Two-photon direct laser writing enables the fabrication of shape-changing microstructures that can be exploited in stimuli responsive micro-robotics and photonics. The use of Liquid Crystalline Networks (LCN) allows to realize 3D micrometric objects that can contract along a specific direction in response to stimuli, such as temperature or light. In this paper, the fabrication of free-standing LCN microstructures is demonstrated as graphical units of a smart tag for simple physical and optical encryption. Using an array of identical pixels, information can be hidden to the observer and revealed only upon application of a specific stimulus. The reading mechanism is based on the shape-change of each pixel under stimuli and their color that combine together in a two-level encryption label. Once the stimulus is removed, the pixels recover their original shape and the message remains completely hidden. Therefore, an opto-mechanical equivalent of an "invisible ink" is realized. This new concept paves the way for introducing enhanced functionalities in smart micro-systems within a single lithography step, spanning from storage devices with physical encryption to complex motion actuators.
Keywords: direct laser writing; gray-scale lithography; liquid crystalline networks; physical information encryption; shape-changing materials.
© 2023 The Authors. Small published by Wiley-VCH GmbH.