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Hydrogel, Iron crosslinking, Extracellular polysaccharide (EPS), Alteromonas macleodii Mo169, Blue biotechnology

Abstract

Hydrogels are three-dimensional structures of hydrophilic nature capable of holding large quantities of water without dissolving (Darge et al., 2019; Yang et al., 2022). Although hydrogels can be fabricated from synthetic polymers, hydrogels based on polymers of natural origin such as polysaccharides  have several advantageous properties, including enhanced biological activity, excellent biocompatibility, low immunogenicity, predictable biodegradability, and physical similarities with tissues (Darge et al., 2019; Jiang et al., 2020; Wang et al., 2020; Yegappan et al., 2018). Moreover, the abundance of  arboxyl, hydroxyl, or amine groups in polysaccharide structures provides a great platform for post-modification and functionalization (Li and Lin, 2021). Thus, these soft biomaterials hold great potential in a wide range of applications, such as scaffolds in tissue engineering, vehicles for drug delivery,  biomaterials for wound management, contact lenses, and immunomodulation (Darge et al., 2019; Hu and Xu, 2020; Jiang et al., 2020; Kang et al., 2019; Yang et al., 2022; Yegappan et al., 2018).

Different physical and chemical crosslinking methodologies can be employed in the preparation of polysaccharide hydrogels (Dave and Gor, 2018). One of those strategies, ionotropic gelation, is based on the formation of coordination bonds between the polymer’s negatively charged functional groups  and metal cations (e.g., K+, Ca2+, Zn2+, Mg2+, Cu2+, or Fe3+) [5–8,10]. Recently, polysaccharide-based hydrogels crosslinked with the trivalent iron cation have attracted interest due to their remarkable properties that include high mechanical stability, stimuli-responsiveness, and enhanced  absorptivity.

In this study, a Fe3+ crosslinked hydrogel was prepared using the biocompatible extracellular polysaccharide (EPS) secreted by the marine bacterium Alteromonas macleodii Mo169. The hydrogel preparation conditions were investigated to obtain homogenous and structurally stable hydrogels. The  impact of Fe3+ and EPS concentrations on the hydrogels’ strength was evaluated through response surface methodology (RSM) and the obtained hydrogels were characterized in terms of their composition, morphology, mechanical, and swelling properties. Hydrogels with mechanical strengths (G’)  ranging from 0.3 kPa to 44.5 kPa were obtained as a result of the combination of different Fe3+ (0.05–9.95 g L–1) and EPS (0.3–1.7%) concentrations. All the hydrogels had a water content above 98%. Three different hydrogels, named HA, HB, and HC, were chosen for further characterization. With  strength values (G’) of 3.2, 28.9, and 44.5 kPa, respectively, these hydrogels can meet the strength requirements for several specific applications. Their mechanical resistance increased as higher Fe 3+ and polymer concentrations were used in their preparation (the compressive hardness increased from 8.7 to 192.1 kPa for hydrogel HA and HC, respectively). In addition, a tighter mesh was noticed for HC, which was correlated to its lower swelling ratio value compared to HA and HB. Overall, this study highlighted the potential of these hydrogels for tissue engineering, drug delivery, or wound  healing applications.

Acknowledgement

This work was supported by Research Unit on Applied Molecular Biosciences – UCIBIO (UIDP/04378/2020 and UIDB/04378/2020), Associate Laboratory Institute for Health and Bioeconomy - i4HB (LA/P/0140/202019), LEAF—Linking Landscape, Environment, Agriculture and Food— Research Center  (UIDP/04129/2020), through national funds from FCT - Fundação para a Ciência e a Tecnologia, I.P. D.A. acknowledges FCT I.P. for PhD Grant SFRH/BD/140829/2018.

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