Molecular Gels for Tissue Engineering - Soft Fibrillar Materials: Fabrication and Applications - page 150

nucleases. Furthermore, compared to other synthetic polymers like poly(lactic acid)
or polyethylene glycol, there will be fewer issues associated with biocompatibility
as DNAs are present naturally in the human body.
The ability to engineer and design specific sequences on the DNA gelators further
expanded the usage of chemically cross-linked DNA hydrogels in a variety of re-
search fields, including tissue engineering. Due to their diverse applications in the
medical and pharmaceutical industries and their unique characteristic of being able
to be disintegrated easily, a property that is shared with the molecular gels, enzy-
matically cross-linked DNA hydrogels therefore warrant the attention of this review.
Um et al . have devised an enzyme-catalyzed assembly of large scale three-
dimensional DNA hydrogel with branched DNAs of varying shapes [86]. Branched
DNAs in the shapes of T, Y, and X have all demonstrated the ability to gelate
solvents with the aid of DNA T4 ligase. The gels formed from different branched
DNA molecules at various concentrations possessed significantly different physical
and mechanical properties. For instance, X-DNA hydrogel had the strongest
tensile modulus and the lowest tensile strength compared to Y-DNA and T-
DNA hydrogels. Morphology observation revealed different internal structures for
different hydrogels, with X-DNA gel comprising two flat stripes tangled into a
knot, Y-DNA gels having branched fibers, and X-DNA possessing puckers-like
scales. Further visualization revealed that X-DNA gel contained standardized and
well-controlled nanoscale holes of 12.3 1.3 nm, which were absent in both Y-DNA

and T-DNA hydrogels. In addition to varying physical and mechanical properties,
the hydrogels formed also differed in terms of their biodegradability. Daily DNA
mass loss analysis indicated that X-DNA hydrogel had the slowest degradation
rate over a period of two weeks. Loading the gels with DNA-binding chemical
entities like camptothecin was shown to confer a protective effect on the gel by
prolonging their degradation. To investigate the drug delivery potentials of the
hydrogels, insulin was loaded into the gels and its release profile was determined.
Insulin was released in a slow and sustained manner, with those loaded into the
X-DNA hydrogels having the slowest rate of release, which coincided with the slow
degradation rate demonstrated by the X-DNA hydrogels.
The use of DNA ligase in catalyzing the gelation process eliminated the need
for organic solvents, extreme pH, high temperature, and long reaction time. This
allowed for increased efficiency and decreased cytotoxicity in live mammalian cells
encapsulation, and, in this paper, CHO cells incorporated in situ into the X-DNA
hydrogels were able to remain viable after three days of incubation, as demonstrated
in Figure 4.11. The positive result of this cell study exhibited not only the DNA
hydrogels' biocompatibility, but also their promising potential to be used in tissue
engineering as three-dimensional cell culture platforms. The ability to disintegrate
and to digest DNA gels with nucleases enables possible cell retrieval after cell
culture, further reinforcing their usefulness in tissue engineering.
In another report, Park et al . have successfully fabricated a cell-free protein-
producing gel that can produce proteins 300 times more efficiently than existing
solution processes [87]. Ligation of the gene of interest, in this case the renilla
luciferase gene, with X-DNA connectors by DNA ligase yielded a protein-producing

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