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Liquid crystal elastomers (LCEs) are well-recognized for programmable, large strain, and reversible shape changes in response to external stimuli. However, so far, there are several issues preventing the use of this class of materials in practical engineering applications such as actuators and electronics. The first part of the dissertation focuses on synthesis and processing strategies to expand capabilities of LCEs for actuator applications. Engineering application of LCEs are often limited by poor static and dynamic mechanical properties, e.g., modulus (~10 MPa), toughness (~10 MPa), blocking stress (~500 kPa), and work capacity (~300 kJ/m3 ). Also, these materials require high temperatures (typically above 100 °C) to undergo shape change. This work enables significant improvement in mechanical properties of LCEs by combining liquid crystallinity and semi-crystallinity. By developing novel synthesis and processing methods, crystallized LCEs are capable of not only enhanced static mechanical properties, including modulus (~350 MPa) and toughness (~40 MPa) but also improved dynamic mechanical properties, including blocking stress (~1.3 MPa) work capacity (~730 kJ/m3 ). This work also describes two routes to create multi-responsive LCE actuators that overcome the need to externally heat the material to high temperatures. We show high speed (~380 rpm) torsional actuation in response to chemical stimuli. Moreover, we provide a facile way to create programmed LCEs and carbon nanotubes (CNTs) composites. The LCE/CNT composites utilize visible light or electricity to trigger high-speed bending (~1 s) or uniaxial actuation (work capacity ~100 kJ/m3 , 2.5 times higher than mammalian muscles). The second part of the dissertation discusses electronic applications of LCEs. As current micro-electronic fabrication requires 2D flat substrates for photolithography processing, resulting devices are limited in 2D geometry which has minimal strain tolerance. Also, polymer-based biomedical electronics, e.g., neural interfaces, have significant issue to achieve long-term reliable encapsulation in the physiological condition. This work enables to process electronics on programmed 2D LCE substrates, then morph to desired 3D structures. The 3D electronics on LCE substrates provide strain tolerance up to 100% of deformation. We further show various examples of 3D electronics including strain tolerant capacitors and temperature sensing antenna enabled by LCE substrates. In the end, we briefly discuss current and on-going research to utilize LCEs for reliable packaging for advanced biomedical devices, e.g., deployable neural probes.
This text is a primer for liquid crystals, polymers, rubber and elasticity. It is directed at physicists, chemists, material scientists, engineers and applied mathematicians at the graduate student level and beyond.
Preparation of Liquid Crystalline Elastomers, by F. Brömmel, D. Kramer, H. Finkelmann Applications of Liquid Crystalline Elastomers, by C. Ohm, M. Brehmer und R. Zentel Liquid Crystal Elastomers and Light, by Peter Palffy-Muhoray Electro-Opto-Mechanical Effects in Swollen Nematic Elastomers, by Kenji Urayama The Isotropic-to-Nematic Conversion in Liquid Crystalline Elastomers, by Andrija Lebar, George Cordoyiannis, Zdravko Kutnjak und Bostjan Zalar Order and Disorder in Liquid-Crystalline Elastomers, by Wim H. de Jeu und Boris I. Ostrovskii
Soft actuator is a promising candidate for replacing a traditional rigid materials-based actuator when the actuating system requires human compatibility, large degree of freedom for the motion, low fabrication cost, and simple body structure. Among many soft materials, liquid crystal elastomer (LCE) is one of the most advantageous soft active material due to their large macroscopic deformability coupled with molecular level anisotropy. Patterning of LCE with precise control of molecular alignment can generate diverse actuations. In addition, different types of actuation of LCE can be induced by various external stimuli such as heat or light. In this study, we demonstrate radially patterned LCE with predesigned stretch field using a strain engineering technique which is facile, effective, and does not require any sophisticated setup. The radially patterned LCE exhibits fully reversible undulating deformation upon heating or swelling, attributed to the constrained expansion of radially patterned LCE in hoop direction. By applying the strain engineering technique, we design different LCE structures which exhibit diverse actuations like bending, rolling, crawling, or jumping. Incorporation of carbon nanotube (CNT) in the LCE allows photoresponsivity of LCE-CNT composite due to the photothermal effect of CNT. We prepare LCE-CNT rod with molecular alignment in its longitudinal direction which shows heliotropic behavior with multi-directional bending under the light irradiation rather than conventional uni- or bi-directional bending. The bending is induced by the contraction gradient of LCE-CNT rod in thickness which is maximized on the surface towards light, so the bending direction can be tuned by controlling the position of light source. Using the similar LCE or LCE-CNT rod, we show unusual rolling phenomena in which the LCE or LCE-CNT rod keeps rolling while maintaining its initial curvature in the same direction continuously induced by simply placing them on a homogeneously hot flat surface or under the visible light irradiation. Such non-intuitive autonomous rolling phenomena is induced by coupling of inhomogeneously distributed supporting force and gravity, which is triggered by continuous bending deformation of the rod during rolling. We also design a light-driven soft robot based on an arch shape LCE-CNT structure with magnet pieces on each end that performs crawling, squeezing, and jumping motions inspired by deformation traveling of inchworm locomotion and power amplification mechanism of jumping fly larva. The soft robot can perform different motions by switching its shape between arch and closed loop shape under different light irradiation modes, which enables fully reversible biomimetic motions.
