Effects of post-processing methods on chitosan-genipin hydrogel properties

Biopolymer based hydrogel materials are attractive options for a variety of medical applications, including drug delivery and tissue regeneration because of their innate biomimetic material properties. While biopolymers are typically selected for a specific application based off of their chemical properties; the overall material structure of the resulting hydrogel ultimately relates to its ability to function for its intended application. In view of this, it is imperative that the impact of commonly used drying procedures on hydrogel physical properties is well understood. Herein, the effects of post-synthesis drying techniques: air-drying and lyophilization, on genipin crosslinked chitosan hydrogel physical structure were studied. Chitosan-genipin hydrogels synthesized out of either 15 kDa MW or 50-190 kDa MW chitosan were either air-dried (AD), flash-frozen (FF) and then lyophilized, or step-down frozen (SD) and then lyophilized. Environmental scanning electron microscopy (ESEM) was employed to evaluate the resulting hydrogels physical structure as a function of chitosan molecular weight and drying condition. ESEM images revealed the presence of larger microscale pores within the SD samples compared to FF samples, but both treatments yielded the induction of micropore with sizes ranging between 9-400 μm in diameter into the hydrogels. Traditional hydrogel swelling studies were performed to assess the resulting hydrogels swelling profile as a function of chitosan molecular weight and drying treatment. Lyophilized hydrogels showed a five-fold increase in swelling ratio compared to AD hydrogels indicating a change in morphology due to the drying process. The results demonstrate that regardless of polymer molecular weight, post-processing technique had a strong correlation with hydrogel porosity.