Abstract:
Optogenetics is a novel technique enabling control of neural activity through light. Neurons can be genetically manipulated to express light-sensitive proteins (opsins) such that illuminating the tissue with light of a particular wavelength only activates the target neuronal populations. This specificity is useful in applications involving the peripheral nervous system (PNS) because it reduces the risk of side-effects that accompany electrical stimulation. In the PNS, nerves move with the surrounding tissue, so rigid optical fibres commonly used in the central nervous system (CNS) risk damaging the delicate nerve bundles. Therefore, there is a need for fully implantable simulators, that are both flexible and biocompatible, to deliver light to mobile tissue in the PNS for chronic experimentation. This thesis proposes a soft, flexible light guide made of silicone that couples light from a hermetically-sealed light source to the stimulation site on a nerve. The chosen light source, a surface-mount LED, emitted blue light at a mean intensity of 9.12 mW, and the chosen light guide comprised of two optically-clear, extensible silicones, Solaris and OE39. TracePro, a Monte Carlo ray-tracing program, modelled the propagation of light through various light guide designs and determined that an LED butt-coupled to a Solaris light guide produced the best efficiency of 28.51 %. This efficiency is equivalent to a light intensity of 1.16 mW/mm2, just meeting the intensity threshold required to activate the opsin, channelrhodopsin-2 (1 mW/mm2). Fabrication and experimental measurements of the best light guide designs found that LEDs embedded inside the Solaris light guide produced the best efficiency of 9.94 %. The addition of an OE39 cladding reduced the efficiency to 9.35 %. Embedded-LED OE39-cladded light guides submerged in water (considered to have a close refractive index to brain tissue) suffered only a 2.73 % drop in optical power relative to air. In another experiment, a 1.5 mm slice of chicken breast tissue (considered to have similar optical properties to brain tissue) absorbed 50.23 % of the light delivered by the light guide. This thesis confirms that a flexible silicone light guide can efficiently deliver light from a miniaturised light source to tissue.