Chemical reaction steers spatiotemporal self-assembly of supramolecular hydrogels.

Supramolecular structures are widespread in living system, which are usually spatiotemporally regulated by sophisticated metabolic processes to enable vital biological functions. Inspired by living system, tremendous efforts have been made to realize spatiotemporal control over the self-assembly of supramolecular materials in synthetic scenario by coupling chemical reaction with molecular self-assembly process. In this review, we focused on the works related to supramolecular hydrogels that are regulated in space and time using chemical reaction. Firstly, we summarized how spatially controlled self-assembly of supramolecular hydrogels can be achieved via chemical reaction-instructed self-assembly, and the application of such a self-assembly methodology in biotherapy was discussed as well. Second, we reviewed dynamic supramolecular hydrogels dictated by chemical reaction networks that can evolve their structures and properties against time. Third, we discussed the recent progresses in the control of the self-assembly of supramolecular hydrogels in both space and time though a reaction-diffusion-coupled self-assembly approach. Finally, we provided a perspective on the further development of spatiotemporally controlled supramolecular hydrogels using chemical reaction in the future.

Localized self-assembly of macroscopically structured supramolecular hydrogels through reaction-diffusion.

Localized molecular self-assembly has been developed as an effective approach for the fabrication of spatially resolved supramolecular hydrogels, showing great potential for many high-tech applications. However, the fabrication of macroscopically structured supramolecular hydrogels through molecular self-assembly remains a challenge. Herein, we report on localized self-assembly of low molecular weight hydrogelators through a simple reaction-diffusion approach, giving rise to various macroscopically patterned supramolecular hydrogels. This is achieved on the basis of an acid-catalyzed hydrazone supramolecular hydrogelator system. The acid was pre-loaded in a polydimethylsiloxane (PDMS) substrate, generating a proton gradient in the vicinity of the PDMS surface after immersing the PDMS in the aqueous solution of the hydrogelator precursors. The acid dramatically accelerates the in situ formation and self-assembly of the hydrazone hydrogelators, leading to localized formation of supramolecular hydrogels. The growth rate of the supramolecular hydrogels can be easily tuned through controlling the concentrations of the hydrogelator precursors and HCl. Importantly, differently shaped supramolecular hydrogel objects can be obtained by simply changing the shapes of PDMS. This work suggests that reaction-diffusion-mediated localized hydrogelation can serve as an approach towards macroscopically structuralized supramolecular hydrogels, which may find potential applications ranging from tissue engineering to biosensors.

Spatiotemporal control over self-assembly of supramolecular hydrogels through reaction-diffusion.

Supramolecular self-assembly is ubiquitous in living system and is usually controlled to proceed in time and space through sophisticated reaction-diffusion processes, underpinning various vital cellular functions. In this contribution, we demonstrate how spatiotemporal self-assembly of supramolecular hydrogels can be realized through a simple reaction-diffusion-mediated transient transduction of pH signal. In the reaction-diffusion system, a relatively faster diffusion of acid followed by delayed enzymatic production and diffusion of base from the opposite site enables a transient transduction of pH signal in the substrate. By coupling such reaction-diffusion system with pH-sensitive gelators, dynamic supramolecular hydrogels with tunable lifetimes are formed at defined locations. The hydrogel fibers show interesting dynamic growing behaviors under the regulation of transient pH signal, reminiscent of their biological counterpart. We further demonstrate a proof-of-concept application of the developed methodology for dynamic information encoding in a soft substrate. We envision that this work may provide a potent approach to enable transient transduction of various chemical signals for the construction of new colloidal materials with the capability to evolve their structures and functionalities in time and space.

Dynamic supramolecular hydrogels mediated by chemical reactions.

Supramolecular self-assembly in a biological system is usually dominated by sophisticated metabolic processes (chemical reactions) such as catalysis of enzymes and consumption of high energy chemicals, leading to groups of biomolecules with unique dynamics and functions in an aqueous environment. In recent years, increasing efforts have been made to couple chemical reactions to molecular self-assembly, with the aim of creating supramolecular materials with lifelike properties and functions. In this feature article, after summarising the work of chemical reaction mediated supramolecular hydrogels, we first focus on a typical example where dynamic self-assembly of molecular hydrogels is activated by in situ formation of a hydrazone bond in water. We discuss how the formation of the hydrazone-based supramolecular hydrogels can be controlled in time and space. After that, we describe transient assembly of supramolecular hydrogels powered by out-of-equilibrium chemical reaction networks regulated by chemical fuels, which show unique properties such as finite lifetime, dynamic structures, and regenerative capabilities. Finally, we provide a perspective on the future investigations that need to be done urgently, which range from fundamental research to real-life applications of dynamic supramolecular hydrogels.

