Construction and Application of Self-Assembled Biomaterials Based on Polyphenols

Abstract

Self-assembly is a “bottom-up” strategy that utilizes noncovalent supramolecular interactions (e.g., van der Waals forces, π-π stacking, hydrophobic interactions, electrostatic forces, and hydrogen bonding) to organize building blocks into high-order hierarchical structures. Polyphenol-based biomaterials formed through self-assembly exhibit diverse functions, demonstrating significant potential in biomedical applications such as antibacterial coatings, tissue adhesives, therapeutic delivery, sensing, and bioimaging.

In this thesis, polyphenol-based nanocoatings and membrane-bound protocells were designed and fabricated using supramolecular self-assembly strategies. Tannic acid (TA), a polyphenolic compound, served as the molecular building block, assembling with 3-aminopropyltriethoxysilane (APTES) or polyethylene glycol (PEG) to form nanoparticles or coacervates. Conformal and morphology-controllable nanocoatings and fluidic membrane-bound protocells (FMPs) were separately constructed from these nanoparticles and coacervates. The thesis is structured as follows:

Chapter 1 reviews various self-assembly building blocks, including small molecules, polymers, biomolecules, and nanoparticles, as well as recent developments in polyphenol-based self-assembled biomaterials.

In Chapter 2, a polyphenolic supramolecular self-assembly strategy was explored for fabricating functional materials. The assembly of nanoparticles onto microstructured surfaces to create conformal and morphology-controllable nanocoatings has attracted significant interest but remains challenging to achieve through a one-pot continuous self-assembly method. In this chapter, we developed polyphenol-based nanoparticles via electrostatic interactions between APTES and TA molecules. By introducing additional APTES into the dispersion, APTES/TA nanoparticles were further assembled into shapes such as ellipsoidal, dumbbell-shaped, and spherical. The growth and deformation mechanisms of these nanoparticles were thoroughly investigated. Through electrostatic interactions and potential differences, negatively charged APTES/TA nanoparticles and positively charged APTES molecules continuously assembled on various substrates, including microparticles, PET substrates with diverse microstructures, and irregular cells. By controlling the deformation of APTES/TA nanoparticles, nanocoatings with tunable morphologies were successfully fabricated, with thickness regulated by assembly time. A model was proposed to explain this continuous self-assembly process, where the potential difference between the substrate surface and the dispersion served as the driving force. Additional APTES not only sustained this potential difference but also facilitated nanoparticle deformation, enabling the formation of nanocoatings with diverse morphologies in a single-step process. This simple and versatile strategy shows promise for fabricating morphology-controllable nanocoatings with antibacterial and thrombosis-resistant properties. Additionally, this method can be employed to modify and camouflage organisms, enhancing their biocompatibility and versatility while preserving structural integrity.

In Chapter 3, using this polyphenolic supramolecular self-assembly strategy, FMPs with multifunctional properties were developed. Protocells are constructed to imitate complex biological processes, such as molecular recruitment, confined chemical reactions, spatial separation, and information communication. Traditional protocells, including liposomes, colloidosomes, polymersomes, proteinosomes, coacervates, vesosomes, and capsosomes, are typically formed through self-assembly of lipids, inorganic colloids, amphiphilic block copolymers, protein-polymer nanoconjugates, macromolecules undergoing liquid-liquid phase separation, and vesicular compartments with high-order architectures. However, their construction remains challenging due to unstable membranes and limited functionality. In this chapter, polyphenol-based membrane-bound protocells with stable and multifunctional membranes were constructed. Membrane-free TA/PEG coacervate droplets were generated via liquid-liquid phase separation (LLPS) driven by hydrogen bonding and were transformed into membrane-bound protocells by introducing a polyvinylpyrrolidone (PVP) aqueous solution. This process enhanced stability and prevented aggregation and coalescence. The resulting FMPs, although polydisperse, could be size-separated through gradient centrifugation. FRAP analysis confirmed the liquid-like nature of the membrane, which allowed selective molecular permeability: small molecules like fluorescein and Rhodamine 6G could cross, whereas macromolecules like FITC-dextran (Mw ≥ 4 kDa) could not. The TA component on the membrane endowed FMPs with antioxidant properties. Enzymes such as GOx and HRP were separately encapsulated inside and on the membrane, enabling cascade reactions. Interaction with ferric ions further stabilized the protocells via coordination between TA and Fe³⁺. Additionally, leveraging electrostatic interactions, the membrane could be functionalized with Pt/CeO₂ nanozymes through interfacial self-assembly. This functionalization provided self-cascade catalytic activity for uric acid degradation. The FMPs developed in this study offer a platform for creating stable and multifunctional protocells with broad potential in biomedical applications.

Chapter 4 outlines the conclusions and novel contributions of the two projects, along with recommendations for future experimental improvements.

In summary, this thesis explored the polyphenolic supramolecular self-assembly strategy for fabricating functional biomaterials. Using this approach, APTES/TA nanoparticles were developed to construct conformal, morphology-controllable nanocoatings on substrates with various microstructures through a one-pot continuous self-assembly method. This simple and versatile technique enables surface modification of substrates with different shapes without causing damage. Additionally, FMPs were constructed by assembling TA, PEG, and PVP on the surface of coacervate droplets. The fluidic membrane not only enhanced stability but also allowed post-functionalization. By integrating membranization with liquid-liquid phase separation (LLPS), these membrane-bound protocells successfully mimicked functions of living cells. The findings in this thesis, including the development of polyphenolic nanocoatings and protocells, provide valuable insights into the design of advanced self-assembled biomaterials. These results demonstrate the potential of polyphenolic supramolecular self-assembly as a simple and general strategy for diverse emerging applications.

Date of Award	16 Dec 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Chia-hung CHEN (Supervisor)