Polymer Artificial Muscles Research Articles

Coil Formation and Biomimetic Performance Characterization of Twisted Coiled Polymer Artificial Muscles

A biological muscle's force is nonlinearly constrained by its current state (force, length, and speed) and state history. To investigate if artificial muscles can mimic (i.e. biomimetic) the complete mechanical state spectrum of biological muscles, this study uses a novel method to characterize twisted coiled polymer actuators (TCPAs) mechanically. Thus, comprehensive and reproducible test procedures are established to verify artificial muscle biomimetics regarding stress, strain, and strain rate combinations intrinsic to biological muscle. A rheometer performs novel high‐precision mechanical characterization methods to comprehensively verify biomimetic performance. Sample twist level, torque, length, force, and temperature are controlled and measured during twist‐induced coiling, heatsetting/annealing, and mechanical testing. TCPAs are formed from linear low‐density polyethylene monofilament. Linear low‐density polyethylene (LLDPE) TCPAs generate larger stresses than biological muscle through the entire spectrum of strains—contracting more than 40%, exerting more than 0.3 MPa at rest length, and withstanding tension of 8 MPa without damage. Thus, the LLDPE TCPAs attain biological muscle performance statically, but additional tests are required to assess this dynamically. The mechanical performance of LLDPE TCPAs enables biomimetic actuation with an intelligent control and measurement system. Their high‐throughput textile manufacturability positions them for advanced biomechatronic applications—including prosthetics and exoskeletons.

A new model of thermo-mechanical actuation for twisted and coiled polymer muscles with initial curvature

The twisted and coiled polymer (TCP) artificial muscle is one type of novel soft actuator for mimicking natural skeletal muscle that can provide large linear and torsional actuation and energy density. Twisting and coiling are the pivotal steps in fabricating TCP muscles. The influence of twisting on the actuation response of TCP muscles has been extensively investigated recently. However, the influence of coiling remains unclear. Based on the finite strain theory, we establish a new thermo-mechanical actuation model for TCP muscles with initial curvature. The theoretical predictions based on the model align well with the finite element simulations, accurately capturing the actuation response of thermally-activated TCP muscles. It is revealed that twisting contributes positively to the actuation, while coiling has a passive effect. Geometrical parameters, such as the helix radius and helix angle, can effectively regulate the actuation performance of TCP muscles. Furthermore, an optimal bias angle is identified that maximizes both the recovery torque and the linear actuation. This study sheds light on the structural optimization design of TCP muscles.

Effect of temperature softening on the actuation performance of twisted and coiled polymer muscles

Twisted and coiled polymer (TCP) artificial muscles represent a promising class of actuators capable of emulating natural skeletal muscle, offering substantial linear actuation and energy density. The actuation characteristics of the thermally-activated TCP muscle and twisted fiber are characterized experimentally, emphasizing the influence of the temperature-softening effect. We find that the recovered torque of twisted fiber increases monotonically with temperature while showing a non-monotonical variation with the fiber bias angle. The variations of actuation strain and end torque with three structural parameters of TCP muscles, i.e., fiber bias angle, helical angle, and spring index, are comprehensively investigated. An optimal bias angle and spring index are present. A significant temperature-softening effect of precursor fiber is observed; a polymer becomes softer and more pliable as the temperature increases. A thermo-mechanical model for predicting the effect of temperature-softening on TCP muscle is established and validated with experimental data. This work facilitates precise control and guides the structural optimization design of the thermally-activated TCP muscle.

Environmentally Driven Intelligent Textiles That Reversibly Actuate to Provide Large Change in Porosity, Loft, Shape, or Their Combination

AbstractTextiles that intelligently respond to changes in the environment by reversibly providing dramatic changes in structure for comfort, safety, or appearance are widely sought. In order to facilitate applications development, here novel intelligent textile structures are described and developed by using polymer artificial muscles and examples of structures and mechanisms are provided that can be used in the future. These actuating textiles, which deploy a host of muscle yarn and fiber types, can change their porosity with or without any change in fabric lateral dimension or can change their loft or can either bend or fold upon temperature change. The presented intelligent textiles provide a porosity increase up to 38.3% from room temperature to 40 °C, while they exhibit a decrease in porosity up to 26.1% from room temperature to −5 °C. Textiles that can bend at an angle of 60° at 40 °C, while exhibiting a complete roll‐up at 70 °C, are also presented. An important way to change the direction of actuation produced by environmental exposure is to transition from homochiral fibers to heterochiral fibers. Other more complicated yarn types can usefully deploy chirality effects, such as yarns that are twisted and coiled, or yarns that are twisted, coiled, and then plied.

