Transparent photothermal (PT) materials absorbing NIR light have attracted strong interest for wireless soft robots, photothermal therapy, sensors, and wearable optoelectronic devices. Since PT materials convert light energy to heat energy, they can function as highly programmable wireless heaters and PT actuators, depending on the intensity and wavelength of the light source. However, transparent PT materials have been rarely researched because of the difficulty in absorption separation between the visible and NIR region in a molecule. Furthermore poor solubility of NIR dyes limit their application as a transparent film. Herein we present a transparent NIR dye having a transparency at visible region over 90 %. With a transparent binder, the NIR dye was fabricated as an NIR absorbing film, which showed high NIR photothermal conversion efficiency over 75 %. Taking advantage of the high photothermal effect of the film, we were able to generate a transparent photothermal heater that could be applied into desalination and actuation. The mechanism and application potential for the photothermal effect will be discussed based on the optical properties of the NIR dyes.
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Thin films of conjugated polymers (CPs) were explored for photothermal pattering of active materials. Due to the semiconductive nature of the CP films, CP film was heated upon NIR exposure via photothermal conversion. Various photothermal patterns were generated from the light pattern generated from POLs. The distance from the POL to the CP substrate was controlled to obtain the target light patterns with the maximum overlap or separation among the diffracted lights. From the light patterns we were able to generate micro thermal patterns to large area thermal pattern, and multiple thermal patterns. The photothermal patterns were used to pattern active layers such as cell sheets which are sensitive to heat. Using thermally responsive interfacing layer (TRI) made of collagen, cell sheets of various shapes were harvested to provide a non-invasive method of live cell sheet preparation. When photothermal CPs were coated on a thermally expandable TRI, the bilayer was folded within few sec of NIR exposure, to generate a complex 3D structure. The degree of folding and reversibility of folding were dependent on the thickness of both CP layer and TRI layer.