Encapsulation of Active Substances in Natural Polymer Coatings

Abstract

Already used in the food, pharmaceutical, cosmetic, and agrochemical industries, encapsulation is a strategy used to protect active ingredients from external degradation factors and to control their release kinetics. Various encapsulation techniques have been studied, both to optimise the level of protection with respect to the nature of the aggressor and to favour a release mechanism between diffusion of the active compounds and degradation of the barrier material. Biopolymers are of particular interest as wall materials because of their biocompatibility, biodegradability, and non-toxicity. By forming a stable hydrogel around the drug, they provide a 'smart' barrier whose behaviour can change in response to environmental conditions. After a comprehensive description of the concept of encapsulation and the main technologies used to achieve encapsulation, including micro- and nano-gels, the mechanisms of controlled release of active compounds are presented. A panorama of natural polymers as wall materials is then presented, highlighting the main results associated with each polymer and attempting to identify the most cost-effective and suitable methods in terms of the encapsulated drug.

Keywords: biopolymers; formulation; hydrogels; ionic gelation; protection.

Figure 1

Different types of microcapsule architectures: (A) simple microcapsule, (B) microsphere, (C) multiwall microcapsule, (D) multicore microcapsule, (E) irregular microcapsule, (F) assembly of microcapsules, polymer layer are represented in black and dark grey and the light grey represents the active substance, inspired by [20].

Figure 2

Scheme of the spray-drying process for encapsulation of active ingredients, (A) tank containing the sprayed mixture: polymer and active ingredient, (B) pump to feed the mixture in the system, (C) spray nozzle, (D) heater to heat up the airflow, (E) chamber, (F) cyclone separator, and (G) spray dried capsules, inspired by [4].

Figure 3

Scheme of the extrusion process, (A) Syringe, (B) Polymer (alginate) solution, (C) Syringe needle, (D) Gelling bath (with calcium chloride), and (E) Syringe pump, inspired by [11].

Figure 4

Mechanism of microencapsulation formation by the coacervation method, (A) suspension of the core material (dark grey circles) in the liquid phase (light grey background), (B) suspension of the polymer (small black circles) in the liquid phase, (C) adsorption of the polymer material onto the core material, and (D) gelation and solidification of the microcapsule wall (black layer surrounding the dark grey circles), inspired by [15].

Figure 5

Mechanism of gum-based micro- and nanoparticle formations: Ionotropic gelation, inspired by [56].

Figure 6

Mechanism of gum-based micro- and nanoparticle formations: Covalent cross-linking, inspired by [56].

Figure 7

Mechanism of gum-based micro- and nanoparticle formations: Polyelectrolyte complexation, inspired by [56].

Figure 8

Mechanism of gum-based micro- and nanoparticle formations: Drug or hydrophobic agent/polymer conjugation with self-assembly, inspired by [56].

Figure 9

Mechanism of gum-based micro- and nanoparticle formations: Self-assembly of amphoteric molecular compounds, inspired by [56].

Figure 10

Mechanisms of the release of active substances, from polymer coatings, (A) initial encapsulation system, (B) fragmentation, (C) swelling, (D) diffusion, (E) degradation, and (F) dissolution, inspired by [4].

Figure 11

Plot of the hydrodynamic radius of poly (N-isopropylacrylamide-co-acrylic acid) polymer microgel according to the temperature at different pH values. Reproduced with permission from Farooqi et al., Arab. J. Chem.; published by Elsevier, 2017 [83].

Figure 12

Molecular structure of sodium alginate.

Figure 13

“Egg box” model for calcium alginate, Reproduced with permission from Finotelli et al., Polimeros; published by ABPol, 2017 [90].

Figure 14

Molecular structure of pectin: homogalacturonan structure.

Figure 15

Molecular structure of amylose.

Figure 16

Molecular structure of amylopectin.

Figure 17

Molecular structure of κ-carrageenan.

Figure 18

Stability and release characteristics of release of phage encapsulated inside microcapsules by in vitro digestion, (A) pure ALG microcapsules. (B) AC microcapsules AC1–AC9 represent polysaccharide mixtures in different proportions. (AC1) 1%ALG and 0.15%CG, (AC2) 1.5%ALG and 0.15%CG, (AC3) 2%ALG and 0.15%CG, (AC4) 1%ALG and 0.3%CG, (AC5) 1.5%ALG and 0.3%CG, (AC6) 2%ALG and 0.3%CG, (AC7) 1%ALG and 0.45%CG, (AC8) 1.5%ALG and 0.45%CG, (AC9) 2%ALG, and 0.45%CG. Reproduced with permission from Zhou et al., Front. Microbiol.; published by Frontiers, 2022 [109].

Figure 18

Stability and release characteristics of release of phage encapsulated inside microcapsules by in vitro digestion, (A) pure ALG microcapsules. (B) AC microcapsules AC1–AC9 represent polysaccharide mixtures in different proportions. (AC1) 1%ALG and 0.15%CG, (AC2) 1.5%ALG and 0.15%CG, (AC3) 2%ALG and 0.15%CG, (AC4) 1%ALG and 0.3%CG, (AC5) 1.5%ALG and 0.3%CG, (AC6) 2%ALG and 0.3%CG, (AC7) 1%ALG and 0.45%CG, (AC8) 1.5%ALG and 0.45%CG, (AC9) 2%ALG, and 0.45%CG. Reproduced with permission from Zhou et al., Front. Microbiol.; published by Frontiers, 2022 [109].

Figure 19

Molecular structure of high acyl gellan.

Figure 20

Molecular structure of low acyl gellan.

Figure 21

Percentage change in bead size (%) after 24 h of immersion in different pH solutions. Values are shown with the standard deviation. Adapted from [40].

Figure 22

Molecular structure of xanthan gum.

Figure 23

Molecular structure of gum arabic.

Figure 24

Molecular structure of guar gum.

Figure 25

Molecular structure of agarose.

Figure 26

Maltodextrin (left) and cyclodextrin (right) molecular structures.

Figure 27

Morphology of lime essential oil microparticles: Encapsulation by spray drying method of lime essential oils in whey protein concentrate, whey protein blended/maltodextrin DE5 (WM5), whey protein blended/maltodextrin DE10 (WM10), and whey protein blended/maltodextrin DE10 (WM20). Reproduced with permission from Campello et al., Food Res. Int.; published by Elsevier, 2018 [134].

Figure 28

Molecular structure of locust bean gum.

Figure 29

Characterisation of the controlled release of Capecitabine in vitro, plasma drug concentration versus time profile of the Capecitabine encapsulated in locust bean gum/alginate microbeads, vertical bars represent mean ± S.D. (standard deviation), the total number of values is 6. Reproduced with permission from Upadhyay et al. Mater. Sci. Eng. C; published by Elsevier, 2019 [7].

Figure 30

Molecular structure of chitin and chitosan.

Figure 31

Molecular structure of gelatin.

Figure 32

Molecular structure of casein.

Figure 33

Total phenolic content of blueberry polyphenol-protein matrices, the scavenging capacity of different combinations of blueberry polyphenol-protein matrices obtained by different entrapment methods: Freeze drying, oven drying, and spray drying (total phenolic content (TPC) calculated as mg gallic acid equivalent). Bars with different letters (a,b,c) are significantly different by Tukey’s test (2-way ANOVA) test, p < 0.01. Reproduced with permission from Correia et al. Food Chem.; published by Elsevier, 2017 [163].