Shape Morphing Directed by Spatially Encoded, Dually Responsive Liquid Crystalline Elastomer Micro‐Actuators

Abstract

have been developed to control the inplane director field, including mechanically stretching/shearing, [23][24][25] guiding by surface patterns, [16,[26][27][28] and use of electric or magnetic field. [29] Out-of-plane bending of a flat LCE film can be realized when there is a gradient in the director field induced by different surface anchoring on top and bottom of the film [30,31] or in mechanical properties along the film thickness. [32][33][34] So far few have shown control of both in-plane and out-of-plane orientations of LC mesogens with complex director fields in the 3D space, locally and globally. Using a top and bottom substrate with planar microchannels oriented in different directions has shown such potentials. [28] However, the thickness of films (up to 100 µm) is limited by the decreasing molecular interactions away from the surface, causing deviation of the mesogens from the aligned direction along film depth. 3D printing allows for arbitrary arrangement of mesogens in 3D space, however, the printing resolution is typically on the order of hundreds of micrometers to millimeters. Thus, only simple geometries are printed, and it is time consuming to build layer-by-layer. While the intrinsic anisotropy of LC mesogens enables reversible actuation, it also constrains the choice of materials, fabrication, and spatial control of the shape morphing, due to the coupling between these aspects.Incorporating LCEs with other materials provides a feasible pathway to expand the materials library for shape morphing. A bilayer-structure of LCE and hydrogel with different elasticity has been demonstrated. [35,36] However, a strong bonding between the two layers is essential, otherwise, it will lead to a small strain [35] and poor reversibility. [36] Microparticles made by freeze-milling the bulk LCE film have been embedded in a conventional elastomer, polydimethylsiloxane (PDMS), followed by alignment under an external magnetic field to create macroscopic "monodomains" for actuation. [37] The use of discrete microparticles allows for decoupling of the enabling components [38] from the matrix itself. Nevertheless, those microparticles are not uniform in size nor anisotropic in shape. They are only aligned along an in-plane direction and a bilayer structure is still needed to achieve out-of-plane bending. Furthermore, the magnetic field-induced mesogen alignment is mostly limited to side-chain LC monomers due to high viscosity of mainchain system. Main-chain LCE microparticles [39][40][41][42][43][44] (Young's modulus E, ≈ 10's MPa) or side-chain LCE micro-actuators [45,46] Liquid crystalline elastomers (LCEs) with intrinsic molecular anisotropy can be programmed to morph shapes under external stimuli. However, it is difficult to program the position and orientation of individual mesogenic units separately and locally, whether in-plane or out-of-plane, since each mesogen is linked to adjacent ones through the covalently bonded polymer chains. Here, dually responsive, spindle-shaped micro-actuators are synthesized from ...

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“…(c) Photograph of the setup and calculated field strength distribution of the uniform magnetic field and (d) bright-field optical microscopy images showing the reorientation of the microparticles under the magnetic field. Adapted with permission from refs and . Copyrights 2022 and 2023 Wiley-VCH GmbH, respectively.…”

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