Smart hydrogels responsive to light are made up of networks of polymers consisting of groups that are responsive to light, for example, photochromic components. These hydrogels alter their physical and/or chemical characteristics, such as elasticity, form, viscosity, and rate of bulging, when illuminated with light. Integration of photochromic components into hydrogels have been achieved chemically (i.e., covalently) or physically (i.e., non-covalently) or through many other w'ays of cross- linking [42-44]. In the advanced display units, optical switches, and devices of ophthalmic drug release, hydrogels susceptible to light exhibit promising uses. The stimulus of light could be immediately applied and with significant precision is released in distinct doses, making these hydrogels susceptible to light. In engineering as well as biochemical areas, the enhancement of hydrogels susceptible to light is crucial because of their capability of immediate release when stimulus is administered. Hydrogels susceptible to light can be categorized into visible light-sensitive and UV-sensitive hydrogels . A simple and direct way for the construction of PNIPAM/ graphene oxide (GO) nanocomposite hydrogels is by in situ /-irradiation- facilitated polymerization of an aqueous solution of N-isopropylacrylamide and GO. The association of GO with PNIPAM brought about outstanding photothermal characteristics, where the reversible phase change in hydrogel was distantly influenced through subjection or non-subjection of laser. The outstanding photothermal susceptibility exhibited by nanocomposite hydrogels stimulated by light can broadly promise uses in biomaterials as well as microdevices. Exceptional characteristics applicable for the alterations in gels when required was showed by composite polypeptide (PC10P) hydrogel with gold nanorods. These gels went through immediate thermal changes through use of outer near-infrared (NIR) light, thereby imparting a method to control drug delivery . When light is turned on, the components of azo are in the cis configuration, and the polymers of PAzo go through an alteration in structure to attain a coil form. As a consequence, the portion invaded by the PAzo chains rises, resulting in impetuous alteration in curvature and through the membrane ‘curling’ eruption of the huge vesicles. Griffin and co-workers produced smart hydrogels that are responsive to light, including photodegradable ortho-nitro- benzyl (o-NB) groups in the macromer backbone through redox polymerization. The apparent rate constants of the degradation were quantified using photorheology (at 370 nm, 10 mW/cm2). The disintegration amount was enhanced excitingly when the count of aryl ethers on the o-NB group was reduced or altered by changing the functionality from primary to secondary at the benzylic site. The outcomes also showed that the hydrogels could be utilized to encase and discharge human mesenchymal stem cells (hMSCs) without impairing survivability of cells . UCNPs, i.e., light- reactive hybrid up-conversion nanoparticles, were formed by Yan et al. (2012) from hydrogel systems, which characterized the first demonstration of how to apply the multi-photon effect of UCNPs to activate many structural changes in photophobic hydrogels. This study helps in the continuous-wave NIR light (980 nm) application, which later helps in the freedom of large biomolecules like enzymes and proteins entangled into the hydrogels and also helps in the induction of gel-sol transition.
Electro-Responsive Smart Hydrogels
Electrically responsive smart hydrogels are proficient in performing mechanical work which includes expansion, elongation, contraction, and bending under the effect of an electric field which depends on the shape and its position in accordance with the electrodes . When a hydrogel is placed perpendicular and parallel to the electrodes, shrinkage and bending are observed respectively. Bending of hydrogel is widely used for the making of mechanical devices, including artificial muscles, valves, switches, soft actuators, and molecular machines . Reversible shrinkage of hydrogels is studied mainly for drug delivery. An electric field as the external stimulus offers certain advantages such as precise control with regard to the current magnitude, the duration of the electric pulses, the interval between pulses, etc.
Evidence has been reported on the use of electric currents in the form of ionotopho- resis and electroporation in the field of transdermal drug delivery. Electro-responsive hydrogels are constructed from the polymers which consist of comparatively high concentrations of ionizable groups, similar to the pH-responsive hydrogels. Synthetic and naturally (chondroitin sulfate, hyaluronic acid, and agarose) occurring polymers, separately as well as in combination, have been explored for this purpose. Synthetic polymers applied are mostly (meth)acrylate based. Overall, the polymers which can be conducting in nature can be called electrically responsive polymers. For example, polythiophene shows inflammation, dwindling, or twisting when an external electric field is applied. Electro-responsive hydrogels have become attractive amongst different other hydrogels because they have high usage in controlled drug delivery . In an incompletely hydrolyzed polyacrylamide gel, an electro-responsive separation and contraction phase have been detected. They also deswell, which may be due to the electrophoretic pressure gradient . Osada and Hasebe in 1985 observed the same effect for water-swollen poly(2-acrylamido-2-methyl-l-propanesulfonic acid) gel with 30% loss of absorbed water when the electric field is applied . The use of chitosan gels as matrices can also assess electrically modulated drug delivery . In electrification studies, release-time outlines for neutral, anionic, and cationic drugs from hydrated chitosan gels were examined as a function of time in reaction to different currents . Similarly, chondroitin-4-sulfate hydrogels are potential matrices enabling electro-controlled peptide and protein delivery. 3D semi-interpen- etrating networks were developed by PAA and fibrin as electro-responsive smart hydrogels from free radical polymerization . Cross-linking between the two can be achieved by using initiators (ammonium persulfate, tetramethylethylenediamine) and an accelerator (N,N-methylenebisacrylamide). The electrical hydrogel stimulus ensured an enhanced penetration of cells and configuration inside the tissue, which can be useful in improving culture and seeding conditions for development of vascular grafts. Improvement in migration of cells and perfusion of cell through the culture medium during the scaffold after smearing a continuous stimulus pattern can be observed .