Hydrogel Polymers
Hydrogel is a three-dimensional network of hydrophilic polymers that can hold a large amount of water while keeping its structure due to crosslinks. Hydrogel has the ability to absorb at least 20% of the total weight of water, (Duan & Jiang, 2017; Sen et al., 2020; Wang et al., 2019) and it is called “superabsorbent” (Chen et al., 1999) when it absorbs more than 95% of the total weight. Hydrogels swell in water and shrink in the absence of water (Budianto & Amalia, 2020) as this is the most characteristic property of hydrogels. Xerogels (dried hydrogel) become much smaller in size than the swollen hydrogel.
The crosslinking of polymer chains in hydrogels usually involves chemical or physical processes. Covalent bonds connect the polymer chains in the chemical crosslinked hydrogel. Thus, it is difficult to change the shape of such gels while polymer chain, which is physically entangled, are connected through non-covalent bonds. (Maitra & Shukla, 2014) The connection points of two chains are known as “crosslinks”. These points could be small chemical bridges of molecular weight much smaller than the crosslinked polymer chains. Crosslinks may also be an association of macromolecular chains due to Van der Waals forces or an aggregate formed via hydrogen bonds. The crosslinked hydrogel can be formed by:
1. Radiation: electron beams, (Hietala et al., 1997) gamma rays, (Kademani et al., 2006) x-rays, (Sundholm et al., 1996) or U.V. light (Hu & Xia, 2004) to activate a polymer site for reaction with another polymeric region.
2. Chemical Crosslinking: This type of crosslinking requires a bifunctional, low molecular weight crosslinking agent. (Abed et al., 2006) Figure 1 shows the main types of crosslinking reactions: (Arias et al., 1993)
Figure 1. Schematic representation for the common crosslinking reactions (addition type)
Other chemical reactions can also crosslink polymers with other functional groups. For example, Albumin and Gelatin can be crosslinked with aldehyde. (Bolto, 1995; Di Silvio et al., 1994) In addition, some hydrogels are formed by physical interaction between polymer chains. These interactions include hydrogen bonding, Van der Waals, and ionic and hydrophobic interactions. (Maitra & Shukla, 2014) Figure 2 shows the main interaction:
Figure 2. The major four types of interaction forces which control the behaviour of most responsive gels