Iron oxide nanoparticles dispersed within a thermally responsive poly(N-isopropylacrylamide) (PNIPAm) hydrogel matrix effectively convert the photo energy of visible light of modest intensity into thermal energy, providing the efficient means to trigger changes in volumetric swelling of hydrogels. However, long irradiation time (on the order of minutes) and modest volume change limit their applications that need fast response and/or large volume change. In this work, we found that the degree of volume change triggered by light could be maximized by adjusting the lower critical solution temperature (LCST) of the hydrogels. On the basis of the evidence in this investigation, we can develop highly responsive hydrogels that show rapid and significant light-induced volume change, which could be achieved by incorporating a hydrophobic N,N-diethylacrylamide moiety in the PNIPAm network. This enhanced responsiveness led to the successful application of this material in a remote-controllable microvalve for microfluidic devices operated by light illumination within a few seconds.

In order to achieve a hydrogel capable of programmable volume change, poly(N-isopropylacrylamide)graf t-methylcellulose hydrogel (PNIPAm-g-MC) was prepared through the grafting of PNIPAm onto a MC backbone and simultaneous cross-linking of the chains. PNIPAm-g-MC exhibited large thermal hysteresis in its volume change, which results from the stable hydrophobic junctions between the MC strands formed during heating. By combining photothermal magnetite nanoparticles as a heat transducer with the prepared hydrogel, programmable volume phase transition between the shrunken and swollen state could be triggered by visible light irradiation and excessive cooling, respectively. Based on this programmable feature, a bilayer actuator capable of static bending was fabricated. The developed programmable hydrogels are expected to provide a platform for the next generation of origami, microvalves, and drug delivery systems.

Extensive research efforts have been devoted to the development of hydrogel microfibers for tissue engineering, because the vascular structure is related to the transport of nutrients and oxygen as well as the control of metabolic and mechanical functions in the human body. Even though stimuli-responsive properties would enhance the potential applicability of hydrogel microfibers for artificial tissue architectures, previous studies of their fabrication have not considered changes in the microfibers in response to external stimuli. In this work, we prepared temperature-responsive poly(N-isopropylacrylamide) (PNIPAm) microfibers with controlled shapes and sizes by the in situ photo-polymerization of aqueous monomers loaded in calcium alginate templates generated from microcapillary devices. We found that the shape and size of the hydrogel microfibers could be controlled by adjusting the injection positions of the solutions and varying the diameters of the inner capillary, respectively. We further fabricated light-responsive materials by incorporating photothermal magnetite nanoparticles (MNPs) within the temperature-responsive PNIPAm hydrogel microfibers. Because the MNPs incorporated into the PNIPAm microfibers generated heat upon the absorption of visible light, we could demonstrate volume changes in the microfibers triggered by both visible light irradiation and temperature.

To overcome the slow kinetics of the volume phase transition of stimuli-responsive hydrogels as platforms for soft actuators, thermally responsive comb-type hydrogels were prepared using synthesized poly(N-isopropylacrylamide) macromonomers bearing graft chains. Fast responding light-responsive hydrogels were fabricated by combining a comb-type hydrogel matrix with photothermal magnetite nanoparticles (MNP). The MNPs dispersed in the matrix provide heat to stimulate the volume change of the hydrogel matrix by converting absorbed visible light to thermal energy. In this process, the comb-type hydrogel matrix exhibited a rapid response due to the free, mobile grafted chains. The comb-type hydrogel exhibited significantly enhanced light-induced volume shrinkage and rapid recovery. The comb-type hydrogels containing MNP were successfully used to fabricate a bilayer-type photo-actuator with fast bending motion.

On-demand release from stimuli-responsive hydrogels has received great attention due to an increasing clinical need. Here, we have prepared spherical hydrogel beads showing visible light-induced volume change at body temper-ature. By spray injection of the monomer solution using the alginate templ-ating method, hybrid beads of several hundred micrometers, consisting of temperature-responsive poly(N-isopropylacrylamide-co-vinyl-2-pyrrolidinone) hydrogel and magnetite nanoparticles (MNP), are produced. MNP dispersed in the hydrogel matrix absorbed visible light and generated heat, increasing the temperature of the matrix and resulting in shrinkage of the beads proportional to light intensity. It is demonstrated that light-induced volume change of dexamethasone-loaded hybrid beads result in on-demand and localized release of the drug by exposure to moderate visible light. As a potential application of the light-sensitive hybrid hydrogel beads, a transdermal patch is developed that incorporates drug-loaded hydrogel beads in multiple drug reservoirs, achieving enhanced release of a model drug when exposed to visible light. This platform should be applicable to on-demand, sequential, and long-term release of drugs via light exposure.