A New Photo-Responsive Hydrogel With Photo-Rewritable Shape-Morphing and Phototunable Conductivity

Nature creatures constantly evolving shapes and properties to realize various functions and become adaptive to the changing environment. In the past several decades, intensive efforts have been given to develop materials with tunable properties. Among those, hydrogels because of their good biocompatibility, large deformation, versatile stimuli-responsiveness, have attracted significant attention among researchers in many engineering fields. Traditional shape morphing hydrogels rely on structural implementation of inhomogeneity inside the material during fabrication to realize predetermined complex shape change upon stimuli activation. Although intriguing, most of these strategies can only realize one predetermined configurational change, which is pre-set during fabrication and not reprogrammable. Similarly, traditional hydrogel electronics are made by incorporating mobile ions into the hydrogel to create circuits with predetermined functions and properties. The lack of tunability and reconfigurability of hydrogel electronics limits their application where adaptivity and multifunctionality are desired.

In recent years, several systems with rewritable shape-morphing capabilities have been demonstrated, including electrothermal and photothermal gels, reversible ion-printing gels, and photochemical gels. Among those, the photochemical hydrogels offer the best spatial and temporal control. However, for most photo-responsive hydrogels, the photoactivation and deformation processes are simultanious. The light pattern needs to be continuously adaptive to the deforming gel during the morphing process, which is impractical for complex 3D morphing. In this work, a general scheme of shape and conductivity reconfigurable hydrogel using photo-ionizable molecules is proposed. Two photocleavable molecules that can form a reactive ion couple upon light activation are incorporated into one hydrogel. This reaction between the ion couples consumes the products of the two photocleavage reactions, which not only locks the photo-ionizable molecules in the activated states but also drives the reversible photochemical reaction forward and improves the photochemical reaction efficiency. Through the coordination between the two photo-ionizable molecules, the new photo-responsive gel can decouple the photopatterning process with the morphing process and meanwhile control the local conductivity even if the light pattern is removed. Partial conversion of the photosensitive molecules can be achieved with different light exposure, making it possible to realize a continuously tunable swelling and conductivity property to a broader range. Also, taking advantage of the reaction reversibility, the photo-responsive molecules can be recovered from the activated state with a recovery stimulus. Then the hydrogel can be rewritten into a new shape or create new conductive paths with another light pattern. Based on the proposed general scheme, a specific example is given by incorporating the triphenylmethane leucohydroxide (Photobase) and 2-nitrobenzaldehyde (Photoacid) molecules into a polyacrylamide hydrogel. The re-programmable morphing and the reconfigurable conductivity with precise gradient control are also demonstrated.