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.

Transient and Dissipative Host-Guest Hydrogels Regulated by Consumption of a Reactive Chemical Fuel

The transient self-assembly of molecules under the direction of a consumable fuel source is fundamental to biological processes such as cellular organization and motility. Such biomolecular assemblies exist in an out-of-equilibrium state, requiring continuous consumption of high energy molecules. At the same time, the creation of bioinspired supramolecular hydrogels has traditionally focused on associations occurring at the thermodynamic equilibrium state. Here, hydrogels are prepared from cucurbit[7]uril host-guest supramolecular interactions through transient physical crosslinking driven by the consumption of a reactive chemical fuel. Upon action from this fuel, the affinity and dynamics of CB[7]-guest recognition are altered. In this way, the lifetime of transient hydrogel formation and the dynamic modulus obtained are governed by fuel consumption, rather than being directed by equilibrium complex formation.

Self-Regulating Colloidal Co-Assemblies That Accelerate Their Own Destruction via Chemo-Structural Feedback

Biological self‐assemblies self‐ and cross‐regulate each other via chemical reaction networks (CRNs) and feedback. Although artificial transient self‐assemblies have been realized via activation/deactivation CRNs, the transient structures themselves do mostly not engage in the CRN. We introduce a rational design approach for chemo‐structural feedback, and present a transient colloidal co‐assembly system, where the formed co‐assemblies accelerate their destruction autonomously. We achieve this by immobilizing enzymes of a deactivating acid‐producing enzymatic cascade on pH‐switchable microgels that can form co‐assemblies at high pH. Since the enzyme partners are immobilized on individual microgels, the co‐assembled state brings them close enough for enhanced acid generation. The amplified deactivator production (acid) leads to an almost two‐fold reduction in the lifetime of the transiently formed pH‐state. Our study thus introduces versatile mechanisms for chemo‐structural feedback.

Dissipative Organization of DNA Oligomers for Transient Catalytic Function

The development of synthetic non-equilibrium systems opens doors for man-made life-like materials. Yet, creating distinct transient functions from artificial fuel-driven structures remains a challenge. Building on our ATP-driven dynamic covalent DNA assembly in an enzymatic reaction network of concurrent ATP-powered ligation and restriction, we introduce ATP-fueled transient organization of functional subunits for various functions. The programmability of the ligation/restriction site allows to precisely organize multiple sticky-end-encoded oligo segments into double-stranded (ds) DNA complexes. We demonstrate principles of ATP-driven organization into sequence-defined oligomers by sensing barcode-embedded targets with different defects. Furthermore, ATP-fueled DNAzymes for substrate cleavage are achieved by transiently ligating two DNAzyme subunits into a dsDNA complex, rendering ATP-fueled transient catalytic function.

Programmable Transient Supramolecular Chiral G-quadruplex Hydrogels via a Chemically Fueled Non-Equilibrium Self-assembly Strategy

The temporal and spatial control of natural systems has aroused great interest in the creation of synthetic mimics. Operating with boronic ester-based dynamic covalent chemistry and coupling it with an internal pH feedback system, herein, we developed a new chemically fueled reaction network to design non-equilibrium supramolecular chiral G-quadruplex hydrogels with programmable lifetime from minutes, to hours, to days, as well as high transparency and conductivity, excellent injectability and rapid self-healability. The cycle system can be controlled via in-situ kinetically-controlled formation and dissociation of dynamic boronic ester bonds between cis-diols of guanosine (G) and 5-fluorobenzoxaborole (B) under chemical fuels (KOH and 1,3-propanesultone), leading to the formation of a precipitate-solution-gel-precipitate cycle under non-equilibrium conditions. A combined experimental-computational approach revealed that the underlying mechanism of the non-equilibrium self-assembly involves aggregation and disaggregation of right-handed helical G-quadruplex superstructure. With consecutive cycles of fuel addition, the non-equilibrium system can be easily refueled at least 6 cycles without obvious loss in the rheological moduli of the transient hydrogels. The proposed dynamic boronic ester-based non-equilibrium self-assembly strategy offers a new option to design next-generation adaptive and interactive smart materials.

Feedback and Communication in Active Hydrogel Spheres with pH Fronts: Facile Approaches to Grow Soft Hydrogel Structures

Compartmentalized reaction networks regulating signal processing, communication and pattern formation are central to living systems. Towards achieving life-like materials, we compartmentalized urea-urease and more complex urea-urease/ester-esterase pH-feedback reaction networks into hydrogel spheres and investigate how fuel-driven pH fronts can be sent out from these spheres and regulated by internal reaction networks. Membrane characteristics are installed by covering urease spheres with responsive hydrogel shells. We then encapsulate the two networks (urea-urease and ester-esterase) separately into different hydrogel spheres to devise communication, pattern formation and attraction. Moreover, these pH fronts and patterns can be used for self-growing hydrogels, and for developing complex geometries from non-injectable hydrogels without 3D printing tools. This study opens possibilities for compartmentalized feedback reactions and their use in next generation materials fabrication.