Hydration of gelatin molecules studied with terahertz time-domain spectroscopy

Gelatin is an irreversible hydrolyzed form of collagen having similar amino acid composites with its parent collagen. The hydrogen atoms on the side chains of the long peptide strings can weakly bond with their surrounding gelatin as well as water molecules in aqueous environment, forming a cross-linked 3D matrix. Gelatin has been used as a model for soft tissue phantoms in several medical imaging studies including in terahertz imaging. In the terahertz regime, the dielectric property of the gelatin relies largely on its hydration status and the relationship has been shown to be non-linear. It is therefore essential to study the change of the dielectric properties with respect to the hydration status so as to accurately mimic the properties of fresh biological tissues in the terahertz spectrum. In this work, we studied the hydration status of gelatin with a terahertz time-domain spectroscopy system in the reflection setup. Gelatin gels with different molar weights were prepared with the weight concentration varying from 0 to 33.3%. The complex dielectric constants of the samples were calculated and fitted with an effective medium approximation model. The results provide a quantitative knowledge on the total number of the water molecules affected by each gelatin molecule and how the hydration status is influenced by the concentration and the molar weight.