Magneto-Photochemically Responsive Liquid Crystal Elastomer for Underwater Actuation

Abstract

The quest for small-scale, remotely controlled soft robots has led to the exploration of magnetic and optical fields for inducing shape morphing in soft materials. Magnetic stimulus excels when navigation in confined or optically opaque environments is required. Optical stimulus, in turn, boasts superior spatial precision and individual control over multiple objects. Herein, we bring these two methodologies together and present a monolithic liquid crystal elastomer (LCE) system that synergistically combines magnetic and photochemical actuation schemes. The resultant composite material showcases versatile possibilities for underwater actuation, and we demonstrate robotic functionalities where the optical and magnetic response can be leveraged in different tasks (object gripping and object translocation, respectively) or where light can be used as a control signal to tune the magnetically induced actuation. Combining these two remote actuation methods offers powerful, dual-mode control in wireless, small-scale robotics, especially in submersed environments due to their isothermal nature.

Keywords: azobenzene; liquid crystal elastomer; magnetoresponsive; shape morphing; soft actuator.

Figure 1

Material characterization. (a) Chemical structures of molecules utilized in the fabrication of magneto-photoresponsive LCE. (b) Top: Polarized optical microscopic images of the LCE–MMP composite (ρ = 1:4) at 0 and 45° angles between the molecular director and the polarizer. Bottom: Photographs of prepared films with varying MMP:LCE ratios. Scale bars: 200 μm (top), 5 mm (bottom). (c) Uniaxial contraction during heating from room temperature to 200 °C. Inset: A schematic representation of the deformation of a planar-aligned LCE, where L₀ represents the original length, L is the contracted length, and n is the director orientation. (d) Stress–strain curves of the planar LCE–MMP composites with varying MMP/LCE ratios, parallel (top) and perpendicular (bottom) to the director orientation.

Figure 2

Photoactuation of the LCE–MMP composite. (a) Photographs illustrating the photochemical deformation process of the LCE-MMP composite (ρ = 1:4) under UV (365 nm, 120 mW cm^–2) and blue (460 nm, 180 mW cm^–2) light illumination. Scale bar: 3 mm. (b) Bending angle of the planar LCE strip upon irradiation with UV and blue light. (c) Bending angle of the LCE strip upon exposure to various UV light intensities and subsequent blue light irradiation. Filled symbols along the lines indicate the periods of light irradiation: purple-filled symbols correspond to the periods when UV light was turned on and blue-filled symbols correspond to the periods when blue light was turned on. Empty symbols denote periods when the light was turned off. (d) Cyclic photoinduced bending and unbending of LCE-MMP composite film upon alternating irradiation with 365 nm (95 mW cm^–2) and 460 nm (180 mW cm^–2). (e) Temperature recorded under different UV light intensities (365 nm) for the pristine LCE and the LCE-MMP composite (ρ = 1:4). Error bars indicate a standard deviation of n = 3 measurements.

Figure 3

Magnetoresponse of the LCE–MMP composite. (a) Magnetic hysteresis curves of MMP and LCE-MMP composite. (b) Top: Schematic representation of the magnetization process. Bottom: Optical images of the LCE-MMP composite (ρ = 1:4) under applying the magnetic field. Scale bar: 5 mm. (c) Bending angle as a function of the magnetic field. Inset: indication of the strip bending angle for actuation measurements.

Figure 4

Magneto-photoinduced object translocation. (a) Schematic illustration of the sequential steps involved in the translocation process using a star-shaped LCE actuator. (b) Sequence of photographs showing the real-time actuation of the soft robot: at rest, bending upon UV exposure to hold an object, movement under the influence of a magnetic field, and release of the object upon blue light illumination. Scale bar: 5 mm.

Figure 5

Photocontrolled oscillations of magneto-photoresponsive LCE cilia under oscillating magnetic field. (a) Schematic representation and sequential photographs depicting different actuation stages of the LCE cilia under sinusoidal magnetic field upon UV light exposure. The stages showcase the transition from initial oscillation without light (Stage 0) to photochemical control over the oscillation amplitudes (Stages 1 to 3). (b) Tip displacement in the X direction of the left, middle, and right cilia over time as they respond to the applied magnetic field and light stimuli, illustrating distinct oscillatory behaviors across the different stages of no light illumination (Stage 0), UV illumination (Stages 1 to 3), blue-light illumination (Stage 4), and retainment of the initial (Stage 5). (c) Amplitude quantification of the cilia oscillations during each stage, demonstrating orthogonal photocontrol over magneto-induced cilia oscillations. Scale bar: 1 cm.