Ocular prosthetic device or prosthetic eyes is a type of implantable device, designed based on the construct of functional electrical stimulation ( FES ) to partially rehabilitate the vision map of the unsighted eyes. Optical image acquisition, signal processing and nervus stimulation can be realized with this device ( Zhou et al. , 2006 ) .
To plan an effectual prosthetic oculus foremost requires a thorough apprehension of assorted facets such as the physiology of the human oculus, the procedures involved in the formation of ocular perceptual experience, the causes of vision loss, the different attacks in reconstructing vision every bit good as the design methods, stuffs and other considerations.
The human oculus consists of assorted constituents which can be classified harmonizing to their maps. It is a pressurized domain with constituents that perform three chief maps ( Weiland et al. , 2006 ) :
Optical constituent which consists of the cornea, student, vitreous organic structure and lens to concentrate an image on the retina.
Sensory constituent which consists of rods and cones at the dorsum of the retina near the choroid that maps as photoreceptors that captures incident photons.
Nervous constituents which consists of bipolar nervous cells in the interior retina to change over light information into nervous electrical signals.
Figure 1: The Human Eye [ J3 ]
It is known that retinitis pigmentosa ( RP ) and age-related macular devolution ( AMR ) are the two prima causes of sightlessness ( Liu et al. , 2000 ) . These diseases result in the devolution of the photoreceptors of the retina while a significant sum of retinal ganglion cells remain unaffected. Since most of the ocular damage job Begin in the retina, a thorough apprehension of retina physiology is important in the design of a device that can potentially reconstruct some grade of vision to the unsighted patient.
The retina is the neural front terminal of the ocular system – the image detector. It has a thickness of about 0.5 millimeters and it lines the dorsum of the oculus. It offers the first measure in border sensing and color analysis of an image and consists of five distinguishable beds ( Stetten, 2000 ) :
Photoreceptor layer – These are photosensitive pigments, to change over incident photons into chemical energy, made up of rod and cone cells. Cones map in bright visible radiation and rods frailty versa. There are about 125 million receptors in each oculus.
Bipolar cells – First degree of information processing. Its map is to react at the boundary line between dark and light countries. Hence, it demonstrates “ centre-surround ” receptive field, intending that if a little point on the retina elicit a bright response, the environing elicit an opposite response. The “ Centre ” response comes from the photoreceptors while the “ environment ” response from the horizontal cells.
Horizontal cells – Provide surround response to the bipolar cells.
Amacrine cells – Involved in the sensing of gesture.
Ganglion cells – Here is where most of the informations compaction occurs. These cells are triggered by bipolar cells, and the nerve cells merely fire when there is meaningful information ( sensing of boundary line between light and dark ) . The ganglion cell axons ( about 1 million of them ) jointly form the ocular nervus.
Ocular input begins when light enters the oculus, passes through a series of crystalline beds ( cornea, aqueous wit, lens and vitreous organic structure ) and encounters the photoreceptors at the dorsum of the retina. The receptors connect through an intermediate bed of crystalline nervus cells to the ganglion cells and leave the retina through the ganglion cells axons or ocular nervus ( as illustrated in Figure 2 ) which extends to the ocular cerebral mantle of the encephalon. The ocular nervus is the lone beginning of ocular input to the encephalon ( Wang et al. , 2005 ) . However, how the encephalon processes the input into complex and elaborate ocular perceptual experiences is still non to the full understood to this twenty-four hours.
Figure 2: The Pathway in the Procedure of Visual Perception ( Wyatt et al. , 1996 )
As there are over 100 million photoreceptors and ganglion cells in the oculus, it is impossible to replace them. However, there is a possibility of utilizing whatever that remains to reconstruct some grade of vision.
Approachs to the Design of Prosthetic Eye Implants
It has been known since the late sixtiess that blind worlds can comprehend electrically elicited phosphenes in response to either retinal or cortical stimulations ( Weiland et al. , 2006 ) . A phosphene is an entoptic phenomenon ( ocular effects whose beginning is within the oculus itself ) characterized by the experience of seeing visible radiation without light really come ining the oculus. Phosphenes can be straight induced by mechanical, electrical, or magnetic stimulation of the retina or ocular cerebral mantle every bit good as by random fire of cells in the ocular system.
