Adaptive Millimeter-Wave and THz Antenna Devices Based on Dielectric Elastomer Actuators

In this paper we propose a new approach to fiber optic voltage sensors via voltage-controlled Liquid Crystals (LC), which would allow direct measurement of up to 400 kV/m electric fields at multiple points. In addition, the electro-optical behavior of deformed helix ferroelectric (DHF) liquid crystal in reflective mode is described and tested. The electrically controlled reflectance has been measured at sub-kilohertz driving voltage frequency for different polarizations of the incident light. Full Text: PDF References Y.V. Izdebskaya, V.G. Shvedov, A.S. Desyatnikov, W. Krolikowski and Y.S. Kivshar, "Soliton bending and routing induced by interaction with curved surfaces in nematic liquid crystals", Opt. Lett. 35, 1692-1694 (2010). CrossRef Z. Ge, S. Gauza, M. Jiao, H. Xianyu and S.T. Wu, "Electro-optics of polymer-stabilized blue phase liquid crystal displays", Appl. Phys. Lett. 94, 101104 (2009). CrossRef Z. Brodzeli, F. Ladouceur, L. Silvestri, A. Michie, V. Chigrinov, G. Q. Guo, E. P. Pozhidaev and A. D. Kiselev, "Liquid crystal-based hydrophone arrays", APOS (2012). CrossRef T. Tanigawa, Y. Sakakibara, S. Fang, T. Sekikawa and M. Yamashita, "Spatial light modulator of 648 pixels with liquid crystal transparent from ultraviolet to near-infrared and its chirp compensation application", Opt. Lett. 34, 1696-1698 (2009). CrossRef K. H. Yang and T. C. Chieu, "Transport Properties of Ions in Ferroelectric Liquid Crystal Cells", Jpn. J. Appl. Phys. 28, 2240-2246 (1989) CrossRef I.C. Khoo and S.T. Wu, "Optics and Nonlinear Optics of Liquid Crystals", World Scientific, Singapore (1993). V.G. Chigrinov, "Liquid Crystal Devices: Physics and Applications", Artech House, Boston: London, (1999). A.D. Kiselev, E.P. Pozhidaev, V.G. Chigrinov and H.S. Kwok, "Polarization-gratings approach to deformed-helix ferroelectric liquid crystals with subwavelength pitch", Phys. Rev. E 83, 31703 (2011). CrossRef N. A. Clark and S. T. Lagerwall, "Submicrosecond bistable electro-optic switching in liquid crystals", Appl. Phys. Lett. 36, 899-901 (1980). CrossRef L.A. Beresnev, V.G. Chigrinov, D.I. Dergachev, E.P. Pozhidaev, J. Funfshilling and M. Schadt, "Deformed helix ferroelectric liquid crystal display: A new electrooptic mode in ferroelectric chiral smectic C liquid crystals", Liq. Cryst. 5, 1171-1177 (1989). CrossRef Z. Brodzeli, H.K. Bal, V.G. Chigrinov, A. Murauski, and F. Ladouceur, "Electrical energy harvesting device for current/voltage fibre-based sensors", Optical Fibre Technology (ACOFT), 2010 35th Australian Conference on Fibre Technology, 1-2, (2010). DirectLink X. Zhao, F. Boussaid, A. Bermak and V.G. Chigrinov, "High-resolution thin ?guest-host? micropolarizer arrays for visible imaging polarimetry", Opt. Exp. 19, 5565-5573 (2011). CrossRef X.H. Li, A. Murauski, A. Muravsky, P.Z. Xu, H.L. Cheung, and V. Chigrinov, "Grayscale Generation and Stabilization in Ferroelectric Liquid Crystal Display," J. Display Technol. 3, 273-279 (2007). CrossRef E.P. Pozhidaev, S. Torgova, M. Minchenko, C. Yednak, A. Strigazzi and E. Miraldi, "Phase modulation and ellipticity of the light transmitted through a smectic C* layer with short helix pitch", Liq. Cryst. 37, 1067-1081, (2010). CrossRef E.P. Pozhidaev and V.G. Chigrinov, "Bistable and multistable states in ferroelectric liquid crystals", Crystallography Reports. 51, 1030-1040 (2006). CrossRef Q. Guo, F. Fan, T. Du, and V.G. Chigrinov, "Electrooptic Response of FLC: Effect of Alignment Materials", SID'11 Conference, P-144, Los Angeles, USA, May, (2011). CrossRef G. Hegde, P. Xu, E.P. Pozhidaev, V.G. Chigrinov, and H.S. Kwok, "Electrically controlled birefringence colours in deformed helix ferroelectric liquid crystals", Liq. Cryst. 35, 1137-1144 (2008). CrossRef