Effective On-Line Performance Modulation and Efficient Continuous Preparation of Ultra-Long Twisted and Coiled Polymer Artificial Muscles for Engineering Applications.

Artificial muscle is a kind of thread-like actuator that can produce contractile strain, generate force, and output mechanical work under external stimulations to imitate the functions and achieve the performances of biological muscles. It can be used to actuate various bionic soft robots and has broad application prospects. The electrically controlled twisted and coiled polymer (TCP) artificial muscles, with the advantages of high power density, large stroke and low driving voltage, while also being electrolyte free, are the most practical. However, the relationship between the muscle performances and its preparation parameters is not very clear yet, and the complete procedure of designing and preparing TCP muscles according to actual needs has not been established. Besides, current preparation approaches are very time-consuming and cannot make ultra-long TCP muscles. These problems greatly limit wide applications of TCP artificial muscles. In this study, we studied and built the relationship between the actuating performances of TCP muscles and their preparation parameters, so that suitable TCP muscles can be easily designed and prepared according to actual requirements. Moreover, an efficient preparation method integrating one-step annealing technique has been developed to realize on-line performance modulation and continuous fabrication of ultra-long TCP muscles. By graphically assembling long muscles on heat-resist films, we designed and produced a series of fancy soft robots (butterfly, flower, starfish), which can perform various bionic movements and complete specific tasks. This work has achieved efficient on-demand preparation and large-scale assembly of ultra-long TCP muscles, laying solid foundations for their engineering applications in soft robot field.

A variable stiffness soft robotic manipulator based on antagonistic design of supercoiled polymer artificial muscles and shape memory alloys

This paper introduces a soft robotic manipulator with variable stiffness capability. A novel compact hybrid and antagonistic actuation method is proposed, inspired by the muscle structure of creatures/organs such as octopus and human wrist. The manipulator consists of three pairs of shape memory alloy (SMA) springs–supercoiled polymer (SCP) strings in an antagonistic configuration. Both SMA spring and SCP string are thermally actuated. When the two types of actuators are driven, they are tensioned to increase stiffness, and the variable stiffness feature of the manipulator is further amplified by the antagonistic configuration of the actuators. A kinematic modeling of the manipulator is conducted, extending the classic constant curvature approach to six-actuators case. The effectiveness of this hybrid antagonistic variable stiffness mechanism and in reducing hysteresis are demonstrated experimentally. The performance of the manipulator is further validated by integrating a three-finger gripper for grasping and an endoscope for monitoring at its end, showing the potential of the proposed design for application in minimally invasive surgery (MIS).

Edible, optically modulating, shape memory oleogel composites for sustainable soft robotics

The increasing impact of technology disposal and e-waste on the environment has driven robotics scientists to develop more sustainable robots. Biodegradable and edible shape-memory polymer artificial muscles represent an important robotic component that enables environmentally modulated deformation and movement. The need for finer behavioral control of artificial muscles also requires proprioceptive functionality. Here we report the first edible shape-memory oleogel composites that monolithically couple shape-memory actuation and color-change sensing within one material. The shape recovery and color change capabilities were characterized and two sustainable soft robots, a robotic gripper and a biomimetic flower, were constructed to demonstrate their potential for the development of sustainable and edible robotics.

Design, Implementation, and Observer-based Output Control of a Super-coiled Polymer-Driven Two Degree-of-Freedom Robotic Eye.

The prevalence of ineffective corrective surgeries for ocular motor disorders calls for a robotic eye platform in aiding ophthalmologists to better understand the biomechanisms of human eye movement. This letter presents the first hardware design and implementation of a 2-DOF robotic eye driven by super-coiled polymer (SCP) artificial muscles. While our previous work designed and simulated a deep deterministic policy gradient (DDPG) learning-based controller that requires full-state feedback of the SCP-driven robotic eye, measuring the temperature states of the slender SCPs is generally impractical for the ubiquitously aimed robot. To address this predicament, this letter proposes a reduced-order state observer to estimate the temperature of SCPs given the kinematic measurements. Combining the designed observer and the learning-based controller, the closed-loop output feedback control is implemented on the robotic eye prototype to examine its performance on three classical types of eye movements: visual fixation, saccadic pursuit, and smooth pursuit. The experimental results are presented which successfully validate the observer-based output control of the SCP-driven robotic eye.

