Biomedical Engineering Reference
In-Depth Information
responsive gelation as mentioned earlier. The mechanism through which
these materials undergo gelation is different from that corresponding to syn-
thetic polymers. In these cases, denaturation of the triple helical confirma-
tion in gelatin and double helical confirmation in polysaccharides results in
a temperature-triggered gelation [284, 285]. Both gelatin and polysaccharides
have been extensively investigated as tissue engineering scaffolds, as men-
tioned earlier [213, 284].
6.3
Self-Assembling Proteins
Another effective approach towards minimally invasive tissue engineering is
via molecular self-assembly of proteins and protein-amphiphiles. The nov-
elty of this approach is that the starting aqueous solution of proteins un-
dergoes self-assembly-driven in situ gelation with respect to temperature,
pH, and chemical triggers in the presence of biological fluids [117, 118, 286-
290]. This self-assembly is generally mediated by secondary forces such as
ionic interactions (e.g. electrostatic interactions), hydrogen bonds, hydropho-
bic interactions, and van der Waals interactions [291]. In addition to their
in situ gelation capability, these protein-based hydrogels also provide the
necessary biochemical cues to support cell proliferation and tissue forma-
tion [60, 287, 292].
Petka and coworkers creatively developed an artificial triblock protein,
with a central hydrophilic peptide segment and terminal leucine zipper
domains on both the ends, using recombinant DNA technology [118].
This protein undergoes reversible gelation with respect to changes in tem-
perature and pH due to the dimerization and higher order aggregate
formations of the leucine zipper motifs [118]. In another study, Wang
et al. created another thermoreversible hydrogel hybrid containing both
a synthetic polymer and an artificial protein segment where the two dif-
ferent synthetic polymers, N -(2-hydroxypropyl)-methacrylamide and N -
( N 9, N 9-dicarboxymethylaminopropyl)
methacrylamide,
were
crosslinked
using coiled-coil protein-folding motifs [119].
Artificial elastin-like proteins (created by DNA recombinant technology)
that undergo a reversible inverse phase separation were investigated as
an injectable scaffold for cartilage tissue engineering by Betre and cowor-
kers [293]. These elastin-like polypeptides enabled the encapsulated chondro-
cytes to retain their phenotype and produce extracellular matrix components
of the cartilage such as sulfated glycosaminoglycan and collagen. Below their
transition temperature, elastin-like polypeptides are soluble in water. Increas-
ing the temperature leads to phase separation resulting in aggregation of
polypeptides [293]. The LCST of elastin-like polypeptides can be controlled
by varying the aminoacid sequence [294]. Urry and co-workers demonstrated
that elastin-like polypeptides have mechanical properties reminiscent of na-
 
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