value for intercalation based on the neighbor exclusion model. Sorption of EtBr by
the cryogels with 90 % efficiency, observed at r < 0.25, is thus attributed to the
intercalation of EtBr within the double helical DNA strands. Moreover, the intercalation of EtBr into DNA changes the rotation angle between adjacent base pairs
and unwinds the helix, resulting in lengthening of the DNA strand [124, 125]. This
process also increases the viscosity of DNA solutions due to the increasing hydrodynamic size of DNA strands [126]. The extent of the increase in viscosity depends
on the number of EtBr molecules bound per nucleotide; this behavior is also a
characteristic of all DNA intercalators. Thus, the initial swelling of DNA gels at
r < 0.3 may originate from the increasing length of DNA network chains, which
decreases the elastically effective crosslink density of the cryogels. We should
mention that the disintegration of DNA hydrogels at r ¼ 0.10–0.15 could also be
related to this phenomenon (Fig. 16a). Lengthening and stiffening of the helix due
to intercalated DNA decreases the crosslink density of the hydrogels [48], leading
to their dissolution in EtBr solutions. In contrast, cryogels are stable in EtBr
solutions due to the dense pore walls formed during cryo-concentration.
The swelling response of DNA cryogels to changes in EtBr concentration
between 3 and 300 μM also suggests that they can be used to detect
DNA-binding substrates in aqueous solutions. On the other hand, it was reported
that at high r ratios (r > 0.25), after the primary sites of DNA have been filled, a
secondary binding process occurs arising from electrostatic interactions between
phosphate residues and EtBr molecules attached outside the helix. As a consequence, in this range of r, the cryogel deswells with increasing concentration of
EtBr, which is similar to the behavior of polyelectrolyte hydrogels immersed in
aqueous solutions of increasing salt concentration. The results thus show that DNA
cryogels can be used as a specific sorbents to remove carcinogenic agents from
aqueous solutions. Moreover, the response of DNA cryogels to changes in EtBr
concentration between 3 and 300 μM also suggests that they can be used to detect
DNA-binding substrates in aqueous solutions.
5.2 Fibroin Cryogels as Mechanically Strong Scaffolds
Silk fibroin derived from Bombyx mori is a fibrous protein exhibiting extraordinary
material properties such as good biocompatibility, biodegradability, high strength
and toughness, and ease of processability [127–129]. Silk fibroin has been used for
cell culture, wound dressing, drug delivery, enzyme immobilization, and as a
scaffold for bone tissue engineering [130, 131]. Silk fibroin has a blocky structure
consisting of less-ordered hydrophilic blocks and crystallizable hydrophobic blocks
[132–135]. Hydrophilic blocks provide solubility in water and are responsible for
fibroin elasticity and toughness, whereas hydrophobic blocks form intermolecular
β-sheet structures leading to the insolubility and high strength of fibroin. Several
techniques have been developed to produce porous fibroin scaffolds such as freeze–
thawing, porogen leaching, gas foaming, electrospinning, and freeze-drying
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O. Okay and V.I. Lozinsky
the cryogels with 90 % efficiency, observed at r < 0.25, is thus attributed to the
intercalation of EtBr within the double helical DNA strands. Moreover, the intercalation of EtBr into DNA changes the rotation angle between adjacent base pairs
and unwinds the helix, resulting in lengthening of the DNA strand [124, 125]. This
process also increases the viscosity of DNA solutions due to the increasing hydrodynamic size of DNA strands [126]. The extent of the increase in viscosity depends
on the number of EtBr molecules bound per nucleotide; this behavior is also a
characteristic of all DNA intercalators. Thus, the initial swelling of DNA gels at
r < 0.3 may originate from the increasing length of DNA network chains, which
decreases the elastically effective crosslink density of the cryogels. We should
mention that the disintegration of DNA hydrogels at r ¼ 0.10–0.15 could also be
related to this phenomenon (Fig. 16a). Lengthening and stiffening of the helix due
to intercalated DNA decreases the crosslink density of the hydrogels [48], leading
to their dissolution in EtBr solutions. In contrast, cryogels are stable in EtBr
solutions due to the dense pore walls formed during cryo-concentration.
The swelling response of DNA cryogels to changes in EtBr concentration
between 3 and 300 μM also suggests that they can be used to detect
DNA-binding substrates in aqueous solutions. On the other hand, it was reported
that at high r ratios (r > 0.25), after the primary sites of DNA have been filled, a
secondary binding process occurs arising from electrostatic interactions between
phosphate residues and EtBr molecules attached outside the helix. As a consequence, in this range of r, the cryogel deswells with increasing concentration of
EtBr, which is similar to the behavior of polyelectrolyte hydrogels immersed in
aqueous solutions of increasing salt concentration. The results thus show that DNA
cryogels can be used as a specific sorbents to remove carcinogenic agents from
aqueous solutions. Moreover, the response of DNA cryogels to changes in EtBr
concentration between 3 and 300 μM also suggests that they can be used to detect
DNA-binding substrates in aqueous solutions.
5.2 Fibroin Cryogels as Mechanically Strong Scaffolds
Silk fibroin derived from Bombyx mori is a fibrous protein exhibiting extraordinary
material properties such as good biocompatibility, biodegradability, high strength
and toughness, and ease of processability [127–129]. Silk fibroin has been used for
cell culture, wound dressing, drug delivery, enzyme immobilization, and as a
scaffold for bone tissue engineering [130, 131]. Silk fibroin has a blocky structure
consisting of less-ordered hydrophilic blocks and crystallizable hydrophobic blocks
[132–135]. Hydrophilic blocks provide solubility in water and are responsible for
fibroin elasticity and toughness, whereas hydrophobic blocks form intermolecular
β-sheet structures leading to the insolubility and high strength of fibroin. Several
techniques have been developed to produce porous fibroin scaffolds such as freeze–
thawing, porogen leaching, gas foaming, electrospinning, and freeze-drying
140
O. Okay and V.I. Lozinsky