If this era is ever named after a type of material, it may well become the era of smart materials. Compliant structures with integrated sensors and actuators reacting autonomously to changes in the environment via intelligent feedback control loops allow for entirely new solutions, e.g. in light-weight structures. A promising technology meeting the requirements for new actuator systems are the dielectric elastomers (DE). The simple working principle, inexpensive materials and unique features like the scalability, compliance, and light weight make this class of the electro-active polymers (EAP) attractive for many applications. The combination of large active deformations uni- or biaxially and the noiseless expansion, is unique among the available actuator technologies.<br /> The present thesis is divided into three parts: The first part treats the subject of up-scaling planar membrane dielectric elastomer actuators. Designs for large-scale actuators were developed. The verification that the material scales after theory is provided from measurements on a planar agonist-antagonist test-rig. The strain and force difference at activation were predicted with existing material models and parameters and compared to the experimental results. The mechanical behaviour of the large-scale actuators can be predicted with classical non-linear continuum mechanics, considering viscoelasticity. Active deformation as well as blocking force scale with the dimensions of the actuator and with the number of layers respectively. Electrical properties, such as capacity and serial resistance of the electrodes were analysed and found to scale according to theory as well.<br /> The specific material used as dielectric, VHB 4910, is well-known and characterized thoroughly. It has shown the largest active strain and the highest energy density of all the materials used so far in DE actuators. On the other hand it features several disadvantages, such as a high viscosity, and a limited range of temperature due to a high glass transition temperature. Therefore, in a second part of the thesis, two other materials, introduced more recently, were evaluated. The first material consists of pre-stretched VHB 4910 that is modified with an interpenetrating TMPTMA network, resulting in a composite material of two intermingled elastomers, one under tension and one under compression. The second material is a silicone with corrugated silver electrodes. The three materials were characterized under comparable testing conditions and their performance as actuators was evaluated in an isotonic activation test and on an agonist-antagonist hinge configuration.<br /> In the third part, a proof-of-concept for the large-scale VHB 4910 actuators was done on a biomimetic airship with fish-like propulsion. The airship is propelled by the interaction of a wave motion pattern of the body and caudal fin, resulting in a Karman-type vortex street in the wake of the airship. Bending the helium-inflated body and a caudal fin is realized with large planar DE actuators in agonist-antagonist configurations. The focus of the design process was on the DE actuators as artificial muscles. In a first flight test, the transfer of the fish-like movement from water to air was verified. Valuable experimental experience was gathered concerning large-scale DE actuators, which can be used for future large-scale structures in active and adaptive systems.

It is common knowledge that when an elastomer (rubber) is stretched, its length will bemaintained if its two ends are fixed. Here, it is serendipitously found that whenan elastomer isslowly elongated further to achieve buckling under such conditions, the final length is much longer than the pre-stretched length. This allows the designof untethered autonomous synthetic-material-based soft robots that do not need any other chemical or electrical energy sources or external stimuli after the pre-strain is installed. Once the growth starts, the elongation continues to proceed even when the applied force is removed. Moreover, the elastomer, after growing, eventually forms a robust soft actuator that can be reshaped for different purposes. Few synthetic materials can grow like this, so far. This investigation shows that the material has an uncommon liquid crystal phase. Contrary to normal liquid crystals, it becomes birefringent only at high temperatures. The formation and the reshaping of the resulting soft actuators relate to a usually unnoticed reversible reaction. The work is promising to promote further understanding of dynamic covalent chemistry and liquid crystal elastomers, as well as to foster new designs and high-impact applications of bioinspired sustainable soft actuators in areas other than soft robots.

