Mechanical behaviour of contractile gels based on light-driven molecular motors

Scalable Approach to Molecular Motor-Polymer Conjugates for Light-Driven Artificial Muscles

The integration of molecular machines and motors into materials represents a promising avenue for creating dynamic and functional molecular systems, with potential applications in soft robotics or reconfigurable biomaterials. However, the development of truly scalable and controllable approaches for incorporating molecular motors into polymeric matrices has remained a challenge. Here, it is shown that light-driven molecular motors with sensitive photo-isomerizable double bonds can be converted into initiators for Cu-mediated controlled/living radical polymerization enabling the synthesis of star-shaped motor-polymer conjugates. This approach enables scalability, precise control over the molecular structure, block copolymer structures, and high-end group fidelity. Moreover, it is demonstrated that these materials can be crosslinked to form gels with quasi-ideal network topology, exhibiting light-triggered contraction. The influence of arm length and polymer structure is investigated, and the first molecular dynamics simulation framework to gain deeper insights into the contraction processes is developed. Leveraging this scalable methodology, the creation of bilayer soft robotic devices and cargo-lifting artificial muscles is showcased, highlighting the versatility and potential applications of this advanced polymer chemistry approach. It is anticipated that the integrated experimental and simulation framework will accelerate scalable approaches for active polymer materials based on molecular machines, opening up new horizons in materials science and bioscience.

Accounts of applied molecular rotors and rotary motors: recent advances

Molecular machines are nanoscale devices capable of performing mechanical works at molecular level. These systems could be a single molecule or a collection of component molecules that interrelate with one another to produce nanomechanical movements and resulting performances. The design of the components of molecular machine with bioinspired traits results in various nanomechanical motions. Some known molecular machines are rotors, motors, nanocars, gears, elevators, and so on based on their nanomechanical motion. The conversion of these individual nanomechanical motions to collective motions via integration into suitable platforms yields impressive macroscopic output at varied sizes. Instead of limited experimental acquaintances, the researchers demonstrated several applications of molecular machines in chemical transformation, energy conversion, gas/liquid separation, biomedical use, and soft material fabrication. As a result, the development of new molecular machines and their applications has accelerated over the previous two decades. This review highlights the design principles and application scopes of several rotors and rotary motor systems because these machines are used in real applications. This review also offers a systematic and thorough overview of current advancements in rotary motors, providing in-depth knowledge and predicting future problems and goals in this area.

Supramolecular Gel-to-Gel Transition Induced by Nanoscale Structural Perturbation via the Rotary Motion of Feringa's Motor

Supramolecular rather than covalent molecular engineering on Feringa motors can provide an alternative toolkit for tuning the properties of motorized materials through appropriate supramolecular structural perturbations, which are underexplored. Herein, a multicomponent supramolecular gel system is successfully prepared by employing an ultra-low molecular weight gelator and a modulator-Feringa motor. The electron microscopic, spectroscopic, and rheological data revealed that the morphology and mechanical properties of the gel can be tuned via a crystallographic mismatch branching (CMB) mechanism simply by adding varied amounts of motor modulators. Notably, the rotary motion of the motor is preserved in such a multicomponent gel system, and the morphology and rheology of the gel can be further altered by the motor's rotary motion that promotes the structural perturbation, resulting in seldomly seen gel-to-gel transition events. The work shown here offers prospects to utilize a supramolecular perturbation strategy to deliver responsiveness from molecular motors to the corresponding bulk materials.

Bending Actuation of Hydrogels through Rotation of Light-Driven Molecular Motors

The unidirectional rotation of chemically crosslinked light-driven molecular motors is shown to progressively shift the swelling equilibrium of hydrogels. The concentration of molecular motors and the initial strand density of the polymer network are key parameters to modulate the macroscopic contraction of the material, and both parameters can be tuned using polymer chains of different molecular weights. These findings led to the design of optimized hydrogels revealing a half-time contraction of approximately 5 min. Furthermore, under inhomogeneous stimulation, the local contraction event was exploited to design useful bending actuators with an energy output 400 times higher than for previously reported self-assembled systems involving rotary motors. In the present configuration, we measure that a single molecular motor can lift up loads of 200 times its own molecular weight.

The Influence of Strain on the Rotation of an Artificial Molecular Motor

In artificial small‐molecule machines, molecular motors can be used to perform work on coupled systems by applying a mechanical load—such as strain—that allows for energy transduction. Here, we report how ring strain influences the rotation of a rotary molecular motor. Bridging the two halves of the motor with alkyl tethers of varying sizes yields macrocycles that constrain the motor's movement. Increasing the ring size by two methylene increments increases the mobility of the motor stepwise and allows for fine‐tuning of strain in the system. Small macrocycles (8–14 methylene units) only undergo a photochemical E/Z isomerization. Larger macrocycles (16–22 methylene units) can perform a full rotational cycle, but thermal helix inversion is strongly dependent on the ring size. This study provides systematic and quantitative insight into the behavior of molecular motors under a mechanical load, paving the way for the development of complex coupled nanomachinery.

