Chemical-Fuel-Driven Assembly in Macromolecular Science: Recent Advances and Challenges

Forging out-of-equilibrium supramolecular gels

The design of supramolecular hydrogels comprising aligned domains is important for the fabrication of biomimetic materials and applications in optoelectronics. One way to access such materials is by the self-assembly of small molecules into long fibres, which can be aligned using an external stimulus. Out-of-equilibrium supramolecular gels can also be designed, where pre-programmed changes of state can be induced by the addition of chemical fuels. Here we exploit these dynamic properties to form materials with aligned domains through a 'forging' approach: an external force is used to rearrange the underlying network from random to aligned fibres as the system undergoes a pre-programmed gel-to-sol-to-gel transition. We show that we can predictably organize the supramolecular fibres, leading to controllable formation of materials with aligned domains through a high degree of temporal control.

Temporally programmed polymer - solvent interactions using a chemical reaction network

Out of equilibrium operation of chemical reaction networks (CRNs) enables artificial materials to autonomously respond to their environment by activation and deactivation of intermolecular interactions. Generally, their activation can be driven by various chemical conversions, yet their deactivation to non-interacting building blocks remains largely limited to hydrolysis and internal pH change. To achieve control over deactivation, we present a new, modular CRN that enables reversible formation of positive charges on a tertiary amine substrate, which are removed using nucleophilic signals that control the deactivation kinetics. The modular nature of the CRN enables incorporation in diverse polymer materials, leading to a temporally programmed transition from collapsed and hydrophobic to solvated, hydrophilic polymer chains by controlling polymer-solvent interactions. Depending on the layout of the CRN, we can create stimuli-responsive or autonomously responding materials. This concept will not only offer new opportunities in molecular cargo delivery but also pave the way for next-generation interactive materials.

Chemical fuels for molecular machinery

Chemical reaction networks that transform out-of-equilibrium 'fuel' to 'waste' are the engines that power the biomolecular machinery of the cell. Inspired by such systems, autonomous artificial molecular machinery is being developed that functions by catalysing the decomposition of chemical fuels, exploiting kinetic asymmetry to harness energy released from the fuel-to-waste reaction to drive non-equilibrium structures and dynamics. Different aspects of chemical fuels profoundly influence their ability to power molecular machines. Here we consider the structure and properties of the fuels that biology has evolved and compare their features with those of the rudimentary synthetic chemical fuels that have so far been used to drive autonomous non-equilibrium molecular-level dynamics. We identify desirable, but context-specific, traits for chemical fuels together with challenges and opportunities for the design and invention of new chemical fuels to power synthetic molecular machinery and other dissipative nanoscale processes.

Dissipative Acid-Fueled Reprogrammable Supramolecular Materials

Smart materials reversibly changing properties in response to a stimuli are promising for a broad array of applications. In this article, we report the use of trichloroacetic acid (TCA) as fuel to create new types of time-controlled materials switching reversibly from a gel to a solution (gel-sol-gel cycle). Applying various neutral amines as organogelators, TCA addition induces amine protonation, switching the system to a solution, while TCA decarboxylation over time enables a return to the initial gel state. Consequently, the newly obtained materials possess interesting time-dependent properties applied in the generation of remoldable objects, as an erasing ink, as chiroptical switches, or for the generation of new types of electrical systems.

Using Rheology to Understand Transient and Dynamic Gels

Supramolecular gels can be designed such that pre-determined changes in state occur. For example, systems that go from a solution (sol) state to a gel state and then back to a sol state can be prepared using chemical processes to control the onset and duration of each change of state. Based on this, more complex systems such as gel-to-sol-to-gel and gel-to-gel-to-gel systems can be designed. Here, we show that we can provide additional insights into such systems by using rheological measurements at varying values of frequency or strain during the evolution of the systems. Since the different states are affected to different degrees by the frequency and/or strain applied, this allows us to better understand and follow the changes in state in such systems.

