Overtwisting and Coiling Highly Enhances Strain Generation of Twisted String Actuators

Twisted string actuators (TSAs) have exhibited great promise in robotic applications by generating high translational force with low input torque. To further facilitate their robotic applications, it is strongly desirable but challenging to enhance their consistent strain generation while maintaining compliance. Existing studies predominantly considered overtwisting and coiling after the regular twisting stage to be undesirable non-uniform and unpredictable knots, entanglements, and coils formed to create an unstable and failure-prone structure. Overtwisting would work well for TSAs when uniform coils can be consistently formed. In this study, we realize uniform and consistent coil formation in overtwisted TSAs, which greatly increases their strain. Furthermore, we investigate methods for enabling uniform coil formation upon overtwisting the strings in a TSA and present a procedure to systematically "train" the strings. To the authors' best knowledge, this is the first study to experimentally investigate overtwisting for TSAs with different stiffnesses and realize consistent uniform coil formation. Ultra-high molecular-weight polyethylene (UHMWPE) strings form the stiff TSAs whereas compliant TSAs are realized with stretchable and conductive supercoiled polymer (SCP) strings. The strain, force, velocity, and torque of each overtwisted TSA was studied. Overtwisting and coiling resulted in approximately 70% strain in stiff TSAs and approximately 60% strain in compliant TSAs. This is more than twice the strain achieved through regular twisting. Lastly, the overtwisted TSA was successfully demonstrated in a robotic bicep

Compliant, Large-Strain, and Self-Sensing Twisted String Actuators with Applications to Soft Robots

The twisted string actuator (TSA) is a rotary-to-linear transmission system that has been implemented in robots for high force output and efficiency. The basic components of a TSA are a motor, strings, and a load (to keep the strings in tension). The twisting of the strings shortens their length to generate linear contraction. Due to their high force output, energy efficiency, and compact form factor, TSAs hold the potential to improve the performance of soft robots. Currently, it is challenging to realize high-performance soft robots because many existing soft or compliant actuators exhibit limitations such as fabrication complexity, high power consumption, slow actuation, or low force generation. The applications of TSAs in soft robots have hitherto been limited, mainly for two reasons. Firstly, the conventional strings of TSAs are stiff and strong, but not compliant. Secondly, precise control of TSAs predominantly relies on external position or force sensors. For these reasons, TSA-driven robots are often rigid or bulky.To make TSAs more suitable for actuating soft robots, compliant, large-strain, and self-sensing TSAs are developed and applied to various soft robots in this work. The design was realized by replacing conventional inelastic strings with compliant, thermally-activated, and conductive supercoiled polymer (SCP) strings. Self-sensing was realized by correlating the electrical resistance of the strings with their length. Large strains are realized by heating the strings in addition to twisting them. The quasi-static actuation and self-sensing properties are accurately captured by Preisach hysteresis operators. Next, a data-driven mathematical model was proposed and experimentally validated to capture the transient decay, creep, and hysteretic effects in the electrical resistance. This model was then used to predict the length of the TSA, given its resistance. Furthermore, three TSA-driven soft robots were designed and fabricated: a three-fingered gripper, a soft manipulator, and an anthropomorphic gripper. For the three-fingered gripper, its fingers were compliant and designed to exploit the Fin Ray Effect for improved grasping. The soft manipulator was driven by three TSAs that allowed it to bend with arbitrary magnitude and direction. A physics-based modeling strategy was developed to predict this multi-degree-of-freedom motion. The proposed modeling approaches were experimentally verified to be effective. For example, the proposed model predicted bending angle and bending velocity with mean errors of 1.58 degrees (2.63%) and 0.405 degrees/sec (4.31%), respectively. The anthropomorphic gripper contained 11 TSAs; two TSAs were embedded in each of the four fingers and three TSAs were embedded in the thumb. Furthermore, the anthropomorphic gripper achieved tunable stiffness and a wide range of grasps

