Strategies to Functionalize the Anionic Biopolymer Na-Alginate without Restricting Its Polyelectrolyte Properties

Abstract

The natural anionic polyelectrolyte alginate and its derivatives are of particular interest for pharmaceutical and biomedical applications. Most interesting for such applications are alginate hydrogels, which can be processed into various shapes, self-standing or at surfaces. Increasing efforts are underway to functionalize the alginate macromolecules prior to hydrogel formation in order to overcome the shortcomings of purely ionically cross-linked alginate hydrogels that are hindering the progress of several sophisticated biomedical applications. Particularly promising are derivatives of alginate, which allow simultaneous ionic and covalent cross-linking to improve the physical properties and add biological activity to the hydrogel. This review will report recent progress in alginate modification and functionalization with special focus on synthesis procedures, which completely conserve the ionic functionality of the carboxyl groups along the backbone. Recent advances in analytical techniques and instrumentation supported the goal-directed modification and functionalization.Polymers 2020, 12, 919 2 of 23 Physicochemical and Polyelectrolyte Characteristics of AlginateThe composition, internal structure, and molar mass of the alg backbone govern the functional properties [9]. The solubility of alg in water depends on the pH value and the type of the counterions of the carboxyl groups. At pH < 3, both the M-and G-structures will precipitate as alginic acid. However, alternating M-and G-structures precipitate at lower pH values compared to the alg containing Polymers 2020, 12, 919 3 of 23 more homogeneous block structures. Neutralization of the alginic acid occurs at pH > 4, where it is converted into its corresponding salt [10]. Na-alg is an example of a water-soluble alg.Na-alg behaves in aqueous solution as a typical linear homogeneously charged polyelectrolyte. The chain conformation and consequently the solution viscosity strongly depend on the ionic strength, i.e., on both the alg concentration and the simultaneous presence of low molar mass salt. The intramolecular electrostatic repulsion between the neighboring negatively charged carboxyl groups of each monomer unit forces alg molecules into an extended random coil conformation in aqueous solution [11]. This results in highly viscous solutions even at relatively low alg concentration. The dynamic viscosity increases exponentially with the molar mass, while the intrinsic flexibility of the alg chains in solution increases in the order GG < MM < MG [12]. On the other hand, the selectivity for cation-binding and gel-forming properties strongly depends on the composition and sequence. Divalent cations preferably bind to the G-blocks. The ability to form ionotropic gels is based on this selective binding of cations. Mixing solutions of Na-alg with solutions of cationic polyelectrolytes yields precipitates of polyelectrolyte complexes.The rheological behavior of Na-alg solutions is important for a number of technologies and depends on both the concentration and th...

Functionalization of an Alginate-Based Material by Oxidation and Reductive Amination

“…However, the main limitation regarding these applications lies especially on low biocompatibility and poor mechanical properties, preventing the utilization of unmodified alginates for sophisticated biomedical areas such as cell immobilization [ 23 , 24 ]. Taking into account these drawbacks, functionalization of alginate-based materials emerges as an excellent option to enhance the quality and long-term stability of alginates without restricting the action of some specific properties characteristic of them [ 25 ]. Furthermore, the physical properties of alginate-based materials can be improved while avoiding the incorporation of other materials, such as carbon nanomaterials [ 26 , 27 ] or nanocellulose [ 28 ].…”

Recent advances in starch‐based films toward food packaging applications: Physicochemical, mechanical, and functional properties

“…The energy requirements, multiple processing steps, and poor atom economy are in direct conflict with the principles of green chemistry and currently preclude this approach as a sustainable solution (Anastas & Warner, 1998; Mendieta, Vallejos, Felissia, Chinga‐Carrasco, & Area, 2020). The production of agropolymers, on the other hand, has considerably improved atom economy and limits the number of necessary chemical transformations but poses its own set of challenges, such as the reliance on heterogeneous or harsh reaction conditions due to inherently poor solubility and high crystallinity of many biopolymers (Bakshi, Selvakumar, Kadirvelu, & Kumar, 2020; Onwukamike, Grelier, Grau, Cramail, & Meier, 2019; Szabo, Gerber‐Lemaire, & Wandrey, 2020; Zhang et al., 2011).…”

pH-Sensitive Alginate/Carboxymethyl Chitosan/Aminated Chitosan Microcapsules for Efficient Encapsulation and Delivery of Diclofenac Sodium

“…The pH sensitivity of the formulated microcapsules was clearly observed with raising the pH medium from pH 1.2 to pH 7.4 ( Figure 4 b). The swelling profiles were greatly enhanced at pH 7.4 as a result of deprotonation of carboxylic groups of Alg and CMCs which leads to promoting the electrostatic repulsion between the COO − anions, and consequently, the swelling degree increased [ 53 ]. Although Alg microcapsules were rapidly swelled and recorded the highest value of 3050% after 3 h, they were completely destroyed in the swelling medium (saline phosphate buffer; pH 7.4) with increasing time beyond 3 h. This action is due to the higher affinity of Ca 2+ ions for phosphate ions than alginate, resulting in the dissolution of alginate microcapsules.…”