1. The Artificial Retina

2. Light enters the eye through the transparent cornea, passing through the pupil at the center of the iris.
The lens adjusts to focus the light on the retina, where it appears upside down and backward.
Receptor cells on the retina send information via the optic nerve to the visual cortex.

3. The left and right eyes each send information to both the left and the right brain hemisphere

5. Photosensitive cells called rods and cones in the retina convert incident light energy into signals that are carried to the brain by the optic nerve.
In the middle of the retina is a small dimple called the fovea or fovea centralis. It is the center of the eye's sharpest vision and the location of most color perception.
When light falls on the retina, it creates a photochemical reaction in the rods and cones at the back of the retina. The reactions then continue to the bipolar cells, the ganglion cells, and eventually to the optic nerve.

6. Retina 120 million rods and 5 million cones
A thin layer (about 0.5 to 0.1mm thick) of light receptor cells covers the inner surface of the choroid.
The focused beam of light is absorbed via electrochemical reaction in this pinkish multilayered structure..
Rods process low level light but do not process color
Cones process color

7. Fovea Centralis/Macula
The eye receives data from a field of about 200 degrees but the acuity over most of that range is poor.
To form high resolution images, the light must fall on the fovea, and that limits the acute vision angle to about 15 degrees.
In low light, this fovea constitutes a second blind spot since it is exclusively cones which have low light sensitivity.
At night, to get most acute vision one must shift the vision slightly to one side, say 4 to 12 degrees so that the light falls on some rods.
Gsu.edu

8. Since the fovea provides the sharpest and most detailed information, the eyeball is continuously moving, so that light from the object of primary interest falls on this region. ...the rods are multiply connected to nerve fibers, and a single such fiber can be activated by any one of about a hundred rods.
By contrast, cones in the fovea are individually connected to nerve fibers. The actual perception of a scene is constructed by the eye-brain system in a continuous analysis of the time-varying retinal image."(Hecht)

9. Pathology of the Eye
Two most common pathologies are age-related macula degeneration (ARMD) and retinitis pigmentosa (RP)
ARMD
Slow degeneration of the photoreceptor cells culminating in death of these cells
Distorted central vision
700,000 new cases in US per year RP
Associated with over 100 genetic defects
Strikes rods first resulting in poor night and peripheral visions
Eventually effects cones
1/4000 people in US have RP
ARMD is more prevalent and RP is generally more severe at a younger age

10. ARMD
There are two forms of ARMD, atrophic (commonly known as dry) and neovascular (commonly known as wet).
All ARMD begins as the atrophic form, in which the nourishing outer layer of the retina withers, or atrophies.
Approximately 90 percent of ARMD remains in this form and progresses slowly.
In the remaining 10 percent, new blood vessels begin to grow erratically within the choroid, the blood-rich membrane that nourishes the retina.
These blood vessels are thin and fragile, and bleed easily.
The resulting hemorrhages cause the retina to swell, distorting the macula and accelerating the loss of cells.
Nih.gov

11. RP Retinitis pigmentosa can run in families.
The disorder can be caused by a number of genetic defects.
The cells controlling night vision (rods) are most likely to be affected. However, in some cases, retinal cone cells are damaged the most.
The main sign of the disease is the presence of dark deposits in the retina.
The main risk factor is a family history of retinitis pigmentosa.
It is an uncommon condition affecting about 1 in 4,000 people in the United States.

12. Artificial Sight Creating a sense of vision by electrically
activating neural cells
in the visual system
Other visual pathways
can be stimulated as well
such as the optic nerve
and visual cortex
Passive devices rely on incident light for power whereas
Active devices have an external power source

13. Which Site is Best for Sight
Retinal implants have advantages
It’s best to place the implant as peripherally to the CNS as possible
Reduces chance of serious infection
Takes advantage of existing signal processing ability to reduce mechanical processing
May make learning easier by involving more potentially plastic systems
More accessible
The retina has a physical mapping system that is easier to understand and decipher

14. Retinal Prostheses Basics
Retinal prosthesis replaces function of the photoreceptors and detects light
There must be viable cells in the inner retina
Signal from prosthetic detected by inner retinal cells– generally via electrical impulses
Chemical signals that replicate neurotransmitter function are also being proposed
Safe, biocompatible, effective and able to withstand the watery, salty eye environment
electrodes would be needed to allow blind patients to read and recognize faces – not yet achieved

15. Retinal Implant Location: Subretinal vs. Epiretinal

16. Subretinal
A microphotodiode array is placed between the inner and outer layers of the retina, between the bipolar cell layer and the retinal pigment epithelium
Concept is to directly replace native photoreceptors with artificial silicon-based photodiodes

