Artificial Muscles

Course material for "Artificial Muscles" e-course

Feasibility of integrating multiple types of electroactive polymers to develop a biomimetic inspired muscle actuator

The focus of this project is to see if it is possible to integrate multiple EAP materials in an electro- mechanical system to produce a closer representation of a biological muscle with smooth varying motion. In this preliminary study, two common types of EAPs, ionic and dielectric, were investigated to determine their mechanical and electrical properties in order to assess their potential to be combined into a working artificial electromechanical muscle prototype at a later time. A conceptual design for an artificial electromechanical muscle was created with biomimetic relationships between EAP materials and the human bicep muscle. With the assistance of the Rochester General Hospital, a human arm model, isolating the bicep muscle, was created to calculate mechanical characteristics of the bicep brachii. From the human arm model, bicep muscle characteristics were compared to those of the dielectric EAP because of the ability for the EAP to output relatively high force and strain during actuation. It was found that the current state of the art of EAPs is a long way from making this a reality due to their limiting force output and voltage requirements. The feasibility of developing an artificial electromechanical muscle with EAP actuators is not possible with current technology

Characterization, fabrication, and analysis of soft dielectric elastomer actuators capable of complex 3D deformation

Inspired by nature, the development of soft actuators has drawn large attention to provide higher flexibility and allow adaptation to more complex environment. This thesis is focused on utilizing electroactive polymers as active materials to develop soft planar dielectric elastomer actuators capable of complex 3D deformation. The potential applications of such soft actuators are in flexible robotic arms and grippers, morphing structures and flapping wings for micro aerial vehicles. The embraces design for a freestanding actuator utilizes the constrained deformation imposed by surface stiffeners on an electroactive membrane to avert the requirement of membrane pre-stretch and the supporting frames. The proposed design increases the overall actuator flexibility and degrees-of-freedom. Actuator design, fabrication, and performance are presented for different arrangement of stiffeners. Digital images correlation technique were utilized to evaluate the in-plane finite strain components, in order to elucidate the role of the stiffeners in controlling the three dimensional deformation. It was found that a key controlling factor was the localized deformation near the stiffeners, while the rest of the membrane would follow through. A detailed finite element modeling framework was developed with a user-material subroutine, built into the ABAQUS commercial finite element package. An experimentally calibrated Neo-Hookean based material model that coupled the applied electrical field to the actuator mechanical deformation was employed. The numerical model was used to optimize different geometrical features, electrode layup and stacking sequence of actuators. It was found that by splitting the stiffeners into finer segments, the force-stroke characteristics of actuator were able to be adjusted with stiffener configuration, while keeping the overall bending stiffness. The efficacy of actuators could also be greatly improved by increasing the stiffener periodicity. The developed framework would aid in designing and optimizing the dielectric elastomer actuator configurations for 3D prescribed deformation configuration. Finally, inspired by the membrane textures of bat wings, a study of utilizing fiber reinforcement on dielectric elastomer actuators were conducted for the mechanical and the coupled electromechanical characteristics. Woven fibers were employed on the surface of actuator membrane with different pre-deformed configurations. Experimentally, actuator stiffness changes were measured for up to four orders of magnitude. The orientation of embedded fibers controlled the level and the triggered phase of stiffness changes. A trade-off between the actuator stiffness and stroke could be controlled during the fabrication stage by the fiber orientation and the prestretch level of the base elastomer membrane. A simplified model using small-strain composite laminate theory was developed and accurately predicted the composite actuator stiffness. Additionally, compliant edge stiffeners were found had to present a marked overall effect on actuator electromechanical response. The developed simplified analytical solutions using Timoshenko-bimaterial laminate solution and composite laminate theory, as well as the developed finite element framework can be utilized in addressing more complex 3D deformation patterns and their electromechanical response

