Elastomers Grow into Actuators.

Synthesis-Processing-Property Relationships in Thermomechanics of Liquid Crystal Elastomers

More than 130,000 people have strokes each year in the United States [1]. Of these victims, 76% are left with disabilities that cost the nation over $54 billion in lost work and medical fees. One prominent disability is upper extremity hemiplegia, which occurs among approximately 50% of stroke sufferers [2]. Robotic technology has the potential to provide an automated platform for controlled rehabilitation and assisted, task-oriented therapy. Several systems have been designed to assist in patient articulation of an impaired hand using rigid robotic components. While these products have been successful in articulating the pure bending motions of the four fingers, the limited capabilities of rigid technologies fail to reproduce the complicated motion path [5] of the thumb during opposition grasp (see Figure 1). This is the most important articulation for normal hand function and specifically for picking up everyday objects. To date, robotic systems for thumb rehabilitation have not been widely investigated [3] apart from some recent work [4] that used a multi-joint rigid robot.

Liquid crystal elastomers (LCEs) are shape-morphing materials whose large and reversible shape transformations are caused by the coupling between the mobile anisotropic properties of liquid crystal (LC) units and the rubber elastic of polymer networks. Their shape-changing behaviors under certain stimuli are largely directed by the LC orientation; therefore, various strategies have been developed to spatially modulate the LC alignments. However, most of these methods are limited as they require complex fabrication technologies or have intrinsic limitations in applicability. To address this issue, programmable complex shape changes in some LCE types, such as polysiloxane side-chain LCEs, thiol-acrylate main-chain LCEs, etc., were achieved by using a mechanical alignment programming process coupled with two-step crosslinking. Here, we report a polysiloxane main-chain LCE with programmable 2- and 3D shape-changing abilities that were created by mechanically programming the polydomain LCE with two crosslinking steps. The resulting LCEs exhibited a reversible thermal-induced shape transformation between the initial and programmed shapes due to the two-way memory between the first and second network structures. Our findings expand on the applications of LCE materials in actuators, soft robotics, and smart structures where arbitrary and easily programmed shape morphing is needed.

The benefits of soft robotic actuation have been recognized and soft actuators are increasingly used. Soft robots are generally lightweight, inexpensive, and highly compliant. They are often used for interaction with small objects which only require relatively low force output. Nevertheless, large-scale applications can still benefit from the advantages of soft actuators if force output is sufficiently large. Some soft actuation has been employed in large-scale applications, but the area remains greatly underexplored. This article presents the Soft Torque-generating Actuation System (STAS) a lean soft actuation architecture capable of producing relatively large torque output while maintaining compliant interaction with the environment. Design and fabrication of the system are described, and a theoretical model is introduced for predicting torque output. The model is validated against measured torque output, and the system is further characterized in terms of power output. The model is shown to adequately predict torque output at low actuator angles. The system is shown to require only 30.0 kPa to achieve a large torque output of 10 N-m. The system also reaches a peak power output of 1.29 Watts. Finally, a versatile application of STAS is shown through a wearable joint-assistive device. The STAS is a promising architecture for a new type of high force actuation using soft actuators.

Soft robots are inherently compliant and adaptive, therefore they are promising candidates for interacting with humans. However robotic manipulators utilizing soft actuators are often constrained by a series of actuator performance limitations. In this work we design a novel linear soft robotic actuator with significantly improved performances over the existing products, achieving 300% deformation ratio, quasi-constant output force over a wide motion range, while maintaining passive compliance and adaptability. Moreover, the novel actuator is less prone to friction, and could be fabricated using inject molding and 3D printing, hence having high repeatability at very low cost. An analytical model was developed to characterize the actuator behavior and provide a guideline for actuator design according to performance specifications. A 6 DOF soft manipulator was designed and fabricated utilizing the novel soft actuator. The manipulator arm had a serial kinematic structure with a biomimetic wrist and was driven by 12 soft actuators mounted onto the arm links. With 1.2m workspace radius and 1kg payload, the working air pressure could be as low as 1bar. Preliminary results have shown the validity of the novel soft actuator and manipulator designs, as well as the strong potential of soft robots in human-oriented applications.

