BIOMATERIALS FOR BIOPRINTING - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 132

6.3.2.2 Functional Group Cross-linking
6.3.2.2.1 Schiff-base Formation
During a Schiff-base formation, the polymer with an amine side chain attaches itself to a carbonyl group
via a nucleophilic addition to form a functional group containing a carbon-nitrogen double bond with
the nitrogen atom connecting to another carbon chain. Such chemical methods can be done to cross-
link natural polymers such as collagen and other proteins using glutaraldehyde. However, the excessive
use of aldehyde may be toxic to the body and induce reactions within the body ( Tan et al., 2009 ). These
linkages are also unstable in low pH, thus these chemical bonds may break if foreign body response is
triggered at the site of transplantation ( Walt and Agayn, 1994 ).
6.3.2.2.2 Michael Type Addition
Michael addition is another nucleophilic addition reaction to produce hydrogels. In this reaction, a
carbanion is attached to an unsaturated carbonyl compound. The nucleophile or carbanion usually con-
tains an electron-withdrawing group that makes the hydrogen more reactive. The acceptor usually
contains a ketone or an amine. These reactions usually require a milder reaction condition and allow
more tunable properties, thus making them more suitable for cell encapsulation purposes ( Fairbanks
et al., 2009; Metters and Hubbell, 2005 ). However, excess or unreacted thiol polymers in the group may
be cytotoxic to the cells ( Di Monte et al., 1984 ).
6.3.2.2.3 Native Chemical Ligation
Native chemical ligation of thioesters is one of the naturally occurring chemical reactions. They help in
the synthesis of numerous cellular components, such as peptides ( Paramonov et al., 2005 ) and lipids ( Re-
ulen et al., 2007 ), and can easily be synthesized. Some of the typical functional groups involve the use of
N-terminal cysteine and aldehydes ( Yeo et al., 2004 ). They are relatively nonreactive to aminolysis, but
reacts readily to a thiol group through transesterification to form a new thioester. These reactions are usu-
ally highly substrate- specific, efficient in cross-linking, and occur in natural conditions. However, they
have complicated synthesis procedures as they involve protecting and deprotecting the peptides.
6.3.2.2.4 Click Chemistry
Click Chemistry is not a specific chemical reaction but one of the reactions created to mimic naturally
occurring polymerization ( Crescenzi et al., 2007 ). Developed to aid in the discovery of new pharma-
ceuticals ( Hein et al., 2008 ), this reaction has been described as modular, wide in scope, high-yield,
stereo-specific, and unable to generate offensive byproducts. However, the use of such chemistry may
prove toxic as copper ions are used as catalysts in the process. The use of copper causes the formation
of reactive oxygen species in the body ( Li and Trush, 1993 ). Research efforts are underway to develop
a copper-free method for click chemistry (Baskin et al., 2007; Orski et al., 2010 ).
6.4 HYDROGELS IN BIOPRINTING
Hydrogels have drawn significant attention in bioprinting and tissue engineering due to their high
water content. Networks of hydrophilic polymer chains are cross-linked through chemical or physi-
cal interaction. Hydrogels formed through chemical bonds, such as covalent bonds, are permanent
and irreversible. On the other hand, hydrogels formed through physical interactions (e.g. hydrogen