Fabrication and characterization of nanometer thin films for low-voltage DEAs

Nanometer-thin films are the essential components of a low-voltage dielectric elastomer actuator (DEA). Comprising of two electrodes sandwiching a dielectric elastomeric material DEAs have evoked versatile materials research. Before choosing the materials used to manufacture low-voltage DEAs one should carefully consider the targeted application. This project aims at finding new techniques to realize nanometer-thin films to obtain low-voltage DEAs with possible future application as artificial muscle to treat urinary incontinence. Therefore, the materials used should be highly biocompatible. Two promising materials are gold and polydimethylsiloxane (PDMS) both show high biocompatibility due to their inherent chemical inertness. Additionally, their physical properties exhibit most of the desired qualities such as for example high electric conductivity for gold and high elasticity for the PDMS. To deposit metal electrodes with nanometer scale thickness, the most frequently used techniques are radio frequency (RF) magnetron sputtering, thermal- or electron beam evaporation, chemical vapor deposition (CVD) methods and electrochemical deposition. Deposition from the liquid/dissolved state by applying a potential between the conducting substrate and a counter electrode, as done in electrochemical techniques, is not applicable for DEA production since the metal has to be deposited on the dielectric elastomer. Regarding the listed deposition methods from the gaseous phase only the physical vapor deposition (PVD) was considered with focus on thermal evaporation and RF magnetron sputtering. The electromechanical properties of simple one layer DEAs with either sputtered or evaporated gold electrodes were investigated taking advantage of the bending of asymmetric planar DEA structures on a flexible substrate. The bending of these cantilever-like structures is induced by applying a voltage. It was found that the actuation at the same voltage was up to 39 % higher for the RF magnetron sputtered actuator compared to the thermally evaporated one. This finding will have a big impact on the stiffness of future multi-stack actuators (cp. Section 2.1). Considering the fabrication of elastomeric nanometer-thin films two methods were established and proven to lead to obtain the targeted nanometer scale in film thickness. Both methods, electro-spray deposition (ESD) and organic molecular beam deposition (OMBD), have advantages and disadvantages regarding the applicable materials, deposition rates, costs and up scaling. In the following sections each method and the corresponding findings will be discussed in more detail. The in house built electro-spray deposition system, which can be coupled to a spectroscopic ellipsometer (SE) to acquire inter alia real time data of film growth, was evaluated as a possible method for the creation of nanometer-thin elastomeric PDMS films. Therefore, the appropriate deposition mode, solvent and pre-polymer had to be identified. Since the aim was to fabricate multi-stack actuators it had to be considered that the conducting substrate needed for direct current (dc) experiments could not be assured throughout the whole manufacturing process. Therefore, it was decided on using the alternating current mode (ac). This mode, according to literature, prevents surface charge accumulation on non-conducting substrate due to neutralization by incoming opposite charged species. As a solvent ethyl acetate was chosen since it dissolves PDMS pre-polymer chains and it is not poisonous to humans. Considering the pre-polymer the commonly used two components PDMS Elastosil 745 A/B was first applied. After deposition and subsequent heat treatment for curing the Elastosil was still a viscous liquid. This finding was attributed to the reduction/oxidation of the Pt catalyst by the applied electric field of ± 5 kV (18 Hz). At this point it was decided to use vinyl-terminated PDMS which has been approved to work with UV curing in OMBD. Having determined the fundamentals to obtain a stable electro-spray and a curing process to manufacture PDMS films, investigations on deposition parameters towards optimization of the resulting films were conducted. First of all the influence of the deposition rate on the resulting film morphology was studied applying in-situ SE, atomic force microscopy (AFM) and interferometry. The results revealed that the surface roughness of the deposited films increases with increasing deposition rate but smoothed to values in the same range by UV irradiation for all deposition rates. The obtained surface roughnesses vary between 0.20 and 0.28 nm determined by atomic force microscopy on areas of 25 μm2 and between 2 and 20 nm on an area of 0.72 mm2 as obtained by interferometry for deposition rates between 0.02 to 5.54 nm/s. With thicknesses in the scale of a few hundreds of nanometer to micrometer these films qualify for use in DEA manufacturing (cp. Section 2.2). In a further study of the electro-spray deposition the focus was put on the film growth mechanism of the deposited droplets/islands. This investigation was based on quasi-dynamic observations of the deposited and subsequently cured PDMS islands. Techniques used to evaluate the film growth ranged