There are four different attacks ( Zhou et al. , 2006 ) used in the design of ocular prosthetic device to excite nervousnesss in different locations of the oculus:
Sub-retinal prosthetic device
Epi-retinal prosthetic device
Ocular nervus prosthetic device
Cortical prosthetic device
The first experimental work started with electrical stimulation of the ocular cerebral mantle utilizing a grid of big surface electrode. It was performed by Giles Brindley in 1968. An 80-electrode device was implanted on the ocular cortical surface of a 52-year old topic, blind from terrible glaucoma and retinal withdrawal in the left oculus. With this system, the patient was able to see light points in 40 different locations in the ocular field, showing that at least 50 % of the electrodes were functional. However, there are several drawbacks – big stimulation currents were needed and the phosphenes observed by the topic were rather big. Ocular nervus has besides been experimented but without bring forthing any favorable consequences because of the complex construction of the ocular nervus which is about 1 – 2 millimeter in diameter but packed with 1.2 million fibres within it. Stimulation of a individual electrode can bring forth multiple percepts in the ocular field and consequence in the unwanted perceptual experience of big fuzz ( Weiland et al. , 2006 ) .
Hence, recent researches on the design of ocular prosthetic device have been narrowed down to two attacks – sub-retinal and epi-retinal prosthetic device. An epi-retinal implant is placed on the interior restricting membrane of the retina while a subretinal implant is inserted in the photoreceptor bed. Because of its thin surface and broad country, the retina allows for the implant of a microelectrode array and more localised electrical stimulation to bring forth a more elaborate phosphene form. The retinal receptive field belongingss are good understood. Therefore, electrical stimulation of an country in the retina produces the perceptual experience of visible radiation in predictable locations.
Table 1 compares the two attacks and inside informations the advantages and disadvantages in footings of image processing, device size, surgery and fond regard of device to the retina ( Weiland et al. , 2006 ) .
Disadvantage: If ganglion cells are stimulated, so the nonlinear image processing performed in the retina may necessitate to be replicated
Advantage: If the bipolar cells can be activated and retinal web is integral, the sub-retinal stimulator is closer to the bipolar cells and can utilize more of retinal processing
Advantage: Vitreous pit provides ample infinite to suit device with minimum break to retina
Disadvantage: Sub-retinal infinite is limited, therefore big constituents must be placed outside oculus orbit
Advantage: Surgical process of the vitreous pit can be performed easy
Disadvantage: Surgery may be complicated as it involves nearing the retina from the dorsum of the oculus
Attachment of device to the retina
Disadvantage: Design of epi-retinal electrode array must conform to curvature of retina without doing harm to the retinal inner liners
Advantage: It has been by experimentation proven that the retina can keep a 3-mm-diameter sub-retinal device in topographic point without doing retinal withdrawal
Table 1: Comparison between Epi-retinal And Sub-retinal Implants in Footings of Design Consideration
Aspects of Design and Models of Prosthetic Eyes
Basic Concept of A Retinal Prosthesis
Basically, all retinal prosthetic devices ( both epi-retinal and sub-retinal ) have three chief systems:
Optical image acquisition – a micro camera made from biocompatible stuffs, which can be either charge-coupled device ( CCD ) which is an electronic array displacements and accumulate charges at one corner of the array or complementary metal-oxide semiconducting material engineering ( CMOS ) image detector with transistors at each pel to magnify and travel electrical charges utilizing traditional wires. This is where visible radiation from the image is captured and converted into electrical signals and maps as replacement for the loss of photoreceptor cells.
Signal ( Data ) processing – this acts like a microprocessor in the retinal prosthetic device and it is capable of executing maps such as transition, elaboration, rectification and transition of the electrical signals acquired from the optical image acquisition system.
Nerve stimulation – in the retinal prosthetic device, ocular perceptual experience is formed when the retinal nerve cells – ganglion cells ( epi-retinal ) or photoreceptors ( sub-retinal ) are electrically stimulated utilizing an implantable microelectrode array. The presence of more electrodes will bring forth a more distinguishable image.
These three constituents can be either extra-ocular or intra-ocular, depending on the design of the prosthetic device. An extra-ocular device is placed outside of the oculus whereas an intra-ocular device is implanted within the oculus. Data transmittal between the three systems mentioned supra is done either through overseas telegram or radio connexion. Besides, there is besides a demand for power supply, which could be an external battery or generated through other agencies.
Figure 3: Basic Concept of a Retinal Prosthesis
Examples of Retinal Prosthesis
Three different theoretical accounts which are taken from ongoing research in this field will be discussed.
