Light-fuelled freestyle self-oscillators.

Self-oscillation is a phenomenon where an object sustains periodic motion upon non-periodic stimulus. It occurs commonly in nature, a few examples being heartbeat, sea waves and fluttering of leaves. Stimuli-responsive materials allow creating synthetic self-oscillators fuelled by different forms of energy, e.g. heat, light and chemicals, showing great potential for applications in power generation, autonomous mass transport, and self-propelled micro-robotics. However, most of the self-oscillators are based on bending deformation, thereby limiting their possibilities of being implemented in practical applications. Here, we report light-fuelled self-oscillators based on liquid crystal network actuators that can exhibit three basic oscillation modes: bending, twisting and contraction-expansion. We show that a time delay in material response dictates the self-oscillation dynamics, and realize a freestyle self-oscillator that combines numerous oscillation modes simultaneously by adjusting the excitation beam position. The results provide new insights into understanding of self-oscillation phenomenon and offer new designs for future self-propelling micro-robots.

An Artificial Nocturnal Flower via Humidity-Gated Photoactuation in Liquid Crystal Networks.

Beyond their colorful appearances and versatile geometries, flowers can self-shape-morph by adapting to environmental changes. Such responses are often regulated by a delicate interplay between different stimuli such as temperature, light, and humidity, giving rise to the beauty and complexity of the plant kingdom. Nature inspires scientists to realize artificial systems that mimic their natural counterparts in function, flexibility, and adaptation. Yet, many of the artificial systems demonstrated to date fail to mimic the adaptive functions, due to the lack of multi-responsivity and sophisticated control over deformation directionality. Herein, a new class of liquid-crystal-network (LCN) photoactuators whose response is controlled by delicate interplay between light and humidity is presented. Using a novel deformation mechanism in LCNs, humidity-gated photoactuation, an artificial nocturnal flower is devised that is closed under daylight conditions when the humidity level is low and/or the light level is high, while it opens in the dark when the humidity level is high. The humidity-gated photoactuators can be fueled with lower light intensities than conventional photothermal LCN actuators. This, combined with facile control over the speed, geometry, and directionality of movements, renders the "nocturnal actuator" promising for smart and adaptive bioinspired microrobotics.

Light-Driven, Caterpillar-Inspired Miniature Inching Robot.

Liquid crystal elastomers are among the best candidates for artificial muscles, and the materials of choice when constructing microscale robotic systems. Recently, significant efforts are dedicated to designing stimuli-responsive actuators that can reproduce the shape-change of soft bodies of animals by means of proper external energy source. However, transferring material deformation efficiently into autonomous robotic locomotion remains a challenge. This paper reports on a miniature inching robot fabricated from a monolithic liquid crystal elastomer film, which upon visible-light excitation is capable of mimicking caterpillar locomotion on different substrates like a blazed grating and a paper surface. The motion is driven by spatially uniform visible light with relatively low intensity, rendering the robot "human-friendly," i.e., operational also on human skin. The design paves the way toward light-driven, soft, mobile microdevices capable of operating in various environments, including the close proximity of humans.

Self-Regulating Iris Based on Light-Actuated Liquid Crystal Elastomer.

The iris, found in many animal species, is a biological tissue that can change the aperture (pupil) size to regulate light transmission into the eye in response to varying illumination conditions. The self-regulation of the eye lies behind its autofocusing ability and large dynamic range, rendering it the ultimate "imaging device" and a continuous source of inspiration in science. In optical imaging devices, adjustable apertures play a vital role as they control the light exposure, the depth of field, and optical aberrations of the systems. Tunable irises demonstrated to date require external control through mechanical actuation, and are not capable of autonomous action in response to changing light intensity without control circuitry. A self-regulating artificial iris would offer new opportunities for device automation and stabilization. Here, this paper reports the first iris-like, liquid crystal elastomer device that can perform automatic shape-adjustment by reacting to the incident light power density. Similar to natural iris, the device closes under increasing light intensity, and upon reaching the minimum pupil size, reduces the light transmission by a factor of seven. The light-responsive materials design, together with photoalignment-based control over the molecular orientation, provides a new approach to automatic, self-regulating optical systems based on soft smart materials.

A light-driven artificial flytrap.

The sophistication, complexity and intelligence of biological systems is a continuous source of inspiration for mankind. Mimicking the natural intelligence to devise tiny systems that are capable of self-regulated, autonomous action to, for example, distinguish different targets, remains among the grand challenges in biomimetic micro-robotics. Herein, we demonstrate an autonomous soft device, a light-driven flytrap, that uses optical feedback to trigger photomechanical actuation. The design is based on light-responsive liquid-crystal elastomer, fabricated onto the tip of an optical fibre, which acts as a power source and serves as a contactless probe that senses the environment. Mimicking natural flytraps, this artificial flytrap is capable of autonomous closure and object recognition. It enables self-regulated actuation within the fibre-sized architecture, thus opening up avenues towards soft, autonomous small-scale devices.

Dual Effect of Manganese Oxide Micromotors: Catalytic Degradation and Adsorptive Bubble Separation of Organic Pollutants.

Manganese oxide (MnO2 ) based micromotors exhibiting a dual effect, that is, catalytic degradation and adsorptive bubble separation, were employed for water remediation. The dual effect of MnO2 microparticles led to a greater than 90 % of decolorization of non-biodegradable organic dyes in just 1 h, without the need for external agitation or bubble generation. These findings suggest high potential of MnO2 micromotors for decontamination of organic pollutants from wastewaters or natural water reserves.

Manganese Oxide-Based Chemically Powered Micromotors.

Chemically powered micromotors represent an exciting research area in nanotechnology. Such artificial devices are typically driven by catalytic bubble formation, taking place at the solid-liquid interface. Platinum has been most frequently used for the fabrication of different micromotors due to its superior catalytic efficiency. Other materials typically suffer from slow speeds and require very high concentrations of chemical fuel. Here, we report preparation and characterization of fast moving micromotors based on manganese oxide (MnO2) with different geometrical shapes (tubes, rods, and spheres). On the basis of the results, the prepared micromotors reached the highest speeds (up to ∼900 µm s(-1) in 10% H2O2) reported to date for any MnO2-based micromotors. Moreover, they moved by bubble propulsion even at very low concentrations of peroxide fuel. Thus, MnO2 represents a promising material for the preparation of micromotors for various biomedical or environmental applications, where high speeds are desired.