Nanomechanics of Exoskeletal Components in Invertebrates

Abstract

Exoskeleton is defined as an external skeleton that protects and supports animals’ body in invertebrates. The exoskeletons compose a structural frame from musculature to support body, giving physical protection, resistance to mechanical loads and desiccation, and defense against predators and infectants. The most plentiful component of exoskeletons is chitin which is the second abundant naturally occurring biopolymer by living organisms after cellulose. The molecular structure of chitin is very similar with cellulose by assembling complex of tight hydrogen bonds and hydrophobic networks. Since, chitin dissolves neither in common organic solvents nor aqueous solutions due to the tight hydrogen bonds and hydrophobic networks, practical applications of chitin still have limitations. The exoskeleton of invertebrates enhances mechanical strength through cross-linking of chitin with polyphenolic compounds, which are the major components of exoskeleton. Unlike bones of the vertebrates, the exoskeleton is composed of minimal amount of minerals and mainly in the form of composites of polyphenolic compounds and chitin nanofibers. As a result, the exoskeleton acquires high stiffness and hardness in both the dry and wet conditions. This strategy of the exoskeleton can be a model system for the bioinspired fabrication. Thus, thorough understanding of the intermolecular interactions among exoskeletal components at the molecular level is very significant for future applications. The main goal of this thesis is elucidating the molecular interaction mechanisms of the main exoskeletal components (e.g., chitin, chitosan, catecholamines) using a surface forces apparatus (SFA). SFA is a powerful instrument used to characterize the fundamental intermolecular forces such as electrostatic forces, van der Waals forces, capillary forces, hydrophobic interactions, bio-specific interactions, metal coordination forces as well as friction force in aqueous conditions. The SFA was a perfect method to investigate the interaction forces between the aforementioned exoskeleton building blocks which also can guide further applications in biomedicine field. Prior to nanomechanical studies, installation of SFA was performed as a part of Ph. D. thesis for the first time in Korea. The thesis is composed of two main topics: 1. Intermolecular force measurement of chitin and chitosan using an SFA, 2. Nanomechanical study of polyphenolic compound mediated self-polymerization coatings. Chapter I contains the general introduction of exoskeleton, the major exoskeletal components, surface forces apparatus, and the research objectives. In chapter II, the molecular interactions of chitin and chitosan were measured using SFA which depend on the molecular weight of chitosan/chitin, contact time, pH of the intervening solution, and degree of acetylation. Strong cohesion and adhesion of high molecular weight chitosan (HMW chitosan, 120,000 – 150,000 Da) were measured at pH 3.0 as molecular rearrangement due to good chain flexibility of chitosan. On the other hand, low molecular weight chitosan (LMW chitosan, 5,000 Da) showed weak intermolecular interactions which suggest the good solubility of LMW chitosan within physiological pH range. And adhesion to mica surface was reduced as DA increased at both HMW and LMW chitosan. Cohesion between LMW chitosan increased as DA increased, while cohesion between HMW chitosan reduced. In chapter III, the intermolecular interactions of catechol-based polyphenolic coatings - poly(norepinephrine) and poly(pyrocatechol) coatings were investigated using SFA and the origin of strong attraction of poly(catecholamine) coatings was defined as various physical interactions such as surface salt displacement by the primary amine, π-π stacking, and cation-π interaction.