Irreversible and Self-Healing Electrically Conductive Hydrogels Made of Bio-Based Polymers

Abstract

Electrically conductive materials that are fabricated based on natural polymers have seen significant interest in numerous applications, especially when advanced properties such as self-healing are introduced. In this article review, the hydrogels that are based on natural polymers containing electrically conductive medium were covered, while both irreversible and reversible cross-links are presented. Among the conductive media, a special focus was put on conductive polymers, such as polyaniline, polypyrrole, polyacetylene, and polythiophenes, which can be potentially synthesized from renewable resources. Preparation methods of the conductive irreversible hydrogels that are based on these conductive polymers were reported observing their electrical conductivity values by Siemens per centimeter (S/cm). Additionally, the self-healing systems that were already applied or applicable in electrically conductive hydrogels that are based on natural polymers were presented and classified based on non-covalent or covalent cross-links. The real-time healing, mechanical stability, and electrically conductive values were highlighted.

Keywords: conjugated polymers; electrically conductive hydrogel; renewable polymers; self-healing hydrogel.

Figure 1

(a) Conjugated system of p-orbitals and (b) alternating double and single bonds enabling p-orbitals overlapping in poly(acetylene).

Figure 2

The electric conductivity of materials in S/cm unit.

Figure 3

The chemical structures of different conductive polymers.

Figure 4

Oxidation polymerization of aniline.

Figure 5

The polymerization of pyrrole via (a) cation radical coupling and (b) chain-growth mechanism.

Figure 6

Cis/trans isomers of polyacetylene.

Figure 7

Proposed mechanism of thiophene oxidative-polymerization via ferric chloride.

Figure 8

Proposed chemical structure of the electrically conductive PT salt.

Figure 9

S-shaped curve for the effective electrical conductivity. Reproduced from Ref. [111] with permission from the Royal Society of Chemistry.

Figure 10

Preparation of carboxymethyl cellulose (CMC)/ polyaniline (PANI) conductive hydrogel.

Figure 11

Hydrogel/fiber conductive scaffold that is based on PANI/PCL electrospun fiber and an oxidized polysaccharide/gelatin/graphene composite.

Figure 12

Electrically conductive scaffold that is based on regenerated cellulose and PANI: (a) Transmission electron microscope image of the cross-section of the conductive side of PANI/ regenerated cellulose; (b) The proposed mechanism of the polymerization reaction and the hydrogen bonding between cellulose/aniline/phytic acid. Reprinted with permission from reference [128].

Figure 13

(a) Scheme of hyaluronic acid (HA) modification by pendant pyrrole moieties to produce PyHA and (b) schematic drawing of the oxidative polymerization of pyrrole in the presence of PyHA. Reprinted with permission from reference [32].

Figure 14

The mechanisms of self-healing hydrogels. Reprinted with permission from reference [141].

Figure 15

The chemical reaction to imine formation.

Figure 16

Systematic diagram of a self-healing conductive hydrogel that was based on imine bonds/PPy. Reprinted with permission from Ref. [143].

Figure 17

Borax dissociation in water and its cross-linking with polyols.

Figure 18

Self-healing bonds in dopamine-modified hyaluronic acid-based hydrogel reported in Ref. [122].

Figure 19

Principle of self-healing based on reversible Diels-Alder reaction.

Figure 20

The formation of a disulfide bond through the coupling of two thiols.

Figure 21

The chemical structure of thiolated-HA; n = 2, HA-DTPH; n = 3, HA-DTBH. Reproduced with permission of Ref. [154].

Figure 22

Schematic diagram of the electrically conductive self-healing hydrogel that was based on chitosan, polyacrylic acid, PPy, and ferric ions. Reproduced with permission from Ref. [145].

Figure 23

Schematic diagram of the self-healing mechanism of the silk-fibroin-based hydrogel. Reproduced with permission of Ref. [147].