HBS-1.2: Lightweight Socially Assistive Robot with 6-Ply Twisted Coiled Polymer Muscle-Actuated Hand

In this paper, a new socially assistive robot (SARs) called HBS-1.2 is presented, which uses 6-ply twisted and coiled polymer (TCP) artificial muscles in its hand to perform physical tasks. The utilization of 6-ply TCP artificial muscles in a humanoid robot hand is a pioneering advancement, offering cost effective, lightweight, and compact solution for SARs. The robot is designed to provide safer human–robot interaction (HRI) while performing physical tasks. The paper explains the procedures for fabrication and testing of the 6-ply TCP artificial muscles, along with improving the actuation response by using a Proportional-Integral-Derivative (PID) control method. Notably, the robot successfully performed a vision-based pick and place experiment, showing its potential for use in homecare and other settings to assist patients who suffer from neurological diseases like Alzheimer’s disease. The study also found an optimal light intensity range between 34 to 108 lumens/m2, which ensures minimal variation in calculated distance with 95% confidence intervals for robust performance from the vison system. The findings of this study have important implications for the development of affordable and accessible robotic systems to support elderly patients with dementia, and future research should focus on further improving the use of TCP actuators in robotics.

Continuous textile manufacturing method for twisted coiled polymer artificial muscles

Twisted, coiled polymer actuators (TCPAs) are a promising type of fiber-based actuators with high energy density, low material costs, and good recyclability; however, current manufacturing methods limit the length and stability of TCPAs, hampering their potential for large-scale textile applications. To overcome this limitation, we propose a textile manufacturing method based on the false-twisting principle, allowing for continuous and rapid production of highly twisted monofilaments. Additionally, this process enables the plying of two or more twisted monofilaments together, as well as the integration of wires for heating and sensing purposes. The resulting twist-stable plies can then be mandrel-coiled and annealed to create a new class of TCPAs with three superimposed levels of helicity, in contrast to the usual two levels. In this study, we investigate the impact of the additional helix level and various factors, including twist density, annealing temperature, cooling speed, and chirality, on the contractility of these TCPAs. Furthermore, due to the twist-stability of the plied yarns, they can be processed on standard textile machines, enabling the manufacture of TCPAs with multiple active yarns that can form contracting artificial muscles using a circular braiding machine. Our key findings reveal that the twisted monofilament coils can contract up to 60%, and higher twist density leads to improved performance for monofilament TCPAs. Notably, this phenomenon is not observed in plied-yarn TCPAs, where varying levels of twist on the monofilament and yarn helix level result in enhanced contractile performance. Overall, this work presents a novel textile manufacturing method for producing twist-stable TCPAs with good contractile performance, providing insights into the design and fabrication of advanced fiber-based actuators for potential applications in large-scale textiles, robotics, and biomedical devices.

Fuel-Driven Redox Reactions in Electrolyte-Free Polymer Actuators for Soft Robotics.

Polymers that undergo shape changes in response to external stimuli can serve as actuators and offer significant potential in a variety of technologies, including biomimetic artificial muscles and soft robotics. Current polymer artificial muscles possess major challenges for various applications as they often require extreme and non-practical actuation conditions. Thus, exploring actuators with new or underutilized stimuli may broaden the application of polymer-based artificial muscles. Here, we introduce an all-solid fuel-powered actuator that contracts and expands when exposed to H2 and O2 via redox reactions. This actuator demonstrates a fully reversible actuation magnitude of up to 3.8% and achieves a work capacity of 120 J/kg. Unlike traditional chemical actuators, our actuator eliminates the need for electrolytes, electrodes, and the application of external voltage. Moreover, it offers athermal actuation by avoiding the drawbacks of thermal actuators. Remarkably, the actuator maintains its actuated position under load when not stimulated, without consuming energy (i.e., catch state). These fuel-powered fiber actuators were embedded in a soft humanoid hand to demonstrate finger-bending motions. In terms of two main actuation metrics, stress-free contraction strain and blocking stress, the presented artificial muscle outperforms reported polymer redox actuators. The fuel-powered actuator developed in this work creates new avenues for the application of redox polymers in soft robotics and artificial muscles.

Characteristic Analysis of Heterochiral TCP Muscle as a Extensile Actuator for Soft Robotics Applications

A soft actuator is an essential component in a soft robot that enables it to perform complex movements by combining different fundamental motion modes. One type of soft actuator that has received significant attention is the twisted and coiled polymer artificial muscle (TCP actuator). Despite many recent advancements in TCP actuator research, its use as an extensile actuator is less common in the literature. This works introduces the concept of using TCP actuators as thermal-driven extensile actuators for robotics applications. The low-profile actuator can be easily fabricated to offer two unique deformation capabilities. Results from the characterization indicate that extensile actuators, made with various rod diameters and under different load conditions, display remarkable elongation deformation. Additionally, a proof-of-concept soft-earthworm robot was developed to showcase the potential application of the extensile actuator and to demonstrate the benefits of combining different types of motion modes.

