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(如视网膜色素变性)和老年性黄斑变性(AMD)是发达国家最常见的致盲原因。这些破坏性的情况以感光细胞退化为特征。然而，视网膜神经节细胞(RGC，视网膜的输出细胞)确实存活下来，并保持与大脑视觉区域的连接，因此在适当的条件下，通过植入式刺激设备进行直接RGC电刺激可以在盲人患者中诱导光感知(荧光素)。世界各地的几个组织正在努力改进视网膜植入物的技术，但在视网膜假体设备成为帮助盲人患者恢复视力的现实方法之前，仍有许多技术挑战需要解决。电极材料的选择是非常重要的。电极必须是生物相容的，并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。通过与以色列特拉维夫大学的Yael Hanein博士的合作，我们最近发现，碳纳米管(CNTs)在视网膜植入技术中比更传统的电极材料具有巨大的优势，因为它们具有高度的生物相容性，并且具有非常大的表面积，这使得它们对于电刺激非常有效。碳纳米管的一个非常吸引人的特征是，它们可以被功能化，以改变/提高其生物学性能，这也是我们计划在这个项目中研究的。我们将使用经过Hanein博士修改的碳纳米管电极，以便它们在用光刺激时可以产生电流，而不是用外部刺激设备。Hanein博士已经成功地将碳纳米管与光敏量子点(QD)连接起来。量子点是微小的晶体；当用适当颜色的光(波长；点的大小决定了精确的激发波长)激发时，它们会产生电流(由于电子在晶体中的运动)。该方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(MEAs)中)对视网膜节细胞的光刺激是否可以驱动视网膜节细胞达到激发阈值。我们将使用能够吸收不同波长的光的量子点--紫外线、蓝色、绿色和红色--在小鼠和人类的视网膜上模拟视锥和棒光感受器。这些实验将使用CRX-/-小鼠营养不良的视网膜进行，在出生后6个月，光感受器完全退化。该项目的另一个重要的新颖方面是，我们将首次使用Hanein博士在柔性基板上制造的MEAs。植入物的灵活性对于在体内沿着眼睛的曲率更好地耦合到视网膜非常重要，尽管我们不打算在这个项目中使用完整的眼睛，但重要的是从硬质(硅)到柔性MEA，为未来的工作做准备。我们将用外部设备电刺激电极，以建立刺激电极附近视网膜节细胞阈值刺激的参数，并将这些反应与光刺激获得的反应进行比较。如果成功，这种方法可能会彻底改变目前神经假体的设计策略，特别是视网膜植入物。事实上，不需要外部刺激，所需的电流将在碳纳米管中根据环境光(不需要更强大的光源，如激光)被量子点吸收而内在地产生。

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.