Elastomer Tube Research Articles

Self-spinning of liquid crystal elastomer tubes under constant light intensity

Self-oscillating motion have the capacity to autonomously converting ambient power into repetitive motion without requiring an additional control unit, and designing more self-oscillating can broaden their utilization in energy extraction, robotic systems, and sensors. However, cyclic self-oscillating motions often cause structural instability and increase friction. To address these challenges, we creatively developed a zero-energy-mode self-spinning liquid crystal elastomer (LCE) tube-mass system under constant light intensity. By proposing a nonlinear dynamic model and using fourth-order Runge-Kutta method, the computational findings suggest that the LCE tube stays stationary when exposed to vertical light while develops into a zero-energy-mode self-spinning state under non-vertical light. The self-spinning state is self-sustained through harvesting ambient light energy, helping counteract the damping loss. In addition, the self-spinning frequency is controllable by tuning the light angle, contraction coefficient, light intensity, elastic modulus, radius, and damping coefficient. The translational damping has no impact on the self-spinning frequency, and the elastic modulus does not affect the X-axis displacement of the free end. The proposed self-spinning LCE tube system, differing from numerous existing self-oscillating systems, offers advantages like zero-energy-mode motion, structural simplicity, and controllability across multiple parameters, promising expanded design opportunities for applications such as motors, soft robotics, energy collectors, micro-machines, and beyond.

Electrofluids with Tailored Rheoelectrical Properties: Liquid Composites with Tunable Network Structures as Stretchable Conductors.

Flexible and stretchable electronics require both sensing elements and stretching-insensitive electrical connections. Conductive polymer composites and liquid metals are highly deformable but change their conductivity upon elongation and/or contain rare metals. Solid conductive composites are limited in mechanoelectrical properties and are often combined with macroscopic Kirigami structures, but their use is limited by geometrical restraints. Here, we introduce "Electrofluids", concentrated conductive particle suspensions with transient particle contacts that flow under shear that bridge the gap between classic solid composites and liquid metals. We show how Carbon Black (CB) forms large agglomerates when using incompatible solvents that reduce the electrical percolation threshold by 1 order of magnitude compared to more compatible solvents, where CB is well-dispersed. We analyze the correlation between stiffness and electrical conductivity to create a figure of merit of first electrofluids. Sealed elastomeric tubes containing different types of electrofluids were characterized under uniaxial tensile strain, and their electrical resistance was monitored. We found a dependency of the piezoresistivity with the solvent compatibility. Electrofluids enable the rational design of sustainable soft electronics components by simple solvent choice and can be used both as sensor and electrode materials, as we demonstrate.

Energy harvesting with dielectric elastomer tubes: active and (responsive materials-based) passive approaches

Mechanical to electrical energy conversion is a well-established energy transduction approach. However, cases in which a mechanical energy source is not available call for new approaches to harvest electrical energy. In the present study, we demonstrate energy harvesting in soft dielectric elastomer (DE) tubes. Broadly, energy harvesting is obtained through inflation of the tube, electrical charging of the DE layer, and deflation, which results in a decrease in capacitance and an increase in voltage. We propose two methods to mechanically charge (or inflate) the system: (1) active, in which the tube is inflated through the application of mechanical pressure, and (2) passive, in which a passive cylindrical component placed inside the DE tube deforms radially in response to an environmental stimulus such as thermal excitation or water uptake and inflates the DE tube. To demonstrate passive charging, we consider gels as the passive component and employ well-known models with the properties of the commonly employed DE VHB 4910 to simulate the mechanical response of the system and estimate the harvested electrical energy. Our findings reveal that energy-densities in the order of ∼10–50 mJ cm–3 can be harvested. The proposed approach and the inclusion of a passive component to mechanically charge the system opens new opportunities to generate energy in environments lacking traditional mechanical energy sources.

Semi-analytical modeling of electro-strictive behavior in dielectric elastomer tube actuators

Dielectric elastomer tube actuators (DETAs) facilitate versatile soft robotic motions when activated by electric fields. However, optimizing their performance necessitates understanding complex deformation behaviors under different electrical loading patterns. While prior analytical models provide valuable insights, many rely on assumptions like infinite-length and uniform conditions, limiting their ability to capture experimentally-observed nonuniform deformations. This paper presents a semi-analytical approach permitting both radial and longitudinal electrostatic effects by modeling a dielectric tube of effectively infinite-length. It also incorporates the crucial compression-torsion behavior for soft actuator designs. We validate the model against finite element simulations, achieving excellent agreement. Our efficient technique successfully predicts intricate deformation phenomena in DETAs under combined electrical, mechanical, and geometric effects. Results show the model effectively captures axial and twisting deformations, overcoming limitations of linear twist angle assumptions. This analytical framework offers a powerful tool for optimizing next-generation soft actuators across diverse cutting-edge engineering and robotic applications.