Coiled Polymer Artificial Muscles Having Dual-Mode Actuation with Large Stress Generation

Coiled polymer artificial muscles with both large tensile stroke and giant force generation are needed for practical applications in robotics, soft exosuits, and prosthesis. However, most polymer yarn artificial muscles cannot generate a large force or stress. Here, we report an inexpensive Twisted and Coiled Polymer artificial muscle (TCP) that performs both large isobaric and isometric contractions. This TCP can generate a tensile stroke of 20.1% and a specific work capacity of up to 1.3 kJ kg−1 during temperature changes from 20 to 180 °C. Moreover, the nylon yarn artificial muscle produced a reversible output stress of 28.4 MPa, which is 100 times larger than human skeletal muscle. A robot arm and a simple gripper were made to demonstrate the isobaric actuation and isometric actuation of our TCP muscle, repectivley. Thus, the polymer artificial muscles with dual-mode actuation show potential applications in the field of robotics, grippers, and exoskeletons and so on.

A Novel Variable Stiffness and Tunable Bending Shape Soft Robotic Finger Based on Thermoresponsive Polymers

In this study, a soft robotic finger with variable stiffness and tunable bending shape capability is proposed. The finger employs shape memory polymers (SMP) as the variable stiffness element and polyimide (PI) electrothermal films as the flexible heaters for segmental heating. SMP and PI electrothermal films constitute the variable stiffness structure. The SMP part exhibits a reversible transition from glass state to rubber state during the heating/cooling process. The whole transition process results in an elastic modulus range of three orders of magnitude. The finger can obtain multiple bending shapes and adapt to the outer contours of diverse objects because of the segmental stiffness modulation capability of the variable stiffness structure. Supercoiled polymer artificial muscles (SCPAMs) are used as actuators for the finger. SMPs and SCPAMs are thermoresponsive and can be activated by thermal heating with electricity, making the whole finger structure compact. Experimental results show that the stiffness of the finger at room temperature (25 °C) is 10.54 times that at 70 °C when the actuators are on. The performance of the finger is further validated via the sequential motion demonstration and grasping tests with variable stiffness and bending shape control capabilities, showing the proposed design’s potential in soft grippers and robotic design.

Design and Application of a Twisted and Coiled Polymer Driven Artificial Musculoskeletal Actuation Module.

Twisted and coiled polymer (TCP) artificial muscles can exhibit unidirectional actuation similar to skeletal muscles. This paper presents a TCP driven artificial musculoskeletal actuation module that can be used in soft robots. This module can contract in the axis direction, and the contraction displacement and force can be controlled easily. The main body of the actuation module consists of TCP muscles and leaf springs, and the deformation of the module is actuated by the TCP muscles. A prototype was made to test the performance of the module. The design and experimental results of the module are presented. The module can provide contraction motion. Results show that the module can provide a contraction force of 0.7 N with displacement of approximately 6.8 mm at 120 °C when exposed to electrical power of 24 V. The proposed artificial musculoskeletal actuation module can potentially be applied in biomimetic robots and the aerospace field.

Multi‐factor influence on the properties of polymer fiber‐based artificial muscles

AbstractThermosensitive polymer artificial muscle fiber filaments made of two polymers, cyclic olefin copolymer elastomer (COCe) and polyethylene (PE), have the ability to sense changes generated by the external environment temperature. To optimize the thermosensitive polymer performance, this work investigates the effects of heating temperature, tensile load, strain rate, and mix proportion applied to COCe‐PE polymers on mechanical properties of the resulting fiber‐based muscle. The fiber filament deformation, stress distribution, and penetration characteristics are compared with multifactor coupling parameters utilizing a computational fluid dynamics simulation model. The simulation results show that although increasing the heating temperature has a small effect on the deformation of the fiber filaments, the interpenetration between the materials is better improved with the increase of temperature. Increasing the tensile load and strain rate can effectively increase the fiber filament deformation and improve the interpenetration between materials. The mix proportion of the two materials has a large effect on the plasticity and brittleness of the materials, and the different mix proportion also have a profound effect on the interpenetration. This study could provide some guidelines for the manufacture of artificial muscle fiber filament.

