When a material is introduced in vivo, its surface will be the first contact with the biological environment. Due to the importance of cellular interactions that occur at the material-biological interface, numerous attempts were made to functionalize bioactive molecules to make surfaces biologically compatible and functional. Yet, available biofunctional materials for surface modification still endorse intrinsic problems such as surface delamination, complications in preparation method, difficulty in synthesis of coating materials, and protein denaturation if proteins are load-ed. In effort to find appropriate novel biofunctional materials, we synthesized a mussel-inspired polymer derivative via chemical conjugation of catechol onto chitosan in order to make a sticky polymer that can serve as surface modifying material. Furthermore, we screened candidate molecules with reactive head groups and identified a small molecule that is multifunctional (adhesive toward surface and has bioactivity). In this thesis, we were able to develop bioactive surfaces using the bio-inspired functional materials.
In chapter 1, general background on surface modification methods and mussel-inspired surface modification were described. Then, a brief introduction of catechol-conjugated polymers followed by importance of implant surface modification with hemophilic molecules were summarized. Finally, the aim of the thesis was remarked.
In chapter 2, a simple method to prepare water-soluble chitosan derivative by conjugation of an enediol group called catechol was described. Chitosan was functionalized with a catechol-containing compound, 3,4-dihydroxyhydrocinnamic acid, by a carbodiimide coupling method which resulted in chitosan？catechol conjugates (Chi-C) with highly water-soluble property (up to 60 mg mL？1 at pH 7.0). Furthermore, the chitosan？catechol conjugates were shown to be adhe-sive, due to the intrinsic adhesive properties of catechol and can be easily form hydrogel at oxidative pH.
In chapter 3, The sticky mucoahdesive propery of the synthesized Chi-C was characterized. We demonstrated the mucoadhesive property of Chi-C using several tools and showed that the gastrointestinal (GI) tract retention time of Chi-C prolonged after catechol-modification, which we be-lieve is due to formation of catechol mediated-crosslinking with amines and thiols of mucin. The results indicated that catechol modification of mucoadhesive polymers may possibly lead to a new generation of mucoadhesive polymers for mucosal drug delivery.
In chapter 4, Chi-C was used as a cationic polymer for fabrication of a Layer-by-Layer (LbL) film. A phenomenon called Self-Enhancement in the thickness of an LbL Film (SELF) was detected in the fabricated film composed of Chi-C and catechol-conjugated heparin (Hep-C). The SELF process resulted in a film with thickness of ~104 nm (~10 μm) after deposition of 20 bi-layers. This is the fastest growth of an organic film without alternating the external pH of the polymer solutions. We revealed that methyl-π interactions between the catechol group (aromatic ring, π) and the acetyl $(-CH_3)$ group were responsible for SELF phenomenon. After deposition of 20 bilayers, a free-standing film that can serve as a high-capacity protein reservoir was formed and we showed that this film can be used to encapsulate a large amount of green fluorescent protein (GFP) and recombinant human bone morphogenic protein-2 (rhBMP-2).
In chapter 5, a new biofunctional molecule was screened among twenty-one commercially available molecules with a phosphonic acid functional group which for P-O-metal coordination bond with titanium surface. Among these molecules, we reported that vitamin B6, pyridoxal 5′-phosphate (PLP), exhibited superhydrophilicity and hemophilicity, further enhancing pro-liferation, migration, and differentiation of mammalian cells and promoted in vivo osteointegra-tion. Unlike other molecules used so far in surface modification, PLP was shown to bind to a variety of biomacromolecules due to presence of an aldehyde group. In fact, RGD and BMP-2 derived osteogenic peptide were successfully tethered to PLP coated surfaces in one step. The mechanism of PLP in in vivo healing and proliferation of mammalian cells is now under study.
In this thesis, novel biofunctional materials were synthesized and their effects as surface coat-ing materials were studied. We synthesized a mussel-inspired, adhesive polymer derivative that can serve as a surface coating for biomedical devices and identified a small molecule that has multiple functions including adhesive property, cell proliferation, and in vivo healing. Because these biofunctional materials exhibit unique and biologically necessary functions, these materials can be useful tools for a wide range of biomedical applications.