Hydrogels CLASSIFICATION BASED ON TYPE OF CROSS-LINKING Hydrogels can

Hydrogels categorizable into two groups based on their natural or synthetic source.
Hydrogels can be arranged from polymers derived from nature or synthetic. Examples of
natural polymers such as collagen, gelatin, fibrin, silk, agarose, hyaluronic acid, chitosan,
dextran and alginate have been employed for hydrogel preparations. Hydrogels based on
synthetic polymers such as poly(ethylene glycol) (PEG), Poly(lactic acid) (PLA), poly(2-
hydroxypropyl methacrylamide) (pHPMAm), poly(vinyl alcohol) (PVA) and
poly(hydroxyethyl methacrylate) (pHEMA) offer great variability in controlling polymer
chemical structure and architecture.6

1.2.2 – CLASSIFICATION CONSIDERING POLYMERIC COMPOSITION

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a) Homopolymeric hydrogels: polymer network which are derived from a single species of
monomer, which is the basic structural unit contain of any polymer network .
Homopolymers have cross-linked skeletal structure dependent on the nature of the
monomer and polymerization method.

b) Copolymeric hydrogels: more distinct monomer species with at least one hydrophilic
component, combined in a random, block or alternating configuration along the chain of
the polymer network .

(c) Multipolymer: These are also called as interpenetrating polymeric hydrogel (IPN), an
important class of hydrogels and made of two independent cross-linked synthetic and
natural polymer component, limited in a network form. In semi-IPN hydrogel, one
component is a cross-linked polymer and other component is a non-crosslinked polymer.

1.2.3 – CLASSIFICATION BASED ON CONFIGURATION

(a)Amorphous (non-crystalline).
(b)Semicrystalline: Amorphous and crystalline phase which is a complex mixture.
(c)Crystalline.
1.2.4 – CLASSIFICATION BASED ON TYPE OF CROSS-LINKING

Hydrogels can be divided into two groups on the basis of their chemical or
behaviour of the cross-link junctions. Chemically cross-linked networks have stable
junctions, while physical networks have temporary junctions that results from either
polymer chain complicacy or physical interactions such as ionic interactions, hydrogen
bonds or hydrophobic interactions.

1.2.5 – CLASSIFICATION BASED ON PHYSICAL APPEARANCE

Hydrogels appearance as matrix, film, or microsphere depends on the technique of
polymerization involved in the preparation process.Hydrogels appearance as matrix, film
or microsphere is dependent on the procedure of polymerization employed in the
formulation process. 7

a)Nonionic (neutral).

(b) Ionic (including anionic or cationic).

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

(d) Zwitterionic (polybetaines) consisting of 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.