Synthesis and application of smart gel material modified silica microspheres for pH-responsive hydrophilicity in liquid chromatography

Inspired by porous smart gel materials, we designed pH-responsive polymer-modified silica microspheres as liquid chromatography stationary phase materials by a one-step strategy. The free radicals generated by the oxidation of dopamine are used to initiate the cross-linking polymerization of functional monomers. At the same time, the good adhesion of dopamine enables the polymer to be modified on silica. The hydrophilicity of this new stationary phase can change in response to the pH of the mobile phase and the stationary phase has weaker hydrophilicity under acidic (pH = 3.78) mobile phase conditions and stronger hydrophilicity under neutral mobile phase conditions. The hydrophilicity difference of the stationary phase leads to the selectivity difference in separation. To evaluate the chromatographic performance of this new stationary phase, 10 oligosaccharides and 9 nucleosides/bases were separated on this stationary phase. This paper will provide good guidance for us to achieve more pH-responsive hydrophilic/hydrophobic stationary phases in the future.

Optoregulated force application to cellular receptors using molecular motors

Progress in our understanding of mechanotransduction events requires noninvasive methods for the manipulation of forces at molecular scale in physiological environments. Inspired by cellular mechanisms for force application (i.e. motor proteins pulling on cytoskeletal fibers), we present a unique molecular machine that can apply forces at cell-matrix and cell-cell junctions using light as an energy source. The key actuator is a light-driven rotatory molecular motor linked to polymer chains, which is intercalated between a membrane receptor and an engineered biointerface. The light-driven actuation of the molecular motor is converted in mechanical twisting of the entangled polymer chains, which will in turn effectively “pull” on engaged cell membrane receptors (e.g., integrins, T cell receptors) within the illuminated area. Applied forces have physiologically-relevant magnitude and occur at time scales within the relevant ranges for mechanotransduction at cell-friendly exposure conditions, as demonstrated in force-dependent focal adhesion maturation and T cell activation experiments. Our results reveal the potential of nanomotors for the manipulation of living cells at the molecular scale and demonstrate a functionality which at the moment cannot be achieved by other technologies for force application.

Molecular scale force application in physiological environments is important for studying mechanotransduction. Here, the authors use a molecular machine to apply forces at cell-matrix and cell-cell junctions using light to trigger twisting actuation which then pulls on cell membrane receptors.

Molecular Pumps and Motors

Pumps and motors are essential components of the world as we know it. From the complex proteins that sustain our cells, to the mechanical marvels that power industries, much we take for granted is only possible because of pumps and motors. Although molecular pumps and motors have supported life for eons, it is only recently that chemists have made progress toward designing and building artificial forms of the microscopic machinery present in nature. The advent of artificial molecular machines has granted scientists an unprecedented level of control over the relative motion of components of molecules through the development of kinetically controlled, away-from-thermodynamic equilibrium chemistry. We outline the history of pumps and motors, focusing specifically on the innovations that enable the design and synthesis of the artificial molecular machines central to this Perspective. A key insight connecting biomolecular and artificial molecular machines is that the physical motions by which these machines carry out their function are unambiguously in mechanical equilibrium at every instant. The operation of molecular motors and pumps can be described by trajectory thermodynamics, a theory based on the work of Onsager, which is grounded on the firm foundation of the principle of microscopic reversibility. Free energy derived from thermodynamically non-equilibrium reactions kinetically favors some reaction pathways over others. By designing molecules with kinetic asymmetry, one can engineer potential landscapes to harness external energy to drive the formation and maintenance of geometries of component parts of molecules away-from-equilibrium, that would be impossible to achieve by standard synthetic approaches.

Helix Inversion Controlled by Molecular Motors in Multistate Liquid Crystals

Unravelling the rules of molecular motion is a contemporary challenge that promises to support the development of responsive materials and is likely to enhance the understanding of functional motion. Advances in integrating light-driven molecular motors in soft matter have led to the design and realization of chiral nematic (cholesteric) liquid crystals that can respond to light with modification of their helical pitch, and also with helix inversion. Under illumination, these chiral liquid crystals convert from one helical geometry to another. Here, a series of light-driven molecular motors that feature a rich configurational landscape is presented, specifically which involves three stable chiral states. The succession of chiral structures involved in the motor cycle is transmitted at higher structural levels, as the cholesteric liquid crystals that are formed can interconvert between helices of opposite handedness, reversibly. In these materials, the dynamic features of the motors are thus expressed at the near-macroscopic, functional level, into addressable colors that can be used in advanced materials for tunable optics and photonics.