How to Easily Make Self-Sensing Pneumatic Inverse Artificial Muscles.

Wearable mechatronics for powered orthoses, exoskeletons and prostheses require improved soft actuation systems acting as 'artificial muscles' that are capable of large strains, high stresses, fast response and self-sensing and that show electrically safe operation, low specific weight and large compliance. Among the diversity of soft actuation technologies under investigation, pneumatic devices have been the focus, during the last couple of decades, of renewed interest as an intrinsically soft artificial muscle technology, due to technological advances stimulated by applications in soft robotics. As of today, quite a few solutions are available to endow a pneumatic soft device with linear actuation and self-sensing ability, while also easily achieving these features with off-the-shelf materials and low-cost fabrication processes. Here, we describe a simple process to make self-sensing pneumatic actuators, which may be used as 'inverse artificial muscles', as, upon pressurisation, they elongate instead of contracting. They are made of an elastomeric tube surrounded by a plastic coil, which constrains radial expansions. As a novelty relative to the state of the art, the self-sensing ability was obtained with a piezoresistive stretch sensor shaped as a conductive elastomeric body along the tube's central axis. Moreover, we detail, also by means of video clips, a step-by-step manufacturing process, which uses off-the-shelf materials and simple procedures, so as to facilitate reproducibility.

Bulging of dielectric elastomer tubes considering residual stress and viscoelasticity

This paper investigates the impacts of residual stress and viscoelasticity on the bifurcation and post-bifurcation behaviors of dielectric elastomer tube actuator (DETA) with finite thickness. A theoretical model with residual stress is developed to analyze the electromechanical (EM) behavior of a DETA under a combined effect of voltage, internal pressure, and axial force loadings. A mathematical derivation of the bifurcation condition for the DETA under EM coupling loadings is presented, and a coexistence phase transition condition for steady bugle propagation is established. The effects of voltage, axial force, and residual stress on the bifurcation and coexistence phase transition conditions are studied. To validate the theoretical predictions, finite element (FE) simulations are performed on a closed-ended DETA subjected to the EM loadings. The results reveal a competitive relationship between the localized bugling and electric breakdown (EB). The large residual stress can inhibit the localized bugling and enhance the stability of the tube. Additionally, we explore the bifurcation behavior of the viscoelastic DETA, revealing that the viscoelastic effect significantly influences the critical pressure of bifurcation and the loading/unloading paths. It is observed that snap-back instability can occur during the unloading process under high loading rates or voltages. This work provides valuable insights into the finite deformation and bifurcation behaviors of the DETA under complex EM coupling loadings.

Soft Self-Healing Robot Driven by New Micro Two-Way Shape Memory Alloy Spring.

Soft robotic bodies are susceptible to mechanical fatigue, punctures, electrical breakdown, and aging, which can result in the degradation of performance or unexpected failure. To overcome these challenges, a soft self-healing robot is created using a thermoplastic methyl thioglycolate-modified styrene-butadiene-styrene (MG-SBS) elastomer tube fabricated by melt-extrusion, to allow the robot to self-heal autonomously at room temperature. After repeated damage and being separated into several parts, the robot is able to heal its stiffness and elongation to break to enable almost complete recovery of robot performance after being allowed to heal at room temperature for 24 h. The self-healing capability of the robot is examinedacross the material scale to robot scale by detailed investigations of the healing process, healing efficiency, mechanical characterization of the robot, and assessment of dynamic performance before and after healing. The self-healing robot is driven by a new micro two-way shape-memory alloy (TWSMA) spring actuator which achieved a crawling speed of 21.6cm/min, equivalent to 1.57 body length per minute. An analytical model of the robot is created to understand the robot dynamics and to act as an efficient tool for self-healing robot design and optimization. This work therefore provides a new methodology to create efficient, robust, and damage-tolerant soft robots.

