hydrogen bonding. Due to these cross-linked knots, the polymer chains are still
bonded, instead of being separated, while they are in contact with water or other
organic solvents. Meanwhile, the surrounding water or organic solvent molecules
may enter the space within the molecular assembly leading to the swelling of the
molecular network. The polymer species and density of cross-linking knots play a
key role in determining the mechanical properties of polymer gels. In most cases,
the elasticity and mechanical strength of a polymer gel increase with the density of
cross-linked knots.
5.1.2 Traditional Polymer Gels
Polymer gels are conventionally prepared via permanent cross-linking of high
molecular weight polymer chains. Among various species of polymer gels, siliconebased gels and hydrogels have been most intensively studied materials [10–12].
Silicones, also referred to as polysiloxanes, are commonly used polymers with
applications in lubricants, thermal and electrical insulation, adhesives and sealants.
The chemical composition of silicones consists of long backbone chains with
alternating silicon and oxygen atoms (ÁÁÁ–Si–O–Si–O–Si–O–ÁÁÁ). The most commonly used siloxane is linear silicone polydimethylsiloxane (PDMS), which is a
silicone oil (Fig. 5.2a). By varying the chain lengths, side groups and cross-linking
of the –Si–O– backbones, silicone oils can be converted into gel-phase materials.
Figure 5.2b–d presents PDMS backbones functionalized with different chemical
groups such as non-reactive methyl groups and reactive methyl groups. This versatility of the –Si–O– backbones leads to an abundant range of silicone-based gel
materials.
Polymer hydrogels are a class of polymer gels that have a wide range of
biomedical uses. Instead of dissolving in water, hydrogels swell due to the
encapsulation of water within the 3D network made of cross-linked polymer chains.
Dating back to the late 1950s, Wichterle and Lim from the Prague Institute of
Chemical Technology developed the first hydrophilic gels for biological applications using copolymers of ethylene dimethacrylate and 2-hydroxyethyl methacrylate [13]. On the basis of these hydrogels, the first soft contact lenses made of
polyacrylamide were introduced in the 1970s [14]. The 3D networks composed of
cross-linked polyacrylamide chains can absorb water forming hydrogels. By
weight, the hydrogels used for soft contact lenses contain water ranging from
approximately 38–79% [15]. The water incorporated in the hydrogels results in the
softness and flexibility of the contact lenses. Also, oxygen in the air is allowed to
pass through the water content of the hydrogels ensuring a healthy environment for
human eyes. However, the mechanical stability and clarity of vision related to the
soft contact lenses may be decreased with the increase in the percentage of water
content within the hydrogels.
5.1 Introduction to Polymer Gels
155
bonded, instead of being separated, while they are in contact with water or other
organic solvents. Meanwhile, the surrounding water or organic solvent molecules
may enter the space within the molecular assembly leading to the swelling of the
molecular network. The polymer species and density of cross-linking knots play a
key role in determining the mechanical properties of polymer gels. In most cases,
the elasticity and mechanical strength of a polymer gel increase with the density of
cross-linked knots.
5.1.2 Traditional Polymer Gels
Polymer gels are conventionally prepared via permanent cross-linking of high
molecular weight polymer chains. Among various species of polymer gels, siliconebased gels and hydrogels have been most intensively studied materials [10–12].
Silicones, also referred to as polysiloxanes, are commonly used polymers with
applications in lubricants, thermal and electrical insulation, adhesives and sealants.
The chemical composition of silicones consists of long backbone chains with
alternating silicon and oxygen atoms (ÁÁÁ–Si–O–Si–O–Si–O–ÁÁÁ). The most commonly used siloxane is linear silicone polydimethylsiloxane (PDMS), which is a
silicone oil (Fig. 5.2a). By varying the chain lengths, side groups and cross-linking
of the –Si–O– backbones, silicone oils can be converted into gel-phase materials.
Figure 5.2b–d presents PDMS backbones functionalized with different chemical
groups such as non-reactive methyl groups and reactive methyl groups. This versatility of the –Si–O– backbones leads to an abundant range of silicone-based gel
materials.
Polymer hydrogels are a class of polymer gels that have a wide range of
biomedical uses. Instead of dissolving in water, hydrogels swell due to the
encapsulation of water within the 3D network made of cross-linked polymer chains.
Dating back to the late 1950s, Wichterle and Lim from the Prague Institute of
Chemical Technology developed the first hydrophilic gels for biological applications using copolymers of ethylene dimethacrylate and 2-hydroxyethyl methacrylate [13]. On the basis of these hydrogels, the first soft contact lenses made of
polyacrylamide were introduced in the 1970s [14]. The 3D networks composed of
cross-linked polyacrylamide chains can absorb water forming hydrogels. By
weight, the hydrogels used for soft contact lenses contain water ranging from
approximately 38–79% [15]. The water incorporated in the hydrogels results in the
softness and flexibility of the contact lenses. Also, oxygen in the air is allowed to
pass through the water content of the hydrogels ensuring a healthy environment for
human eyes. However, the mechanical stability and clarity of vision related to the
soft contact lenses may be decreased with the increase in the percentage of water
content within the hydrogels.
5.1 Introduction to Polymer Gels
155