Optical characterization by a fiber-optic system of UV-induced refractive index modulation in gelatin–methacrylamide hydrogels

Abstract Photopatternable hydrogels with controllable refractive index (RI) have gained increasing attention as a tool for biophotonic calibration, optical phantoms, and studies of light–matter interaction in soft matter. Here, we investigate UV-induced photopolymerization in gelatin-methacrylamide (MAM) hydrogels as a low-toxicity, optically transparent platform enabling spatial RI modulation. The hydrogels were synthesized using non- or low-toxic components and characterized by fiber-optic refractometry, validated against an Abbe refractometer with deviations below 1%. UV irradiation at 365 nm produced controlled RI modulation within the biologically relevant range of 1.32–1.36 at 1310 nm, exhibiting a reproducible parabolic RI–time dependence (R 2 = 0.996). Dynamic light scattering and Raman spectroscopy reveal that residual dissolved oxygen may inhibit early-stage photopolymerization, leading to reduced hydrodynamic polymer dimensions and a nonlinear optical response at short exposure times. Following oxygen depletion, polymer growth proceeds uniformly, resulting in a predictable increase of RI with irradiation time. These results establish gelatin-MAM hydrogels as a physically accessible system for refractive index engineering in soft matter. The demonstrated optical tunability and transparency make this platform suitable for biophotonic phantoms, fiber-optic sensing validation, and investigations of photochemically driven refractive index modulation in hydrated materials.