Stimuli Responsive Hydrogels

Stimuli Responsive Hydrogels

Stimuli-responsive hydrogels are polymers that undergo swelling due to their sensitivity to environmental factors. The cross-linked polymer networks that form hydrogels undergo a physicochemical transition due to an external stimulus such as a change in pH levels, temperature, and light. The response to stimuli often manifests as a change in surface charge, hydrophobicity, and swelling and de-swelling (Priya James, John, Alex, & Anoop, 2014).  

The swelling of a pH-responsive hydrogel is the result of changes in the concentration of H+ ions present in the surrounding system. Ionizable acidic or alkaline groups attached to the polymer gives it this unique characteristic. The extent to which swelling occurs is determined by the degree to which the attached functional groups are able to be ionized (Samchenko, Ulberg, & Korotych, 2011). Deprotonation and swelling of an anionic hydrogel is greater when the external pH level is greater than the pKA level of the ionizable functional group. This is because as the electrostatic repulsion between the chains grows as ionization increases, which results in greater swelling of the gel (Kasapis, Norton, & Ubbink, 2009). In contrast, cationic hydrogels are able to swell significantly better when the surrounding pH is lower as amino groups on the gel protonate at lower pH.

Light-responsive hydrogels experience physical changes based on the intensity of the ultraviolet or visible light. The responsiveness of UV-sensitive hydrogels is due to the implementation of a leuco derivative molecule, bis (4-dimethylamino) phenyl methyl leucocyanide, into the hydrogel’s network structure. Triphenylmethane leuco derivatives dissociate into ion pairs when exposed into ultraviolet irradiation producing triphenylmethyl cations, causing an increase in osmotic pressure within the gel and inducing swelling (Singh et al., 2010). Visible light-sensitive hydrogels are produced by incorporating trisodium salt of copper chlorophyllin, light-sensitive chromophore, into a thermosensitive polymer. When exposed to light, the chromophore absorbs the incoming rays and dissipates them throughout the structure via radiationless transitions. Since the polymer hydrogel is thermosensitive, the heat causes a swelling behaviour (Vernon, 2011).

Temperature-responsive hydrogels are polymers that react to temperature with changes in their volumetric size. Most hydrogels have a lower critical solution temperature (LCST), and collapse if their temperature goes beyond their LCST (Yaszemski, 2004). This is because thermo-sensitive polymer chains consist of either moderately hydrophobic groups or mixtures of hydrophobic and hydrophilic segments. Hydrogen bonding between water and the hydrophilic segments causes enhanced dissolution at lower temperatures (Kim, Bennet, Marimuthu, & An, 2012). As the temperature rises, hydrophobic segments are strengthened, causing the hydrogels to shrink and collapse due to the inter-polymer chain associations. Hydrogels that display this behavior are referred to as thermophobic. Hydrogels that swell upon heating are called thermophilic, however thermophilic behavior in water is uncommon for polymer gels (Mah & Ghosh, 2013).