Recently, the team of Professor Chen Jiawen of South China Normal University and Professor Ben L. Feringa of the University of Groningen in the Netherlands collaborated to design a bionic liquid crystal elastomer (LCEs) based on 3D printing technology. In this work, the long-range directional order and layered structure of liquid crystals and the unidirectional rotation of light-driven molecular motors are combined, and the molecular motors are established on the backbone of liquid crystal oligomers, and then these oligomers are used as inks to print different forms of liquid crystal elastomers by 3D printing technology. The resulting LCEs are capable of performing multiple types of locomotion under UV light, including bending, spiral winding, petal opening and closing, and butterfly flapping, paving the way for future designs of responsive materials with stronger complex actuation capabilities.
Figure 1: 3D printing technology to construct a liquid crystal elastomer containing a molecular motor
Background
Exercise is very important in nature because it maintains various basic functions of all living systems. These movements are driven by biomolecular motors, which convert chemical energy when exposed to external stimuli and amplify along multiple length scales, ultimately performing specific activities and functions. At present, the movement of artificial molecular machines is well controlled at the molecular level, but the movement of translating molecular motion along multiple scales and amplifying it to induce macroscopic dimensions is still limited. Liquid crystal elastomers (LCEs) are an important method for constructing adaptive soft materials. LCE has a large strain at break and a lower modulus of elasticity at room temperature, which provides more room for order-disorder transitions, resulting in larger reversible deformations when subjected to certain stimuli. What's more, LCEs can be quickly manufactured through additive manufacturing methods such as 3D printing to create complex objects with larger dimensions. In the preparation of LCE by 3D printing technology, a viscous ink composed of non-cross-linked liquid crystal oligomers is extruded through a printing nozzle, and the liquid crystal oligomers are spontaneously arranged along the printing path under the shear stress generated during the extrusion process. By design, the local arrangement of liquid crystals can be well organized, resulting in a system with multifunctional characteristics. To date, thermoactive LCE systems fabricated by 3D printing technology have shown large reversible shape changes, while light-responsive LCE systems remain largely unexplored.
Highlights of this article:
Based on the above research background, the team of Professor Chen Jiawen of South China Normal University and Professor Ben L. Feringa of the University of Groningen in the Netherlands embedded a light-responsive molecular motor 1 into the backbone of LC oligomers through a thiol-Michael addition reaction. The liquid crystal oligomers obtained by screening ratios show typical shear-thinning and temperature-responsive rheological properties and can be used as inks for 3D printing. After optimizing the printing parameters, photoactive LCE objects with a wide range of morphologies and sizes were prepared with pre-defined printing paths, including racemic or chiral motors. Biomimetic functions are achieved using these systems, including spiral curling, petal closure, and butterfly wing flipping.
Fig. 2 (A) Chemical structure of the motor used to synthesize liquid crystal oligomers, the liquid crystal monomer RM 82, and the chain extender EDDET. (B-D) Rheological properties of the resulting LC oligomers. (E) POM image of a 3D-printed LCE strip.
At room temperature, the molecular motor 1 was able to achieve a fast full 360-degree unidirectional rotational motion by irradiating it with ultraviolet light, which was in line with the purpose of preparing a fast-response system, so the authors mixed it with the liquid crystal monomer RM 82 and the chain extender EDDET to prepare the backbone LC oligomer as the ink (Figure 2A). To further investigate the effect of chain extenders on the properties of the prepared oligomers, acrylates and chain extenders (1:0.60, 1:0.75, 1:0.80, 1:0.85, 1:0.90) were screened at different molar ratios and their effects were evaluated using 1H-NMR and DSC studies. The results show that with the increase of mercaptan content, the phase change temperature of the oligomers decreases, and the viscosity of the system increases. To ensure the alignment of liquid crystals and the quality of the printed pattern, the operating temperature of 3D printing is usually 10 °C lower than the phase change temperature of the ink. Therefore, the authors ended up using a material system with a molar ratio of 1:0.90 (acrylate:mercaptan) in this study.
Fig.3 Photo-driven behavior of a racemic and enantiomer pure motor 1 in a 3D printed LCE film.