Asymmetric bending enables rapid and efficient crawling motions in a soft robotic system. However, achieving asymmetric motion in soft robots with temperature‐responsive liquid crystal elastomers (LCEs) necessitates complex Joule heating configurations or precise temperature regulation. Herein, a simple electroless plating method is presented for patterning asymmetric temperature gradients on paper substrates, thereby facilitating the development of Joule heating, caterpillar‐inspired soft robots. Cellulose‐based paper substrates provide distinct advantages due to their porous structure, which enables facile electrode deposition via solution‐based processes and offers high mechanical deformability. Leveraging these material properties, Cu electrodes are asymmetrically deposited onto paper substrates by varying electrode widths. This variation induces differences in electrical resistance, generating significant temperature gradients across the substrate. Furthermore, it is demonstrated that temperature gradients could be modulated by adjusting the extent of Cu enhancement in the electrode patterns. it is fabricated energy‐efficient soft robots capable of directional crawling at a low actuation voltage of 0.5 V by integrating the paper substrate with LCEs in a bilayer architecture. The simplicity of the electrode patterning process, combined with the eco‐friendly nature of paper‐based actuators, presents a promising approach for the scalable and sustainable fabrication of soft robots with complex locomotion capabilities.

From Responsive Molecules to Interactive Materials.

Smart materials adapt to, rather than resist, changes to their environment. In Nature, a variety of smart materials in the biological system demonstrate unique and potential functions, which appear as norm in the design of smart synthetic materials. Liquid crystals are very useful in the engineering of smart synthetic materials because of their well-ordered and controllable structure, anisotropy, and high sensitivity to external stimuli. These properties promote the engineering of synthetic materials to display smart and potential functions comparative to those of biological materials. The research presented in this thesis describes strategies to develop new, functional and smart molecular materials by amplifying molecular motion, which is intrinsically limited to the nanoscale, up to the macroscopic level of functional materials. A special focus is given to molecular motion that is triggered by light as an external stimulus, and induces a change in chiral structure of cholesteric liquid crystals at molecular level, which is eventually amplified by cooperativity of the liquid crystals into either a mechanical or an optical output. This work also provides insights into the mechanisms of amplification of molecular movement. While stabilizing the twisted organization of liquid crystals by in-situ polymerization, photo-induced molecular switching lead to disorder in the liquid crystal polymer network, which in turn created strain and was eventually transformed into mechanical motion at the macroscale (chapter 3 and 4), including both helical motion (chapter 3 and 4) and bending motion (chapter 4) which were shown to arise from a twisted geometry. Alternatively, starting from a ground state in which the twisting of a chiral liquid crystal is hampered, amplification of molecular motion was achieved by releasing this frustration, which was translated into original properties for this system (chapter 5 and 6). These investigations also point out that the amplification of molecular motion can be manipulated to reach different ranges, from the microscale (chapter 5 and 6) to the macroscale (chapter 3 and 4), by using an appropriate choice of irradiation with light (with either a local- or a spatial- focus). In addition to amplifying motion, we showed that chirality, an essential property of molecular matter, can be translated (e.g. chapter 3; molecular chirality indirectly amplified into macroscopic chiral shapes) or suppressed (e.g. chapter 5 and 6; supramolecular chirality is suppressed using homeotropic confinement) at different scales. Changes of material design, such as architecture, chemical constitution, fabrication and controlled operation have impact on the amplification of molecular motion, molecular chirality as well as other intrinsic molecular properties to present variously at the level of functional materials (e.g. mechanical output in chapter 3 (helical motion) and chapter 4 (helical- and bending motion with slow shape relaxation), and optical output in chapter 5 (single stimuli-responsiveness) and chapter 6 (dual stimuli-responsiveness)). Notably, taking inspiration from the design of biological materials in nature can lead to effective strategies in engineering new, smart and sophisticated function of materials (e.g. chapter 3). Overall, we have developed light-responsive liquid crystal polymer networks that are capable of performing light-induced helical motion and some of them also retain their light-induced shape. The complex and versatile helix-based behavior they display suggests prospective use in applications such as soft robotics and microfluidic technology. Moreover, light-responsive chiral liquid crystals show original optical properties that could be used for the development of re-writable and photonic technologies.

