Ultrasonic synthesis of stable oil filled microcapsules using thiolated chitosan and their characterization by AFM and numerical simulations

An experimental protocol has been developed for synthesizing stable core-shell microcapsules using a biopolymer, chitosan, lacking cross-linkable thiol functional groups. In the first step, thiol moieties were introduced into the backbone of chitosan using dl-N-acetylhomocysteine thiolactone (AHT). In the second step, AHT-modified chitosan shelled microcapsules, encapsulating an oil core, were successfully prepared using high intensity 20 kHz ultrasound. The size of chitosan and AHT modified chitosan microcapsules was found to be in the range of 1-15 μm. The thickness of the microcapsule shell increased with an increase in thiol content. The mechanical properties of microcapsules were evaluated by subjecting the microcapsules to compressive forces by colloidal probe AFM. The stiffness and the Young's modulus of the shell of microcapsules were determined by analyzing the force versus indentation data using Reissner's theory for indentation of thin elastic shells. The stiffness of AHT modified chitosan microcapsules was found to be higher than unmodified chitosan microcapsules. The viability of microcapsules to be embedded into processed food, pharmaceutical and cosmetic products was tested via numerical simulations. The confined capsule in the micro-channel was subjected to linear shear and uniform flows. We used finite element numerical simulations to determine the deformation of microcapsules in flow as a function of shear rate and thickness of the shell. The deformation of capsule was found to be linear with an increase in the shear rate. The deformation decreased with an increase in the thickness of the shell. Based on the simulations, we predict that the microcapsules would survive processing conditions and shear rates used in industrial applications.