Programming Deformations of 3D Microstructures: Opportunities Enabled by Magnetic Alignment of Liquid Crystalline Elastomers

Abstract

Synthetic structures that undergo controlled movement are crucial building blocks for developing new technologies applicable to robotics, healthcare, and sustainable self-regulated materials. Yet, programming motion is nontrivial, and particularly at the microscale it remains a fundamental challenge. At the macroscale, movement can be controlled by conventional electric, pneumatic, or combustion-based machinery. At the nanoscale, chemistry has taken strides in enabling molecularly fueled movement. Yet in between, at the microscale, top-down fabrication becomes cumbersome and expensive, while bottom-up chemical self-assembly and amplified molecular motion does not reach the necessary sophistication. Hence, new approaches that converge top-down and bottom-up methods and enable motional complexity at the microscale are urgently needed. Synthetic anisotropic materials (e.g., liquid crystalline elastomers, LCEs) with encoded molecular anisotropy that are shaped into arbitrary geometries by top-down fabrication promise new opportunities to implement controlled actuation at the microscale. In such materials, motional complexity is directly linked to the built-in molecular anisotropy that can be "activated" by external stimuli. So far, encoding the desired patterns of molecular directionality has relied mostly on either mechanical or surface alignment techniques, which do not allow the decoupling of molecular and geometric features, severely restricting achievable material shapes and thus limiting attainable actuation patterns, unless complex multimaterial constructs are fabricated. Electromagnetic fields have recently emerged as possible alternatives to provide 3D control over local anisotropy, independent of the geometry of a given 3D object. The combination of magnetic alignment and soft lithography, in particular, provides a powerful platform for the rapid, practical, and facile production of microscale soft actuators with field-defined local anisotropy. Recent work has established the feasibility of this approach with low magnetic field strengths (in the lower mT range) and comparably simple setups used for the fabrication of the microactuators, in which magnetic fields can be engineered through arrangement of permanent magnets. This workflow gives access to microstructures with unusual spatial patterning of molecular alignment and has enabled a multitude of nontrivial deformation types that would not be possible to program by any other means at the micron scale. A range of "activating" stimuli can be used to put these structures in motion, and the type of the trigger plays a key role too: directional and dynamic stimuli (such as light) make it possible to activate the patterned anisotropic material locally and transiently, which enables one to achieve and further program motional complexity and communication in microactuators. In this Account, we will discuss recent advances in magnetic alignment of molecular anisotropy and its use in soft lithography and related fabrication approaches to create LCE microactuators. We will examine how design choices-from the molecular to the fabrication and the operational levels-control and define the achievable LCE deformations. We then address the role of stimuli in realizing the motional complexity and how one can engineer feedback within and communication between microactuator arrays fabricated by soft lithography. Overall, we outline emerging strategies that make possible a completely new approach to designing for desired sets of motions of active, microscale objects.

Figure 1

Fabrication and alignment methods for liquid crystalline elastomer (LCE) microstructures—their applicability and deficiencies: (a) An LCE undergoes a directional deformation determined by its molecular anisotropy (director n), order parameter, and network architecture. (b) Comparison of LC alignment methods and fabrication strategies through (i) mechanical force, (ii) topographical patterning, and (iii) magnetic fields. Magnetic field alignment, as a volumetric method, is decoupled from the sample geometry and allows for arbitrary choice of director orientation within molded 3D shapes (particularly at the micron scale), unlike approaches (i) and (ii).

Figure 2

Magnetic alignment of LC mesogens enables arbitrary director orientations in LCEs, thereby significantly expanding achievable microactuator deformations. (a) The molecular structure of the mesogens and magnetic field strength affect the mesogens’ propensity for alignment. (b) Various magnetic field patterns are accessible through changing arrangements of magnets. (c) Magnetic field alignment is compatible with a wide range of geometries and manufacturing methods. (d) Fabrication is easily scalable to multiple shapes and (micro)structure arrays. (e) Region-specific alignment by multistep polymerization and discontinuous director fields. (f) Through chemical modifications, LCEs can be actuated by various stimuli.

