PhD Thesis Defence Presentations - Sofia Falia Saravanou

The design of multifunctional three-dimensional (3D) networks offers significant advantages in the field of bioapplications. These systems, often referred to as "smart" networks, respond to stimuli such as changes in temperature, pH, light, etc. Under these conditions, hydrogels can undergo phase transitions, shifting from a liquid state to a 3D network and can swell, shrink, bend, and more. Specifically, the 3D networks that can revert to their initial state in the absence of external stimuli are called "self-healing" materials. Thus, the design and modification of polymer chains contribute to the development of intelligent hydrogels that respond to external factors and can be used as drug carriers, controlling the release rate of active substances or in tissue engineering applications.

The aim of this dissertation is the development of "smart" hydrogels using the natural polysaccharide sodium alginate (NaALG) for potential use in bioapplications. For this purpose, the following systems are developed and studied in terms of their viscoelastic properties. Initially, sodium alginate is grafted with thermoresponsive side chains based on poly-N-isopropylacrylamide, enriched with the hydrophobic comonomer N-tert-butylacrylamide, forming NaALG-g-P(NIPAM-co-NtBAM). Then, a doubly grafted sodium alginate is developed, incorporating both the thermoresponsive chains mentioned above and boronic acid groups (BA), which respond to pH changes and glucose concentration, NaALG-g1-P(NIPAM-co-NtBAM)-g2-BA. Finally, a semi-interpenetrating network is created by simply mixing the two separately grafted polymers, NaALG-g-P(NIPAM-co-NtBAM) and NaALG-g-BA. The mechanical properties of these networks are further enhanced by the addition of calcium chloride (CaCl₂). These networks can form through the following crosslinking mechanisms, depending on the grafted groups attached to the alginate polysaccharide:

1. Hydrophobic associations of the thermoresponsive side chains at temperatures above the critical gelation temperature (~24 °C),

2. Formation of boronate esters between boronic acid and the diol groups in sodium alginate at physiological pH,

3. Ionic interactions between the carboxyl groups of alginate and calcium ions.

The resulting hydrogels demonstrate excellent biocompatibility and non-toxicity, as shown in studies using cell lines such as human embryonic kidney cells (HEK 293T) and human periosteum-derived cells (hPDC). These hydrogels exhibit injectability, 3D printability, and self-healing capabilities.

The hydrogels based on NaALG-g-P(NIPAM-co-NtBAM) and the semi-interpenetrating networks of NaALG-g-P(NIPAM-co-NtBAM)/NaALG-g-BA are used as 3D-printed materials that support the formation of cell spheroids. In particular, the hydrogel formed by simply mixing NaALG-g-P(NIPAM-co-NtBAM) and NaALG-g-BA promotes the formation of boronate esters between the boronic groups and the diols present in the cell glycocalyx, thereby enhancing cell aggregation. These hydrogels can be used in tissue engineering or preclinical drug screening.

The hydrogel formed from the doubly grafted polysaccharide NaALG-g₁-P(NIPAM-co-NtBAM)-g₂-BA exhibits injectable properties and is used as a drug delivery system in the presence of glucose. The glucose molecule, which contains diols, competes with the boronate esters formed within the 3D network. This hydrogel can be applied in controlled drug delivery systems for Type I diabetes treatment.