Liquid crystal elastomers (LCEs), as an intriguing class of soft active materials, exhibit excellent actuation performances and biocompatible properties, as well as a high degree of design flexibility, which have been of increasing interest in many disciplines. This review summarizes recent developments in this inspiring area, providing an overview of fabrication methods, design schemes, actuation mechanisms, and diverse applications of LCEs. Firstly, two-stage and one-pot synthesis methods, as well as emerging fabrication techniques (e.g., 3D/4D printing and top-down microfabrication techniques) are introduced. Secondly, the design and actuation mechanisms are discussed according to the different types of stimuli (e.g., heat, light, and electric/magnetic fields, among others). Thirdly, the representative applications are summarized, including soft robotics, temperature/strain sensors, biomedical devices, stretchable displays, and smart textiles. Finally, outlooks on the scientific challenges and open opportunities are provided.
Soft robotics offer advantages over their rigid counterparts due to the intrinsic softness of their consisting materials, soft robotic matter. When equipped with programmable shape morphing and controllable function, soft robotics are best qualified for interaction with delicate objects, exploration of unknown terrains, and large, impact-resistant deformations. Towards this goal, new materials and fabrication methods are needed to create actuators with programmable shape- morphing behavior akin to human muscles. Liquid crystal elastomers (LCEs) are soft materials comprised of anisotropic liquid crystal mesogen molecules, which when aligned, give rise to reversible contraction with high energy density when heated above their nematic-to-isotropic transition temperature (TNI). However, the ability to produce LCE actuators with programmed director alignment in arbitrary, bulk forms is a grand challenge. The focus of my Ph.D. thesis is to create programmable LCE actuators through the integration of design, synthesis, and multi-material 3D printing methods. Towards this goal, solvent-free, oligomeric LCE inks were synthesized that incorporate rigid mesogens along their backbone as well as photopolymerizable groups at the chain ends. By varying the molecular composition of these oligomeric species, LCE inks with the appropriate viscoelastic response were designed for high operating temperature-direct ink writing (HOT-DIW), an extrusion-based 3D printing method. By tailoring polymer backbone and crosslinking chemistries of our LCE inks, their TNI could be varied from 92°C to 127°C after printing and UV cross-linking, and enable custom thermal response. We further demonstrated that patterned LCEs with programmed director alignment along the print path were produced when printing in the nematic phase. These 3D LCEs exhibit large reversible contractility and high specific energy density. Our integrated approach allows for prescribed LCE alignment in arbitrary geometric motifs. Building on this seminal advance, we created untethered soft robotic matter that repeatedly shape-morphs and self-propels in response to thermal stimuli through passive control. Specifically, we designed and printed active LCE hinges with orthogonal director alignment that interconnect rigid polymeric tiles. These hinges can be programmed as mountain or valley folds to produce reversible active origami structures. Moreover, in a single structure, we programmed hinges made of LCEs with disparate TNI to enable sequential folding and demonstrated untethered, reversible sequential folding in soft, active origami for the first time. We further demonstrated a self- compacting prism with a modular geometric locking mechanism capable of sequential folding with three temperature-specific, stable configurations. To enable the informed design of untethered robotic matter, LCE hinge bending angle and torque can be prescribed by geometry and LCE chemistry. We then exploited their exemplary performance by programming LCE hinges into the "rollbot", an exemplar self-propelling structure with passive control. Specifically, we designed a pentagonal prism with low TNI LCE hinges and propellers with high TNI LCE hinges, informed by our torque and bending angle characterization, enabling reversible reconfiguration and self- propulsion across a heated surface. To expand upon these capabilities, ewe developed a novel method of 3D printing aligned LCE filaments with embedded, coaxial liquid metal by co-extrusion of LCE and liquid metal through a core-shell nozzle. Our innervated LCE (iLCE) fibers are electrothermally heated well above TNI with programmable and predictable heat generation through the core of the filament, which resulted in large, prescriptible contractile strains akin to those of our neat 3D printed LCEs. The iLCE fibers enable self-sensing of actuation through the resulting change of resistance with respect to actuation strain, where a change of resistance is directly predictable from strain. Moreover, our iLCEs exhibited reliable reversible actuation and considerable work output, which combined with self-sensing capabilities allows for closed loop control. Specifically, our actuators automatically reach target resistance and strain values rapidly and repeatedly despite large bias load perturbations. As a final demonstration, we patterned iLCEs with a spiral printpath to demonstrate programmable 3D shape morphing. Analogous to iLCE fibers, these spiral iLCEs were electrothermally heated, exhibited self-sensing, and were regulated with closed loop control. In summary, we have developed a new platform for creating soft robotic matter through the design, synthesis, and assembly of LCE inks, which can be seamlessly integrated with structural, sensing, and functional materials. Our platform may be harnessed for applications including soft robotics, reconfigurable electronics, adaptable structures, and well beyond.
