Evaluation of penetration depth of near-infrared irradiation generated by tunable ultra-short pulsed laser in ex vivo samples of mouse head

Optogenetic research has opened up the possibility to control neurons that will help detect and treat neurological diseases in the early stage. Treatment of dysfunctions requires exposure to a partial neural network accessible through the absorption of opsins or phytochromes expressed in the brain matter. The use of II-NIR USP lasers makes it possible to non-linear activate and deactivate photoactuators in neuronal cells through the skull. The possible obstacles for noninvasive stimulation are the limits in light penetration depth, scattering and absorption by biological tissues. This research aimed to investigate light propagation and penetration depth in skin, skull and brain matter of mouse head. To evaluate the light transmittance in brain tissues, we developed an experimental setup with a tunable ultra-short pulsed laser source operating at the wavelength range of 1.1-1.2 μm. This spectrum range corresponds to the spectra of nonlinear absorption of opsins/phytochromes and matches the second biological window where laser irradiation can penetrate the skin and skull bone without damaging and overheating them. The experimental results demonstrate that under certain conditions, the ultra-short pulsed laser radiation can reach a penetration depth with required power that will be sufficient for non-linear activation of opsins/phytochromes in the brain of living animals. These results could support applications of II-NIR USP laser in non-invasive optogenetics, photobiomodulation of the brain functioning and even neurological disorders diagnostics.