A review for Hydrogel A:
Preparation, characterization, and applications

Journal of Advanced Research
Volume 6, Issue 2, March 2015, Pages 105-121



Abstract

Hydrogel products constitute a group of polymeric materials, the hydrophilic structure of which renders them capable of holding large amounts of water in their three-dimensionalnetworks. Extensive employment of these products in a number of industrial and environmental areas of application is considered to be of prime importance. As expected, natural hydrogels were gradually replaced by synthetic types due to their higher water absorption capacity, long service life, and wide varieties of raw chemical resources. Literature on this subject was found to be expanding, especially in the scientific areas of research. However, a number of publications and technical reports dealing with hydrogel products from the engineering points of view were examined to overview technological aspects covering this growing multidisciplinary field of research. The primary objective of this article is to review the literature concerning classification of hydrogels on different bases, physical and chemical characteristics of these products, and technical feasibility of their utilization. It also involved technologies adopted for hydrogel production together with process design implications, block diagrams, and optimized conditions of the preparation process. An innovated category of recent generations of hydrogel materials was also presented in some details.

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Introduction

The materials of interest in this brief review are primarily hydrogels, which are polymer networks extensively swollen with water. Hydrophilic gels that are usually referred to as hydrogels are networks of polymer chains that are sometimes found as colloidal gels in which water is the dispersion medium .

Researchers, over the years, have defined hydrogels in many different ways. The most common of these is that hydrogel is a water-swollen, and cross-linked polymeric network produced by the simple reaction of one or more monomers. Another definition is that it is a polymeric material that exhibits the ability to swell and retain a significant fraction of water within its structure, but will not dissolve in water. Hydrogels have received considerable attention in the past 50 years, due to their exceptional promise in wide range of applications . They possess also a degree of flexibility very similar to natural tissue due to their large water content.

The ability of hydrogels to absorb water arises from hydrophilic functional groups attached to the polymeric backbone, while their resistance to dissolution arises from cross-links between network chains. Many materials, both naturally occurring and synthetic, fit the definition of hydrogels.

During last two decades, natural Hydrogels were gradually replaced by synthetic hydrogels which has long service life, high capacity of water absorption, and high gel strength. Fortunately, synthetic polymers usually have well-defined structures that can be modified to yield tailor able degradability and functionality. Hydrogels can be synthesized from purely synthetic components. Also, it is stable in the conditions of sharp and strong fluctuations of temperatures .

Recently, hydrogels have been defined as two- or multi-component systems consisting of a three-dimensional network of polymer chains and water that fills the space between macromolecules. Depending on the properties of the polymer (polymers) used, as well as on the nature and density of the network joints, such structures in an equilibrium can contain various amounts of water; typically in the swollen state, the mass fraction of water in a hydrogel is much higher than the mass fraction of polymer. In practice, to achieve high degrees of swelling, it is common to use synthetic polymers that are water-soluble when in non-cross-linked form.

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Hydrogels may be synthesized in a number of “classical” chemical ways. These include one-step procedures like polymerization and parallel cross-linking of multifunctional monomers, as well as multiple step procedures involving synthesis of polymer molecules having reactive groups and their subsequent cross-linking, possibly also by reacting polymers with suitable cross-linking agents. The polymer engineer can design and synthesize polymer networks with molecular-scale control over structure such as cross-linking density and with tailored properties, such as biodegradation, mechanical strength, and chemical and biological response to stimuli.

96a1fa3f4e9c7e38b26bb1747afef74f.pngMorphology of hydrogel prepared by single-opening mold. (a)–(f) Micrographs of hydrogels: (a) CLSM image, c(CS) = 0.1 wt.%, cross section; (b) SEM image, c(CS) = 0.5 wt.%, longitudinal section; (c) CLSM images, c(CS) = 4.0 wt.%, longitudinal section; (d) SEM images, c(CS) = 3.0 wt.%, longitudinal section; (e) and (f) Fluorescence microscope images, bright field, colored by rhodamine for visibility, cross section and longitudinal section, respectively, c(CS) = 3.0 wt.%; (g) Schematic diagram of longitudinal and cross section of hydrogel, the longitudinal section is perpendicular to the primary hydrogel layer, while the cross section is parallel to it.

Classification of hydrogel products

The hydrogel products can be classified on different bases as detailed below:

Classification based on source

Hydrogels can be classified into two groups based on their natural or synthetic origins .

Classification according to polymeric composition

The method of preparation leads to formations of some important classes of hydrogels. These can be exemplified by the following:

(a)

Homopolymeric hydrogels are referred to polymer network derived from a single species of monomer, which is a basic structural unit comprising of any polymer network. Homopolymers may have cross-linked skeletal structure depending on the nature of the monomer and polymerization technique.

(b)

Copolymeric hydrogels are comprised of two or more different monomer species with at least one hydrophilic component, arranged in a random, block or alternating configuration along the chain of the polymer network.

(c)

Multipolymer Interpenetrating polymeric hydrogel (IPN), an important class of hydrogels, is made of two independent cross-linked synthetic and/or natural polymer component, contained in a network form. In semi-IPN hydrogel, one component is a cross-linked polymer and other component is a non-cross-linked polymer.

 

Classification based on configuration

The classification of hydrogels depends on their physical structure and chemical compositioncan be classified as follows:

(a)Amorphous (non-crystalline).

(b)Semicrystalline: A complex mixture of amorphous and crystalline
phases.

(c)Crystalline.

 

Classification based on type of cross-linking

Hydrogels can be divided into two categories based on the chemical or physical nature of the cross-link junctions. Chemically cross-linked networks have permanent junctions, while physical networks have transient junctions that arise from either polymer chainentanglements or physical interactions such as ionic interactions, hydrogen bonds, or hydrophobic interactions.

Classification based on physical appearance

Hydrogels appearance as matrix, film, or microsphere depends on the technique of polymerization involved in the preparation process.

Classification according to network electrical charge

Hydrogels may be categorized into four groups on the basis of presence or absence of electrical charge located on the cross-linked chains:

(a).Nonionic (neutral).

(b).Ionic (including anionic or cationic).

(c).Amphoteric electrolyte (ampholytic) containing both acidic and basic groups.

(d).Zwitterionic (polybetaines) containing both anionic and cationic
groups in each structural repeating unit.

 

Hydrogel-forming natural polymers include proteins such as collagen and gelatine and polysaccharides such as starch, alginate, and agarose. Synthetic polymers that form hydrogels are traditionally prepared using chemical polymerization methods.





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