Emerging classes of soft robotics with flexible actuation, intelligent sensibility, and biomimetic functionality are driving significant advances in academic researches and commercial applications. Such new generation robotics relies heavily on important breakthroughs in soft matter engineering and flexible actuation systems. Unlike conventional rigid robots, soft robots generally have unique sporting styles and manufacturing strategies. Recently, a series of newly developed soft materials—gallium‐based liquid metals (LMs) are found to display dramatical roles in making soft machines and robotics with their unusual properties in soft robotic actuation and self‐driven field. With superior merits of both high stretchability and electroconductivity over conventional soft materials, LM is increasingly innovated as flexible sensors and actuators in constructing intelligent soft robots. Typical advances in LM enabled soft machines and robotics are summarized and interpreted, herein, with special focus on the driving principles of the LM objects, the preparation methods of LM flexible electronics for making soft robots, and the applications of LM in biomedicines and intelligent robots, together with the consideration of key challenges facing LM‐based soft robotics and its future prospects.

From Responsive Molecules to Interactive Materials.

The development of smart actuators that integrate robust mechanical properties with rapid responsiveness remains a critical challenge for advancing artificial muscles. To address this, this study incorporates lignin—a renewable resource—into liquid crystalline elastomer (LCE) to fabricate a high‐performance photothermal‐responsive artificial muscle material. By leveraging the inherent aromatic conjugated structure of lignin as an efficient photothermal unit, we successfully integrated it with mesogens to construct a liquid crystalline network featuring a nanoscale microphase‐separated architecture. This network incorporates lignin‐based phenolic carbamate bonds and disulfide bonds concurrently, forming a dual dynamic crosslinking architecture that significantly enhances the mechanical properties of LCE, achieving a tensile strength of 21.5 MPa and an elongation at break exceeding 754%, thereby substantially surpassing conventional single‐network LCEs. It also confers outstanding photothermal response, enabling rapid, self‐actuated reversible deformation (33% strain). As a light‐driven artificial muscle, it achieves 40% reversible strain and 30.0 kJ/m 3 output power density. This work presents a novel strategy for developing high‐performance light‐driven LCEs through the integration of biomass lignin as both a reinforcing phase and an intrinsic photothermal converter. This approach not only enhances the material's performance but also offers promising pathways for the design and fabrication of next‐generation soft robots and artificial muscles.

Conventional robots are constructed with rigid parts and materials, restricting their ability to deform freely and adapt to the external environment. Performing tasks such as handling delicate materials and navigating narrow pathways is challenging for these typical robots made of stiff materials. However, there is a growing demand for such functions in various industries. Recent advances in soft robotics have garnered considerable attention as solutions to these challenges. Soft robotic systems in various forms present a systematic solution for performing unique tasks. The goal of soft robotics research is to develop advanced technologies, incorporating innovative robotic mechanisms based on soft materials and structures to enhance adaptability. Soft robots find broad applications in diverse fields, including industry, healthcare, and field exploration. Soft component technology plays a crucial role in configuring soft robots. Soft actuators introduce new types of motion and operation mechanisms, while soft sensors measure the deformation of the actuators with a tiny stiffness change. These advancements in soft actuators and sensors encourage researchers to explore new robotic designs and applications. Additionally, the component technology enables the integration of soft and typical robots into hybrid B Youngsu Cha

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.

Due to their light weight, flexibility, and low energy consumption, ionic electroactive polymers have become a hotspot for bionic soft robotics and are ideal materials for the preparation of soft actuators. Because the traditional ionic electroactive polymers, such as ionic polymer-metal composites (IPMCs), contain water ions, a soft actuator does not work properly upon the evaporation of water ions. An ionic liquid polymer gel is a new type of ionic electroactive polymer that does not contain water ions, and ionic liquids are more thermally and electrochemically stable than water. These liquids, with a low melting point and a high ionic conductivity, can be used in ionic electroactive polymer soft actuators. An ionic liquid gel (ILG), a new type of soft actuator material, was obtained by mixing 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4), hydroxyethyl methacrylate (HEMA), diethoxyacetophenone (DEAP) and ZrO2 and then polymerizing this mixture into a gel state under ultraviolet (UV) light irradiation. An ILG soft actuator was designed, the material preparation principle was expounded, and the design method of the soft robot mechanism was discussed. Based on nonlinear finite element theory, the deformation mechanism of the ILG actuator was deeply analyzed and the deformation of the soft robot when grabbing an object was also analyzed. A soft robot was designed with the soft actuator as the basic module. The experimental results show that the ILG soft robot has good driving performance, and the soft robot can grab a 105 mg object at an input voltage of 3.5 V.

