Environment–Stimuli Response of Chitosan-Based Hydrogels - Chitosan Based Hydrogels: Functions and Applications - page 232

hydrogels may also respond to temperature. These stimuli-responsive properties of
chitosan-based hydrogels have been widely investigated and explored for the biomedical
field, especially for drug-controlled release.
5.2 pH Response
The presence of amino groups in chitosan (p K a = 6.2-6.5) results in pH-sensitive character.
The pH sensitivity of a chitosan-based hydrogel is a change in volume of the hydrogel in
response to pH changes in the surrounding medium, caused by the presence of weakly
acidic or basic character. One characteristic phenomenon of pH-sensitive hydrogels is the
dynamic swelling corresponding to different pH values of the surrounding medium. The
swelling takes place because of ionization by acid or base transfer from the surrounding
bulk solution. The acid or base character of the bulk solution causes the gels to swell or
shrink [2]. Various methods have been employed for elucidating the swelling behavior of
pH-sensitive chitosan-based hydrogels; they include weighing of hydrogel slabs, calculat-
ing the volume change by measuring the diameter of hydrogel discs, and spectroscopic
and microelectro-mechanical methods.
The mechanism of water transport could be diffusion, convection, or both during the
swelling process. Water and solute molecules travel within the gel by the osmotic pressure
gradient (due to the water concentration gradient) between the inside and outside of the
hydrogels. From the structural point of view, all pH-sensitive hydrogels have either acidic
or basic groups; these groups can ionize and develop fixed charges on the polymer net-
work, which results in electrostatic repulsive forces, responsible for pH-dependent
swelling/shrinkage of the hydrogels. For cationic chitosan-based hydrogels, amino groups
are protonated below the p K a of chitosan, leading to swelling of the hydrogel at a pH below
the chitosan p K a because of a large osmotic swelling force due to the presence of ions ( cf.
Figure 5.1). Small changes in pH (especially in the vicinity of p K a ) can result in significant
changes in the mesh size of the chitosan hydrogel networks [3]. Considering that the p K a
of chitosan is ~6.5, at pH > p K a , chitosan is found to form dissociated precipitates rather
than a massive hydrogel because the aggregation of chitosan polymers occurred too rap-
idly and locally [4]. Recently, the pH-sensitive chitosan-based hydrogels are prepared via
chemical modification or composites with other compounds. Different chitosan-based
hydrogel networks have specific pH responses. The swelling and pH responsiveness can
Figure 5.1
The pH-responsive swelling of chitosan-based hydrogels.