(162ab) Influence of Degree of Acetylation on Physical Properties of Biomimetic Chitin Films

Chitin and its deacetylated derivative chitosan are important polysaccharides that are found in the exoskeletons of invertebrates and play an essential role in their structural properties. Chitin is one of the critical structural components in the invertebrate’s exoskeleton, influencing its strength and rigidity. In insects, chitin interacts with various proteins and catechols (dopamine derivatives), where it is organized into hierarchical structures. The precise combination allows the material of the exoskeleton, known as cuticle, to display a wide range of mechanical properties, from relatively soft, flexible materials to sturdy, rigid materials. Understanding the role that chitin and these other biomolecules play in giving rise to these varying properties in insect cuticle has been a long-term focus of research and provides inspiration for the development of chitin-based biomimetic composite materials that are described in the present work.

Chitin is a low-cost and sustainable material obtained primarily from shellfish. Most chitin is deacetylated to produce chitosan, which can be dissolved in mild acidic conditions and thus is more readily used for many applications, from drug delivery and tissue engineering to food packaging. The difference between chitin and chitosan is the degree of deacetylation of the glucosamine subunit, with chitin being dominantly acetylated, and chitosan being dominantly deacetylated. The degree of acetylation impacts the solubility, elasticity, strength, and net charge.

Most literature focuses on the use of highly deacetylated chitosan though, biologically chitin is the dominant material, with a degree of acetylation of 70% or higher. Therefore, it is of interest to understand the properties not just at the ends of the spectrum, but also to understand the features as the material changes from highly acetylated chitin to highly deacetylated chitosan. Transitioning from chitin which self-assembles into nanofibrils, that organize into a crystalline structure to chitosan which is an amorphous material that lacks an organized structure. These changes in structure are hypothesized to have an impact on not only the viscoelastic behavior of a pure polysaccharide film, but also how it affects protein and dopamine interactions.

Therefore, we present the physical characterization of chitin and chitosan as a function of the degree of acetylation both as pure films and as composite materials utilizing dopamine and protein. As the exoskeleton of beetles is not made of pure chitin and is a complex multi-component material, we examined chitin materials utilizing a combination of proteins and dopamine in order to mimic this design.

In this study, thin films were made by the dissolution of chitosan in mild acetic acid, followed by the controlled addition of acetic anhydride to achieve a desired degree of acetylation. The resulting solution was cast into a film via solvent evaporation on a glass slide. A range of tests including dynamic mechanical analysis, fracture tests, stress relaxation, and thermogravimetric analysis were used to carry out a comprehensive assessment of composite chitin-based films over a range of acetylation. The results showed that both the storage and loss moduli reached a maximum at intermediate acetylation, before decreasing as the degree of acetylation continued to increase. Additionally, the polymers showed no significant changes in the tan δ, nor in the residual water content as the degree of acetylation increased, which indicates that results are not due simply to changes in hydrophobicity. These results show that the degree of acetylation is an important parameter to control when utilizing chitin or chitosan, with a non-linear relationship between it and the viscoelastic. This study shows that intermediate acetylation between chitin and chitosan could be more useful than either of the extremes are by themselves. In addition, the effects of combining chitin/chitosan with polydopamine were examined. Polydopamine derivatives are part of the native cuticle of beetles, and polydopamine coatings have received much attention over the last decade for its ability to control interactions between drugs and cells. At this point, our work to date has shown that the properties are dominated by the degree of acetylation and not the presence of polydopamine.