4D Printing of Supramolecular Liquid Crystal Elastomer Actuators Fueled by Light

Recent years have seen major advances in the additive manufacturing of stimuli-responsive materials, also known as “4D printing,” among which liquid crystal elastomers (LCEs) play an important role. However, during fabrication photo-crosslinking of the LCEs is required, but this step is time-consuming and efficient polymerization is challenging, especially in the case of light-responsive materials. In this work, the first light-fueled supramolecular LCEs suitable for the direct ink writing (DIW) of soft actuators are synthesized in which a photopolymerization step is not needed. With the responsive supramolecular material, 3D-printed objects are fabricated by exploiting the thermoreversible interplay of the hydrogen-bonding physical cross-links. After printing, the supramolecular LCE shows reversible shape changes in response to light and is capable of bending and lifting a load. Through the combined photothermal and photochemical contributions of the incorporated azobenzene, the actuators can be triggered both in air and water. The freedom provided by DIW allows for the printing of complex responsive objects, as demonstrated by fabricating re-entrant honeycomb and spiral director structures. This approach of printing light-responsive supramolecular soft actuators opens avenues toward the application of “smart” and sustainable materials for additive manufacturing without the requirement of photo-crosslinking.