The development of hydrogels for soft contact lenses has stimulated extensive
interest to study smart polymer hydrogels that are responsive to external stimuli
such as light [16], electric field [17], pH [18] and temperature [19]. Traditional
pathways to synthesize hydrogels include cross-linking copolymerization and
cross-linking of hydrophilic polymer precursors. These methods achieve easy
preparation of hydrogels but do not help achieve exact control over chain length,
sequence or three-dimensional structure of hydrogels. To date, the exploration of
new methods to design and synthesize smart hydrogels has always been an
attractive research topic. Scientists have made efforts to synthesize hydrogels or
gelation building blocks by using genetic engineering methods. In addition, a large
number of associative building blocks have been designed for the self-assembly of
hydrogel structures. The research goal is to achieve a rational design of hydrogels
that can respond to external stimuli rapidly by adopting a combination of these
methods.
Stimuli-responsive hydrogels have been formed from numerous molecular
building blocks. The performance of hydrogels has been found to be closely related
to the molecular arrangement of polymer components. Typical polymer components include triblock and diblock copolymers [20]. Triblock copolymers can be
self-assembled into stimuli-sensitive hydrogels by protein engineering [21].
Diblock copolymers composed of polyoxyethylene and polyoxybutylene can also
be used for hydrogel materials at high polymer concentrations [22].
Apart from bulk hydrogels, hybrid materials combining hydrogel and solid
surfaces have also attracted extensive research interests [23, 24]. Zhao and
co-workers demonstrated a range of hybrid systems composed of hydrogel and
non-porous surfaces [25]. In this reported work, the solid surfaces of glass, silicon
wafer, titanium, aluminium and mica ceramic were first functionalized with 3(trimethoxysilyl) propyl methacrylate. Subsequently, the long chain polymer network of polyacrylamide or polyethylene glycol diacrylate was covalently
cross-linked to the silanes. This design enhances the interaction between the
resulting hydrogel layers and underlying surfaces. The tough bonding was mainly
due to the high values of the intrinsic adhesion and the excellent mechanical
properties of the bulk hydrogels. The hybrid hydrogel-surface systems can open up
new opportunities to prepare hydrogel-based electronic and microfluidic systems.
5.2 Fundamental Aspects
5.2.1 Concepts
Depending on the nature of cross-linking between polymer chains, polymer gels
can be categorized into two groups: chemical gels and physical gels. Chemical gels
include the cross-linking of polymer chains via chemical reactions such as radical
vinyl polymerization and stepwise polycondensation. In contrast to chemical gels,
5.1 Introduction to Polymer Gels
157
interest to study smart polymer hydrogels that are responsive to external stimuli
such as light [16], electric field [17], pH [18] and temperature [19]. Traditional
pathways to synthesize hydrogels include cross-linking copolymerization and
cross-linking of hydrophilic polymer precursors. These methods achieve easy
preparation of hydrogels but do not help achieve exact control over chain length,
sequence or three-dimensional structure of hydrogels. To date, the exploration of
new methods to design and synthesize smart hydrogels has always been an
attractive research topic. Scientists have made efforts to synthesize hydrogels or
gelation building blocks by using genetic engineering methods. In addition, a large
number of associative building blocks have been designed for the self-assembly of
hydrogel structures. The research goal is to achieve a rational design of hydrogels
that can respond to external stimuli rapidly by adopting a combination of these
methods.
Stimuli-responsive hydrogels have been formed from numerous molecular
building blocks. The performance of hydrogels has been found to be closely related
to the molecular arrangement of polymer components. Typical polymer components include triblock and diblock copolymers [20]. Triblock copolymers can be
self-assembled into stimuli-sensitive hydrogels by protein engineering [21].
Diblock copolymers composed of polyoxyethylene and polyoxybutylene can also
be used for hydrogel materials at high polymer concentrations [22].
Apart from bulk hydrogels, hybrid materials combining hydrogel and solid
surfaces have also attracted extensive research interests [23, 24]. Zhao and
co-workers demonstrated a range of hybrid systems composed of hydrogel and
non-porous surfaces [25]. In this reported work, the solid surfaces of glass, silicon
wafer, titanium, aluminium and mica ceramic were first functionalized with 3(trimethoxysilyl) propyl methacrylate. Subsequently, the long chain polymer network of polyacrylamide or polyethylene glycol diacrylate was covalently
cross-linked to the silanes. This design enhances the interaction between the
resulting hydrogel layers and underlying surfaces. The tough bonding was mainly
due to the high values of the intrinsic adhesion and the excellent mechanical
properties of the bulk hydrogels. The hybrid hydrogel-surface systems can open up
new opportunities to prepare hydrogel-based electronic and microfluidic systems.
5.2 Fundamental Aspects
5.2.1 Concepts
Depending on the nature of cross-linking between polymer chains, polymer gels
can be categorized into two groups: chemical gels and physical gels. Chemical gels
include the cross-linking of polymer chains via chemical reactions such as radical
vinyl polymerization and stepwise polycondensation. In contrast to chemical gels,
5.1 Introduction to Polymer Gels
157