In order to study the photodynamics of LCE films, the authors first designed a parallel-oriented LCE film (5 mm×30 mm), which was subjected to 365 nm irradiation after polymerization, and the film showed a bending motion towards the light source. The saturation bending motion is completed in 2 seconds, and the film returns to its initial position immediately after the light is turned off. In addition, by subsequently switching the light source on and off, multiple cycles can be carried out without significant fatigue of the system. To further confirm that the observed actuation of the LCE film containing motor 1 is mainly due to the rotation and shape change of the motor, the authors placed the LCE film in water to rule out the influence of heat on it. And the actuation velocity of the sample observed after UV irradiation is similar to that in air, which clearly indicates that the deformation of the film is driven by the rotational motion of the embedded motor rather than the photothermal effect. In addition, the authors added an enantio-pure motor (containing 1 mol% (R)-1 or (S)-1) to the system, with (R)-1 band showing left-handed helical motion and (S)-1 band showing right-handed helix motion (Fig. 3D, 3F).
The above experimental data demonstrate the uniqueness of light-driven molecular motors, where all key functions, including photoactuators and chiral dopants, are embedded in the single-molecule structure. The light-triggered, unidirectional rotating motor is amplified along multiple length scales at the nanoscale, ultimately resulting in a right- or left-hand spiral curl of the 3D printed LC band.
Fig.4 Schematic diagram of the helical deformation of the double-layer LCE strip at 365 nm and the surface shape change of the double-layer LCE strip due to the uneven spatial curvature.
To achieve more complex movements, a double-layer strip at 45°/135° to the long axis was designed. Under irradiation at 365 nm, the bilayer bands show a helical motion (as shown in Figure 4A), which is attributed to the design of the bilayer. Because LCE strips containing uniaxial orientation motors are capable of shrinking and bending along a predetermined direction (orientation direction), when two identical layers are joined together at an angle, each layer actuates in its preferred direction, causing the LC strips to deform in different directions. As a result, the bending curvature is spatially unevenly distributed, resulting in incompatible strains and eventually out-of-plane deformation of the fringes. This movement lays the groundwork for the further construction of biomimetic materials.
Fig.5 Light-responsive biomimetic behavior of a 3D printed LCE object.
Then, in order to mimic the locomotor behavior of living organisms, two photosensitive flowers were designed and printed. After the preset flowers have been printed, cured, and dried, the petals are completely closed within 60 seconds under UV irradiation, and return to their original open state when the light source is turned off (Figure 5B), which simulates the opening and closing process of a natural flower. When the printing orientation of the petals was changed to 45°/135° to the long axis of the bilayer structure (Figure 5C), the petals spirally closed under UV light (Figure 5D).
Finally, a true light-responsive 3D object was constructed in size of 55 mm× 55 mm, × 30 mm. As shown in Figure 5E, the authors designed a hybrid system for biomimetic butterflies, with polycaprolactone (PCL) in the body because of its high modulus and ability to support the three-dimensional model, and a photoactive LCE containing a molecular motor for flapping wings. According to the programming, the butterfly's wings can be reversibly flipped under the illumination of ultraviolet light (Figure 5F), i.e., the flapping motion associated with the butterfly's flight motion is successfully simulated.
The work was published as a Research Article in the Journal of the American Chemical Society. The first author is Long Guiying, a Ph.D. student at South China Normal University, and the corresponding authors are Professor Chen Jiawen from South China Normal University and Professor Ben L. Feringa from the University of Groningen. This research work has been strongly supported by the National Key R&D Program of China, the Guangzhou Science and Technology Project, and the Guangdong Provincial Key Laboratory of Optical Information Materials and Technology.
Article Details:
Guiying Long1,2, Yanping Deng1, Wei Zhao3, Guofu Zhou1,3, Dirk J. Broer3,4, Ben L. Feringa*,1,2, Jiawen Chen*,1，Photo-responsive bio-mimetic functions by light-driven molecular motors in 3D printed liquid crystal elastomers.
Link to article
https://doi.org/10.1021/jacs.4c01642
Source: Frontiers of Polymer Science