Reconfigurable antennas, with the ability to radiate more than one pattern at different frequencies and polarizations, are necessary in modern telecommunication systems. The requirements for increased functionality (e.g., direction finding, beam steering, radar, control, and command) within a confined volume place a greater burden on today's transmitting and receiving systems. Reconfigurable antennas are a solution to this problem. This paper discusses the different reconfigurable components that can be used in an antenna to modify its structure and function. These reconfiguration techniques are either based on the integration of radio-frequency microelectromechanical systems (RF-MEMS), PIN diodes, varactors, photoconductive elements, or on the physical alteration of the antenna radiating structure, or on the use of smart materials such as ferrites and liquid crystals. Various activation mechanisms that can be used in each different reconfigurable implementation to achieve optimum performance are presented and discussed. Several examples of reconfigurable antennas for both terrestrial and space applications are highlighted, such as cognitive radio, multiple-input-multiple-output (MIMO) systems, and satellite communication.

Reconfigurable antennas are capable of dynamically altering their frequency, polarization, and radiation properties in a controlled and reversible manner. They modify their geometry and behaviour to maximize the antenna performance in response to changes in their surrounding conditions. To implement a dynamical response, they employ different mechanisms such as PIN diodes, varactors, radio-frequency microelectromechanical systems (RF-MEMS), field effect transistors (FETs), parasitic pixel layers, photoconductive elements, mechanical actuators, metamaterials, ferrites, and liquid crystals. These mechanisms enable intentional distribution of current on the antenna surface producing reversible modification of their properties. This paper presents the design process and applications of reconfigurable antennas. The activation mechanisms of reconfigurable antennas, and their design and operation optimization are discussed. The latest advances on reconfigurable metamaterial engineering, and the current trends and future directions relating to reconfigurable antennas are reviewed. Finally, the applications of reconfigurable antennas in cognitive radio, multi-input multi-output (MIMO) systems, satellite communications, and biomedical devices are highlighted.

A review: The influence of graphene material integration in antenna characteristics in the presence of bias for fifth and sixth generation wireless communication application

Liquid crystal elastomers (LCEs) have shown great potential as soft actuating materials in soft robots, with large actuation strain and fast response speed. However, to achieve the unique features of actuation, the liquid crystal mesogens should be well aligned and permanently fixed by polymer networks, limiting their practical applications. The recent progress in the 3D printing technologies of LCEs overcame the shortcomings in conventional processing techniques. In this study, the relationship between the 3D printing parameters and the actuation performance of LCEs is studied in detail. Furthermore, a type of inchworm-inspired crawling soft robot based on a liquid crystal elastomeric actuator is demonstrated, coupled with tilted fish-scale-like microstructures with anisotropic friction as the foot for moving forwards. In addition, the anisotropic friction of inclined scales with different angles is measured to demonstrate the performance of anisotropic friction. Lastly, the kinematic performance of the inchworm-inspired robot is tested on different surfaces.

Liquid Crystal (LC) is an anisotropic liquid material which flows like a liquid, but at the same time its molecules have an orientational order like in the solid state [1]. Thus, LC is a promising dielectric material for designing reconfigurable devices at microwave frequencies. In order to optimize the design of reconfigurable microwave devices, accurate values of the dielectric permittivity and the loss tangent of LCs are needed. However, new LCs are not well characterized at these frequencies because of its recent use for microwave applications. Therefore, the characterization in this frequency range is required for its practical use within microwave components and devices [2]. In this work, the split-cylinder resonator method has been used for the characterization of LCs at two frequency points, i.e. 5 and 11 GHz. The method is based on the measurement of the resonance frequency and quality factor of the two states of the LC molecules for extracting the complex dielectric permittivity [3]. For achieving these two states, no electric or magnetic fields are needed, just the cell must be turned 90º inside the cavity. The dielectric properties (permittivity and loss tangent) of four different LC samples, GT3-23002 from Merck and QYPD193, QYPD142, and QYPD036 from Qingdao QY Liquid Crystal Co, have been obtained. The highest values of the dielectric anisotropy are presented for the samples QYPD036 and QYPD193, together with the highest values of the corresponding loss tangent parameters. Furthermore, it is observed for all the LCs that the loss tangent decreases and the dielectric anisotropy increases at higher frequencies, which must be taken into account in the development of reconfigurable microwave devices.

