This paper presents a review of electrothermal micro-actuators and applications. Electrothermal micro-actuators have been a significant research interest over the last two decades, and many different designs and applications have been investigated. The electrothermal actuation method offers several advantages when compared with the other types of actuation approaches based on electrostatic and piezoelectric principles. The electrothermal method offers flexibility in the choice of materials, low-cost fabrication, and large displacement capabilities. The three main configurations of electrothermal actuators are discussed: hot-and-cold-arm, chevron, and bimorph types as well as a few other unconventional actuation approaches. Within each type, trends are outlined from the basic concept and design modifications to applications which have been investigated in order to enhance the performance or to overcome the limitations of the previous designs. It provides a grasp of the actuation methodology, design, and fabrication, and the related performance and applications in cell manipulation, micro assembly, and mechanical testing of nanomaterials, Radio Frequency (RF) switches, and optical Micro-Electro-Mechanical Systems (MEMS).

The exceptional actuation properties of liquid crystal elastomers (LCEs) have made these materials highly attractive for various emerging applications such as soft robotics and artificial muscles. The large strain gradients occurring under thermal stimuli induce bending and curling of initially flat LCE films. Due to the complex physics behind the spontaneous deformation in nematic liquid crystal elastomers, there is no single universal finite element-based method for the simulation of the behaviors of LCE actuators. In this work, we developed a simple layered 2D model for modeling and simulation of the bending and curling characteristics of LCE beams based on the gradient of the temperature-dependent equivalent thermal expansion. The appropriate parameters were derived by measuring the radius of curvature of the LCE film aligned unidirectionally at one surface produced on a rubbed Kapton film. It was found that in a large range of thicknesses (12–134 μm) of the LCE beams, the equivalent thermal expansion coefficients tend to approach a similar value. It was demonstrated and experimentally validated that the thermal expansion model is very effective in predicting the nonlinear curling behavior of LCE beams of various thicknesses. Remarkably, the model is also capable of simulating the rolling behavior of LCE beams with tapered thickness variation. The proposed method offers good flexibility in terms of the geometric shape and expansion parameters, computational efficiency, and accuracy.

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polymers, which normally exhibit uniform expansion under the thermal stimulus, LCE offers a unique advantage of implementing complex deformation shapes with a monolithic structure composed of a single layer. [16] The actuation function in the LCE-based structures is dictated by the internal molecular alignment pattern. Large reversible deformations in thermal LCE actuators arise from the rapid anisotropic shrinking (up to 400% of strain) [17] along the director due to the reduction in the LC order, and also due to a large anisotropic coefficient of thermal expansion (CTE) of the LCE material. [18] In the last two decades, the ability to locally control the orientation of mesogens in photo-crosslinkable LCEs has opened a significant path for microstructured actuation. [19,20] Not only the anisotropic actuation properties, but also the mechanical properties of the material can be spatially programmed. [21,22] Thermal LCE-based actuators have been studied in much detail in the literature. However, most of the research and applications have been focused on the out-of-plane bending of the flat LCE layers, whereas the in-plane actuators are mainly based on simple uniform longitudinal contraction. When the orientation of the mesogens is varied through the thickness of the LCE layer, as a result it can produce reversible bending out of the substrate plane on actuation. Different alignment techniques, such as rubbed or micropatterned surfaces [23][24][25][26] and photoalignment layers, [10,[27][28][29] have been used very effectively for creating LCE layers with twisted and splayed LC directors. It was demonstrated that a difference in temperature [30] or degree of photopolymerization [31] between the top and the bottom surfaces of the LCE can also create the required gradient in properties perpendicular to the substrate. On the other hand, it is more challenging to implement an in-plane gradient in properties which is necessary to achieve lateral bending movement of the structures at elevated temperatures.The possibility of lateral bending actuation mode would bring significant advancement to the applications of LCE in microrobotics and micromanipulation. For instance, the majority of the soft LCE-based inching and crawling robots demonstrated to date can only perform unidirectional locomotion. [25,26,[32][33][34][35][36][37][38] Recently, it has been shown that the motion of LCE soft robots

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