from AFM images to select appropriate pre-polymer molecular weight, optical micrograph segmentation to spectroscopic ellipsometry. The most convenient pre-polymer molecular weight, from the four investigated in this study, turned out to be 6,000 g/mol. Furthermore, studies of the deposited and cured islands of this pre-polymer revealed an average height of 30 nm. During the early stages of deposition a 3D growth is observed which eventually, with increasing deposition time, turns into a 2D growth. With a flow rate of 267 nL/s and an average deposition time of 155 s a confluent layer with a thickness of about 91 nm, which still exhibits a rough surface, can be obtained. Prolonging the deposition time will smoothen the surface to a scale of a few nanometers (cp. Section 2.3). OMBD deposition, possible after assembling a small ultra-high vacuum (UHV) chamber, was applied to get a proof of concept for thermal evaporation, deposition and UV curing of PDMS pre-polymer chains. In a later stage a more elaborate UHV chamber was assembled with e.g. a mounted SE to conduct sophisticated investigations. Based on the structure of a standard (DMS-V05) pre-polymer, approved for thermal evaporation, a new pre-polymer was synthesized. The new poly((chloropropyl)methylsiloxane-co-dimethylsiloxane) copolymer showed higher dielectric permittivity and higher break down strength in liquid state. Therefore, a comparison study of film growth with in-situ curing as well as their resulting films after post deposition cure was conducted. The results suggest the use of the new copolymer for low-voltage DEA application since it has enhanced dielectric and elastic properties. Due to the inherent higher polarity a different growth mode during the early deposition stages could be detected by real time SE. The resulting films showed an increased surface roughness by a factor of two but still in the subnanometer scale for an area of 5 μm × 5 μm as determined by AFM (cp. Section 2.4). These results show that a major step towards low-voltage DEA has been accomplished with this work. Salient points The investigation on the impact of compliant metal electrode preparation for DEA application revealed an increase of actuation of 39 % for RF magnetron sputtered as compared to thermally evaporated gold electrodes. Imaging the surface of the deposited electrodes using AFM revealed the origin of this discrepancy of actuation. The micro-structure of the thermally evaporated electrode shows circular cluster formation with heights of about 20 nm whereas the sputtered electrode has a smoother surface with randomly distributed cracks in the range of 100 to 200 nm in width (cp. Section 2.1). Without the technical support of Yves Pellmont for thermal evaporation of gold and the assistance on the AFM of Monika Schönenberger this investigation would not have been possible. Sections 2.2 and 2.3 deals with the ac electro-spray deposition of vinyl terminated PDMS in a 5 vol. % ethyl acetate solution. It was shown that during deposition the surface roughness of the deposited film proportionally depends on the flow rate. During UV curing the films seem to recover and end up in a similar surface roughness for all flow rates. Furthermore, the growth in early stages is rather three dimensional whereas in more advanced deposition times a 2D growth was observed using spectroscopic ellipsometry. Deposited and cured single circular islands show a mean height of about 30 nm observed after 13 s of deposition while a confluent layer with a rather high surface roughness is obtained within an averaged deposition time of 155 s with a mean height of 91 nm. The observed surface roughness is decreasing with successive time evolution after 155 s. Based on Dr. Gabor Kovacs (Empa, Dübendorf) idea electro-spray deposition of PDMS was evaluated. Due to valuable support of Tino Töpper (SE data analysis), Bekim Osmani (AFM morphology determination) and Dr. Hans Deyhle (Segmentations of the optical micrographs) these investigations could be evaluated. After the proof of concept with the assembled small UHV chamber the new sophisticated UHV chamber (assembled by Dr. Vanessa Leung) allowed for the investigation of polymeric materials in UHV conditions. A commercially available pre-polymer (DMS-V05) was compared to a newly synthesized pre-polymer (found and optimized by Dr. Frederikke Bahrt Madsen, DTU) similar in structures to the DMS-V05. Due to the incorporated copolymer an permittivity increase of 33 % at 100 Hz was measured as well as an enhanced dielectric break down strength of 25 %. During early stages of growth two varying mechanisms were observed using real time SE. The presence of the copolymer increased the dipole of the pre-polymer molecule which causes this initial island growth observed compared to a 2D growth of DMS-V05. Nano-indentation experiments after the same UV irradiation time showed Young’s moduli differing in a factor of about two. These enhancements of dielectric and mechanical properties lead to an increase of 4.6 for the according figure of merit (cp. Section 2.4). Again the AFM (Bekim Osmani) and SE (Tino Töpper) measurements and data analysis lead to the evaluation of the collected data from the manufactured samples. Additional support for the dielectric measurements was given by Liyun Yu (DTU)