Photoactivated Artificial Molecular Machines that Can Perform Tasks

Research on artificial photoactivated molecular machines has moved in recent years from a basic scientific endeavor toward a more applicative effort. Nowadays, the prospect of reproducing the operation of natural nanomachines with artificial counterparts is no longer a dream but a concrete possibility. The progress toward the construction of molecular-machine-based devices and materials in which light irradiation results in the execution of a task as a result of nanoscale movements is illustrated here. After a brief description of a few basic types of photoactivated molecular machines, significant examples of their exploitation to perform predetermined functions are presented. These include switchable catalysts, nanoactuators that interact with cellular membranes, transporters of small molecular cargos, and active joints capable of mechanically coupling molecular-scale movements. Investigations aimed at harnessing the collective operation of a multitude of molecular machines organized in arrays to perform tasks at the microscale and macroscale in hard and soft materials are also reviewed. Surfaces, gels, liquid crystals, polymers, and self-assembled nanostructures are described wherein the nanoscale movement of embedded molecular machines is amplified, allowing the realization of muscle-like actuators, microfluidic devices, and polymeric materials for light energy transduction and storage.

Structural properties of contractile gels based on light-driven molecular motors: a small-angle neutron and X-ray study

The detailed structure of active polymer gels built by integrating light-driven rotary molecular motors as reticulation units in polymer networks is discussed as a function of gel composition. Upon light-irradiation, the collective rotation of molecular motors is translated into the macroscopic contraction of the gels through polymer chains twisting. The major role of the characteristic ratio c/c* (c* being the overlap concentration of the polymer-motor conjugates before crosslinking) on the contraction efficiency is exploited. Combined small-angle neutron and X-ray scattering experiments reveal the importance of heterogeneities in the macroscopic contraction process: the mesh size of the network increases under irradiation in the whole range of c/c*, an increase that is maximal for c/c* = 1; i.e. at higher contraction efficiency. Furthermore, the mesh size of the network reaches equilibrium within a short period of time, while the heterogeneities increase in size untill the end of the contraction process. Finally, the significant motorized twisting of polymer chains within the network allows to foresee the design of new storage energy systems.

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

Precise control over molecular movement is of fundamental and practical importance in physics, biology, and chemistry. At nanoscale, the peculiar functioning principles and the synthesis of individual molecular actuators and machines has been the subject of intense investigations and debates over the past 60 years. In this review, we focus on the design of collective motions that are achieved by integrating, in space and time, several or many of these individual mechanical units together. In particular, we provide an in-depth look at the intermolecular couplings used to physically connect a number of artificial mechanically active molecular units such as photochromic molecular switches, nanomachines based on mechanical bonds, molecular rotors, and light-powered rotary motors. We highlight the various functioning principles that can lead to their collective motion at various length scales. We also emphasize how their synchronized, or desynchronized, mechanical behavior can lead to emerging functional properties and to their implementation into new active devices and materials.

Covalently Trapped Triarylamine-Based Supramolecular Polymers

C₃ -Symmetric triarylamine trisamides (TATAs), decorated with three norbornene end groups, undergo supramolecular polymerization and further gelation by π-π stacking and hydrogen bonding of their TATA cores. By using subsequent ring-opening metathesis polymerization, these physical gels are permanently crosslinked into chemical gels. Detailed comparisons of the supramolecular stacks in solution, in the physical gel, and in the chemical gel states, are performed by optical spectroscopies, electronic spectroscopies, atomic force microscopy, electronic paramagnetic resonance spectroscopy, X-ray scattering, electronic transport measurements, and rheology. The results presented here clearly evidence that the core structure of the functional supramolecular polymers can be precisely retained during the covalent capture whereas the mechanical properties of the gels are concomitantly improved, with an increase of their storage modulus by two orders of magnitude.

Mechanical adaptability of artificial muscles from nanoscale molecular action

The motion of artificial molecular machines has been amplified into the shape transformation of polymer materials that have been compared to muscles, where mechanically active molecules work together to produce a contraction. In spite of this progress, harnessing cooperative molecular motion remains a challenge in this field. Here, we show how the light-induced action of artificial molecular switches modifies not only the shape but also, simultaneously, the stiffness of soft materials. The heterogeneous design of these materials features inclusions of free liquid crystal in a liquid crystal polymer network. When the magnitude of the intrinsic interfacial tension is modified by the action of the switches, photo-stiffening is observed, in analogy with the mechanical response of activated muscle fibers, and in contrast to melting mechanisms reported so far. Mechanoadaptive materials that are capable of active tuning of rigidity will likely contribute to a bottom-up approach towards human-friendly and soft robotics.

Photo- and Redox-Driven Artificial Molecular Motors

Directed motion at the nanoscale is a central attribute of life, and chemically driven motor proteins are nature's choice to accomplish it. Motivated and inspired by such bionanodevices, in the past few decades chemists have developed artificial prototypes of molecular motors, namely, multicomponent synthetic species that exhibit directionally controlled, stimuli-induced movements of their parts. In this context, photonic and redox stimuli represent highly appealing modes of activation, particularly from a technological viewpoint. Here we describe the evolution of the field of photo- and redox-driven artificial molecular motors, and we provide a comprehensive review of the work published in the past 5 years. After an analysis of the general principles that govern controlled and directed movement at the molecular scale, we describe the fundamental photochemical and redox processes that can enable its realization. The main classes of light- and redox-driven molecular motors are illustrated, with a particular focus on recent designs, and a thorough description of the functions performed by these kinds of devices according to literature reports is presented. Limitations, challenges, and future perspectives of the field are critically discussed.