Concentration and thickness dependent optical transparency in biological tissues via refractive-index modulation

We present a quantitative framework for inducing optical transparency in biological tissue samples by utilizing an absorbing dye to modulate the refractive index. Ou et al. found that loading a strongly absorbing dye into biological tissue enhances transparency via refractive-index modulation at wavelengths that are off-resonance to the dye's absorption peak [Science385, 6869 (2024)10.1126/science.adm6869]. In this study, we designed an optical platform that enables a precisely controlled manifold of the tissue thickness for repeatable transmission measurements. Chicken breast samples soaked in tartrazine-water solutions for a range of concentrations (1-400 mM) exhibited concentration- and thickness-dependent transparency enhancements. A threshold concentration (∼100 mM for thin and ∼400 mM for thick tissues) marked the transition from reduced to enhanced transmission, with enhancement factors of up to 26-fold when chicken breast tissue was soaked in a 200 mM tartrazine solution. These results agree with the Kramers-Kronig dispersion model, linking the narrow-band absorption peak of tartrazine at 428 nm with refractive-index modulation and scattering suppression in the near-IR region. Using our optical platform, we can determine the dye loading required to obtain a specific level of optical transparency for a given thickness of tissue sample.