Biomedical Engineering Reference
In-Depth Information
the characteristics of collagen are distinct from those of synthetic polymers, specifically in its mode
of interaction in the body. More than 20 different types of collagen have been found, but their basic
structures are the same, namely, consisting of three polypeptide chains. These three chains wrap
around one another, thereby creating a three-stranded rope structure. The strands are held together by
covalent bonds and hydrogen. Stable collagen fibers can immediately form as a result of strands self-
aggregating. Furthermore, the mechanical properties of collagen fibers can be enhanced by introduc-
ing chemical cross-linkers such as carbodiimide and glutaraldehyde (Lee et al., 2002). Collagen is
naturally degraded by metalloproteases, hence, the degradation process can be locally controlled by
cells in the engineered tissue.
6.4.1.2 Gelatin
Gelatin, a protein-based polymer, is derived through partial hydrolysis of collagen. Chemical pretreat-
ment followed by heat treatment disorganizes collagen protein structure, resulting in helix-to-coil
transition and conversion into soluble gelatin. Gelatin has a lower antigenicity compared to collagen
(
Ratner et al., 2013
). It undergoes gelation during a change in temperature. However, gelatin has been
modified to photopolymerizable hydrogel by the addition of the methylacrylate group (GelMA). Sev-
eral studies have used GelMA for bioprinting
(Billiet et al., 2014; Duan et al., 2013; Soman et al., 2013;
Visser et al., 2013
).
6.4.1.3 Fibrin
Fibrin is a biopolymer formed through thrombin-mediated cleavage of fibrinogen, which resulted in
self-associated, insoluble fibrin monomer
(Ahmed et al., 2008
). This naturally occurring material has
been used as injectable scaffolds and cell delivery vehicles (
Dare et al., 2007
). The major advantage is
that fibrinogen can be autologously obtained from the plasma, which reduces the risks of foreign body
reaction. Moreover, fibrin has good adhesion capabilities. Generally, fibrin is used as a glue to control
bleeding and adhere tissues in surgery. In addition, it has also shown promise in skin grafting and in the
delivery of exogenous growth factors to reduce wound healing time.
6.4.1.4 Alginate
Alginate, a natural polymeric material, is derived from brown seaweed and bacteria. It has a wide
range of medical applications, particularly in drug stabilization and delivery, and cell encapsulation.
It is a linear polysaccharide copolymer of
a
-L-guluronic acid (G) and (1-4)-linked
b
-D-mannuronic
acid (M) monomers (see
Figure 6.1
). The G and M monomers are distributed sequentially in either
alternating or repeating blocks
(Donati and Paoletti, 2009; Johnson et al., 1997
). Under neutral pH,
alginate is a polyanion. It has low toxicity and gels under gentle conditions. The species, location,
and age of seaweed are the influential factors that determine the amount and distribution of each
monomer. Alginate gels are formed as a result of divalent cations, such as Ba
2+
, Ca
2+
, or Sr
2+
, coop-
eratively interacting with G monomer blocks to generate ionic bridges between different polymer
chains. By changing and adjusting the G and M ratio together with the molecular weight of the
polymer chain, the cross-linking density and the resulting mechanical properties can be easily ma-
nipulated. It is noted that ionically cross-linked alginate hydrogels undergo slow and uncontrolled
dissolution rather than following a specific degradation trend. Mass is gradually lost through ion
exchange of calcium and, consequently, individual chains are dissociated, leading to the loss of me-
chanical stiffness over time.
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