Liquid Crystal Elastomer-Based Magnetic Composite Films for Reconfigurable Shape-Morphing Soft Miniature Machines

Abstract

Stimuli-responsive and active materials promise radical advances for many applications. In particular, soft magnetic materials offer precise, fast, and wireless actuation together with versatile functionality, while liquid crystal elastomers (LCEs) are capable of large reversible and programmable shape-morphing with high work densities in response to various environmental stimuli, e.g., temperature, light, and chemical solutions. Integrating the orthogonal stimuli-responsiveness of these two kinds of active materials could potentially enable new functionalities and future applications. Here, magnetic microparticles (MMPs) are embedded into an LCE film to take the respective advantages of both materials without compromising their independent stimuli-responsiveness. This composite material enables reconfigurable magnetic soft miniature machines that can self-adapt to a changing environment. In particular, a miniature soft robot that can autonomously alter its locomotion mode when it moves from air to hot liquid, a vine-like filament that can sense and twine around a support, and a light-switchable magnetic spring are demonstrated. The integration of LCEs and MMPs into monolithic structures introduces a new dimension in the design of soft machines and thus greatly enhances their use in applications in complex environments, especially for miniature soft robots, which are self-adaptable to environmental changes while being remotely controllable.

Keywords: liquid crystal elastomers; magnetic composites; programmable shape-morphing; soft machines; stimuli-responsive materials.

Figure 1

Monolithically integrated magnetic soft liquid crystal elastomer (LCE) composite film for programmable and multiple degrees of freedom (multi‐DOF) shape‐morphing. a) Schematic illustrations of the proposed composite film for an example circular alignment of the director field of the LCE base matrix. The size and volume ratio of hard NdFeB magnetic microparticles (MMPs) are not to scale. MMPs are conceptually represented by black spheres. b) Schematic and experimental observations of the characteristics of the composite films of different MMP concentrations (0:1, 1:4, 1:2, 1:1, and 2:1) with polarized‐light and bright‐field optical microscopy. The shape‐morphing behaviors of different samples triggered by temperature increase were observed and compared. Cross sections of the samples were observed by scanning electron microscopy (SEM). Pseudocolor was added manually based on sharp edges of MMPs. Scale bars are 0.25 mm for the optical microscopy, 20 µm for the SEM images, and 5 mm for the rest.

Figure 2

Behavioral and mechanical characterization of the proposed composite material in different MMP concentrations. a) The transition temperature T _NI between the nematic and isotropic state of the proposed composite material at different MMP concentrations. The value of T _NI was determined by microscopic observation of liquid monomer textures in both heating and cooling processes with a rate of 5 °C min⁻¹. Each column and error bar represent the average and the standard deviation of four measurements, respectively. b) Shape‐morphing of the samples (1 mm × 9 mm × 25 µm size film with a 1:2 mass ratio between MMPs and LCE and splay alignment of mesogens across thickness) at different temperatures. Each data point and error bar represent the average and the standard deviation of three measurements, respectively. c) Measured magnetic hysteresis curves (four‐quadrant BH curves) of the composite materials at different MMP concentrations. d) Measured initial magnetic curves (BH curves) of the composite materials at different MMP concentrations. e–g) Measured stress–strain curves of the composite materials at different MMP concentrations with different director fields: mesogens were aligned: e) along the tensile direction, f) perpendicular to the tensile direction, and g) perpendicular to the tensile direction with a splay formation across the film thickness.

Figure 3

An untethered in situ reconfigurable soft miniature machine that self‐adapts to different environments/terrains by exhibiting distinct locomotion modes. a) The pre‐programmed magnetization profile views of the sheet‐shaped soft machine. b) Schematic illustrations of different locomotion modes on a solid surface in air and inside a viscous fluid exhibited by the proposed machine in response to exerted magnetic fields. c) Video image snapshots from Movie S1 (Supporting Information) of the proposed machine walking on a substrate in air and swimming by helical propulsion inside a viscous liquid. The switching between these two locomotion modes was triggered autonomously by an increase in the environmental temperature (from room temperature to 70 °C).

Figure 4

Experimental demonstrations of a vine‐plant‐inspired filament and a reconfigurable magnetic spring. a) A filament of the proposed composite film twined around a hot needle in its vicinity when it was swaying left and right with the exerted rotating magnetic field. The demonstrated behavior of the composite film mimicked the twining motion of the climbing vine plants (see Movie S2 in the Supporting Information). b) A proof‐of‐concept magnetic spring with reconfigurable damping profiles. The sample made of the proposed composite film resisted the movement along x‐axis (Mode 1). After being exposed to UV light, the sample curled and the deformation persisted once the light was removed. The deformed sample exhibited a different resistance curve along x‐axis (Mode 2). Note that the force–displacement profiles for both modes start from 0 to 10 mm and then come back from 10 to 0 mm. The preceding and receding curves overlap with each other, suggesting no observable hysteresis.