Figure 1: Overview of Hydrogel Optics:

Schematic of the main events behind light going through biological tissues, solid-state silica, and hydrogels. The denser media have inherently higher refractive indexes (n₂), leading to a slowing down of light going through and an angle of refraction (ɵ_r) inferior to that of incidence (ɵ_i). The refractive index difference allows for total internal reflection (TIR) when the incident angle is equal or below the critical angle (ɵ_c), containing light within the denser material. This process leads to coupling losses when there is a high refractive index difference (mismatch), as with silica versus living tissues. The light which is transmitted (T) results from the Incident light (I) minus the light that is reflected (R) and/or scattered (S). The general mathematical equations that connect these different concepts, namely Snell’s Law, are represented on the right panel. An overview of the main properties that make hydrogels more like biological tissues than classical solid-state alternatives is also outlined.