Towards the development of novel retinal implants: electrical and photo-stimulation of dystrophic retinas with carbon nanotube electrodes

Hereditary retinal degeneration (e.g. retinitis pigmentosa) and age-related macular degeneration (AMD) are among the commonest causes of blindness in the developed world . These devastating conditions are characterised by photoreceptor degeneration. Retinal ganglion cells (RGCs, the output cells of the retina) do, however survive and maintain their connections with the brain visual areas, so that under appropriate conditions, direct RGC electrical stimulation via implantable stimulating devices can elicit light perception (phosphenes) in blind patients. Several groups worldwide are working on improving the technology for retinal implants, but many technical challenges remain to be resolved before retinal prosthetic devices will become a realistic approach to help blind patients to regain sight. The choice of electrode material is very important. Electrodes must be biocompatible and capable of delivering enough electrical charge to the tissue. In that respect, the basic hypothesis driving our research is that advanced materials are more performing than conventional ones. Through a collaboration with Dr Yael Hanein (Tel-Aviv university, Israel), we have recently established that carbon nanotubes (CNTs) offer great advantages over more conventional electrode materials for retinal implant technology because they are highly biocompatible and they have a very large surface area, which makes them very efficient for electrical stimulation. One of the very attractive features of CNTs is that they can be functionalised in order to modify/improve their biological performance and this is what we are planning to investigate in this project. We are going to use CNT electrodes that have been modified by Dr Hanein so that they can generate electrical current when stimulated with light rather than with an external stimulating device. Dr Hanein has successfully conjugated CNTs to light-sensitive quantum dots (QDs). QDs are tiny crystals; when excited with light of the appropriate colour (wavelength; the dot size determines the precise excitation wavelength), they generate current (due to movement of electrons within the crystal). The aim of this proposal is to undertake proof-of-principle experiments to demonstrate whether photostimulation of RGCs via QD-CNT electrodes (integrated into planar multielectrode arrays (MEAs)) can drive RGCs to firing threshold. We will use QDs that absorb light at different wavelengths - UV, blue, green and red - emulating cones and rods photoreceptors in mouse and human retina. These experiments will be performed using dystrophic retinas from the Crx-/- mouse, where photoreceptors undergo complete degeneration by 6 months postnatal. Another important novel aspect of this project is that for the first time, we are going to use MEAs fabricated on a flexible substrate by Dr Hanein. Implant flexibility is important for allowing better coupling to the retina in vivo, along the curvature of the eye, and although we are not planning to use intact eyes in this project, it is important to move from hard-based (silicon) to flexible MEAs in preparation for future work. We will stimulate electrodes electrically with an external device to establish the parameters for threshold stimulation of RGCs in the vicinity of the stimulating electrode and we are going to compare these responses to those obtained with photo-stimulation. If successful, this approach could revolutionise current design strategies for neural prosthetics in general, and for retinal implants in particular. Indeed, there would be no need for external stimulation, the necessary current would be intrinsically generated in the CNTs upon ambient light (there is no need for more powerful light sources such as lasers) absorbance by the QDs.

In retinal dystrophic diseases, photoreceptors degenerate, but retinal ganglion cells (RGCs, the output cells of the retina) survive and maintain their connections with visual areas, so that under appropriate conditions, direct RGC electrical stimulation via implantable stimulating devices can elicit light perception (phosphenes) in blind patients. The choice of electrode material is very important for such prosthetic devices (in terms of biocompatibility, electrical performance and durability). We have recently established through collaboration with Yael Hanein (Tel-Aviv Univ.) that carbon nanotubes (CNTs) offer great advantages for retinal implants because they have a large surface area, hence they are very efficient for charge delivery and they encourage neurite growth and entanglement within the CNT complex, yielding very stable interfaces. CNTs can be functionalised to modify/improve their performance. In this project, we are going to explore whether we can stimute RGCs with CNT electrodes conjugated to light-sensitive quantum dots (QDs) that generate electrical responses upon photo-stimulation. Yael Hanein has recently engineered UV-sensitive QD-CNT electrodes and was able to generate photocurrents that could potentially stimulate neurones. Using isolated retinas from the Crx mouse, a model of photoreceptor degeneration, we are going to investigate whether we can elicit action potentials in the RGC layer upon photo-stimulation (using UV, blue, green and red light, emulating cones and rods) of QD-CNT electrodes integrated in planar arrays. We will compare these responses to those obtained with electrical stimulation delivered with an external stimulating device. All experiments will be done on electrode arrays made of flexible substrate, paving the way for future in vivo experiments where flexible devices will facilitate coupling to the retina along the eye curvature.

There are many beneficiaries from this project beyond immediate academics carrying out research on similar or related issues. Both medical engineers and clinicians interested in restoring neural function using neural prosthetics could potentially find a significant benefit in this research because it will pave the way to designing a novel generation of more versatile and more powerful brain-machine interfaces. This is true for all types of neural implants, not only for the retina. Indeed, there are many efforts worldwide in trying to use photostimulation to interfere (stimulate or inhibit) with neural activity in intact networks using optogenetics. The approach we are developing in this project will open many new exciting possibilities in that field. People in the commercial private sector may find great benefit in our research, especially for the development of neural prosthetic devices. In the field of retinal implants, there are several companies worldwide that are developing electrode-based devices (e.g. Second Sight in California, Retina Implant in Germany). We could easily foresee that such companies would be interested in implementing the technology in clinically approved devices once the scientific ground work has proven successful. This project falls within the remit of the BBSRC priorities for funding research for 2010-2015, including fostering international collaborations. One of the top priorities of the BBSRC is 'to lead world-class 21st century bioscience, promoting innovation and realising benefits for society within and beyond the UK'. Our project falls within the Strategic research priority 3, which is to promote 'basic bioscience research to achieve advances for better health and improve the quality of life across the life course, reducing the need for medical and social intervention'. Indeed, through basic bioscience research within the remit of the Strategic Tools and Resources Development Fund, this project has the potential to bring new solutions to alleviate the immense burden of blindness. Although it does not claim to be able to cure blindness, it could nevertheless establish the biological foundation for a novel technological strategy for the development of better retinal prosthetic devices that could help blind patients to regain at least partial sight. A very large proportion of the ageing population suffers from retinal degeneration, leading to various levels of blindness. These blind patients not only have very poor quality of life, they also represent a heavy burden on healthcare and social services. Helping them to regain sight would thus bring immense benefits to the patients themselves and to society in general. We estimate that a successful outcome to this research could lead to the first clinical trials within 5-8 years. The first step, after demonstrating that neuronal photostimulation with QD-CNT electrodes is indeed a realistic approach, would be to undertake animals experiments in vivo, including chronic implantations. The following step would then be the development of clinical applications.