Illustration of an NR-CNT micoelectrode array (red and purple) on a retinal film model (gray network). Exposure to violet light (purple arrow) triggers electrical signals of retina activity (red dashes). Credit: Nano Lett., 14, 6685 (2014)
Researchers from Israel and the United Kingdom have developed a material that could serve as a platform for a wire-free prosthetic retina for people suffering from blindness caused by retina damage—for example, age-related macular degeneration (AMD) (Nano Lett., doi: 10.1021/nl5034304). According to the authors, this is the first nanomaterial-based approach for retinal photostimulation.
People with AMD experience blindness when photoreceptors in their retinas degenerate. Other prosthetic retina designs are based on metallic electrodes and have low spatial resolution, a rigid structure and awkward wires. A new design platform, from a team led by Yael Hanein, Tel Aviv University, Israel, uses special nanomaterials to wirelessly activate the remaining retinal neurons with optical stimulation of photoresponsive surfaces. The flexible material is a combination of semiconductor nanorods (NRs) and carbon nanotubes (CNTs)—two nanomaterials ideal for neurostimulation. NR structure enables efficient light absorbance and charge separation at the NR-CNT interface, and NCTs have suitable neuronal recording and stimulation properties because of their high surface roughness and biomimetic nature.
To create the NR-CNT combo, the researchers developed a new covalent bonding method based on plasma-polymerized acrylic acid to produce CNT films with porous and entangled surfaces. CNT porosity is important for electrochemical functions and to allow adherence to neuronal tissue. NRs were synthesized from cadmium selenide/cadmium sulfide. An antioxidant called tripeptide-glutathione was added to the NT structure to facilitate conjugation with the CNT film.
To test their prosthetic retina concept, the group used light-insensitive embryonic chick retinas at day 14 of development. At this development stage, photoreceptors have not yet developed in the retina—i.e., photoreceptor electrical activity during light exposure does not happen. When the retinas were placed on a NR-CNT microelectrode array, the researchers exposed the system to varying pulses and intensities of violet light (405 nm). After photostimulation, the group observed extracellular electrical signals of retina activity.
A cell viability assay was also conducted to see if nerve cells could survive on the NR-CNT structure. Since the proposed prosthetic retina containing the NR-CNT material would be placed inside the eye, it’s important that the material does not damage the existing nerve cells. The team recorded no differences in cell viability between neurons placed on the NR-CNT microelectrode array and neurons grown in regular cell culture conditions.
Next, the researchers need to optimize the stability of the NR-CNT system platform for long-term and for use for in vivo applications.