The
main purpose of science is to help people obtain a better understanding of the
world and to give us a better future. Researches in neurological field make us
not only get a closer look to the delicacy of neurons, more importantly, help
human kind to solve neuronal diseases. Scientists have been working on
developing prostheses to improve the quality of lives from patients suffering
from neurodegenerative diseases. The development of retinal implants, like other
prostheses, aim to restore vision for the blind. Here I briefly introduce the
principles of retinal prostheses and summarize some representative projects.
Concepts of Retinal Implants
Retinae
are composed by very well organized layers of neurons, the photoreceptor layer,
the inner nuclear layer and ganglion cell layer (check our other article
‘RETINA: OUR RULES AND CELLS WHO VIOLATE THEM’ for more detail). Most of the
retinal implants are designed to benefit patients from retinitis pigmentosa
(RP) or age-related macular degeneration (AMD), whose photoreceptor layers
degenerate eventually causing
irreversible vision loss. However, most of the patients, even after years of
suffering from these diseases, still have the remaining inner cells and ganglion
cells in well contact (Weiland
et al., 2011). To recover the vision,
all we need is to find a good way to compensate the loss of photoreceptors;
that is, a device that can sense light
and send the vision signals to the remaining retinal neurons. Therefore, the
main task of retinal prostheses is to transform the light signals into
electrical signals that retinae can understand.
Retinal implants are
most commonly implemented in three approaches: subretinal, epiretinal and suprachoroidal (Zrenner,
2013).
In
the subretinal approach, the implant is placed right between the pigment
epithelial layer, which is the layer right next to photoreceptor layer, and the
(lost) photoreceptor layer. This kind of implants are usually made by
light-sensitive photodiodes that make them able to transfer the light into
small currents. They play the role of photoreceptors and rely on the remaining
neuronal network for the rest of signal transduction. The advantages of implants
from subretinal side are therefore 1. Easy positioning 2. Directly replace the
damaged photoreceptors 3. No external cameras are required.
However, they currently face the problem
of power supply, meaning they either need
huge amount of light in order to generate sufficient current, usually a
lot higher than the light from nature environment. Patients today need to carry an external power source which
provides sufficient voltage for stimulation.
On
the other hand, epiretinal implants are placed directly on the retinal ganglion
cell layer. As retinal ganglion cells act as the output of the visual signal to
the brain, implants no longer rely on the remaining neural network; instead,
the implant itself directly transfers the images into electrical pulses to the optic
nerve. For that reason, epiretinal implants are accompanied by external cameras.
The electrical stimuli, compared to subretinal approach, act directly onto the ganglion cells or their
axons and could also help patients even there are barely no remaining healthy
cells. Disadvantages are that they are harder to fix on retina since only one
side of the implant is attached to the retina, they need an extra force to
stabilize the position. More importantly, this approach will need the full
understanding of the activities from dozens types of retinal ganglion cells
without activating axons of passage.
In
the suprachoroidal approach the implant is
placed between choroid and sclera, and is similar to subretinal approach;
however stimulating from a larger distance and therefore requiring larger
electrodes. (Luo
and da Cruz, 2014).
Few
Representative Examples
Argus® II
The
Argus® II prosthesis implement a 60 electrodes micro electrode arrays
(MEAs) in an epiretinal fashion. Images are captured by a video processing unit
adapted to eyeglasses, later on sent to the implant in a wireless way. This implant
has
received a CE mark for medical devices (for Europe) and FDA approval
:Currently more than 100 patients have
received these implants.
This
implant help patients with bare or no light perception to increase their
abilities of recognize and discriminate forms, localize targets, detect motion,
and navigate. The best visual acuity is reported by 20/1262.
Alpha-IMS
Each of the Alpha-IMS implants comprises 1500 photosensitive pixels and is implanted
in the subretinal side of the fovea, the area with the highest visual acuity.
The photodiodes capture light and transfer it into stimulation currents, which activate
downstream inner neurons; The external power supply is magnetically attached to
a subdermal internal coil (Stingl et al., 2013). This
implant has also been commercially available in Europe and is going through human
clinical trial. Patients with Alpha IMS implants restore partially the ability
of recognition of objects and help them avoid dangerous obstacles on the road.
The best reported visual acuity is 20/546.
These are the two most advanced examples of retinal
implants. Other ongoing consortia like, Pixium, Boston Retinal Implant or TSIC, Taiwan Sub-retinal Implant Consortium are developing own implants.
(Cheng et al., 2017)
Challenges and Future
Despite
of the few successful cases and
advances made over the last
decade, there are still challenges for retinal implants.. How are the effects
to the chronic stimulation to both the function of the implants and to the
remaining retinal neurons remain unclear. The resolution that implants can
provided is another important issue. Eeven the best implant so far can only
allow patients see objects vaguely. To increase the visual acuity, there are still
a lot of engineering challenges to overcome. Other issues like the significant
remodeling of neural network after photoreceptor degeneration or the lack of
understanding of the interface between retina and the implants are the open
questions to be answered.
Although
there are difficulties to conquer, simple light sensitivity already help blind
people greatly improve their quality of lives. More studies to the fundamental
retinal neurosciences are going to help us explore the possibility and to break
through the boundaries of technology. In the future, implants with better
spatial and temporal resolution can be expected.
Current Status of Projects
(Cheng
et al., 2017)
References
Cheng, D.L.,
Greenberg, P.B., and Borton, D.A. (2017). Advances in Retinal Prosthetic
Research: A Systematic Review of Engineering and Clinical Characteristics of
Current Prosthetic Initiatives. Curr Eye Res
42, 334-347.
Luo, Y.H., and da Cruz, L. (2014). A review and update on the
current status of retinal prostheses (bionic eye). Br Med Bull 109,
31-44.
Stingl, K., Bartz-Schmidt, K.U., Besch,
D., Braun, A., Bruckmann, A., Gekeler, F., Greppmaier, U., Hipp, S.,
Hoertdoerfer, G., Kernstock, C., et al.
(2013). Artificial vision with wirelessly powered
subretinal electronic implant alpha-IMS. Proceedings of the Royal Society
B-Biological Sciences 280, 20130077.
Weiland, J.D., Cho, A.K., and Humayun, M.S. (2011). Retinal
Prostheses: Current Clinical Results and Future Needs. Ophthalmology 118, 2227-2237.
Zrenner, E. (2013). Fighting
blindness with microelectronics. Science translational medicine 5, 210ps216-210ps216.
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2 Comments
Thanks for sharing this information it is really useful for all of us.
Reply Deletemicro pigmentation boston
Your very welcome! :)
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