Figure 3

New types of deformations result from the combination of magnetic alignment and soft lithography: (a) Fluorescence confocal microscopy images of deforming single LCE microplates (250 × 50 × 200 μm) with different director orientations, when heated >125 °C (left), and corresponding finite element simulations for the in-plane and out-of-plane bending (right). (b) Correlation of magnetic field strength during fabrication and the resulting extent of mechanical deformation for a z-aligned microplate, in experiment and theory. (c) The magnetic field direction can be continuously modulated by changing position of the sample or arrangement of permanent magnets, affecting the tilting angle. Left: Calculated polar angles (COMSOL) of the director as a function of the distance d from the magnet center. Right: Experimentally measured microplate tilting angles confirming this trend. (d) Gradual change in tilting angles is experimentally illustrated in an array of microplates. Images adapted with permission from refs (19, 41). Copyright (2018) National Academy of Sciences and Copyright (2021) Wiley, respectively.

Figure 4

A directional stimulus such as light can create transient, dynamically changing bimorphs of activated and nonactivated regions. (a) Illustration of the underlying opto-chemo-mechanical feedback in an LCE micropost with noncollinear geometric axis (G), molecular anisotropy (M), and light direction (L) (1). As light penetrates (2), only a part of the material becomes activated (3), which deforms along the director (4). The deformation changes the subsequent activation step (5), which is made possible by induced transparency (inset top right, in which activation (i and ii) makes the material partially transparent to the activating light, causing deeper penetration and activation (iii and iv)). Depending on the light intensity, the feedback loop can either be completed (gray flow arrow, high intensity), or stopped at the initial deformation (green flow arrow, low intensity). (b) Out-of-plane UV-irradiation of a square micropost (30 × 30 × 150 μm) with tilted director alignment causes a characteristic and finely tunable power stroke. Depending on the light-direction, the same pillar can sway into opposite directions. (c) Motion trajectory as a function of light intensity. Images adapted with permission from ref (48). Copyright (2022) Springer Nature.

Figure 5

Emergence of communication and coordinated movements in arrays of microactuators. (a) Illustration of how shadowing causes a domino effect in ensembles of LCE microposts when slight off-axis illumination exposes the subsequent pillar unevenly. (b) Experimental observation (left) and modeling (right) of the resulting zigzag self-sorted pattern. (c) Experiment (left) and modeling (right), showing interpillar communication when illuminated along the array (top), and absence thereof when illuminated in a diagonal direction (bottom). (d) In more complex cases, programmable “metachronal” waves emerge. Images adapted with permission from ref (48). Copyright (2022) Springer Nature.

Figure 6

Multimodal deformations in jointed compositionally uniform microactuators. (a) Cross-shaped LCE microactuator with different anchoring spots displays different deformation behaviors: (i) when center-anchored, symmetric inward and outward twisting can be invoked for different arms, (ii) while with asymmetric anchoring of one arm, the twisting of the vertical arm couples to the twisting/rotation of the horizontal segment, enabling amplification of the sway motion. Experiments, center; simulations, right. (b) Diverse motions can be also obtained by changing illumination directions (simulations). Photoactuation is depicted by the blue-to-yellow transition. Images adapted with permission from ref (48). Copyright (2022) Springer Nature.

Figure 7

Deformations of base-attached cellular LCE microstructures. (a) Different director alignment causes different deformations both in simulation (top) and experiment (bottom). (b) Domain walls (indicated by the blue dashed line) in the form of a second order buckling pattern in a first-order buckling square lattice. (c) Two-step polymerization enables encoding area-specific buckling patterns. (d) Optical microscopy images (bottom) and transmitted light intensity profile (right) at T = 135 °C > T_NI by an LCE square lattice with mesogenic director gradually changing from −20° to 90°. (e) Combination of multiple stimuli for deforming z-aligned diamond-shaped cellular LCE structures. Images adapted with permission from refs (41, 49). Copyright (2021) Wiley and Copyright (2021) Springer Nature, respectively.