Design, characterisation and control of TCN artificial muscles

Today there are several muscle weaknesses that hinder individuals from fully using their mobility. In attempts to solve these problems, several artificial muscles, so called actuators, have been invented to complement skeletal muscles. Yet, there is not a single actuator that covers all the characteristics of such muscles. In recent years, a new way of manufacturing actuators has made its way into the field. The actuators are manufactured by twisting and coiling silver coated nylon yarn and activated by sending a voltage through them. This thesis covers research on the design, characterisation and control of Twisted and Coiled Nylon (TCN) actuators. It explains the manufacturing process, including the yarn to use, the number of twists to perform for the thread to coil and how to handle the coiled thread. It also describes how to manufacture a longer actuator. The characterisation and control are studied through testing the actuators with a control program written in MATLAB and comparing their behaviour due to several PID parameters together with a bilinear compensation and displacement reference. The project also includes an introduction to a rigidifiable material where the actuators are applied to change the rigidity of a flexible material. In conclusion, the result of the study of the design, characterisation and control shows that the material used, Shieldex® 235/36 dtex 2-ply HCB, does not reach new heights in the research on TCN actuators due to its force-to-strain ratio being lower than the ratio of previously obtained actuators. The actuators can still be used in the rigidifiable material, which gives them a future chance.Ingeniería Biomédic

Capabilities of Conductive Thread Twisted-and-Coiled Actuators as Sarcomeres

Twisted-and-coiled actuators (TCAs) have shown great potential as an artificial muscle for robotics in terms of material cost and production expenses. However, the power-to-force efficiency of these artificial muscles falls short of biological muscles. Soft robotics takes inspiration from biological organisms for more natural movement, and biological mimicry helps increase the efficiency of robotics. Taking inspiration from how sarcomeres are structured in natural muscles, improvements in the energy efficiency of artificial muscles are possible. In this paper, an experiment was designed to analyze the effects on the efficiency of emulating biological sarcomere structures in artificial muscles. Specifically, this experiment used a load cell to capture and compare data between conventional and biological-emulating applications of TCAs under concentric contraction conditions. The experiment used silver-plated sewing thread to fabricate TCAs. While many works have attempted to increase efficiency by changing the material of the TCA, we show that it is also possible to increase efficiency by changing the structure of the TCAs, and the electrical circuit that connects the TCAs. The resulting TCA was approximately seven times as effective as its unchanged counterpart. Additionally, for the same amount of input power, the changed TCA’s contraction is approximately three times as much force as the unchanged TCA. Optimizing the resulting efficiency of this new TCA requires further study of the thermoelectrical properties of the material used for the TCA. Nevertheless, the increased efficiency of changing the structure of the TCA to mimic biological muscles may be worth a new endeavor

The Future is Now in Twisted Coil Polymer Actuators (TCPA)

This thesis aimed to fabricate and test twisted coiled polymer actuators (TCPA) to understand the mechanical and thermal aspects of this artificial muscle fiber. The purpose of this thesis was to find a linear relationship using the LVDT sensor, fabricating TCPA fibers, and interpreting the data. The project tested whether nylon/polymer could be used as a better artificial muscle fiber. This research accomplished three goals: (1) designing and fabricating a system capable of creating supercoiled muscle fibers consistently, (2) calibrating the Linear Variable Differential Transformer (LVDT) and Core, and (3) analyzing/interpreting the data of the Twisted Coiled Polymer Actuators (TCPA) fibers through the sensors. The proposed methods could be used to control the monofilament\u27s twisting and tension through the four steps, measuring, coiling, annealing, and testing the TCPA fibers. This thesis has built a foundation that can be used to fabricate TCPA fibers from nylon and evaluate their mechanical and thermal behavior while being measured within the LVDT sensor. The results provided a better understanding of the mechanical behavior of the TCPA fibers and provided the foundations to optimize a final building block to understanding this artificial muscle fiber

Plasmid DNA and bacterial artificial chromosomes processing for gene therapy and vaccination: studies on membrane sterile filtration.

Plasmids currently applied in clinical trials are generally 20 kb. The filtration performance was affected by the membrane type used and could be improved by addition of NaCl in the formulation buffer. For filtrations performed at constant pressure, permeate flow decayed with time. As predicted from controlled flux experiments, transmission decreased with increasing molecule size. Initial permeate flux was affected by vector size, DNA concentration and operating pressure. Increase in plasmid size and operating pressure led to reduced membrane capacities. The small scale membrane (filtration area = 1 cm2) capacity was used successfully to predict the performance of a larger scale filtration (filtration area = 4 cm2)

Novel amphiphilic block copolymers and their self-assembled injectable hydrogels for gene delivery

