Multivalent Noncovalent Interfacing and Cross-Linking of Supramolecular Tubes

Natural supramolecular filaments have the ability to cross-link with each other and to interface with the cellular membrane via biomolecular noncovalent interactions. This behavior allows them to form complex networks within as well as outside the cell, i.e., the cytoskeleton and the extracellular matrix, respectively. The potential of artificial supramolecular polymers to interact through specific noncovalent interactions has so far only seen limited exploration due to the dynamic nature of supramolecular interactions. Here, a system of synthetic supramolecular tubes that cross-link forming supramolecular networks, and at the same time bind to biomimetic surfaces by the aid of noncovalent streptavidin–biotin linkages, is demonstrated. The architecture of the networks can be engineered by controlling the density of the biotin moiety at the exterior of the tubes as well as by the concentration of the streptavidin. The presented strategy provides a pathway for designing adjustable artificial supramolecular superstructures, which can potentially yield more complex biomimetic adaptive materials

Light-responsive self-assembly towards supramolecular machines

Incorporating artificial molecular motors and switches into self-assembled systems allows transferring and amplifying the forces that these molecules generate, across increasing length scales. Interfacing the mechanics of such molecules with larger self-assembled architectures represents a viable strategy to fight against overwhelming Brownian storm and viscosity of liquid environment. Thus the focus of my research was on developing controllable light-responsive supramolecular systems by integrating molecular photoswitches into self-assembled architectures. Photo-switching was selected as an ideal strategy to alternate between assembling and non-assembling isomers reversibly, repeatedly, with an exquisite control over the timescale of the process, without accumulating chemical waste and in combination with spontaneous reversed switching. The artificial molecular photoswitches involved in this work were azobenzenes and spiropyrans. Upon irradiation with light, azobenzenes offer geometrical changes between cis and trans isomers, whereas spiropyrans provide large differences in their charged character. Chapter 1 provides a general introduction to this thesis. Chapter 2 reviews recent progress light-responsive supramolecular tubular systems. Our motivation to work with tubular systems lies in the versatility displayed by the cellular microtubes that operate in the cell. In Chapter 3, we show how strain builds up in wholly artificial tubules, upon trans-to-cis photo-switching. The light-fueled accumulation of the strain eventually leads to the catastrophic burst of the tubules, in a mechanism that is reminiscent of the disassembly mechanism of cellular microtubules. In Chapter 4, we demonstrate a strategy that allows connecting the tubules discussed in chapter 3, with either surfaces, or with each other. In Chapter 5, we have synthesized a spiropyran-based amphiphile that self-assembles into vesicles, in water. Upon irradiation with light, merocyanine converts into the spiropyran, which leads to the transient and reversible expansion of the vesicles. In Chapter 6, we present efforts towards combining tubes with spiropyran switches, because they would promote tubular growth upon irradiation with light, when the polar protonated merocyanine converts into the spiropyran. Overall, we have introduced a framework for the development of light-controllable nanosystems in water, with a dynamic behavior ranging from assembly, to disassembly and transient growth

Light-fuelled reversible expansion of spiropyran-based vesicles in water

We present the design and synthesis of spiropyran-based dynamic vesicles, for which the building block is the amphiphilic merocyanine isomer. Under irradiation with visible light, the photo-conversion of the protonated and charged merocyanine to the neutral spiropyran form leads to the transient and reversible expansion of these vesicles

Mechanical Adaptability of Artificial Muscles from Nanoscale Molecular Action

The cooperative operation of artificial molecular motors and switches has been amplified in polymer-based approaches that have led to versatile motion at the macroscale. As these active, shape-shifting polymers have become ever more sophisticated in their morphing capabilities, a major remaining challenge is to encode muscle-like mechanical adaptability during their operation and to explore its molecular origin. Here, we describe the mechanical adaptability of materials in which the light-induced action of molecular switches modifies the intrinsic interfacial tension, in a phase heterogeneous design featuring a liquid crystal polymer network swollen by a liquid crystal. When the swelling creates sufficient interfacial tension, light triggers an unprecedented and reversible photo-stiffening, analogous to myosin-powered muscle fibers. These mechanoadaptive materials adjust their stiffness to the task they must perform, also while they move, and display muscle-like behaviour that might contribute significantly to the development of human-friendly and soft robotics

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