A Review on the Design and Hydration Properties of Natural Polymer-Based Hydrogels

Abstract

Hydrogels are hydrophilic 3D networks that are able to ingest large amounts of water or biological fluids, and are potential candidates for biosensors, drug delivery vectors, energy harvester devices, and carriers or matrices for cells in tissue engineering. Natural polymers, e.g., cellulose, chitosan and starch, have excellent properties that afford fabrication of advanced hydrogel materials for biomedical applications: biodegradability, biocompatibility, non-toxicity, hydrophilicity, thermal and chemical stability, and the high capacity for swelling induced by facile synthetic modification, among other physicochemical properties. Hydrogels require variable time to reach an equilibrium swelling due to the variable diffusion rates of water sorption, capillary action, and other modalities. In this study, the nature, transport kinetics, and the role of water in the formation and structural stability of various types of hydrogels comprised of natural polymers are reviewed. Since water is an integral part of hydrogels that constitute a substantive portion of its composition, there is a need to obtain an improved understanding of the role of hydration in the structure, degree of swelling and the mechanical stability of such biomaterial hydrogels. The capacity of the polymer chains to swell in an aqueous solvent can be expressed by the rubber elasticity theory and other thermodynamic contributions; whereas the rate of water diffusion can be driven either by concentration gradient or chemical potential. An overview of fabrication strategies for various types of hydrogels is presented as well as their responsiveness to external stimuli, along with their potential utility in diverse and novel applications. This review aims to shed light on the role of hydration to the structure and function of hydrogels. In turn, this review will further contribute to the development of advanced materials, such as "injectable hydrogels" and super-adsorbents for applications in the field of environmental science and biomedicine.

Keywords: hydration; hydrogels; natural polymers; structure; swelling; water.

Figure 1

Hydrogel classification based on physicochemical properties. Adapted from Reference [19].

Figure 2

Structure of alginic acid and the ionic crosslinking of alginate by multivalent metal cations (Mⁿ⁺). G = α-L-Guluronic acid, and M = β-D-Mannuronic acid residues. Reprinted with permission [51].

Figure 3

Chemical structures of G-block, M-block, and alternating blocks of alginate, where M = mannuronic and G = guluronic units. Reprinted with permission [53].

Figure 4

Formation of a supramolecular hydrogel using amphiphilic β-cyclodextrin host (β-CDA) and guest polymer (HEC-AD, m: n ≈ 1:8) and formation of the supramolecular hydrogel by crosslinking of the polymer chains via host-guest inclusion complexes of the adamantane substituents and the macrocyclic hosts on the surface of β-CD vesicles (β-CDV). Reprinted with permission [57].

Figure 5

Free swelling capacity (FSC) of carboxylic acid-crosslinked monostarch-phosphate (MSP)-based hydrogels and their dependence on various factors: (a) the degree of substitution of phosphate on the MSP, (b) citric acid feed ratio, and (c) type of the carboxylic acid. Panel (d) is an SEM image of the surface of MSP crosslinked with citric acid at a feed ratio of 1:0.015 (w/w), where the highest FSC value was observed. SAH = succinic acid anhydride. Reprinted with permission [68].

Figure 6

Schematic representation of a thermo-responsive functionalized “smart” fabric hydrogel system with dual-function: (i) to coat a drug-loaded hydrogel onto the fabric, and (ii) to apply the atopical drug onto the skin of a patient (iii) to show how the coated fabric works with moisture and drug diffusion across the skin The drug-loaded hydrogel coated onto the fabric releases the drug, which diffuses across the skin of the patient, as the hydrogel undergoes volume phase transition as a function of moisture variations. Reprinted with permission [85].

Figure 7

A schematic representation of a load release in thermo-responsive hydrogels upon a trigger by temperature change. The polymer chains form a hydrogel at T < LCST and collapse at T > LCST; thereby releasing the encapsulated drug. Adapted from Reference [80].

Figure 8

Schematic representation of a general behavior of pH-responsive polymer hydrogel for a drug delivery application. The cationic hydrogel swells at acidic pH and shrinks at basic pH, and releases its cargo. The opposite is true for anionic hydrogels. Adapted from Reference [78].

Figure 9

(a) Preparation of azobenzene-modified dextran (AB-Dex) and cyclodextrin-modified dextran (CD-Dex). (b) Schematic representation of photoresponsive protein release from the trans AB-Dex and CD-Dex hydrogels. Azobenzene moieties isomerize from trans- to cis-forms upon irradiation with UV light, allowing collapse of the hydrogel structure and release of encapsulated proteins. Reprinted with permission [92].

Figure 10

Hydrogels designed with distinct stimuli-responsive crosslinkers (“logic gates”). Chemical crosslinkers enable the hydrogels to respond to specific cues based a simple principle of Boolean logic “YES”, “AND”, or “OR”. (a) The YES-gated crosslinker contains a single stimuli-responsive unit (red). (b) The OR-gated crosslinker contains two different stimuli-responsive units (red and blue) connected in series. The presence of either relevant input cleaves the crosslinker to allow cargo release. (c) The AND-gated crosslinker contains two different stimuli-responsive units (red and blue) connected in parallel. The presence of a single input alone does not fully sever the crosslinker. Adapted from Reference [95].

Figure 11

Chemical structures of (a) starch, (b) cellulose, and (c) chitosan (where a –H on –NH₂ is replaced with an acetyl in the case of chitin).

Figure 12

Schematic view of a crosslinked polymer network. The blue dots indicate crosslink joints, white dots indicate polar active sites, and spaces indicate pores or voids. Hydrophobic sites are shown as rectangles. Adapted from Ref. [22].

Figure 13

The effect of salt, salt type or salt sensitivity factor (f) on (A) WRV; and (B) Swelling of CMC-ECH crosslinked hydrogels as a function of time. Reproduced from Alam et al. [130].

Figure 14

(a) Photographic images of cellulose-based (1 wt.%)/surfactant composite hydrogels produced with various loading levels of cationic surfactants: (i) Control (0 mM); (ii) 1 mM; (iii) 5 mM; (iv) 10 mM of DTAB; (v) 1 mM’ (vi) 5 mM and (vii) 10 mM of CTAB; and (b) ξ-potential values of diluted cellulose (0.1 wt.%)/surfactant systems. Reprinted with permission from [55].

Figure 15

The hydration properties of starch samples with various amylose (%)/amylopectin (%) contents. Japonica (JRS-AA; 20/52), indica (IRS-AA; 24/50), and glutinous (GRS-AA; 1.5/75) rice starch samples showing (A) swelling recyclability; (B) Swelling rate in water; (C) water desorption rate; and (D) moisture absorption rate. Figure reproduced with permission from [45].

Figure 16

(a) Water absorption rate of CMCNa/HEC hydrogels as a function of molar ratio at room temperature; (b) effect of reaction temperature on the swelling of the CMCNa/HEC at the 5/1 mole ratio and variable temperature. Reprinted with permission [75].