Localized Liquid Secretion from a Photopatterned Liquid-Crystal Polymer Skin

Liquid-releasing artificial skins are made from films made of a smectic liquid-crystal polymer network (LCN) photopolymerized in the presence of a photoactive azobenzene chromophore and a liquid-crystal porogen. The nonreactive porogen phase separates during the polymerization process, and the polymer forms a spongy polymer network filled with liquid. The liquid is excreted from the film when exposed to UV light upon conversion of trans-azobenzene to its cis isomer. Here, localized liquid secretion at preset positions at the polymer film is described. The design principle is based on creating a hybrid molecular architecture with both smectic and nonordered isotropic alignments in a continuous LCN coating. This coating is fabricated by a maskwise photopolymerization of the monomer mixture in the smectic phase, followed by a flood exposure at an elevated temperature above the isotropic state of the unpolymerized region. The smectic regions that polymerized during the maskwise exposure are not affected by the heating needed for the second polymerization step of the isotropic area. Upon activation under light illumination, the embedded liquid is exclusively released from the area with the smectic alignment. This approach reveals films of which the excretion process is reversible. The secreted liquid is reabsorbed spontaneously when azobenzene takes its initial trans-form. This process occurs thermally in time or can be accelerated by light irradiation with visible light.