Snap-through instability in rotating dielectric elastomer tubes

Dielectric elastomers (DEs) have been widely used in soft robotics, high-performance actuators, and bioengineering. However, they may suffer from more failure modes than pure elastic elastomers when undergoing rotational motion. In this paper, we present a theoretical analysis of the finite deformation of rotating DE tubes, addressing the challenges associated with rotational motion in DEs. Our study focuses on the combined influence of a radial electric field and an axial pre-stretch. We numerically calculate three different materials, including neo-Hookean, Gent, and Ogden models, for rotating DE tubes and find that the snap-through instability caused by rotation occurs only in tubes with the Ogden model, resulting in dramatic deformation. Moreover, we study the effects of geometric dimensions and external loads and reveal the competition mechanism between the snap-through instability and electrical breakdown of DE tubes under external voltage. Significantly, our work presents practical thresholds that offer comprehensive guidance for designing and controlling of rotating DE tubes, preventing electrical breakdown during rapid deformation through the adjustment of external loads. This study provides novel insights and practical recommendations for the field of nonlinear deformation in rotating DE tubes.

Jacketed elastomeric tubes for passive self-regulation of pulsatile flow

Regulating pulsatile flow is important to achieve optimal separation and mixing and enhanced heat transfer in microfluidic devices, as well as maintaining homeostasis in biological systems. The human aorta, a composite and layered tube made (among others) of elastin and collagen, is an inspiration for researchers who seek an engineering solution for a self-regulation of pulsatile flow. Here, we present a bio-inspired approach showing that fabric-jacketed elastomeric tubes, manufactured using commercially available silicone rubber and knitted textiles, can be used to regulate pulsatile flow. Our tubes are evaluated via incorporation into a mock-circulatory ‘flow loop’ that replicates the pulsatile fluid flow conditions of an ex-vivo heart perfusion (EVHP) device, a machine used in heart transplants. Pressure waveforms measured near the elastomeric tubing clearly indicated an effective flow regulation. The ‘dynamic stiffening’ behavior of the tubes during deformation is analyzed quantitatively. Broadly, the fabric jackets allow for the tubes to experience greater magnitudes of pressure and distension without risk of asymmetric aneurysm within the expected operating time of an EVHP. Owing to its highly tunable nature, our design may serve as a basis for tubing systems that require passive self-regulation of pulsatile flow.

Measurement and analysis of deformation behavior of thermoplastic elastomer tube subjected to biaxial tensile stress under loading and unloading

Measurement and analysis of deformation behavior of thermoplastic elastomer tube subjected to biaxial tensile stress under loading and unloading

Fiber pumps for wearable fluidic systems

Incorporating pressurized fluidic circuits into textiles can enable muscular support, thermoregulation, and haptic feedback in a convenient wearable form factor. However, conventional rigid pumps, with their associated noise and vibration, are unsuitable for most wearables. We report fluidic pumps in the form of stretchable fibers. This allows pressure sources to be integrated directly into textiles, enabling untethered wearable fluidics. Our pumps consist of continuous helical electrodes embedded within the walls of thin elastomer tubing and generate pressure silently through charge-injection electrohydrodynamics. Each meter of fiber generates 100 kilopascals of pressure, and flow rates approaching 55 milliliters per minute are possible, which is equivalent to a power density of 15 watts per kilogram. The benefits in design freedom are considerable, which we illustrate with demonstrations of wearable haptics, mechanically active fabrics, and thermoregulatory textiles.

Osmotic instability in soft materials under well-controlled triaxial stress

Living tissues and some engineering materials contain water. When a wet material loses water, high triaxial tensile stress may build up and cause instability. The mechanism of instability under triaxial tension has attracted great attention, but quantitative study remains an ongoing challenge. Here we develop an experimental method to apply well-controlled triaxial tensile stress and observe osmotic instability in situ. We synthesize a hydrogel in an elastomer tube with strong adhesion between them. The elastomer dissolves minute amount of water, but allows water to diffuse out and places the hydrogel under homogeneous, equal-triaxial, tensile stress. We develop a method to determine the stress as a function of time. The transparent setup enables observation of various types of osmotic instabilities, including cavity nucleation, crack propagation, and surface undulation. Notably, our method enables the measurement of crack speed from ∼10−5 m/s to a limit comparable to the Rayleigh wave speed ∼1 m/s. We observe a large jump in crack speed at a critical energy release rate. This work opens opportunities to study the physics of soft materials under high triaxial tension.