Chemically Fueled Three-State Chiroptical Switching Supramolecular Gel with Temporal Control

The recent discovery of temporally controlled gels opens broad perspectives to the field of smart functional materials. However, to obtain fully operative systems, the design of simple and robust gels displaying complex functions is desirable. Herein, we fuel dissipative gelating materials through iterative additions of trichloroacetic acid (TCA). This simple fuel enables to switch over time an acid/base-dependent commercially available amino acid gelator/DBU combination between three distinct states (anionic, cationic, and neutral), while liberating volatile CO₂ and CHCl₃ upon fuel consumption. Of interest, the anionic resting state of the system is obtained through trapping of 1 equiv of CO₂ through the formation of a carbamate. The system is tunable, robust, and resilient over time with over 25 consecutive sol-gel-sol cycles possible without significant loss of properties. Most importantly, because of the chiral nature of the amino acid gelator, the system features chiroptical switching properties moving reversibly between three distinct states as observed by ECD. The described system considerably enhances the potential of smart molecular devices for logic gates or data storage by adding a time dimension based on three states to the gelating materials. It is particularly simple in terms of chemical components involved, but it enables sophisticated functions.

Urea-Urease Reaction in Controlling Properties of Supramolecular Hydrogels: Pros and Cons

Supramolecular hydrogels are useful in many areas such as cell culturing, catalysis, sensing, tissue engineering, drug delivery, environmental remediation and optoelectronics. The gels need specific properties for each application. The properties arise from a fibrous network that forms the matrix. A common method to prepare hydrogels is to use a pH change. Most methods result in a sudden pH jump and often lead to gels that are hard to reproduce and control. The urease-urea reaction can be used to control hydrogel properties by a uniform and controlled pH increase as well as to set up pH cycles. The reaction involves hydrolysis of urea by urease and production of ammonia which increases the pH. The rate of ammonia production can be controlled which can be used to prepare gels with differing properties. Herein, we show how the urease-urea reaction can be used for the construction of next generation functional materials.

Engineering bioinks for 3D bioprinting

In recent years, three-dimensional (3D) bioprinting has attracted wide research interest in biomedical engineering and clinical applications. This technology allows for unparalleled architecture control, adaptability and repeatability that can overcome the limits of conventional biofabrication techniques. Along with the emergence of a variety of 3D bioprinting methods, bioinks have also come a long way. From their first developments to support bioprinting requirements, they are now engineered to specific injury sites requirements to mimic native tissue characteristics and to support biofunctionality. Current strategies involve the use of bioinks loaded with cells and biomolecules of interest, without altering their functions, to deliverin situthe elements required to enhance healing/regeneration. The current research and trends in bioink development for 3D bioprinting purposes is overviewed herein.

The Transient Covalent Bond in Abiotic Nonequilibrium Systems

Biochemical systems accomplish many critical functions with by operating out-of-equilibrium using the energy of chemical fuels. The formation of a transient covalent bond is a simple but very effective tool in designing analogous reaction networks. This Minireview focuses on the fuel chemistries that have been used to generate transient bonds in recent demonstrations of abiotic nonequilibrium systems (i.e., systems that do not make use of biological components). Fuel reactions are divided into two fundamental classifications depending on whether the fuel contributes structural elements to the activated state, a distinction that dictates how they can be used. Reported systems are further categorized by overall fuel reaction (e.g., hydrolysis of alkylating agents, carbodiimide hydration) and illustrate how similar chemistry can be used to effect a wide range of nonequilibrium behavior, ranging from self-assembly to the operation of molecular machines.

Robust Dynamics of Synthetic Molecular Systems as a Consequence of Broken Symmetry

The construction of molecular robot-like objects that imitate living things is an important challenge for current chemists. Such molecular devices are expected to perform their duties robustly to carry out mechanical motion, process information, and make independent decisions. Dissipative self-organization plays an essential role in meeting these purposes. To produce a micro-robot that can perform the above tasks autonomously as a single entity, a function generator is required. Although many elegant review articles featuring chemical devices that mimic biological mechanical functions have been published recently, the dissipative structure, which is the minimum requirement for mimicking these functions, has not been sufficiently discussed. This article aims to show clearly that dissipative self-organization is a phenomenon involving autonomy, robustness, mechanical functions, and energy transformation. Moreover, it reports the results of recent experiments with an autonomous light-driven molecular device that achieves all of these features. In addition, a chemical model of cell-amplification is also discussed to focus on the generation of hierarchical movement by dissipative self-organization. By reviewing this research, it may be perceived that mainstream approaches to synthetic chemistry have not always been appropriate. In summary, the author proposes that the integration of catalytic functions is a key issue for the creation of autonomous microarchitecture.