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

遗传性视网膜变性（例如视网膜色素变性）和年龄相关性黄斑变性（AMD）是发达国家最常见的致盲原因。这些破坏性的条件的特点是感光细胞变性。然而，视网膜神经节细胞（RGC，视网膜的输出细胞）存活并保持它们与大脑视觉区域的连接，使得在适当的条件下，通过植入式刺激装置的直接RGC电刺激可以在盲人患者中引起光感知（光幻视）。全球有几个组织正在努力改进视网膜植入技术，但在视网膜假体设备成为帮助盲人恢复视力的现实方法之前，仍有许多技术挑战有待解决。电极材料的选择非常重要。电极必须具有生物相容性，并且能够向组织输送足够的电荷。在这方面，推动我们研究的基本假设是先进材料比传统材料性能更好。通过与Yael Hanein博士（以色列特拉维夫大学）的合作，我们最近确定了碳纳米管（CNT）在视网膜植入技术中比传统电极材料具有更大的优势，因为它们具有高度生物相容性，并且具有非常大的表面积，这使得它们非常有效地进行电刺激。碳纳米管的一个非常有吸引力的特点是，它们可以被功能化，以修改/改善其生物性能，这就是我们计划在这个项目中研究的。我们将使用由Hanein博士修改的CNT电极，这样它们就可以在受到光刺激而不是外部刺激设备刺激时产生电流。Hanein博士已经成功地将CNT结合到光敏量子点（QD）上。量子点是微小的晶体;当用适当颜色的光（波长;点的大小确定精确的激发波长）激发时，它们产生电流（由于晶体内电子的运动）。该提案的目的是进行原理验证实验，以证明通过QD-CNT电极（集成到平面多电极阵列（MEA）中）对RGCs进行光刺激是否可以将RGCs驱动到放电阈值。我们将使用量子点吸收不同波长的光-紫外线，蓝色，绿色和红色-模仿老鼠和人类视网膜中的视锥细胞和视杆细胞。这些实验将使用Crx-/-小鼠的营养不良视网膜进行，其中光感受器在出生后6个月发生完全变性。该项目的另一个重要的新颖方面是，我们将首次使用Hanein博士在柔性衬底上制造的MEA。植入物的柔韧性对于允许在体内沿着眼睛的曲率沿着更好地耦合到视网膜是重要的，并且尽管我们不计划在该项目中使用完整的眼睛，但重要的是从硬基（硅）转移到柔性MEA以准备未来的工作。我们将用外部设备电刺激电极，以建立刺激电极附近的RGC阈值刺激的参数，并将这些响应与光刺激获得的响应进行比较。如果成功的话，这种方法可以彻底改变目前神经修复术的设计策略，特别是视网膜植入物。实际上，将不需要外部刺激，在环境光（不需要更强大的光源，例如激光）被QD吸收时，将在CNT中固有地产生必要的电流。

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.