Recent twists in photoactuation and photoalignment control

The design of functional and stimuli-responsive materials is among the key goals of modern materials science

Enhanced photoinduced birefringence in polymer-dye complexes: Hydrogen bonding makes a difference

The authors demonstrate that photoinduced birefringence in azo-dye-doped polymers is strongly enhanced by hydrogen bonding between the guest molecules and the polymer host. The primary mechanism behind the enhancement is the possibility to use high dye doping levels compared to conventional guest-host systems because dye aggregation is restrained by hydrogen bonding. Moreover, hydrogen bonding reduces the mobility of the guest molecules in the polymer host leading to a larger fraction of the induced birefringence to be preserved after the excitation light has been turned off.Peer reviewe

Optically Controlled Latching and Launching in Soft Actuators

Snapping is an abrupt reaction, in which mechanical instability allows the structure to rapidly switch from one stabilized form to another. Snapping is attained through a sudden release of prestored elastic energy. It is perfected by natural species to enhance their preying, locomotion, and reproduction abilities. Recent developments in responsive materials research has allowed the realization of bioinspired snappers and rapidly moving soft robots triggered by external stimuli. However, it remains a grand challenge to reversibly and accurately control the snapping dynamics in terms of, e.g., onset timing and speed of motion. Here, a facile method to obtain light-fueled snapping-like launching with precise control over the elastic energy released and the onset timing is reported. The elastic energy is prestored in a light-responsive liquid crystal elastomer actuator, and the launching event is dictated by releasing the energy through a photothermally induced crystal-to-liquid transition of a liquid-crystalline adhesive latch. The method provides manual control over the amount of prestored energy, motion speed upon multiple launching events, and enables demonstrations such as jumping and catapult motions in soft robots and concerted motions of multiple launchers. The results provide a practical solution for controlled fast motions in soft small-scale robotics.publishedVersionPeer reviewe

Optically controlled grasping-slipping robot moving on tubular surfaces

Stimuli-responsive polymers provide unmatched opportunities for remotely controlled soft robots navigating in complex environments. Many of the responsive-material-based soft robots can walk on open surfaces, with movement directionality dictated by the friction anisotropy at the robot-substrate interface. Translocation in one-dimensional space such as on a tubular surface is much more challenging due to the lack of efficient friction control strategies. Such strategies could in long term provide novel application prospects in, e.g. overhaul at high altitudes and robotic operation within confined environments. In this work, we realize a liquid-crystal-elastomer-based soft robot that can move on a tubular surface through optical control over the grasping force exerted on the surface. Photoactuation allows for remotely switched gripping and friction control which, together with cyclic body deformation, enables light-fueled climbing on tubular surfaces of glass, wood, metal, and plastic with various cross-sections. We demonstrate vertical climbing, moving obstacles along the path, and load-carrying ability (at least 3 × body weight). We believe our design offer new prospects for wirelessly driven soft micro-robotics in confined spacing.publishedVersionPeer reviewe

Complex Fourier Surfaces by Superposition of Multiple Gratings on Azobenzene Thin Films

Diffractive optical elements (DOE) are integral components for lightweight and ultra-thin optical elements due to their ability to manipulate light efficiently and accurately. However, conventional DOEs are static and cannot be altered after fabrication, which hinders their adaptability to changing requirements. To overcome this limitation, the potential of surface patterning on azobenzene thin films to fabricate reconfigurable DOEs is investigated. Using holographic lithography, surface topographies with sinusoidal surface relief gratings (SRG) are created and the superposition of up to 80 SRGs with high accuracy and minimal information loss in subsequent inscriptions is demonstrated. This is enabled by a surface patterning tool combining holographic lithography and digital holographic microscopy. Reconfigurable and adaptive optical elements can improve the efficiency of optical coupling and increase the sensitivity and selectivity of sensors, especially in applications such as near-eye displays and plasmonic sensors. These results demonstrate the ability to create complex azobenzene-based DOEs for advanced photonic applications, where the ability to alter optical elements is of high importance.Peer reviewe

Hydrogen-Bonded Liquid Crystal Elastomers Combining Shape Memory Programming and Reversible Actuation