Dynamic Facial Prosthetics for Sufferers of Facial Paralysis

BackgroundThis paper discusses the various methods and the materialsfor the fabrication of active artificial facial muscles. Theprimary use for these will be the reanimation of paralysedor atrophied muscles in sufferers of non-recoverableunilateral facial paralysis.MethodThe prosthetic solution described in this paper is based onsensing muscle motion of the contralateral healthy musclesand replicating that motion across a patient’s paralysed sideof the face, via solid state and thin film actuators. Thedevelopment of this facial prosthetic device focused onrecreating a varying intensity smile, with emphasis ontiming, displacement and the appearance of the wrinklesand folds that commonly appear around the nose and eyesduring the expression.An animatronic face was constructed with actuations beingmade to a silicone representation musculature, usingmultiple shape-memory alloy cascades. Alongside theartificial muscle physical prototype, a facial expressionrecognition software system was constructed. This formsthe basis of an automated calibration and reconfigurationsystem for the artificial muscles following implantation, soas to suit the implantee’s unique physiognomy.ResultsAn animatronic model face with silicone musculature wasdesigned and built to evaluate the performance of ShapeMemory Alloy artificial muscles, their power controlcircuitry and software control systems. A dual facial motionsensing system was designed to allow real time control overmodel – a piezoresistive flex sensor to measure physicalmotion, and a computer vision system to evaluate real toartificial muscle performance.Analysis of various facial expressions in real subjects wasmade, which give useful data upon which to base thesystems parameter limits.ConclusionThe system performed well, and the various strengths andshortcomings of the materials and methods are reviewedand considered for the next research phase, when newpolymer based artificial muscles are constructed andevaluated.Key WordsArtificial Muscles, facial prosthetics, stroke rehabilitation,facial paralysis, computer vision, automated facialrecognition

A comparative review of artificial muscles for microsystem applications

Artificial muscles are capable of generating actuation in microsystems with outstanding compliance. Recent years have witnessed a growing academic interest in artificial muscles and their application in many areas, such as soft robotics and biomedical devices. This paper aims to provide a comparative review of recent advances in artificial muscle based on various operating mechanisms. The advantages and limitations of each operating mechanism are analyzed and compared. According to the unique application requirements and electrical and mechanical properties of the muscle types, we suggest suitable artificial muscle mechanisms for specific microsystem applications. Finally, we discuss potential strategies for energy delivery, conversion, and storage to promote the energy autonomy of microrobotic systems at a system level

Flexible and stretchable electrodes for dielectric elastomer actuators

Dielectric elastomer actuators (DEAs) are flexible lightweight actuators that can generate strains of over 100%. They are used in applications ranging from haptic feedback (mm-sized devices), to cm-scale soft robots, to meter-long blimps. DEAs consist of an electrode-elastomer-electrode stack, placed on a frame. Applying a voltage between the electrodes electrostatically compresses the elastomer, which deforms in-plane or out-of plane depending on design. Since the electrodes are bonded to the elastomer, they must reliably sustain repeated very large deformations while remaining conductive, and without significantly adding to the stiffness of the soft elastomer. The electrodes are required for electrostatic actuation, but also enable resistive and capacitive sensing of the strain, leading to self-sensing actuators. This review compares the different technologies used to make compliant electrodes for DEAs in terms of: impact on DEA device performance (speed, efficiency, maximum strain), manufacturability, miniaturization, the integration of self-sensing and self-switching, and compatibility with low-voltage operation. While graphite and carbon black have been the most widely used technique in research environments, alternative methods are emerging which combine compliance, conduction at over 100% strain with better conductivity and/or ease of patternability, including microfabrication-based approaches for compliant metal thin-films, metal-polymer nano-composites, nanoparticle implantation, and reel-to-reel production of μm-scale patterned thin films on elastomers. Such electrodes are key to miniaturization, low-voltage operation, and widespread commercialization of DEA

