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.
Two photon lithography (TPL) is a versatile method for the fabrication of photonic structures based on photoresist materials. Structures producing vivid colors in transmission or reflection can be achieved. Combining TPL with smart hydrogels opens the route to reversible sensors for a wide range of external stimuli. The printing resolution for TPL can reach 300 nm. As the scale of the smart hydrogel is decreased from the millimeter to the micron level, its actuation speed can be increased many-fold. Herein, we report on a square spiral shaped vapor responsive hydrogel photonic structure. The structural color is shown to be reproducible and reversible under exposure to water, ethanol, IPA and acetone vapor. The influence of the fabrication laser power on the structure dimension and vapor responsivity are also demonstrated, with structures fabricated using a higher laser power producing a larger vapor sensitive spectral response. Finite difference time domain simulations accurately predict the structural color and confirm expansion of the structure when exposed to the vapors is the dominant contribution for the color transformation. Structures for pattern transformation and encryption are also demonstrated.
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