Thesis title: Nuovi idrogel eco-sostenibili di origine sintetica e naturale per diverse applicazioni
Hydrogels have garnered significant attention across various fields due to their distinctive properties, including high water content, tunable mechanical characteristics, and responsiveness. However, conventional methods for hydrogel synthesis often involve the use of non-renewable, non-biodegradable resources, and harsh chemicals, which raise environmental concerns. In this context, the development of eco-sustainable hydrogels has become increasingly imperative.
Eco-sustainable hydrogels present a promising solution to mitigate the environmental impact associated with traditional hydrogel production by utilizing natural polymers and green synthesis routes. Research in this area can enhance the sustainability and performance of hydrogels, paving the way for greener and more efficient technologies. Therefore, this doctoral thesis contributes to this overarching goal by focusing on the synthesis of organic-inorganic hybrid hydrogels and bacterial cellulose hydrogels.
Polyvinyl alcohol (PVA) was chosen for its exceptional hydro-solubility and biocompatibility, while tetraethyl orthosilicate (TEOS) was selected for its ability to interact with PVA and its application in the sol-gel process, which aligns with principles of green chemistry. The synthetic approach adopted proved effective in forming Class II hybrids as confirmed by Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) analysis. However, challenges regarding the observed fragility and long-term stiffness of the synthesized hybrids emerged due to hydrolysis and condensation of TEOS over time, as evaluated by ATR-FTIR and water holding capacity (WHC) measurements. This underscores the necessity for optimizing synthesis conditions.
The core of the research focused on bacterial cellulose (BC) hydrogels derived from kombucha beverage as a potential source of low-cost bacterial cellulose. The biomass of kombucha was efficiently used as a starter for hydrogel biosynthesis in sweetened tea (the traditional kombucha substrate) and in two low-cost culture media derived from industrial wastes, namely milk whey (MW) and brewery spent grains (BSG). The cellulose yield depended on the substrate composition, with BSG yielding results comparable to those from the sugared substrate. Samples underwent characterization using various analytical techniques, including Field Emission Scanning Electron Microscopy (FE-SEM), ATR-FTIR, X-ray Diffraction analysis (XRD), Simultaneous Thermal Analysis (TG-DSC), and tensile testing. Furthermore, WHC and water release rate (WRR) were estimated. A purification protocol involving sodium hydroxide treatment yielded pure cellulose hydrogels. Samples were characterized by a network microstructure of cellulose nanofibrils forming aggregates, high crystallinity with a prevalent Iα form, and significant WHC. Cellulose produced from different substrates exhibited no substantial differences in crystallinity, chemical structure, thermal behavior, and WRR. However, BC hydrogel samples displayed diversity in microstructural characteristics, surface area, and WHC, attributed to the diversity in the substrate composition.
Two innovative approaches were explored to enhance the BC yield from low-cost sources. The potential of BSG was demonstrated, with enzymatic hydrolysate resulting in a substantial increase in yield compared to hydrothermal extract, reaching 5.4 g/L, while investigation of multi-microorganism fermentation of MW showed no significant increases in cellulose yield.
In addition to biosynthesized samples, four Kombucha cellulosic by-products produced in brewing industries during kombucha fermentation were obtained and characterized to assess their potential as a low-cost source of bacterial cellulose. These materials exhibited substantial uniformity in chemical structure and crystallinity, with variations in microstructural characteristics and WHC due to different production processes. Some samples were further employed in application experiments in two novel fields.
The use of BC hydrogels in the conservation and restoration of cultural heritage as solvent carriers was investigated. Although synthetic hydrogels have dominated this field by far, their high costs and lack of renewability and biodegradability necessitate the investigation of more sustainable formulations. A BC organogel loaded with ethyl acetate was formulated, and experiments on the removal of waxes, a common type of dirt on various surfaces, were conducted. Results from μ-Raman spectroscopy and ATR-FTIR analysis demonstrated good cleaning efficiency for the removal of beeswax from marble and weak removal of microcrystalline wax after a 120-minute treatment. The same kombucha-derived BC hydrogel was loaded with ethylenediaminetetraacetic acid (EDTA) at different concentrations to remove copper corrosion stains from marble, a common stone alteration. Results from μ-Raman spectroscopy and colorimetric analyses attested the effective cleaning of 1% w/v EDTA-loaded BC hydrogel after a 120-minute treatment.
A novel antimicrobial hydrogel was formulated by functionalization of BC hydrogel with ozone, focusing on its potential as a remedy for stone biodeterioration. Ozonated BC hydrogels with different concentrations were obtained, and selected samples were characterized by ATR-FTIR, rotational rheometry, and WHC estimation. These hydrogels were tested against selected biodeteriogenic microorganisms in water suspension, resulting in the complete suppression of their viability after a 10-minute treatment for bacterial spores and a 24-hour treatment for fungal spores. Furthermore, an high activity of the ozonated BC hydrogel was assessed on contaminated marble, brick, and biocalcarenitic stone specimens. The formulation with the highest concentration of ozone exhibited high antimicrobial activity, highlighting its potential as a green and effective antimicrobial treatment with advantages in sustainability and cost-effectiveness.
This research lays the groundwork for future work aimed at refining synthesis techniques and exploring broader applications for eco-sustainable hydrogels.