Synthesis of Hydrophilic/Amphiphilic Copolymers and Conetwork Gels by Nucleophilic Reaction for Biomedical Applications

Amphiphilic copolymers are an important material containing both hydrophilic and hydrophobic components on a single polymer chain. They can self-assemble into a variety of nanostructures, increasing the solubility of hydrophobic substances, modifying surface properties, acting as stabilising emulsifiers, and facilitating controlled release of medicinal products or macromolecules. They are used extensively in medicinal products, materials science, biotechnology applications, environmental management and many more areas which make them essential to a broad variety of industries and research purposes. To advance the use of amphiphilic copolymers as novel materials for biomedical applications, we must understand their capacity to self-assemble and form injectable hydrogels. Several key areas such as their biocompatibility, safety, stability, degradation behaviour and release mechanisms must be characterised in a thorough and robust manner. In addition, to engineer tailored drug delivery profiles and optimize their mechanical properties, synergistic combinations of copolymers must be investigated. The development of a scalable manufacturing process is necessary to join the gap between laboratory research and the clinical use of these materials. In addition, efforts are directed at enhancing the mechanical strength of injectable hydrogels for load-bearing applications and improving their hydrolytic stability to prolong their functional lifespan. These advancements aim to create versatile and effective platforms with broader applications in the biomedical field. Research presented in this thesis focusses on the synthesis of amphiphilic copolymers which can self-assemble in water, suitable for loading of hydrophobic drugs. The synthesized copolymers all contain a tertiary amine which is required for effective crosslinking. Additional crosslinking was achieved by using a crosslinking agent. An additional advantage of the tertiary amine is that it imparts pH responsive nature to the material. After crosslinking, the tertiary amine is converted a quaternary amine having inherent antimicrobial activity. Thus, the formed self-assembly and crosslinked networks were additionally assessed for their pH responsive and antimicrobial activity. The initial study presented in this thesis focussed on the synthesis of linear poly(amidoamines) (PAMAMs) followed by grafting with four different alkyl halides. The copolymer grafted with longer alkyl chain lengths showed the best results in terms of its stability, loading of hydrophobic molecules and pH responsive release. However, these copolymers were not able form injectable hydrogels due to absence of sufficient functional groups available for crosslinking. Therefore, in the next work multiblock copolymers (MBC) were synthesized using polyethylene glycol (PEG) and polycaprolactone (PCL) as a primary block and different primary monoamines/diamines were used. In the library of polymers synthesized, MBC with a short primary diamine show self-assembly behaviour, loading and pH responsive release of hydrophobic molecule. The copolymers were further crosslinked into hydrogel but due to less crosslinking density, the formed crosslinked network is not stable. To tackle this stability, a new approach was used in the next study by using a double network (covalent crosslinking+ ionic crosslinking). Injectable hydrogels were formed by simultaneous creation of ionic bond induced by covalent crosslinking. The formed double network hydrogels were observed to be more stable and demonstrated an improved sustained release profile compared to pure covalent or ionic hydrogels. However, the injectable hydrogels still face the issue of hydrolytic degradation due to the specific crosslinker used. The crosslinker used to crosslink the copolymer contained a halide terminated polyethylene glycol connected to an ester linkage. As this ester group is not hydrolytically stable, this can cause degradation of the hydrogels. Therefore, the subsequent study focussed on the synthesis of a new crosslinker which was structurally identical to the previous crosslinker tested, but with replacement of the ester group with an ether group. The effect of this little change was assessed on properties of the hydrogel, such as strength of the gel, water swelling and loading efficiency. Hydrogels formed using the newly synthesized crosslinker generally displayed identical properties to the previous study, but with reduced hydrolytic degradation. In the final chapter of this thesis, the work based on polymer was extended to evaluate the efficacy of hybrid polymer-lipid nanoparticles. Lipid nanoparticles of cubic symmetry, termed cubosomes are widely used in drug delivery such as antimicrobials and are typically sterically stabilised using a polymer. Commercially available copolymers typically contain polyethylene glycol (PEG) with potential issues due to the anti-PEG antibodies formation in susceptible individuals. Similar concerns were raised with the COVID-19 vaccines which contain lipid nanoparticles stabilised with a PEGylated lipid. In this study, a multifunctional cationic amphiphilic copolymer having antimicrobial properties was synthesised as an alternative PEG-free steric stabiliser for cubosomes. While the quaternized copolymer was capable of stabilizing cubosome lipid nanoparticles, this stability was not retained with long-term storage with the cubosomes displaying changes to their structure and morphology with time. These cubosomes demonstrated enhanced antibacterial activity against E. coli and B. subtilis bacteria. Further research should focus on the synthesis of alternative amphiphilic copolymers as potential steric stabilisers with retained antimicrobial activity but improved capacity to stabilise cubosomes over prolonged storage.