17. Subretinal Advantages Disadvantages:
Utilizes the surviving bipolar cells – the next step in the pathway –Retinal processing can take place
Placing the microphotodiodes between layers on the retina will allow for it to be held in position next to functioning cells
Proximity with existing neurons requires less current and leads to better resolution
Disadvantages:
Limited space
Heat damage due to proximity of device to retinal cells
Ambient light may not be adequate to generate current in this array
Outside power source may be needed

18. Subretinal Artificial Devices
Artificial Silicon Retina (ASR) by Optiobionics
2mm by 25 microns (thinner than a human hair)
3,500 solar cells that convert light into electrical pulses
Implanted in the subretinal space
Powered by ambient light

19. Placement of the ASR Device in the subretinal space
Relative size of the ASR Device
Placement of the ASR Device in the subretinal space

20. The ASR Device
The ASR device works by producing visual signals similar to those produced by the photoreceptor layer
These artificial “photoelectric” signals from the ASR microchip induce biological visual signals in the remaining functional retinal cells which may be processed and sent via the optic nerve to the brain
The microchip is designed to interface and function with a retina that has partial outer retinal degeneration

21. Epiretinal Implanted on the surface of the retina
The implant converts externally captured data to a sequence of electrical stimuli
Stimulates ganglia leading to optic nerve activation

22. Epiretinal Advantages Disadvantages
Minimizes the amount of microelectronics implanted and upgrades are easy to do on the wearable portion thus avoiding future surgery
Heat can be dissipated into the vitreous humor
External control over image processing allowing for customizability, possible better clarity
Disadvantages
Difficulty attaching the implant to the fragile inner retina
Complicated processing due to the stimulation at the ganglion which is output of the retina

23. Epiretinal Artificial Devices
Artificial Retina Component Chip (ARCC)
2 mm by 20 microns
Placed on retinal surface
Secondary device attached to a pair of common eyeglasses directs a laser at the chip's solar cells to provide power
Requires small battery pack

24. The ARCC Device
ARCC is powered by an external laser aimed at a photovoltaic cell implanted on the back of the eye
The laser is mounted on glasses that must be worn for the chip to function
The photosensors on the chip convert the light and images into nerve impulses, much like the normal human retina

25. The ARCC Device
This system is, in essence, a video camera which views an image, sends the information of the pattern of light in the image by laser to the photovoltaic cell, which then stimulates the ganglia of the optical nerve to recreate a partial image
Image is a rough pattern of light and dark areas that provides clues on the shape and size of objects being viewed
The electrodes do not pass current to stimulate the ganglia directly. Instead, the electrodes charge a plate that then stimulates the ganglia. This step is intended to reduce the risk of damage to the retinal tissue from the electrical current

26. The ARCC Device

27. Device Complications and Limitations
Biocompatability
Device materials silicon and silicon oxide (for chip itself), titanium and iridium oxide (for wiring and electrodes)
Device pre-clinical trials and preliminary clinical studies show good biocompatability
Surgical Complications
Incision into eye, draining of vitreous gel creates opportunities for infection
Silicon disc difficult to handle in surgery due to flexibility
Physical Complications related to device and eye
Distance between electrodes and targeted cells can result in crossed signals between electrodes and increase in current required, which can be damaging to tissue
Limit of size and density of implantable chip limits resolutions
Fragility and curvature of retina

28. Device Complications and Limitations
Long term Complications
Replacement of vitreous fluid with saline may cause irritation or damage to retinal surface
Irritation or damage due to long term electrical stimulus, and residual heat
Ionic interactions between retinal cells and metallic electrodes may cause long term degradation to tissue
Limitations
Devices are not expected to produce full, clear vision.
Allows patient to perceive basic shapes, direction of movements, boundaries between contrasting objects

29. Device Complications and Limitations: Subretinal
Surgical Complications
Injection of fluid to create space for device in retina, injection of air to close space create further chance of infection
Physical Complications related to device and eye
Coarse edges may damage retina with eye movement
Risk of device damage also due to movement
Long term Complications
Irritation or damage due to long term electrical stimulus, and residual heat
Limitations
Not yet clear whether solar power is sufficient to create threshold stimulus to retinal cells

30. Device Complications and Limitations: Epiretinal
Surgical Complications
Implant trauma
Physical Complications related to device and eye
Movement may cause detachment of device from retinal surface
Fragility and curvature of retina
Long term Complications
Replacement of vitreous fluid with saline may cause irritation or damage to device
Limitations
Head-mounted cameras do not respond to natural eye movement
Small eye movements may be necessary for image to persist

31. Gene Therapy to Treat Blindness http://www. cbsnews