Real-Time in Situ Investigation of Supramolecular Shape Memory Process by Fluorescence Switching

Shape memory hydrogels (SMHs) that take advantage of supramolecular chemistry cross-links to fix temporary shapes at room temperature are newly developed important shape memory polymers. It is thus highly desirable to explore in detail the in situ formation and real-time spatial distribution of temporary supramolecular cross-links in SMH systems, which can provide more in-depth information about the shape memory mechanism and promote the fabrication of new SMHs. However, related study still remains very challenging. We herein report the development of a special SMH system that involves fluorescent alginate-Eu3+ complexes as switching cross-links. Its coordination-triggered supramolecular shape memory/recovery processes and dynamics could be directly visualized by a high-contrast fluorescence imaging method. Remarkably, this efficient and sensitive fluorescence study enables real-time in situ monitoring of the process, in which Eu3+ ions gradually diffuse into the hydrogel sample and chelate with alginate to form the Eu3+-Alg temporary cross-links. Furthermore, a theoretical model correlating the fluorescence intensities of the SMH system with their shape memory effect (SME) was successfully established, which allows the facile and accurate prediction of both the shape memory and recovery ratios on the basis of quantitative emission spectral results. These details disclosed in this study will thus deepen the understanding of supramolecular shape memory process and mechanisms and accelerate the development of new practical shape memory hydrogels in the future.