Model 1: The MIT-Harvard Device [ J2 ]
Optical image acquisition constituents: CCD camera
Signal processing constituents: signal processing bit mounted on a brace of dark glassess
Nerve stimulation constituents: stimulator bit powered by a photodiode array
Microelectrode array implant location: inner restricting membrane ( epi-retinal )
Power beginning: portable external batteries
Principle of operation:
The image in forepart of the topic will be captured by the bantam CCD mounted on a brace of dark glassess.
The end product from the CCD will be modulated onto a bearer signal from a little, fixed-direction optical maser on the spectacless.
The optical maser beam powers the stimulator bit with the assistance of the photodiode array and at the same time conveys the ocular information.
Stimulator bit directs current to the electrode array to electrically excite specific locations of the ganglion cells.
Advantages: ganglion cells are well-arranged ; easy to entree surgically
Disadvantages: axons could interfere because of its location between the electrodes and the ganglion cells and may ensue in unwanted perceptual experience of a big fuzz
Figure 4: The MIT-Harvard Device ( Collaboration between Massachusetts Institute of Technology Research Lab of Electronics and Harvard Medical School Eye and Ear Infirmary )
Model 2: EPI-RET-3 Implant System [ J4 ]
Optical image acquisition constituents: CMOS image detector
Signal processing constituents: extra-ocular portable computing machine system with transmitter unit mounted on monocle frame
Nerve stimulation constituents: receiving system unit implanted in an unreal lens and a set of 25 three-dimension stimulation electrodes
Microelectrode array implant location: ganglion cell bed ( epi-retinal )
Power beginning: rechargeable internal batteries inside monocle frame
Principle of operation:
The image acquired is processed by the external computing machine system to bring forth simple forms.
The forms are transformed into stimulation pulse sequence.
These informations will modulate the bearer frequence and transmitted wirelessly utilizing inductive yoke method. Power is besides transmitted in the same mode.
The electromagnetic signal is acquired by the receiving system spiral and forwarded to the stimulator bit.
Based on the informations received, the stimulator bit activates the selected electrodes in the upper ganglion cell bed and later triggers phosphenes in the ocular cerebral mantle to make a ocular esthesis.
Advantages: no overseas telegram connexion needed ; 3-D electrode provide good contact to the ganglion cells ;
Disadvantages: an array of 25 electrodes is merely sufficient to arouse images of simple forms
Figure 5: EPI-RET-3 retina implant system ( A undertaking funded by the German Ministry of Education and Research )
Model 3: Hermetically-Encased Wireless Sub-retinal Prosthesis [ J7 ]
Optical image acquisition constituents: separate external hardware constituents required ( non described in this design but is similar to old two theoretical accounts mentioned above )
Signal processing constituents: accountant micro chip
Nerve stimulation constituents: micro-fabricated array of sputtered Ir oxide movie ( SIROF ) electrodes
Microelectrode array implant location: sub-retinal infinite
Power beginning: transmitted wirelessly from external power supply
Principle of operation:
Image is acquired from external optical device.
A computer-based accountant with a user interface determines which electrodes to drive and the stimulation current degree.
Data signal and power is transmitted wirelessly to the implant ( shown in Figure 6 )
The implant receives the power and informations via the receiving system spirals right behind the conjunctiva and processes the informations through the circuitry enclosed in the hermetic instance.
The implant sends electrical stimulation current to the sub-retinal nervus cells ( i.e. photoreceptor bed cells ) through a thin-film micro-fabricated SIROF to make a ocular perceptual experience.
Advantages: big receiving system spirals allows for better information and power telemetry ; hermetic Ti instance enveloping the circuitry has 10-year survivability and suited for longer term nidation ; snaky electrode array that extends to the dorsum of the oculus ball provides easier entree into the sub-retinal infinite.
Disadvantages: hazard of infection due to arrangement of receiving system spirals on the conjunctiva, which is delicate and sensitive
Figure 6: Hermetically-encased radio sub-retinal prosthetic device
Important Factors in the Design of A Prosthetic Eye
External hardware such as monocle causes few jobs to the user. Normally, it is the internal hardware that is implanted in the oculus that creates most complications particularly because of its invasive nature and contact with delicate tissues. Besides, certain signal processing facets besides has to be considered. These are some of the important factors [ J9 ] that need to be taken into history in the design procedure of a functional prosthetic oculus:
Number of channels: The figure of channels in a retinal prosthetic device depends on the size of the stimulation electrodes. Each channel maps likewise to a pel in a digital image. Due to the limited infinite in the retina, bigger electrodes will ensue in fewer available channels. However, most of the recent experiments use less than 25 electrodes which are merely sufficient to make simple ocular perceptual experience such as alphabets and forms.
