Low-toxicity functionalized photopolymer for high-efficiency reflection holography with humidity response.

Reflection holograms stored in functionalized photopolymers offers a powerful approach to optical humidity sensing. In this work, unslanted volume gratings were recorded in Biophotopol, a low-toxicity, water-soluble acrylate-based photopolymer functionalized with 2-methacryloyloxyethyl phosphorylcholine (MPC). After controlled humidification, these gratings reached diffraction efficiencies above 70%, representing the highest values reported for environmentally friendly, non-commercial materials. The addition of MPC enhanced water uptake and produced controlled swelling, which modulated grating period, optical thickness, and Bragg wavelength. Spectral reconstruction, refractive index measurements, 3D profilometry, and structural modeling based on Kogelnik's Coupled Wave Theory confirmed a strong correlation between MPC concentration and humidity-induced structural and optical changes. The dynamic humidity response and opto-mechanical hysteresis under severe hygroscopic stress (up to 80% relative humidity) were also evaluated. The kinetic analysis and hysteresis cycles revealed that while higher MPC concentrations increase sensitivity and water uptake, they also prolong water retention via hydrogen bonding, inducing an opto-mechanical lag during the desorption regime. All these advances position MPC-functionalized Biophotopol as a highly promising material for the future development of holographic humidity sensors, linking sustainable photopolymer design with applied holography in sensing technologies.