Stimuli‐Responsive Tannin–Fe^III Hybrid Microcapsules Demonstrated by the Active Release of an Anti‐Tuberculosis Agent

Abstract

A simple and facile strategy for the creation of ferric tannin microcapsules around a liquid, non-sacrificial core is described. The assembly of the capsules occurs rapidly once ferric tannin complexes are subjected to ultrasonic treatment. The driving forces for the rapid capsule assembly reside in the strategy of adding ferric ions into the initial emulsion, which promotes shell formation and stability through well-known complexation effects. This is the first time that microcapsule assemblies of monomeric tannins like epigallocatechin-3-O-gallate has been demonstrated, which are reportedly unable to form dispersing microcapsules in the absence of a templating metal. The efficacy of the approach is demonstrated by the complete release of a hydrophobic molecule that is active against M. tuberculosis by using Acacia tannin capsules. The release kinetics of the active molecule were of zeroth order over a 12 h time frame.

Lignosulfonate Microcapsules for Delivery and Controlled Release of Thymol and Derivatives

“…The phenolic nature of the actives and, thus, their similarity to the shell-forming structures might interfere with the capsule formation, as well as cause variations in encapsulation efficiencies and/or nature. However, polyphenol-based systems studied so far mixed well with the oily phase, while showing little to no tendencies for the aqueous solution that did not contain the shell material [8,17,21,22]. Thus, this system is different in two respects.…”

“…For the generation of microcapsules from natural polyphenols (NPs), sonochemistry proved to be exceptionally useful, as demonstrated in various studies before [8,12,17,21,22]. Capsules were generated according to a protocol previously validated for SLS [12] starting from an oil-in-water (o/w) emulsion that was obtained by mixing and vigorously shaking a 1/1-mixture (v/v) of the following two solutions: a 5 mg/mL aqueous solution of softwood lignosulfonate (SLS) at pH = 12 and a 5 mg/mL solution of thymol derivative in olive oil spiked with 5% (m/m) of linoleic acid (LA).…”

“…The energy evolved during sonication can, in fact, trigger supramolecular stacking interaction in natural polyphenolic macromolecules, such as tannins or lignins, most noteworthy also without the addition of crosslinking, and thus eventually only additionally stabilizing agents [18][19][20]. This allowed for the generation of different families of capsules for controlled stimuli responsive active release under different conditions [6,8,17,21,22]. Tuning of the sonication power allows for either simple stacking or, under higher power treatments, crosslinks between the macromolecule chains to generate thicker capsules.…”

Tannin-Based Hybrid Materials and Their Applications: A Review

“…Figure 2 schematically depicts the complexation behavior of tannins with metals, precisely for iron, which is the most frequently studied metal ion regarding the complexation behavior with tannins. A tannin–metal mono-complex is formed at pH below 2, whereas a pH of 3–6 gives a bis-complex, and a stable tris-complex is generated at a pH of above 7 [ 42 , 43 ]. Other metal species also show such pH-dependent complex formation, but the exact pH ranges may vary.…”

“…Despite the functionality of tannin–metal hybrids as adsorbent materials, they are also discussed for other applications, for example biomedical ones including drug-delivery agents in organisms [ 42 ]. By adhering to the reaction of tannin and iron, Bartzoka et al [ 42 ], for instance, trialed all different classes of tannins regarding their formation of microcapsules with iron(III).…”