DNA existing in all living cells acts as the carrier of genetic information in their
base sequences. Previous work shows that the entrapment of double-stranded (ds)DNA within a clay hydrogel network enhances its bioresponsivity and degradation
stability [55, 56]. For example, ds-DNA is easily digested by the DNase, whereas it
is efficiently protected from DNase digestion and conserves its biological function in
a clay environment [56]. The protection of ds-DNA provided by clay hydrogel is
possibly important in the life’s origin and sustainability on the earth. Haraguchi
showed that Laponite clay nanoparticles in water serve as a multifunctional crosslinker during the polymerization of hydrophilic monomers such as
N-isopropylacrylamide (NIPAM) or DMAA leading to the formation of highly
stretchable and tough hydrogels [57–63]. Recently, self-healable DNA/clay
nanocomposite hydrogels were fabricated by free-radical polymerization of
DMAA in aqueous solutions of ds-DNA (~2,000 bp, molecular
weight ¼ 1.3 Â 10
6 g mol
À1 ) and Laponite [15]. From the cyclic mechanical tests,
the intermolecular bond strength in the hydrogels was estimated as 2.7 Æ 0.2 kJ mol
À1
which is close to that of H-bonds. It was shown that Laponite nanoparticles contribute to the elastic behavior of DNA/clay hydrogels, whereas their DNA component
promotes the energy dissipation under stress [15]. This is due to the repulsive
interactions between equally charged DNA and surfaces of disk-like Laponite
nanoparticles in water [57, 64, 65], preventing H-bonding between each other so
that DNA can move freely between the nanoparticles contributing to the energy
dissipation.
Although DNA/clay hydrogels have a low modulus and tensile strength in the
kPa range, they are highly stretchable (up to 1,500%) and display the characteristics
of ds-DNA such as its thermally induced denaturation and renaturation behavior
[15]. To highlight this feature, the hydrogels were prepared in the presence of
ethidium bromide (EtBr), which is known to intercalate between ds-DNA base
pairs leading to a higher fluorescence intensity as compared to the single-stranded
(ss)-DNA [66, 67]. Figure 8a shows fluorescence spectra of EtBr in a DNA/clay
hydrogel during a thermal cycle between 25 and 90
C, while the inset shows
temperature dependence of EtBr emission intensity at 600 nm [15]. With increasing
temperature, labeled by 1 to 5 in the figure, the peak intensity decreases, whereas it
again increases after cooling back to 25
C, labeled by 6, revealing denaturationrenaturation of ds-DNA within the gel network. The optical images of a gel sample
under UV light also visualize this conformational transition between ds- and ss-DNA
(Fig. 8b); the yellow-orange color of the gel becomes lighter with increasing
temperature up to 90
C but cooling back to 25
C recovers its initial color. Thus,
ds-DNA in the hydrogels dissociate into single strands when heated above its
melting temperature T m , whereas the double-stranded conformation is recovered
after cooling back below T m .
DNA/clay hydrogels also display an interesting healing mechanism based on the
denaturation and renaturation behavior of ds-DNA encapsulated within the hydrogel
[15]. When the cut regions of a hydrogel specimen are heated above T m of ds-DNA
and then pushed together following by cooling below T m , the hydrogel exhibits a
high healing efficiency. For instance, heating a damaged hydrogel specimen
How to Design Both Mechanically Strong and Self-Healable Hydrogels?
33
base sequences. Previous work shows that the entrapment of double-stranded (ds)DNA within a clay hydrogel network enhances its bioresponsivity and degradation
stability [55, 56]. For example, ds-DNA is easily digested by the DNase, whereas it
is efficiently protected from DNase digestion and conserves its biological function in
a clay environment [56]. The protection of ds-DNA provided by clay hydrogel is
possibly important in the life’s origin and sustainability on the earth. Haraguchi
showed that Laponite clay nanoparticles in water serve as a multifunctional crosslinker during the polymerization of hydrophilic monomers such as
N-isopropylacrylamide (NIPAM) or DMAA leading to the formation of highly
stretchable and tough hydrogels [57–63]. Recently, self-healable DNA/clay
nanocomposite hydrogels were fabricated by free-radical polymerization of
DMAA in aqueous solutions of ds-DNA (~2,000 bp, molecular
weight ¼ 1.3 Â 10
6 g mol
À1 ) and Laponite [15]. From the cyclic mechanical tests,
the intermolecular bond strength in the hydrogels was estimated as 2.7 Æ 0.2 kJ mol
À1
which is close to that of H-bonds. It was shown that Laponite nanoparticles contribute to the elastic behavior of DNA/clay hydrogels, whereas their DNA component
promotes the energy dissipation under stress [15]. This is due to the repulsive
interactions between equally charged DNA and surfaces of disk-like Laponite
nanoparticles in water [57, 64, 65], preventing H-bonding between each other so
that DNA can move freely between the nanoparticles contributing to the energy
dissipation.
Although DNA/clay hydrogels have a low modulus and tensile strength in the
kPa range, they are highly stretchable (up to 1,500%) and display the characteristics
of ds-DNA such as its thermally induced denaturation and renaturation behavior
[15]. To highlight this feature, the hydrogels were prepared in the presence of
ethidium bromide (EtBr), which is known to intercalate between ds-DNA base
pairs leading to a higher fluorescence intensity as compared to the single-stranded
(ss)-DNA [66, 67]. Figure 8a shows fluorescence spectra of EtBr in a DNA/clay
hydrogel during a thermal cycle between 25 and 90
C, while the inset shows
temperature dependence of EtBr emission intensity at 600 nm [15]. With increasing
temperature, labeled by 1 to 5 in the figure, the peak intensity decreases, whereas it
again increases after cooling back to 25
C, labeled by 6, revealing denaturationrenaturation of ds-DNA within the gel network. The optical images of a gel sample
under UV light also visualize this conformational transition between ds- and ss-DNA
(Fig. 8b); the yellow-orange color of the gel becomes lighter with increasing
temperature up to 90
C but cooling back to 25
C recovers its initial color. Thus,
ds-DNA in the hydrogels dissociate into single strands when heated above its
melting temperature T m , whereas the double-stranded conformation is recovered
after cooling back below T m .
DNA/clay hydrogels also display an interesting healing mechanism based on the
denaturation and renaturation behavior of ds-DNA encapsulated within the hydrogel
[15]. When the cut regions of a hydrogel specimen are heated above T m of ds-DNA
and then pushed together following by cooling below T m , the hydrogel exhibits a
high healing efficiency. For instance, heating a damaged hydrogel specimen
How to Design Both Mechanically Strong and Self-Healable Hydrogels?
33