retina. The small size of the micro bumps allows them to rest on the surface of the retina without perforating the retina. In addition, the slight spacing among the extrusions in each micro bump array provides some additional heat dissipation capability. Note that the distance between adjacent sets of micro bumps is approximately 70 microns
