BIOMATERIALS FOR BIOPRINTING - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 133

Table 6.1 Classifications of hydrogel
Classification of hydrogels ( Ratner et al., 2013 )

Physiochemical
structural features Method of preparation Ionic hydrogel Complexation hydrogel

i) Homopolymer
- One type of
hydrophilic monomer
unit
ii) Copolymer
- Crosslinking two
comonomer units
with at least on
hydrophilic monomer
iii) Multi polymer
- Three of more
comonomers
iv) Interpentrating Network
- Intermeshed
network of monomer
with cross-linked
hydrogel of different
mechanism

i) Neutral
- Uncharged
ii) Anionic
- Negative charges
only
iii) Cationic
- Positive charges only
iv) Ampholytic
- Positive and negative
charges

i) Amorphous
- Covalent crosslinks
- Random arrangement
ii) Semicrystalline
- May or may not have
covalent crosslinks
- Self-assembled,
ordered
macromolecular
chains

i) Hydrogen bonds
ii) Hydrophobic group
associations
iii) Affinity Complexes
- Heterodimers
- biotin/streptavidin
- antibody/antigen
- conA/glucose
- PDLA/PLLA
- stereocomplexes
- Cyclodextrin
Inclusion complexes

bonds, ionic forces, physical entanglements of individual polymer chains, polymer crystallites, and
hydrophobic) are reversible under certain conditions. Alternatively, a hydrogel can be formed through
a combination of two or more of the mentioned interactions. There are a number of ways to categorize
hydrogels, based on preparation methods, ionic charge, or physico-chemical structural features, and
these are summarized in Table 6.1 ( Ratner et al., 2013 ). In this section, examples of hydrogels used
are discussed based on their origin, for example, natural vs. synthetic.

6.4.1 NATURAL HYDROGEL
Natural hydrogels have been heavily utilized for tissue engineering because they usually already con-
tain specific bioactive regions that give them good cellular compatibility with the cells of interest. Since
they are usually produced through biological methods, they also tend to have a more defined structure
and a monodispersed molecular weight. They are usually biodegradable and possess mechanical prop-
erties similar to that of the cell's natural ECM. However, they have issues concerning immunogenicity
and are relatively unstable compared to their synthetic counterparts. They also vary in terms of proper-
ties between species, tissue, and batch of production. In addition, characterizing their innate properties
can be difficult due to their complex composition.

6.4.1.1 Collagen
Collagen, a well-known protein, has been extensively used in biomedical applications as it is
the main component of natural ECM and the most abundant protein in mammalian tissues ( Lee
et al., 2001 ). Although a large number of natural and synthetic polymers are used as biomaterials,