Materials that undergo shape morphing in response to external stimuli have numerous applications, e.g., in soft robotics and biomedical devices. Shape memory polymers utilize kinetically trapped states to, typically irreversibly, transfer between a programmed morphed shape and an equilibrium shape. Liquid crystal elastomers (LCEs), in turn, can undergo reversible actuation in response to several stimuli. This study combines the irreversible and reversible shape morphing processes to obtain LCEs that undergo shape-programming via the shape memory effect and subsequent reversible actuation of the programmed shape. This is enabled by an LCE crosslinked via dynamic hydrogen bonds that break at high temperatures and reform upon cooling, endowing the shape memory effect, while mild thermal or photothermal stimulation yields the reversible actuation. Through this combination, proof-of-concept robotic application scenarios such as grippers that can adjust their shape for grabbing different-sized objects and crawling robots that can morph their shape to adapt to constrained spaces, are demonstrated. It is anticipated that this work adds new diversity to shape-programmable soft microrobotics.Peer reviewe

The Halogen Bond

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design

Reconfiguring Gaussian Curvature of Hydrogel Sheets with Photoswitchable Host–Guest Interactions

Photoinduced shape morphing has implications in fields ranging from soft robotics to biomedical devices. Despite considerable effort in this area, it remains a challenge to design materials that can be both rapidly deployed and reconfigured into multiple different three-dimensional forms, particularly in aqueous environments. In this work, we present a simple method to program and rewrite spatial variations in swelling and, therefore, Gaussian curvature in thin sheets of hydrogels using photoswitchable supramolecular complexation of azobenzene pendent groups with dissolved α-cyclodextrin. We show that the extent of swelling can be programmed via the proportion of azobenzene isomers, with a 60% decrease in areal swelling from the all trans to the predominantly cis state near room temperature. The use of thin gel sheets provides fast response times in the range of a few tens of seconds, while the shape change is persistent in the absence of light thanks to the slow rate of thermal cis–trans isomerization. Finally, we demonstrate that a single gel sheet can be programmed with a first swelling pattern via spatially defined illumination with ultraviolet light, then erased with white light, and finally redeployed with a different swelling pattern

Surface-Relief Gratings in Halogen-Bonded Polymer–Azobenzene Complexes: A Concentration-Dependence Study

In recent years, supramolecular complexes comprising a poly(4-vinylpyridine) backbone and azobenzene-based halogen bond donors have emerged as a promising class of materials for the inscription of light-induced surface-relief gratings (SRGs). The studies up to date have focused on building supramolecular hierarchies, i.e., optimizing the polymer–azobenzene noncovalent interaction for efficient surface patterning. They have been conducted using systems with relatively low azobenzene content, and little is known about the concentration dependence of SRG formation in halogen-bonded polymer–azobenzene complexes. Herein, we bridge this gap, and study the concentration dependence of SRG formation using two halogen-bond-donating azobenzene derivatives, one functionalized with a tetrafluoroiodophenyl and the other with an iodoethynylphenyl group. Both have been previously identified as efficient molecules in driving the SRG formation. We cover a broad concentration range, starting from 10 mol % azobenzene content and going all the way up to equimolar degree of complexation. The complexes are studied as spin-coated thin films, and analyzed by optical microscopy, atomic force microscopy, and optical diffraction arising during the SRG formation. We obtained diffraction efficiencies as high as 35%, and modulation depths close to 400 nm, which are significantly higher than the values previously reported for halogen-bonded polymer–azobenzene complexes

Halogen Bonding in Halothiophene Building Blocks

Thiophenes bearing monotopic and symmetric ditopic halogen substituents and asymmetric ditopic halogen and pyridine substituents that act as halogen bond acceptors were evaluated for their halogen bonding interactions with 1,4-diiodotetrafluorobenzene and N-iodosuccinimide. The combinations resulted in C/N-I···X′ (X′ = Cl, Br, I) and C/N-I···NPy (Py = pyridine) interactions, the former characterized by an interaction energy (ΔEint) ranging from −4.4 to −18.7 kJ mol-1 and the latter from −26.3 to −56.0 kJ mol-1. X-ray crystallography studies show that the ditopic asymmetric systems consisting of both halogens and pyridine self-associate through C-X···NPy (X = Cl, Br, I) halogen bonding, and in their optimized structures the energies range from −5.4 to −19.6 kJ mol-1 depending on the type of halogen atom present. The 1H NMR association constants of N-I···NPy halogen bonds range between 1405 and 6397 M-1. The σ-hole strengths of halogens have been useful in describing the interaction energies and solution models.Peer reviewe