Plasmonic Hydrogel Actuators for Octopus-Inspired Photo/Thermo-Responsive Smart Adhesive Patch

Living organisms in nature exhibit remarkable adhesive properties, such as mussels, geckos, tree frosg, and octopuses, inspiring the development of novel adhesive systems for various applications such as skin attachable electronics, biomedical devices, and locomotion devices. Recently, octopus-inspired adhesives have gained attention owing to their ability to achieve high adhesion on various substrates. An octopus’ suckers are sealed at the rim and function by reducing the pressure inside the cavity, thereby creating a pressure difference between the ambient environment and the inner cavity. In this study, we propose a smart adhesive patch that mimics the octopus adhesion mechanism, offering tunable adhesion in response to temperature and near-infrared (NIR) light. This dual responsivity is achieved by integrating plasmonic gold nanostars (GNSs) as NIR light-responsive photothermal materials and poly(N-isopropylacrylamide) (PNIPAM) hydrogels as thermo-responsive materials on a nanohole-patterned polydimethylsiloxane (PDMS) film, mimicking the suction cups of an octopus. The muscle-like, thermo-responsive PNIPAM hydrogel functions as a volumetric actuator to regulate the cavity pressure. At temperatures exceeding the lower critical solution temperature (LCST) of PNIPAM, the hydrogel layer contracts, enlarging the cavity volume and thus reducing the cavity pressure to achieve strong suction adhesion. The expelled water molecules during the process contribute to an increased adhesion effect. The smart adhesive patch demonstrates effective adhesion performance by controlling nanocavity pressure and leveraging both capillary-assisted and chemical adhesion mechanisms.<br/>The smart adhesive patch shows strong adhesion forces up to 134 kPa at 45 °C and large on-off adhesion ratio (~63) through temperature control. During temperature elevation, the GNS adsorbed with the polymer enhances the responsiveness of PNIPAM hydrogel with a synergetic effect of strong mechanical stress and high thermal conductivity. GNS is characterized by a pronounced light absorption peak in the NIR light range and transforms light into heat using the localized surface plasmon resonance (LSPR) effect. The sharp tips of plasmonic GNS amplify localized electromagnetic fields more effectively than other nanoparticle shapes, resulting in a red-shifted LSPR band. The light absorption peak of GNS located in the NIR range effectively absorbs NIR light compared to gold nanospheres. The photothermally excited GNS increase the temperature under NIR light irradiation, resulting in a high adhesion force of 71 kPa at 85 mW cm^-2. The smart adhesive patch can be used on a variety of materials, including plastic, glass, metal, and organ, and it leaves minimal residue after operation due to the switchable adhesion properties, mitigating potential harm to delicate organs. We have further employed our smart adhesive patch to transfer an ultrathin Temperature Coefficient Resistance (TCR) sensor onto the organ of a living mouse. We utilized the smart adhesive patch as a transfer medium to successfully relocate a 3 µm-thick TCR sensor to a living mouse’s liver for real-time temperature monitoring.