High-Performance Polyvinyl Chloride Gel Artificial Muscle Actuator with Graphene Oxide and Plasticizer

A transparent and electroactive plasticized polyvinyl chloride (PVC) gel was investigated to use as a soft actuator for artificial muscle applications. PVC gels were prepared with varying plasticizer (dibutyl adipate, DBA) content. The prepared PVC gels were characterized using Fourier-transform infrared spectroscopy, thermogravimetric analysis, and dynamic mechanical analysis. The DBA content in the PVC gel was shown to have an inverse relationship with both the storage and loss modulus. The electromechanical performance of PVC gels was demonstrated for both single-layer and stacked multi-layer actuators. When voltage was applied to a single-layer actuator and then increased, the maximum displacement of PVC gels (for PVC/DBA ratios of 1:4, 1:6, and 1:8) was increased from 105.19, 123.67, and 135.55 µm (at 0.5 kV) to 140.93, 157.13, and 172.94 µm (at 1.0 kV) to 145.03, 191.34, and 212.84 µm (at 1.5 kV), respectively. The effects of graphene oxide (GO) addition in the PVC gel were also investigated. The inclusion of GO (0.1 wt.%) provided an approximate 20% enhancement of displacement and 41% increase in force production, and a 36% increase in power output for the PVC/GO gel over traditional plasticizer only PVC gel. The proposed PVC/GO gel actuator may have promising applications in artificial muscle, small mechanical devices, optics, and various opto-electro-mechanical devices due to its low-profile, transparency, and electrical response characteristics

3D printed flexure hinges for soft monolithic prosthetic fingers

Mechanical compliance is one of the primary properties of structures in nature playing a key role in their efficiency. This study investigates a number of commonly used flexure hinges to determine a flexure hinge morphology, which generates large displacements under a lowest possible force input. The aim of this is to design a soft and monolithic robotic finger. Fused deposition modeling, a low-cost 3D printing technique, was used to fabricate the flexure hinges and the soft monolithic robotic fingers. Experimental and finite element analyses suggest that a nonsymmetric elliptical flexure hinge is the most suitable type for use in the soft monolithic robotic finger. Having estimated the effective elastic modulus, flexion of the soft monolithic robotic fingers was simulated and this showed a good correlation with the actual experimental results. The soft monolithic robotic fingers can be employed to handle objects with unknown shapes and are also potential low-cost candidates for establishing soft and one-piece prosthetic hands with light weight. A three-finger gripper has been constructed using the identified flexure hinge to handle objects with irregular shapes such as agricultural products

Smart Devices and Systems for Wearable Applications

Wearable technologies need a smooth and unobtrusive integration of electronics and smart materials into textiles. The integration of sensors, actuators and computing technologies able to sense, react and adapt to external stimuli, is the expression of a new generation of wearable devices. The vision of wearable computing describes a system made by embedded, low power and wireless electronics coupled with smart and reliable sensors - as an integrated part of textile structure or directly in contact with the human body. Therefore, such system must maintain its sensing capabilities under the demand of normal clothing or textile substrate, which can impose severe mechanical deformation to the underlying garment/substrate. The objective of this thesis is to introduce a novel technological contribution for the next generation of wearable devices adopting a multidisciplinary approach in which knowledge of circuit design with Ultra-Wide Band and Bluetooth Low Energy technology, realization of smart piezoresistive / piezocapacitive and electro-active material, electro-mechanical characterization, design of read-out circuits and system integration find a fundamental and necessary synergy. The context and the results presented in this thesis follow an “applications driven” method in terms of wearable technology. A proof of concept has been designed and developed for each addressed issue. The solutions proposed are aimed to demonstrate the integration of a touch/pressure sensor into a fabric for space debris detection (CApture DEorbiting Target project), the effectiveness of the Ultra-Wide Band technology as an ultra-low power data transmission option compared with well known Bluetooth (IR-UWB data transmission project) and to solve issues concerning human proximity estimation (IR-UWB Face-to-Face Interaction and Proximity Sensor), wearable actuator for medical applications (EAPtics project) and aerospace physiology countermeasure (Gravity Loading Countermeasure Skinsuit project)