Liquid crystalline network microstructures for stimuli responsive labels with multi-level encryption

Two-photon Direct Laser Writing enables the fabrication of shape-changing microstructures that can be exploited in stimuli responsive microrobotics and photonics. The use of Liquid Crystalline Networks allows to realise 3D micron-sized objects that can contract anisotropically, along a specific direction in response to stimuli, such as temperature or light. In this paper, we demonstrate the fabrication of free-standing LCN microstructures as graphical units of a couvert tag for simple physical and optical encryption. Using an array of identical Liquid Crystalline pixels, information can be hidden to the observer and revealed only upon application for a specific stimulus. The reading mechanism is based on a specific shape-change of each pixel so that once the stimulus is removed and the pixels recover their original shape the message remains completely hidden. We have therefore realised an opto-mechanical equivalent of an "invisible ink". This new concept paves the way for introducing enhanced functionalities in micro-optomechanical systems within a single lithography step, spanning from storage devices with physical encryption to complex motion actuators.