Characterization of bound and free water in gelatin hydrogels and their contribution to THz frequency permittivity

We present reflective THz spectroscopic measurements of gelatin hydrogels with different gelatin bases combined with differential scanning calorimetry to quantify the bound water mass fraction of each sample. Samples were synthesized as either planar, optically thick slabs, or thin films lying atop optically thick water volumes. The analysis shows that the THz permittivity of water bound to gelatin molecules and the total bound water mass fraction vary significantly depending on the gelatin base used, the total water mass, and environmental conditions. These results support thorough characterization bound water in gelatin hydrogel phantoms prior to their us as a calibration target.