Nonlinear deformation and instability of a dielectric elastomer tube actuator

The mechanical response and electromechanical instability (EMI) of a dielectric elastomer tube actuator (DETA) under the coupling loadings of voltage, internal pressure and axial force are studied in this paper. Based on the Gent model and considering the nonlinear relationship between dielectric permittivity and stretches, two different loading conditions are discussed: force tuning and voltage tuning. For the force tuning, it is found that the radius of the DETA first decreases, then increases and finally decreases due to the competition between the axial force and voltage as well as the strain-stiffening property of the Gent model under the large deformation. For the voltage tuning, the effects of axial force, internal pressure and electrostrictive coefficient on the electromechanical response are explored. The electrical breakdown (EB) during the nonlinear deformation of DETA is also analyzed. The stress distribution along the thickness of the DETA is given by the analytical solution, and the two failure modes including EMI and loss of tension for the DETA are studied with axial constraints. It is found that the EMI can be eliminated by changing the stretch in the z-direction and internal pressure, which may also lead to the failure of loss of tension. The competitive relations of the two failure modes are discussed in detail. This work can contribute to the design of DETAs with a large actuation.

Electrically-induced twist in geometrically incompatible dielectric elastomer tubes

Dielectric elastomers (DEs) are materials that experience large deformations in response to electric excitation. In this work, we introduce and investigate a DE-based bi-layer tubular design that can be used as an electrically-activated torsional actuator. Specifically, the bi-layer tube is constructed as follows: an isotropic Gent tube is twisted, inserted into another tube, and the two layers are adhered along the interface, resulting in a bi-layer with residual stress. The two layers are referred to as geometrically incompatible. Next, the bi-layer tube is subjected to an electric field and, due to deformation-induced anisotropy, experiences twist. This work begins with a summary of the governing electro-mechanical equations. The overall electrically-induced twist depends on the ratios between the radii, the stiffnesses, the lock-up stretch parameters, and the permittivities of the two layers. Thus, we employ the Gent model and follow by investigating the role of each one of these four key ratios. To illustrate the feasibility of the proposed design, three bi-layer tubes from commonly used DE materials are studied. We show that twists ranging from ∼25deg /mm to ∼130deg/mm can be achieved in response to voltages of ∼6–15 kV. The behaviors that can be obtained with commonly used DEs highlight the merit of the proposed configuration. The findings and insights from this work can be used in a wide range of applications that require a twist motion, including soft rotational robots, artificial muscles, and soft torsional actuators.

Elastomeric tubes with self-regulated distension

SummaryCompliant elastomer tubing with a fabric “jacket” has been essential in various applications as soft robotic actuators, such as in biomedical exomuscles and massage therapy implements. Here, our study shows that a similar design concept can be an effective strategy in realizing passive regulation in the tube’s distension, as well as in preventing aneurysm-like asymmetric rupture of the tube. A custom hydraulic pressure testing rig was built to perform experiments. The jacketed tubes initially deform rapidly as pressure increases, but a self-regulation behavior suppresses the tube’s continued distension by strain-stiffening of the “jacket”. In addition, highly asymmetric distension, common to elastomeric tubes due to imperfection in fabrication, is prevented dramatically by the “jacket”. A three-dimensional finite element model predicts the distension of all tested tubes quantitatively across the entire experimental pressure ranges and beyond. Incorporating custom-designed kirigami relief patterns in the “jackets” expands the potential of the elastomeric tubes.

Embedded Soft Inductive Sensors to Measure Arterial Expansion of Tubular Diameters in Vascular Phantoms

Measuring diameter change in flexible tubular structures embedded in opaque material is challenging. In this article, we present a soft braided coil embedded in an elastomer tube as a method to continuously measure such a change in diameter. By measuring the inductance change in the braided coil, we estimate the instantaneous diameter with a simple inductance model. In applying this method, we demonstrate that diameter waves in a vascular phantom, a model of a radial artery embedded in a viscoelastic wrist structure, can be recorded continuously. Four sensors were made, and their ability to measure physiologically relevant simulated pulse waves was assessed. Several pressure pulse profiles were generated using a precision digital pump. Inductance of the coil was measured simultaneously as the change in diameter was recorded using an optical laser/mirror deflection measurement. One sensor was then embedded in a vascular phantom model of the human wrist. The diameter of the simulated radial artery was recorded via ultrasound and estimated from coil inductance measurements. The diameter estimates from the inductance model corresponded well with the comparator in both experimental setups. We demonstrate that our method is a viable alternative to ultrasound in recording diameter waves in artery models. This opens opportunities in empirical investigations of physiologically interesting fluid-structure interaction. This method can provide new ability to measure diameter changes in tubular systems where access is obstructed.

A buckling-sheet ring oscillator for electronics-free, multimodal locomotion.