Dynamic Growth of Macroscopically Structured Supramolecular Hydrogels through Orchestrated Reaction-Diffusion.

Living organisms are capable of dynamically changing their structures for adaptive functions through sophisticated reaction-diffusion processes. Here we show how active supramolecular hydrogels with programmable lifetimes and macroscopic structures can be created by relying on a simple reaction-diffusion strategy. Two hydrogel precursors (poly(acrylic acid) PAA/CaCl2 and Na2 CO3 ) diffuse from different locations and generate amorphous calcium carbonate (ACC) nanoparticles at the diffusional fronts, leading to the formation of hydrogel structures driven by electrostatic interactions between PAA and ACC nanoparticles. Interestingly, the formed hydrogels are capable of autonomously disintegrating over time because of a delayed influx of electrostatic-interaction inhibitors (NaCl). The hydrogel growth process is well explained by a reaction-diffusion model which offers a theoretical means to program the dynamic growth of structured hydrogels. Furthermore, we demonstrate a conceptual access to dynamic information storage in soft materials using the developed reaction-diffusion strategy. This work may serve as a starting point for the development of life-like materials with adaptive structures and functionalities.

Bioinspired Self-Resettable Hydrogel Actuators Powered by a Chemical Fuel.

The movements of soft living tissues, such as muscle, have sparked a strong interest in the design of hydrogel actuators; however, so far, typical manmade examples still lag behind their biological counterparts, which usually function under nonequilibrium conditions through the consumption of high-energy biomolecules and show highly autonomous behaviors. Here, we report on self-resettable hydrogel actuators that are powered by a chemical fuel and can spontaneously return to their original states over time once the fuels are depleted. Self-resettable actuation originates from a chemical fuel-mediated transient change in the hydrophilicity of the hydrogel networks. The actuation extent and duration can be programmed by the fuel levels, and the self-resettable actuation process is highly recyclable through refueling. Furthermore, various proof-of-concept autonomous soft robots are created, resembling the movements of soft-bodied creatures in nature. This work may serve as a starting point for the development of lifelike soft robots with autonomous behaviors.

Transient supramolecular hydrogels formed by catalytic control over molecular self-assembly.

The present work shows how transient supramolecular hydrogels can be formed by catalytically controlled molecular self-assembly. Catalysis formation of molecular gelators leads the self-assembly along a kinetically favored pathway, resulting in transient hydrogels. This work demonstrates an effective approach towards pathway-dependent supramolecular materials.

A facile approach to obtain highly tough and stretchable LAPONITE®-based nanocomposite hydrogels.

LAPONITE® sheets have been widely used for the preparation of tough nanocomposite hydrogels for enticing applications; however, their inferior dispersion in aqueous media resulting from electrostatic interactions between the nanosheets remarkably limits further improvements in the mechanical performances of the nanocomposite hydrogels. Here, we show a simple approach to dramatically accelerate the dispersion of LAPONITE® sheets in water, and in turn further improve the mechanical performances of the resulting nanocomposite hydrogels. Upon addition of poly(acrylic acid) (PAA), the electrostatic interactions between the LAPONITE® sheets were effectively reduced due to the adsorption of PAA onto the positively charged edges of the LAPONITE® sheets, thereby accelerating the dispersion of the LAPONITE® sheets in water. On this basis, a series of polyacrylamide (PAAm) hydrogels with a high content of LAPONITE® sheets was prepared, showing excellent tensile strength, stretchability, and anti-fatigue properties. This study will be beneficial for the preparation of LAPONITE®-based nanocomposite hydrogels bearing excellent mechanical properties for new applications.

Mineralized Supramolecular Hydrogels Bearing Tunable Thermo-Responsiveness.

Although a variety of biomimetic mineralized materials have been created in the lab, the vast majority of these manmade examples lack response to external stimuli. Here, mineralized supramolecular hydrogels with on-demand thermo-responsiveness that are formed by a simple, physical crosslinking between amorphous CaCO3 (ACC) nanoparticles and poly(acrylic acid) (PAA) are reported. Upon the addition of Na2 CO3 solution into a mixture composed of PAA and CaCl2 , amorphous ACC nanoparticles are formed in situ and simultaneously crosslinked by PAA chains, giving rise to the mineralized hydrogels. Interestingly, upon tuning the content of the formed ACC, hydrogels with different types of thermo-responsiveness can be easily obtained, and the transparencies of the resulting hydrogels are dramatically changed during the temperature-driven phase transitions. As an application, these thermo-responsive mineralized hydrogels are used to control the exposure of UV light, which is successfully applied to switch fluorescent signals in response to temperature.