Molecular orientation states in the liquid crystal (LC) cell can easily be deformed and change their properties by applying low driving voltage with low power consumption. Those features are also attractive for the various functional devices in the wider frequency regions such as a millimeter wave (MMW), although only the optics applications are well investigated as the display application is put the head of them. Waveguide systems are well known as an excellent way to make a low loss transmission circuit in the ultrahigh frequency region such as microwaves and MMWs, and then the LC devices utilizing the waveguide have been proposed so far. However, the planar waveguide has gathered many attractions for the wider functionality by integration. We adopt here a coplanar waveguide (CPW) substrate to prepare the planar type of LC MMW devices combined with an ITO glass substrate. Since the ITO film acts as a floating electrode to simplify the electrode structure, LC molecules can be driven by applying the voltage only to the CPW substrate. Unfortunately, since the ITO electrode close to the CPW decreases the transmission of the MMW, the molecular orientation effects related with the electrode structure are very important to minimize the electrode area. We prepare the transparent LC cell which has exactly the same electrode structure with the CPW LC cell, and observe the molecular orientation states in detail by using a polarization microscope. It becomes clear that only the narrow area above the signal electrode of the CPW affects strongly to the phase change of the MMW propagation in the CPW type LC cell.

Electromagnetic (EM) soft actuators exhibit exceptional responsiveness to the critical demands of soft robotics, offering advantages such as high actuation speed, low weight, energy efficiency, and electrical controllability. However, conventional designs require external permanent magnets positioned outside the soft structure, constraining practical applications. This study addresses this limitation by developing a novel class of soft EM actuators composed of a silicone matrix with fully embedded copper coils and small magnets, eliminating the need for external magnets. This fully integrated architecture, enabled by additive manufacturing, ensures operation in unstructured environments, as functional elements are monolithically integrated within the actuator. The actuators function in two modes: attraction, inducing 41.7% compression, and repulsion, enabling 47.6% expansion. These actuators demonstrate key performance such as stretchability (continuous operability after 94% strain), scalability (up to 300%), multimodal operation, wearable compatibility (blocking force of 0.15 N), fast response (&gt;1 Hz), low power consumption (2.8 W), lightweight design (18 g), multifrequency capability, and bio‐inspired actuation, wherein compression is achieved through electrical current. These silicone EM actuators demonstrate versatility across several applications, such as flow‐regulating soft valves, fluid mixing devices, high‐speed soft robots (exceeding the 24 BL/s relative speed of cheetahs), and complex 3D structures for controlled contraction and expansion.

Herein, we report a remotely controlled soft robot employing a photoresponsive nanocomposite synthesized from liquid crystal elastomers (LCEs), high elastic form-stable phase change polymer (HEPCP), and multiwalled carbon nanotubes (MWCNTs). Possessing a two-stage deformation upon exposure to near-infrared (NIR) light, the LCE/HEPCP/MWCNT (LHM) nanocomposite allows the soft robot to exhibit an obvious, fast, and reversible shape change with low detection limitations. In addition to the deformation and bending of the LCE molecular chains itself, the HEPCP in the composite material can also be triggered by a reversible solid-liquid transition due to the temperature rise caused by MWCNTs, which further promotes the change of the LCE. In particular, the proposed photodriven LHM soft robot can bend up to 180° in 2 s upon NIR stimulation (320 mW, distance of 5 cm) and generate recoverable, dramatic, and sensitive deformation to execute various tasks including walking, twisting, and bending. With the capacity of imitating biological behaviors through remote control, the disruptive innovation developed here offers a promising path toward miniaturized untethered robotic systems.

Deformable liquid crystal polymers (LCPs) driven by more than one external stimulus have received extensive attention in fields ranging from multifunctional soft robots to bionic actuators. Combining responsive liquid crystal with nonmesogenic responsive groups within polymer offers a versatile way to obtain multiresponsive LCPs. However, the incorporation of nonmesogenic responsive groups causes interruption in the assembly of mesogens and brings a challenge to the alignment of LCPs. Herein, a new method is put forward to facilitate uniform mesogen alignment by exerting water vapor in the film preparation process. Using this method, vapor-assisted alignment, the homeotropic alignment of azobenzene mesogens is achieved in a copolymer containing nonmesogenic poly(ethylene glycol) (PEG). The obtained copolymer films present photodeformation brought by azobenzene isomerization and humidity-responsive deformation resulting from the asymmetric swelling of film surfaces. The dual-responsive smart "blinds" and bionic flower actuators are fabricated to demonstrate the integration of the two different stimuli. This work is anticipated to provide a feasible alignment method for multiresponsive LCPs, showing the potential applications in soft robots, sensors, and biomimetic devices.