Encapsulation in Alginates Hydrogels and Controlled Release: An Overview

Abstract

This review aims to gather the current state of the art on the encapsulation methods using alginate as the main polymeric material in order to produce hydrogels ranging from the microscopic to macroscopic sizes. The use of alginates as an encapsulation material is of growing interest, as it is fully bio-based, bio-compatible and bio-degradable. The field of application of alginate encapsulation is also extremely broad, and there is no doubt it will become even broader in the near future considering the societal demand for sustainable materials in technological applications. In this review, alginate's main properties and gelification mechanisms, as well as some factors influencing this mechanism, such as the nature of the reticulation cations, are first investigated. Then, the capacity of alginate gels to release matter in a controlled way, from small molecules to micrometric compounds, is reported and discussed. The existing techniques used to produce alginates beads, from the laboratory scale to the industrial one, are further described, with a consideration of the pros and cons with each techniques. Finally, two examples of applications of alginate materials are highlighted as representative case studies.

Keywords: alginates; controlled release; encapsulation; hydrogels.

Figure 1

Schematic representation of the $\alpha$-d-mannuronic acid (M) and $\alpha$-l-guluronic acid (G) structure (a) as well as the polymeric sequence of the alginate with the succession of the M and G blocks (b). Reproduced with permission from Thu et al., Colloid and Polymer Science; published by Elsevier, 1996 [26].

Figure 2

¹H NMR of the alginate. Reproduced with permission from Thu et al., Colloid and Polymer Science; published by Elsevier, 1996 [26].

Figure 3

Schematic representation of the different abilities to bind calcium cations depending on the alginate sequence (i.e., MM, MG or GG). Reproduced with permission from Bennacef et al., Food Hydrocolloids; published by Elsevier, 2021 [5].

Figure 4

Schematic representation of different designs of alginate microcapsules. (a) Simple core-shell micro capsule. (b) Matrix micro-capsule. (c) Multi-layer micro-capsule. (d) Assembly of micro-capsules.

Figure 5

Representation of the binding mechanism of calcium divalent cations by alginate in the so-called “egg-box” model.

Figure 6

The chemical equation for the ionic gellation of sodium alginate in the presence of divalent cations.

Figure 7

Scheme of the controlled gelification of the alginate using a ${\mathrm{CaCO}}_3$/gluconolactone (GDL) system (calcium cations are released in solution after the acidic decomposition of ${\mathrm{CaCO}}_3$).

Figure 8

Release curves of pindolol in (a) HCl 0.1M, pH = 1,1 and (b) phosphate buffer, pH = 7.4. (△) drug-free alginates beads; (□) pindolol-containing beads; (×) pindolol-containing glutaraldehyde cross-linked alginate–gelatine beads; (∘) pindolol-containing alginate–gelatine beads; (•) pindolol-containing formaldehyde cross-linked alginate–gelatine beads. Reproduced with permission from Ferreira et al., Journal of Controlled Release; published by Elsevier, 2004 [58].

Figure 9

Raoultella planticola release depending on the alginates and bentonite concentrations. Reproduced with permission from He et al., Applied Clay Science; published by Elsevier, 2015 [64].

Figure 10

Schematic picture of the extrusion technique with a bacteria as the compound of interest. Reproduced with permission from Saberi Riseh et al., International Journal of Molecular Science; published by MDPI, 2021 [36].

Figure 11

Schematic picture of the emulsion technique for bacterial encapsulation. Reproduced with permission from Saberi Riseh et al., International Journal of Molecular Science; published by MDPI, 2021 [36].

Figure 12

Schematic representation of the overall encapsulation process of enzymes in alginates using spray drying. Reprinted with permission from Weng et al. [68], Copyright 2022 American Chemical Society.

Figure 13

Representation of the fluid bed technology. Reproduced with permission from de Vos et al., International Dairy Journal; published by Elsevier, 2010 [17].

Figure 14

Scheme of the application of alginate encapsulation to protect a bio-control agent. Reproduced with permission from Saberi Riseh et al., International Journal of Molecular Science; published by MDPI, 2021 [36].