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.

The soft robotics field investigates biological bodies’ movements to develop soft actuation with similar behavior seen in nature. The skeletal muscles’ movements are vastly mimicked in soft actuators; however, the contractions of smooth muscles are overlooked in soft robotics. Smooth muscles provide peristaltic contractions to the digestive system, such as the stomach, and their design challenges the limits of soft robot development that simulates digestive organs. In this article, we present a novel, bellows-driven soft pneumatic actuator (SPA) with a self-sensing capability that generates linear displacement to simulate a smooth muscle segment in a stomach. This SPA is proposed as a solution for applications such as stomach simulators. The SPA concept, design, fabrication, creep model, and experimental validation are presented in this article. A lumped viscoelastic model presents the actuator's modeling to predict the SPA's displacement from a known applied pressure. The model is successfully validated, which resulted in a maximum displacement of 20.0 mm with a displacement error of 3%. A sensory system embedded in the SPA measures the displacement, and it is found that a force of 0–8.0 N is produced. A polynomial equation calibrates the sensor readings, with a displacement error of 5%. The results show the SPA's capability as a linear soft actuator that can generate a deformation like a smooth muscle segment. To simulate a circular contraction from a stomach with the SPA, we install multi-SPAs in a ring frame that performs contractions. Although the proposed SPA is specifically designed to mimic the smooth muscle, the ring actuator's contractions can be used for other applications such as gripping soft objects.

Continued technological progress in robotic systems has led to more applications where robots and humans operate in close proximity and even physical contact in some cases. Soft robots, which are made of highly compliant and deformable materials, provide inherent safety features unlike conventional robots that are made of stiff and rigid components. Soft robotics is a rapidly developing field exploiting biomimetic design principles, novel sensor and actuation concepts, and advanced manufacturing techniques. In this study, we propose novel 3D printable soft vacuum actuators that are inspired by the sporangium of fern trees. These actuators that are directly manufactured using commercial and affordable fused deposition modeling 3D printers offer many advantages such as high actuation speed (5.54 Hz), long lifetime (123,000 cycles), large payload to weight ratio (∼26), and significant output forces (∼16 N). The behavior of these actuators is accurately predicted, and their performance is optimized using finite element modeling. Furthermore, diverse robotic applications such as locomotion robots (a walking robot moving with an average forward speed of vf = 3.54 cm/s, and a hopping robot called Gongaroo hopping with an average speed of vf = 3.75 cm/s), grippers, and artificial muscles have been established and activated using the new soft actuation concept. Finally, to demonstrate the modularity of the proposed actuation concept, soft actuators with multiple degrees of freedom and variable length are established using a series of 3D printable vacuum hinges.

A method for fabricating long, soft, and reversibly actuatable liquid crystal elastomer (LCE) fibers by using direct ink write (DIW) printing was developed. Here, the LCE was produced based on a two-stage thermal-photo curingreaction between a difunctional acrylate monomer and thiol. The LCE ink, mixed with nanoclay to increase the viscosity, was extruded through a nozzle onto a rotating mandrel to obtain a long fiber. After printing, the fiber was first thermally cured on the mandrel, then mechanically stretched, and photocured to achieve liquid crystal chain alignment for stress-free reversible activation. Upon optimizing the ink viscosity and DIW printing parameters, long fibers (up to 1.5 m long from the laboratory) were obtained. The resulting fiber had a modulus of 2 MPa, 51% actuation strain, and a failure strain of well over 100%. The potential of these fibers for applications was demonstrated. The LCE fibers were knit, sewn, and woven to form a variety of smart textiles. The fiber was also used to mimic bicep muscles with both large activation force and activation strain. By incorporating further intelligent characteristics, such as conductivity and biosensing into a single fiber, the LCE fibers could be potentially used for smart clothing, soft robotics, and biomedical devices.