Size and form of stimulator electrode: With smaller electrodes there can be more channels. However, the smaller the size of the electrode, the higher the electric resistance becomes. This means that the stimulator will necessitate to defy higher conformity electromotive force of up to 10V. As for form, it needs to hold rounded corners to minimise retinal tissue harm when implanted.
Stimulus wave forms: There needs to be big tuning scope and programmability to provide persons with varied threshold. A survey based on coneies suggests that shorter pulsation train in sub-milliseconds is more effectual.
Power and informations telemetry: The transmittal of power or informations from the between the different constituents of the oculus prosthetic device such as power supply, image detector, microcontroller and electrode array is best done wirelessly, utilizing wireless frequence ( RF ) waves to debar possible tissue infection or hurt due to transdermal wires inside the oculus ball. Power input of 100 mW to the implant and information rate of 2 Mbps is sufficient to back up high-density stimulation.
Stimulator current injection: Recent clinical tests [ J3 ] have found that stimulus current in the scope of 300 to 500 AµA per msec is adequate to arouse response from the retinal nerve cells. Safe current bound lessenings with electrode size and frequently expressed in C. It is between 0.9 to 50 North Carolinas.
Flexibility of microelectrode array: A retinal microelectrode array must be flexible to conform to the spherical form of the retina. This can be achieved with the usage of polymer substrate such as polydimethyl siloxane ( PDMS ) and parylene.
Biocompatibility of fiction stuffs: The most critical issue in the design of the oculus prosthetic device is the interface between the electrodes and the nervous tissues. Traditionally, rare metals such as gold, Pt, Ir and Ti nitride are used for doing the stimulation electrodes. However, drawn-out usage of metal parts on these tissues consequences in glioma collection and cicatrix formation. To get the better of such jobs, research [ J10 ] is under manner to replace conventional electrodes with conductive polymers. These polymers should hold high affinity to biological tissues and can excite the nerve cells intracellularly, therefore diminishing the threshold current value.
Surgical processs: All oculus implants require some signifier of surgery, normally on the lens or retina of the oculus. The surgical process to infix an epi-retinal implant is normally done through the vitreous pit which is comparatively easy as compared to a sub-retinal prosthetic device which requires surgery through the back sclerotic coat of the oculus. Another factor to be considered is the hazard during or post-operation such as infection, inflammatory reaction, neo-vascularization or bleeding.
To work towards a fully-functional oculus prosthetic device, foremost it requires a thorough apprehension of the ocular system such as the anatomical construction of the oculus and the maps of the different constituent or beds in the oculus. Following, the attack to concentrate on has to be decided, i.e. the location of the implant. It could be the ocular cerebral mantle, ocular nervousnesss, epi-retinal bed or sub-retinal bed.
A strong cognition in the construct of prosthetic oculus is besides required. Basically, the prosthetic oculus should be able to execute three basic maps i.e. optical image acquisition, signal processing and electrical nervus stimulation. A complete and functional oculus prosthetic device normally consists of both extra-ocular ( such as monocle frame, image detector, power supply, processor circuitry and sender unit ) and intra-ocular ( such as power/data receiving system and microelectrode array ) constituents. Because of its complicated nature, most research workers work on a specific facet of the design, and work collaboratively, with each concentrating on his or her field of expertness such as circuit design, hardware design, package scheduling, telemetry, material choice and implant method. Some of the factors that need to be taken into consideration in the design procedure include figure of channels, size and form of stimulator electrode, stimulus wave form, power and informations telemetry, stimulator current injection, flexibleness of microelectrode array, biocompatibility of fiction stuffs and surgical processs.
To day of the month, most of the research in this country is still in the experimental phase, with largely field tests done on non-human topics. There are besides some clinical tests done on human topics with some promising consequences. Experimental consequences show that functional electrical stimulation of the retinal nervus is a possible manner to reconstruct some grade of utile vision in unsighted topics but there are still tonss of fine-tuning to be done at the design degree before such a device can be implemented successfully.