Formulation of three-dimensional, photo-responsive printing ink: Gold nanorod-hydrogel nanocomposites and their four-dimensional structures that respond quickly to stimuli

Interest in moving from 3D printing to 4D printing, which would enable rapid and precise motion in reaction to a given stimulus, is evolving. Thermally responsive hydrogels are one of the most common types of soft 4D devices. They are capable of changing shape or behavior autonomously in a manner resembling natural movement, and a bio-inspired soft actuator has been fabricated by combining stimuli-responsive nanoparticles with 3D printable hydrogels. We formulated and tested an N-isopropylacrylamide (NIPAm) and polyethylene glycol thiol-coated gold nanorod (P-AuNR) hydrogel nanocomposite as an active ink material for use in the fabrication of heat-sensitive soft actuators for photo-responsive soft robotic motions. Due to their surface plasmon resonance (SPR), those highly dispersible P-AuNR was able to absorb near-infrared (NIR) light and release heat into their surroundings. This system was not immersed in water and reacted quickly and sensitively to NIR exposure. The 3D structure designed and fabricated with these inks was able to deform selectively only in a very localized part in response to non-contact external stimuli, leading to various types of controlled shape-morphing.