Material Extrusion Advanced Manufacturing of Helical Artificial Muscles from Shape Memory Polymer

Rehabilitation and mobility assistance using robotic orthosis or exoskeletons have shown potential in aiding those with musculoskeletal disorders. Artificial muscles are the main component used to drive robotics and bio-assistive devices. However, current fabrication methods to produce artificial muscles are technically challenging and laborious for medical staff at clinics and hospitals. This study aims to investigate a printhead system for material extrusion of helical polymer artificial muscles. In the proposed system, an internal fluted mandrel within the printhead and a temperature control module were used simultaneously to solidify and stereotype polymer filaments prior to extrusion from the printhead with a helical shape. Numerical simulation was applied to determine the optimal printhead design, as well as analyze the coupling effects and sensitivity of the printhead geometries on artificial muscle fabrication. Based on the simulation analysis, the printhead system was designed, fabricated, and operated to extrude helical filaments using polylactic acid. The diameter, thickness, and pitch of the extruded filaments were compared to the corresponding geometries of the mandrel to validate the fabrication accuracy. Finally, a printed filament was programmed and actuated to test its functionality as a helical artificial muscle. The proposed printhead system not only allows for the stationary extrusion of helical artificial muscles but is also compatible with commercial 3D printers to freeform print helical artificial muscle groups in the future.

A Proprioceptive Soft Robot Module Based on Supercoiled Polymer Artificial Muscle Strings.

In this paper, a multi-functional soft robot module that can be used to constitute a variety of soft robots is proposed. The body of the soft robot module made of rubber is in the shape of a long strip, with cylindrical chambers at both the top end and bottom end of the module for the function of actuators and sensors. The soft robot module is driven by supercoiled polymer artificial muscle (SCPAM) strings, which are made from conductive nylon sewing threads. Artificial muscle strings are embedded in the chambers of the module to control its deformation. In addition, SCPAM strings are also used for the robot module’s sensing based on the linear relationship between the string’s length and their resistance. The bending deformation of the robot is measured by the continuous change of the sensor’s resistance during the deformation of the module. Prototypes of an inchworm-like crawling robot and a soft robotic gripper are made, whose crawling ability and grasping ability are tested, respectively. We envision that the proposed proprioceptive soft robot module could potentially be used in other robotic applications, such as continuum robotic arm or underwater robot.

Actuation Mechanisms of a Semicrystalline Elastomer-Based Polymer Artificial Muscle with High Actuation Strain

Semicrystalline elastomer-based artificial muscles (sePAMs) are of high actuation strain generally above 30% and are promising for applications of soft robots, flexible electronics, etc. However, a comprehensive and in-depth understanding of the actuation behavior and mechanisms of the sePAMs is lacking. This obstructs the future design and development of the sePAMs. In this paper, we reported a sePAM with a high actuation strain above 40% under 1.0 MPa. Furthermore, we presented an in-depth analysis of the underlying molecular and thermodynamic mechanisms related to this high actuation strain. It is found that both the entropy-elastic actuation and the crystallization-induced elongation (CIE)/melting-induced contraction (MIC) play important roles in controlling the actuation behavior. More importantly, it is revealed that the primary and direct cause for the CIE is the further elongation of the amorphous section in the stretched network during oriented crystallization. The oriented crystallization induces an entropy increase of the amorphous section, causing the stress of the network to be partly relaxed and the network to be further stretched by the external stress. This entropy increase was confirmed by theoretical calculation. The theory also predicts that an oriented crystallization of about 10% can induce a large elongation of the network with a strain increment of about 80%, which agrees well with the experiments reported in this paper as well as in the literature. This work greatly advances our understanding of the actuation behavior and mechanisms of the sePAMs, opening up new strategies for the future design and development of high-performance polymer actuators.

A Twisted and Coiled Polymer Artificial Muscles Driven Soft Crawling Robot Based on Enhanced Antagonistic Configuration

Twisted and coiled polymer (TCP) actuators are becoming increasingly prevalent in soft robotic fields due to their powerful and hysteresis-free stroke, large specific work density, and ease of fabrication. This paper presents a soft crawling robot with spike-inspired robot feet which can deform and crawl like an inchworm. The robot mainly consists of two leaf springs, connection part, robot feet, and two TCP actuators. A system level model of a soft crawling robot is presented for flexible and effective locomotion. Such a model can offer high-efficiency design and flexible locomotion of the crawling robot. Results show that the soft crawling robot can move at a speed of 0.275 mm/s when TCP is powered at 24 V.