Application of free volume concept and manipulation of network structure parameters for optimum loading of gallic acid in the modified kutki (Panicum sumatrense) millet starch hydrogel matrix

Abstract

Hydrogels have been widely preferred for their use as nutraceutical/drug delivery vehicles due to the easy manipulation of their network parameters. In this study, gallic acid (GA) was used as a nutraceutical for loading into the optimized lysine modified kutki millet starch (Lys9-KMS) hydrogel. Various ethanol-water mixtures (% v/v) were used to tailor the network parameters and attain a suitable mesh size of the matrix that could withhold GA without compromising its structure. The diffusivity of GA in the solvent (DAB) and Lys9-KMS hydrogel (DG) possessed an inverse relationship with the solvent viscosity. The encapsulation efficiency (80.37%) was observed to be maximum in case of an 80% ethanol-water mixture. A simple and useful approximation was used for the estimation of free volume for loading the number of nutraceutical molecules in the hydrogel matrix without compromising structural integrity. Upon modeling the ratio of diffusivity (DG/DAB) of GA in the matrix to that in solvent against the hydration i.e. (∅p1−∅p), where ∅p is polymer volume fraction, the value of the parameter, Vf,w (free volume per molecule of solvent), was determined as 1.74 Ȧ and a final amount of GA that can be loaded into the gel matrix was found to be 32.11 ± 1.02 mg for 300 mg hydrogels. The validity of the proposed approach was verified from diffusivity studies and showed an insignificant difference (p > 0.05). This approach would avoid uneven loading of the nutraceuticals or ingredients in the polymer matrix and would reduce their wastage during the design of functional foods.