This book is the second edition of Soft Actuators, originally published in 2014, with 12 chapters added to the first edition. The subject of this new edition is current comprehensive research and development of soft actuators, covering interdisciplinary study of materials science, mechanics, electronics, robotics, and bioscience. The book includes contemporary research of actuators based on biomaterials for their potential in future artificial muscle technology. Readers will find detailed and useful information about materials, methods of synthesis, fabrication, and measurements to study soft actuators. Additionally, the topics of materials, modeling, and applications not only promote the further research and development of soft actuators, but bring benefits for utilization and industrialization. This volume makes generous use of color figures, diagrams, and photographs that provide easy-to-understand descriptions of the mechanisms, apparatus, and motions of soft actuators. Also, in this second edition the chapters on modeling, materials design, and device design have been given a wider scope and made easier to comprehend, which will be helpful in practical applications of soft actuators. Readers of this work can acquire the newest technology and information about basic science and practical applications of flexible, lightweight, and noiseless soft actuators, which differ from conventional mechanical engines and electric motors. This new edition of Soft Actuators will inspire readers with fresh ideas and encourage their research and development, thus opening up a new field of applications for the utilization and industrialization of soft actuators.
Currently, many smart materials exhibit one or multifunctional capabilities that are being effectively exploited in various engineering applications, but these are only a hint of what is possible. Newer classes of smart materials are beginning to display the capacity for self-repair, self-diagnosis, self-multiplication, and self-degradation. Ultimately, what will make them practical and commercially viable are control devices that provide sufficient speed and sensitivity. While there are other candidates, piezoelectric actuators and sensors are proving to be the best choice. Piezoelectric Actuators: Control Applications of Smart Materials details the authors’ cutting-edge research and development in this burgeoning area. It presents their insights into optimal control strategies, reflecting their latest collection of refereed international papers written for a number of prestigious journals. Piezoelectric materials are incorporated in devices used to control vibration in flexible structures. Applications include beams, plates, and shells; sensors and actuators for cabin noise control; and position controllers for structural systems such as the flexible manipulator, engine mount, ski, snowboard, robot gripper, ultrasonic motors, and various type of sensors including accelerometer, strain gage, and sound pressure gages. The contents and design of this book make it useful as a professional reference for scientists and practical engineers who would like to create new machines or devices featuring smart material actuators and sensors integrated with piezoelectric materials. With that goal in mind, this book: Describes the piezoelectric effect from a microscopic point of view Addresses vibration control for flexible structures and other methods that use active mount Covers control of flexible robotic manipulators Discusses application to fine-motion and hydraulic control systems Explores piezoelectric shunt technology This book is exceptionally valuable as a reference for professional engineers working at the forefront of numerous industries. With its balanced presentation of theory and application, it will also be of special interest to graduate students studying control methodology.
Polymers for Light-Emitting Devices and Displays provides an in-depth overview of fabrication methods and unique properties of polymeric semiconductors, and their potential applications for LEDs including organic electronics, displays, and optoelectronics. Some of the chapter subjects include: • The newest polymeric materials and processes beyond the classical structure of PLED • Conjugated polymers and their application in the light-emitting diodes (OLEDs & PLEDs) as optoelectronic devices. • The novel work carried out on electrospun nanofibers used for LEDs. • The roles of diversified architectures, layers, components, and their structural modifications in determining efficiencies and parameters of PLEDs as high-performance devices. • Polymer liquid crystal devices (PLCs), their synthesis, and applications in various liquid crystal devices (LCs) and displays. • Reviews the state-of-art of materials and technologies to manufacture hybrid white light-emitting diodes based on inorganic light sources and organic wavelength converters.
This book provides a comprehensive overview of various self-assemblies in liquid crystalline polymers and their electrical, optical, mechanical, and flame retardant properties. Liquid crystalline polymers are unique self-assembled, functional soft materials with electrical, magnetic, and thermal responses which find potential applications in numerous areas. As well as providing an overview of their synthesis, self-assembly and dynamics the various applications are also discussed. Such applications as liquid crystalline elastomers, light responsive actuators, optical reflectors, gas barrier films, and even flame retardant polymers will be presented. The book is a useful resource for undergraduates, postgraduates and experienced researchers.