Development of a photoresponsive hydrogel based on azo compound

Hydrogels are materials composed of hydrophilic polymeric chains which retain a huge amount of water. They are advantageous in the biomedical field because they may mimic the characteristics of extracellular matrix (ECM) in terms of physical/chemical properties, and for their biodegradability and biocompatibility. It is also possible to define their composition from scratch based on wide range of available starting materials, promoting different properties such as cell adhesion and proliferation as well as tailoring the mechanical properties. Nowadays hydrogels are well established for a plethora of applications, nevertheless expanding their capabilities is needed, going into the direction of making them able to respond to different stimuli. Such materials are known as smart hydrogels. The aim of this thesis project was to obtain a smart hydrogel based on the incorporation of an azo compound in the structure. Such chemical moieties are well-known for exhibiting light/thermal activated properties of isomerization. A specific azo compound, named AzoPEGMA,was employed. It consists of an azobenzene unit linked to a short hydrophilic methacrylate oligoethylene chain. Interestingly,azobenzene is hydrophobic in its stable trans isomerization, while it becomes more hydrophilic in the cis form. Such compounds are amphiphilic structures in the stable form and capable of spontaneously forming micelles in aqueous solutions. It has also been methacrylated to make it polymerizable and possibly capable of forming physical crosslinks. In this study we aimed at assessing the possibility to fabricate a smart hydrogel with light controllable micelles embedded. The hydrogel was formed employing other photocurable hydrophilic polymers to create the macromolecular structure:polyethylene glycol methyl ether methacrylate (PEGMEMA) as the main monomer and polyethylene glycol diacrylate (PEGDA) as crosslinker. The formulation was then polymerized activating a water soluble photoinitiator with 420nm light irradiation. The properties of the liquid formulation, as well as the successful polymerization and the choice of the optimal quantity of photoinitiator, were studied through rheological tests. The hydrogel’s mechanical properties were investigated through compression tests. FT-IR spectroscopy was employed to assess the polymerization process and the contribution of azoPEGMA to the hydrogel. The TRANS-to-CIS isomerization of azoPEGMA within the hydrogel was achieved using 365nm light irradiation, while the CIS-to-TRANS isomerization was performed with 450nm light. To study the occurred isomerization, UV-VIS spectroscopy was exploited. The presence of the micelles and their behaviour were analysed, first, as a function of the quantity of monomer by observing the turbidity of the formulation itself. Then, it was analysed following polymerization and ultraviolet and visible light irradiation both through optical microscopy and fluorescence. A well-established colorant with solvatochromic behaviour, 9-diethylamino-5H- benzo[alpha]phenoxazine-5-one (Nilered), was used as a dye to assess the micelles’ capabilities of encapsulation and release, to evaluate a possible drug release. Water uptake was examined through swelling and evaporation tests. Preliminary 3D printing tests of the formulation obtained were successfully performed. This project evidenced promising results, but also new scientific questions, paving the way for the development of a photopolymerizable and physically cross-linkable smart hydrogel.