Light-sensitive polypeptide hydrogel and nanorod composites.

Stimuli-responsive hydrogels exhibit structural changes based on changes in local temperature or pH and analytes and are highly valued in fields ranging from controlled drug release to tissue repair and also in microdevices. Hydrogels that respond to external stimuli, such as light or magnetic fields, offer additional advantageswith respect to on-demandand triggered response. With this in mind, we formulated a composite of thermoreversible polypeptide-based hydrogels, formed from a genetically engineered multiblock polypeptide that exhibits a temperature-dependent transition from a solid to liquid state, and gold nanorods. Near-infrared-light (NIR) exposure of the nanorods induces local heating and, consequently, melting of the gels. These networks were explored for the controlled release of a macromolecule and the release profiles were controlled by the extent and timing of light exposure, including stepwise release with intermittent light. The infrared-lightcontrolled dissociation of these hydrogels offers unique opportunities, such as for the delivery of drugs and growth factors with transdermal light exposure or for incorporation of hydrogel actuators in microdevices. Smart biopolymeric hydrogels are a new generation of biomaterials that may exhibit reversible physicochemical changes in response to their environment. These stimuliresponsivegels arefindingapplications inmanyfields, suchas for the delivery of therapeutic molecules, biomedical devices, such as actuators and biosensors/diagnostics, and as scaffolds for tissue engineering and regenerative medicine. A unique feature of such materials is that they undergo significant conformational changes upon variation in one or more physicochemical stimuli such as pH, temperature, analytes, or light. Although many variables have been explored, hydrogels that reversibly respond to temperature changes have been the subject of major investigation over the past two decades. These thermoresponsive systems are typically centered around synthetic polymers suchas poly(N-isopropylacrylamide) (PNIPAm) and its derivatives or biomimetic elastin-based