Locomotion of soft robots typically relies on control of multiple inflatable actuators by electronic computers and hard valves. Soft pneumatic oscillators can reduce the demand on controllers by generating complex movements required for locomotion from a single, constant input pressure, but either have been constrained to low rates of flow of air or have required complex fabrication processes. Here, we describe a pneumatic oscillator fabricated from flexible, but inextensible, sheets that provides high rates of airflow for practical locomotion by combining three instabilities: out-of-plane buckling of the sheets, kinking of tubing attached to the sheets, and a system-level instability resulting from connection of an odd number of pneumatic inverters made from these sheets in a loop. This device, which we call a "buckling-sheet ring oscillator" (BRO), directly generates movement from its own interaction with its surroundings and consists only of readily available materials assembled in a simple process-specifically, stacking acetate sheets, nylon film, and double-sided tape, and attaching an elastomeric tube. A device incorporating a BRO is capable of both translational and rotational motion over varied terrain (even without a tether) and can climb upward against gravity and downward against the buoyant force encountered under water.

Inflation-extension behaviour of 3D printed elastomer tubes and their constitutive description

It is known that arteries in situ are axially prestretched. This can be proved during an autopsy when a cylindrical segment of an artery is removed from a body. The segment retracts to ex situ length, and the ratio of in situ length to ex situ length defines the amount of prestretching. Ex vivo inflation-extension experiments showed that axial prestretching is advantageous from a mechanical point of view because it minimizes the longitudinal movement of an artery during pressure pulse transmission. Furthermore, calculations suggested that axial prestretching decreases variation of axial stress and makes the radial distension of arteries easier. Recently, it has been shown theoretically that axial prestretching of a nonlinear tube increases the volume, which it is able to accommodate during pressurization. The present study complements previous results with the experimental observation of this fact. Tango Plus tubes, made with the help of 3D printing PolyJet technology, were cyclically pressurized. Axial prestretching was induced by hanging weights on the vertically oriented tubes. The results showed that the internal volume of the tubes increased with the elevated axial prestretch. The recorded mechanical responses were utilized in the regression analysis to identify the material parameters of the tubes. It was found that the hyperelastic Ogden model is suitable for describing the mechanical behaviour of the Tango Plus material. Our study demonstrates that 3D printing can be used to produce tubes, which, for example, may serve as a substitute for biological tissues in laboratory experiments. Simultaneously, this study yields the constitutive parameters of the digital material identified at multiaxial stress conditions. Such parameters are necessary, for instance, in the finite element analyses of parts undergoing loading conditions which result in multiaxial stress states.

Self-powered stretchable strain sensors for motion monitoring and wireless control

Smart skins and smart textiles equipped with strain sensors for motion detection are of prime significance for personalized health monitoring, lifestyle and fitness applications. Yet, the dependence of these devices on wired power supplies and rigid batteries limits their use in everyday settings. Here, we report self-powered and highly elastic strain sensors withstanding stretching to 200% for monitoring the human motion. The sensor is based on a torsional-spring-shaped coil of liquid metal wound around an elastomeric tubing and equipped with a tiny piece of a magnetic ring. The energy is harvested from the body motion relying on the Faraday’s law of electromagnetic induction when the coil is exposed to a time-varying magnetic field of the magnetic ring upon the mechanical deformation of the strain sensor. The max short-circuit current is 2 mA, which is much higher than previous work, and the peak power of our device is 20 µW, sufficiently high to drive conventional low-power electronics. We demonstrate the application potential of our sensor for wearable electronics for monitoring the motion of arms and legs during fitness workout and riding bicycle. The sensor can measure motion of fingers and wrist for health applications and establish wireless control of robotic hands.

A Procedure for the Fatigue Life Prediction of Straight Fibers Pneumatic Muscles

Different from the McKibben pneumatic muscle actuator, the straight fibers one is made of an elastomeric tube closed at the two ends by two heads that ensure a mechanical and pneumatic seal. High stiffness threads are placed longitudinally into the wall of the tube while external rings are placed at some sections of it to limit the radial expansion of the tube. The inner pressure in the tube causes shortening of the actuator. The working mode of the muscle actuator requires a series of critical repeated contractions and extensions that cause it to rupture. The fatigue life duration of a pneumatic muscle is often lower than traditional pneumatic actuators. The paper presents a procedure for the fatigue life prediction of a straight-fibers muscle based on experimental tests directly carried out with the muscles instead of with specimens of the silicone rubber material which the muscle is made of. The proposed procedure was experimentally validated. Although the procedure is based on fatigue life duration data for silicone rubber, it can be extended to all straight-fibers muscles once the fatigue life duration data of any material considered for the muscles is known.