Regulation of a Real‐Time Self‐Healing Process in Organogel Tissues by Molecular Adhesives

Abstract

Materials that change their physical properties on mechanical agitation are known as thixotropic materials. In the human body, for example, the functions of protoplasm, red blood cells, synovial fluid, and muscular activities are regulated by thixotropy. [1][2][3] Similarly, the properties of materials such as paints, bentonite clay, cosmetics, and foodstuffs arise from thixotropy. Despite the significant potential of this dynamic phenomenon, there is a lack of materials that can act as a model system to investigate vital natural processes such as muscle thixotropy and nerve fiber regeneration. In this respect, low-molecular-weight organogels [4] (LMOGs), which generally consists of 1D fibrous molecular assemblies, are envisioned to be attractive candidates when provided with thixotropic properties. However, not only are examples of LMOGs that exhibit thixotropy rare, [5][6][7][8][9] but there is also little information related to their aggregation and stability. These aspects circumvent the use of LMOGs for the study of this unique phenomenon.Of the hundreds of functional LMOGs reported to date, only very few exhibit the unique property of thixotropy. In general, LMOGs are extremely sensitive to mechanical stress and these materials irreversibly expel solvent molecules from their network when subjected to flow. On removal of the external force, these materials behave as a solid suspension that loses its original elastic properties. On the other hand, thixotropic gels can disintegrate in solution under an external mechanical stress and can regain their elastic properties upon removal of the stress. This operation can normally be carried out for an infinite number of cycles.Thixotropic LMOGs represent an intriguing and unique class of truly dynamic self-assembled supramolecular systems. Discernible visual insights into the process would be required in order to understand how such self-assembled entities evolve under mechanical stress followed by a resting time. To obtain such "snapshots", the thixotropic gel has to operate under a strict real concentration domain. However, the LMOGs reported to date show thixotopic properties only at concentrations of 10 wt % or more. At these high concentrations, the molecules are closely associated and therefore the reaggregation to the gel state from the disaggregated solution state occurs within seconds, thus rendering these systems unviable for real-time imaging. Hence, from the perspective of a supramolecular chemist, a successful thixotropic LMOG candidate has to fulfill three stringent requirements: firstly, the LMOG has to be an extremely efficient gelator (close to a super-gelator); secondly, it must exhibit the thixotropic property at such low concentrations; and thirdly, the gel has to maintain its original state without undergoing aging or forming crystalline domains during the cycles of breaking and regeneration processes. It is therefore difficult to obtain a supramolecular system that comprises all of these favorable functional features.Herein we report the discove...

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