3.1.4 Disulfide Bonds
Incorporation of dynamic disulfide groups into the main chains of cellulose is
another effective method to obtain reversible cross-linked cellulose-based gels
[149–152]. The disulfide bonds are reversible covalent bonds based on thiol/disulfide dynamic exchange reactions and are sensitive to pH or redox potential
[153, 154], which provide a new promising physiologically compatible strategy
for preparing dissociable materials hydrogels and micelles for drug and gene delivery [155–157].
In 2011, the thiolated hydroxypropyl cellulose derivatives (HPC-SH) were synthesized without destroying the thermo-sensitive property of HPC by Tan et al.
[149]. The cellulose nanogels were fabricated by the self-association of HPC-SH in
the solution at 45
C, and then oxidation of thiol groups to disulfide bonds occurred
to stabilize the associated structure. Neither monomer nor cross-linker was used in
the preparation of HPC nanogels in this approach, and the resultant nanogels showed
both thermo- and redox sensitivities. The cross-linking degree of the nanogels could
be controlled by the substitution degree of thiol groups (–SH) in the thiolated HPC.
The hydrodynamic radius of the nanogels can be tuned by adjusting the degree of
cross-linking and the concentration of HPC-SH concentration and temperature. The
dual stimuli-sensitive nanogels may have potential applications in controlled drug
release, transfer switch device, and sensors. Afterward, Hou and co-workers [151]
presented a novel pH and redox dual-responsive cellulose-based nanogel and applied
the controlled release of agrochemicals. Hydrophobic carboxymethyl cellulose
(HCMC) was prepared by esterification of carboxymethyl cellulose (CMC) and
palmitoyl chloride (PCl), and then aldehyde groups were grafted onto HCMC by
the addition of glyoxal to facilitate the cross-linking reaction. The obtained product
(HCMC-a) was mixed with salicylic acid (SA) and 3,3
0 -dithiobis
(propionohydrazide) (DTP) solutions to form dual-responsive nanogel, which
displayed pH and glutathione (GSH)-triggered release behaviors of SA.
Mao’s group [152] also prepared a cellulose-based multi-responsive hydrogel
containing enamine and disulfide bonds in the same system. The cellulose hydrogel
was obtained by simply mixing aqueous solutions of cellulose acetoacetate (CAA)
and cystamine dihydrochloride (CYS) at room temperature. Because it contained a
pH-responsive enamine moiety and a redox-active disulfide moiety, the obtained
cellulose-based hydrogel displayed dual-responsive properties with tunable release
in response to changes in the pH value and dithiothreitol (DTT) concentration.
The structurally dynamic disulfide bond for the design of reversible bonding
adhesive hydrogel has also attracted a growing interest. Recently, a strong,
rebondable, semicrystalline disulfide nanocomposite network was reported by
Cudjoe et al. [150], where the thiol-endcapped polymer was dynamically crosslinked with thiol-functionalized CNCs by the disulfide bonds. The obtained
nanocomposites that exhibited excellent adhesive properties and increasing temperature from 80 to 150
C resulted in the rebonding with little to no loss in adhesive
Dynamics in Cellulose-Based Hydrogels with Reversible Cross-Links
337
Incorporation of dynamic disulfide groups into the main chains of cellulose is
another effective method to obtain reversible cross-linked cellulose-based gels
[149–152]. The disulfide bonds are reversible covalent bonds based on thiol/disulfide dynamic exchange reactions and are sensitive to pH or redox potential
[153, 154], which provide a new promising physiologically compatible strategy
for preparing dissociable materials hydrogels and micelles for drug and gene delivery [155–157].
In 2011, the thiolated hydroxypropyl cellulose derivatives (HPC-SH) were synthesized without destroying the thermo-sensitive property of HPC by Tan et al.
[149]. The cellulose nanogels were fabricated by the self-association of HPC-SH in
the solution at 45
C, and then oxidation of thiol groups to disulfide bonds occurred
to stabilize the associated structure. Neither monomer nor cross-linker was used in
the preparation of HPC nanogels in this approach, and the resultant nanogels showed
both thermo- and redox sensitivities. The cross-linking degree of the nanogels could
be controlled by the substitution degree of thiol groups (–SH) in the thiolated HPC.
The hydrodynamic radius of the nanogels can be tuned by adjusting the degree of
cross-linking and the concentration of HPC-SH concentration and temperature. The
dual stimuli-sensitive nanogels may have potential applications in controlled drug
release, transfer switch device, and sensors. Afterward, Hou and co-workers [151]
presented a novel pH and redox dual-responsive cellulose-based nanogel and applied
the controlled release of agrochemicals. Hydrophobic carboxymethyl cellulose
(HCMC) was prepared by esterification of carboxymethyl cellulose (CMC) and
palmitoyl chloride (PCl), and then aldehyde groups were grafted onto HCMC by
the addition of glyoxal to facilitate the cross-linking reaction. The obtained product
(HCMC-a) was mixed with salicylic acid (SA) and 3,3
0 -dithiobis
(propionohydrazide) (DTP) solutions to form dual-responsive nanogel, which
displayed pH and glutathione (GSH)-triggered release behaviors of SA.
Mao’s group [152] also prepared a cellulose-based multi-responsive hydrogel
containing enamine and disulfide bonds in the same system. The cellulose hydrogel
was obtained by simply mixing aqueous solutions of cellulose acetoacetate (CAA)
and cystamine dihydrochloride (CYS) at room temperature. Because it contained a
pH-responsive enamine moiety and a redox-active disulfide moiety, the obtained
cellulose-based hydrogel displayed dual-responsive properties with tunable release
in response to changes in the pH value and dithiothreitol (DTT) concentration.
The structurally dynamic disulfide bond for the design of reversible bonding
adhesive hydrogel has also attracted a growing interest. Recently, a strong,
rebondable, semicrystalline disulfide nanocomposite network was reported by
Cudjoe et al. [150], where the thiol-endcapped polymer was dynamically crosslinked with thiol-functionalized CNCs by the disulfide bonds. The obtained
nanocomposites that exhibited excellent adhesive properties and increasing temperature from 80 to 150
C resulted in the rebonding with little to no loss in adhesive
Dynamics in Cellulose-Based Hydrogels with Reversible Cross-Links
337