This work describes the development and investigation of a family of novel smart copolymers as non-viral gene delivery vectors. The copolymers have five blocks, and thus named pentablock, with a central block of a hydrophobic polymer, surrounded by two blocks of a hydrophilic polymer, and capped at each terminal end with cationic polymer blocks, arranged in an architecture to provide temperature and pH sensitivity to the copolymers. They are derived from commercially available triblock Pluronic copolymers. The cationic copolymers can efficiently condense negatively charged plasmid DNA in nanostructures with efficient cellular uptake. The amphiphilic nature of copolymers causes them to exist as micelles in aqueous solutions that help them traverse cellular membranes with minimal cell membrane damage. Intra-cellular trafficking of copolymer/DNA complexes revealed that they are up-taken by the cells predominately via endocytosis and are able to deliver the ferried gene into the nuclei. The copolymers efficiently protect the condensed DNA against degradation by nucleases while their protonation capability at low pH assists them in escape from endosomal vesicles into the cytoplasm. The efficiency of the copolymers to deliver condensed DNA into the cells in vitro was comparable to the commercially available polymeric transfection vectors, and they were also found to be significantly less cytotoxic. Adding non-ionic Pluronic copolymers to the formulation of pentablock copolymer/DNA complexes sterically shielded their surface charge and protected them against aggregation with serum proteins. These stabilized formulations were able to retain their ability to transfect cells even in complete growth media supplemented with serum proteins, warranting efficient transfection efficiency in an in vivo application. The amphiphilic nature of copolymers further permits copolymer/DNA complexes to form thermo-reversible hydrogels at physiological temperatures. At concentrations above 15 wt%, copolymer/DNA complexes existed as solutions at room temperature and formed elastic hydrogels at 37°C that dissolved over seven days in excess buffers to release colloidally stable polyplexes. The system thus permits an injectable aqueous pharmaceutical preparation at room temperature that can be injected subcutaneously in tissues/cavities to form a localized depot in situ, which provides a long-term sustained release of therapeutic genes well protected inside the copolymer/DNA complexes

Anthropomorphic Twisted String-Actuated Soft Robotic Gripper with Tendon-Based Stiffening

Realizing high-performance soft robotic grippers is challenging because of the inherent limitations of the soft actuators and artificial muscles that drive them, including low force output, small actuation range, and poor compactness. Despite advances in this area, realizing compact soft grippers with high dexterity and force output is still challenging. This paper explores twisted string actuators (TSAs) to drive a soft robotic gripper. TSAs have been used in numerous robotic applications, but their inclusion in soft robots has been limited. The proposed design of the gripper was inspired by the human hand. Tunable stiffness was implemented in the fingers with antagonistic TSAs. The fingers' bending angles, actuation speed, blocked force output, and stiffness tuning were experimentally characterized. The gripper achieved a score of 6 on the Kapandji test and recreated 31 of the 33 grasps of the Feix GRASP taxonomy. It exhibited a maximum grasping force of 72 N, which was almost 13 times its own weight. A comparison study revealed that the proposed gripper exhibited equivalent or superior performance compared to other similar soft grippers.Comment: 19 pages, 15 figure

A Bioinspired Bidirectional Stiffening Soft Actuator for Multimodal, Compliant, and Robust Grasping

The stiffness modulation mechanism for soft robotics has gained considerable attention to improve deformability, controllability, and stability. However, for the existing stiffness soft actuator, high lateral stiffness and a wide range of bending stiffness are hard to be provided at the same time. This paper presents a bioinspired bidirectional stiffening soft actuator (BISA) combining the air-tendon hybrid actuation (ATA) and a bone-like structure (BLS). The ATA is the main actuation of the BISA, and the bending stiffness can be modulated with a maximum stiffness of about 0.7 N/mm and a maximum magnification of 3 times when the bending angle is 45 deg. Inspired by the morphological structure of the phalanx, the lateral stiffness can be modulated by changing the pulling force of the BLS. The lateral stiffness can be modulated by changing the pulling force to it. The actuator with BLSs can improve the lateral stiffness about 3.9 times compared to the one without BLSs. The maximum lateral stiffness can reach 0.46 N/mm. And the lateral stiffness can be modulated decoupling about 1.3 times (e.g., from 0.35 N/mm to 0.46 when the bending angle is 45 deg). The test results show the influence of the rigid structures on bending is small with about 1.5 mm maximum position errors of the distal point of actuator bending in different pulling forces. The advantages brought by the proposed method enable a soft four-finger gripper to operate in three modes: normal grasping, inverse grasping, and horizontal lifting. The performance of this gripper is further characterized and versatile grasping on various objects is conducted, proving the robust performance and potential application of the proposed design method