Our standard procedure is to hold at 60°C for 30 minutes with gentle stirring.

Dear Vandhana,

The following are commonly used methods for the preparation of gels. For more details please use the following link:

http://cdn.intechopen.com/pdfs-wm/17237.pdf

Hydrogels: Methods of Preparation,

Characterisation and Applications

Syed K. H. Gulrez, Saphwan Al-Assaf

and Glyn O Phillips

Glyn O Phillips Hydrocolloids Research Centre

Glyndwr University, Wrexham

United Kingdom

4. Methods to produce hydrogel

Cross-linked networks of synthetic polymers such as polyethylene oxide (PEO) (Khoylou & Naimian, 2009), polyvinyl pyrollidone (PVP) (Razzak et al., 2001), polylactic acid (PLA) (Palumbo et al., 2006), , polyacrylic acid (PAA) (Onuki et al., 2008), polymethacrylate (PMA) (Yang et al.), polyethylene glycol (PEG) (Singh et al.), or natural biopolymers (Coviello et al., 2007) such as alginate, chitosan, carrageenan, hyaluronan, and carboxymethyl cellulose (CMC) have been reported. The various preparation techniques adopted are physical crosslinking (Hennink & Nostrum, 2002), chemical cross-linking (Barbucci et al., 2004), grafting

polymerisation (Said et al., 2004), and radiation cross-linking (Fei et al., 2000; Liu et al., 2002b). Such modifications can improve the mechanical properties and viscoelasticity for applications in biomedical and pharmaceutical fields (Barbucci et al., 2004; Nho & Lee, 2005; Rosiak et al., 1995; Rosiak & Yoshii, 1999). The general methods to produce physical and chemical gels are described below.

4.1 Physical cross-linking

There has been an increased interest in physical or reversible gels due to relative ease of production and the advantage of not using cross-linking agents. These agents affect the integrity of substances to be entrapped (e.g. cell, proteins, etc.) as well as the need for their removal before application. Careful selection of hydrocolloid type, concentration and pH can lead to the formation of a broad range of gel textures and is currently an area receiving considerable attention, particularly in the food industry. The various methods reported in literature to obtain physically cross-linked hydrogels are:

4.1.1 Heating/cooling a polymer solution

Physically cross-linked gels are formed when cooling hot solutions of gelatine or

carrageenan. The gel formation is due to helix-formation, association of the helices, and forming junction zones (Funami et al., 2007). Carrageenan in hot solution above the melting transition temperature is present as random coil conformation. Upon cooling it transforms www.intechopen.com Hydrogels: Methods of Preparation, Characterisation and Application 127 

Fig. 3. Gel formation due to aggregation of helix upon cooling a hot solution of carrageenan. to rigid helical rods. In presence of salt (K+, Na+, etc.), due to screening of repulsion of sulphonic group (SO– 3), double helices further aggregate to form stable gels (Figure 3). In some cases, hydrogel can also be obtained by simply warming the polymer solutions that causes the block copolymerisation. Some of the examples are polyethylene oxide-polypropylene oxide (Hoffman, 2002), polyethylene glycol-polylactic acid hydrogel (Hennink & Nostrum, 2002).

4.1.2 Ionic interaction

Fig. 4. Ionotropic gelation by interaction between anionic groups on alginate (COO-) with divalent metal ions (Ca2+).

O

-O2C

O

HO OH

O

O

O

O

-O2C

O

HO OH

O

Ca2+

Progress in Molecular and Environmental Bioengineering 128 – From Analysis and Modeling to Technology Applications Ionic polymers can be cross-linked by the addition of di- or tri-valent counterions. This method underlies the principle of gelling a polyelectrolyte solution (e.g. Na+ alginate-) with a multivalent ion of opposite charges (e.g. Ca2+ + 2Cl-) (Figure 4). Some other examples are

chitosan-polylysine (Bajpai et al., 2008), chitosan-glycerol phosphate salt (Zhao et al., 2009), chitosan-dextran hydrogels (Hennink & Nostrum, 2002).

4.1.3 Complex coacervation

Complex coacervate gels can be formed by mixing of a polyanion with a polycation. The underlying principle of this method is that polymers with opposite charges stick together and form soluble and insoluble complexes depending on the concentration and pH of the respective solutions (Figure 5). One such example is coacervating polyanionic xanthan with polycationic chitosan (Esteban & Severian, 2000; 2001; 1999). Proteins below its isoelectric point are positively charged and likely to associate with anionic hydrocolloids and form polyion complex hydrogel (complex coacervate) (Magnin et al., 2004). 

Fig. 5. Complex coacervation between a polyanion and a polycation.

4.1.4 H-bonding

Fig. 6. Hydrogel network formation due to intermolecular H-bonding in CMC at low pH.

H-bonded hydrogel can be obtained by lowering the pH of aqueous solution of polymers carrying carboxyl groups. Examples of such hydrogel is a hydrogen-bound CMC (carboxymethyl cellulose) network formed by dispersing CMC into 0.1M HCl (Takigami et al., 2007). The mechanism involves replacing the sodium in CMC with hydrogen in the acid

Anionic polymer Cationic polymer Complex coacervate / polyion complex hydrogel

solution to promote hydrogen bonding (Figure 6). The hydrogen bonds induce a decrease of CMC solubility in water and result in the formation of an elastic hydrogel. Carboxymethylated chitosan (CM-chitosan) hydrogels can also prepared by cross-linking in the presence of acids or polyfunctional monomers (2008). Another example is polyacrylic acid and polyethylene oxide (PEO-PAAc) based hydrogel prepared by lowering the pH to form H-bonded gel in their aqueous solution (Hoffman, 2002). In case of xanthan-alginate mixed system molecular interaction of xanthan and alginate causes the change in matrix structure due to intermolecular hydrogen bonding between them resulting in formation of insoluble hydrogel network (2007).

4.1.5 Maturation (heat induced aggregation)

Fig. 7. Maturation of gum arabic causing the aggregation of proteinaceous part of molecules leading to cross-linked hydrogel network. Gum arabic (Acacia gums) is predominately carbohydrate but contain 2-3% protein as an integral part of its structure (Williams & Phillips, 2006). Three major fractions with different

molecular weights and protein content have been identified following fractionation by hydrophobic interaction chromatography with different molecular weights and protein content (Islam et al., 1997). These are arabinogalactan protein (AGP), arabinogalactan (AG) and glycoprotein (GP). Aggregation of the proteinaceous components, induced by heat treatment, increases the molecular weight and subsequently produces a hydrogel form with enhanced mechanical properties and water binding capability (Aoki et al., 2007a; Aoki et al., 2007b). The molecular changes which accompany the maturation process demonstrate that a

hydrogel can be produced with precisely structured molecular dimensions. The controlling feature is the agglomeration of the proteinaceous components within the molecularly disperse system that is present in of the naturally occurring gum. Maturing of the gum leads to transfer of the protein associated with the lower molecular weight components to give larger concentrations of high molecular weight fraction (AGP) (Figure 7). The method has also been applied on to other gums such as gum ghatti and Acacia kerensis for application in denture care (Al-Assaf et al., 2009).

4.1.6 Freeze-thawing

Physical cross-linking of a polymer to form its hydrogel can also be achieved by using freeze-thaw cycles. The mechanism involves the formation of microcrystals in the structure due to freeze-thawing. Examples of this type of gelation are freeze-thawed gels of polyvinyl alcohol and xanthan (Giannouli & Morris, 2003; Hoffman, 2002; 2004).

Hoping this will be helpful,

Rafik