Assessment of the Complex Refractive Indices of Xenopus Laevis Sciatic Nerve for the Optimization of Optical (NIR) Neurostimulation

Despite an increasing interest in the use of light for neural stimulation, there is little information on how it interacts with neural tissue. The choice of wavelength in most of the optical stimulation literature is based on already available light sources designed for other applications. This paper is the first one to report the complex refractive index of the
sciatic
nerve of
Xenopus laevis
, which is a crucial parameter for identifying the optimal wavelength of optical stimuli. The
Xenopus laevis
neural tissue is the most widely used tissue type in peripheral neurostimulation studies. In this paper, the reflectance (
${R}$
) and the transmittance (
${T}$
) of the
sciatic
nerve were measured over a wavelength range of 860–2250 nm, and the corresponding real (
${n}$
) and the imaginary (
${k}$
) refractive indices were calculated using appropriate formulae in a novel way. The reported
${n}$
values were between 1.3–1.44 and the
${k}$
values are of the order of
$\textsf {10}^{-\textsf {5}}$
over the full wavelength range. The absorption coefficient
$\alpha $
was found to be 100–500 cm
$^{-1}$
. Several localized wavelength ranges were identified that can offer a maximized power coupling between potential optical stimuli and the neural tissue (1150–1200 nm, 1500–1700 nm, and 1900–2050 nm). The narrower regions of 1400–1600 nm and 1850–2150 nm were found to exhibit maximized absorbance. Separately, three regions were identified, where the penetration depths are the greatest (950–1000 nm, 1050–1350 nm, and 1600–1900 nm). This paper provides, for the first time, the fundamental specifications for optimizing the parameters of optical neurostimulation systems.