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

Metal ion implanted electrodes for dielectric elastomer actuators

This thesis reports on the successful use of low-energy metal ion implantation to fabricate compliant electrodes for miniaturized dielectric elastomer actuators (DEAs, also known as artificial muscles). DEAs are elastomeric actuators capable of large deformations (above 100% depending on conditions) and which require deformable electrodes. On most of the macroscale DEAs, they are made of carbon powder or grease, which can be easily applied on large uniform surfaces (cm2 to m2). This technology is not applicable to small-size DEAs which require reliable electrodes that can be patterned on a mm-to-µm scale. On the other hand, metallic thin-film deposition can be used to make patterned electrodes, but their maximum strain is limited to that of metals, i.e. 2-3%. The Microsystems for Space Technologies Laboratory (LMTS) has introduced implantation of metallic elements into soft polydimethylsiloxane (PDMS) layers by filtered cathodic vacuum arc, as a means of creating compliant electrodes on elastomers. The incoming metallic particles have an energy between 0.05-5 keV, which leads to a spatial distribution of the implanted elements between the surface of the elastomer, and a depth of 50-60 nm. The implanted atoms form nanometer-size clusters which are in contact but can slide relative to each other, hence keeping a conduction path at large strain. Titanium, palladium and gold implantations were conducted in an experimental implanter. Au-implanted electrodes exhibited the best overall performance, combining low sheet resistance (100-200 Ω/square), high maximum strain before loss of conductivity (175%), and a small impact on the Young's modulus of the PDMS on which they are created (50-100% relative increase). Small-size circular diaphragm dielectric elastomer actuators (∅1.5-3 mm) with Au-implanted electrodes were fabricated and characterized. Out of plane displacements up to 25% of the membrane's diameter were observed. This is a factor 4 increase compared to similar devices using patterned Au thin-film as electrode material, thus demonstrating the outstanding properties of metal-ion implanted layers as compliant electrodes for DEAs

3-dimensional electrode patterning within a microfluidic channel using metal ion implantation

The application of electrical fields within a microfluidic channel enables many forms of manipulation necessary for lab-on-a-chip devices. Patterning electrodes inside the microfluidic channel generally requires multi-step optical lithography. Here, we utilize an ion-implantation process to pattern 3D electrodes within a fluidic channel made of polydimethylsiloxane (PDMS). Electrode structuring within the channel is achieved by ion implantation at a 40° angle with a metal shadow mask. The advantages of three-dimensional structuring of electrodes within a fluidic channel over traditional planar electrode designs are discussed. Two possible applications are presented: asymmetric particles can be aligned in any of the three axial dimensions with electro-orientation; colloidal focusing and concentration within a fluidic channel can be achieved through dielectrophoresis. Demonstrations are shown with E. coli, a rod shaped bacteria, and indicate the potential that ion-implanted microfluidic channels have for manipulations in the context of lab-on-a-chip devices

Inkjet printing of carbon black electrodes for dielectric elastomer actuators

Inkjet printing is an appealing technique to print electrodes for Dielectric Elastomer Actuators (DEAs). Here we present the preparation and ink-jet printing of a carbon black electrode mixture and characterise its properties. Carbon black has been used extensively in the past because it is very compliant; however, it has a high resistance and can be very dirty to work with. In this paper we show that carbon black remains an appropriate electrode material, and when inkjet printed can be used to fabricate devices meeting today’s demanding requirements. DEAs are becoming thinner to decrease actuation voltages and are shrinking in size to match the scale of the devices in the biomedical field, tuneable optics, and microfluidics. Inkjet printing addresses both of these problems. Firstly, Inkjet printing is a non-contact technique and can print on very thin freestanding membranes. Secondly, the high precision of inkjet printers makes it possible to print complex electrode geometries in the millimetre scale. We demonstrate the advantages of inkjet printing and carbon black electrodes by conducting a full characterisation of the printed electrodes. The printed carbon black electrodes have resistances as low as 13kΩ/□, an elastic modulus of approximately 1MPa, and a cyclic resistance swing which increases by 7% over 1500 cycles at 50% stretch. We also demonstrate a DEA with printed carbon black electrodes with a diametral stretch of 8.8% at an electric field of approximately 94V/μm. Finally a qualitative test is conducted to show that the printed carbon black electrode is extremely hardwearing

Small, fast, and tough: Shrinking down integrated elastomer transducers

We review recent progress in miniaturized dielectric elastomer actuators (DEAs), sensors, and energy harvesters. We focus primarily on configurations where the large strain, high compliance, stretchability, and high level of integration offered by dielectric elastomer transducers provide significant advantages over other mm or μm-scale transduction technologies. We first present the most active application areas, including: tunable optics, soft robotics, haptics, micro fluidics, biomedical devices, and stretchable sensors. We then discuss the fabrication challenges related to miniaturization, such as thin membrane fabrication, precise patterning of compliant electrodes, and reliable batch fabrication of multilayer devices. We finally address the impact of miniaturization on strain, force, and driving voltage, as well as the important effect of boundary conditions on the performance of mm-scale DEAs

Small, fast and tough: an overview of our silicone-based actuators

At the Microsystems for Space Technologies Laboratory, EPFL, we design and manufacture miniaturised silicone-based dielectric elastomer actuators (DEAs). We love silicone, because even though the achievable strain is generally smaller than what you can get with acrylic elastomers, they lead to fast devices, which are reliable and have a long lifetime. Because silicones can be bought in their uncrosslinked state and are available in a broad range of Shore hardness, they offer unlimited design flexibility, as they can be casted to any desired thickness. Over the years, we have developed a mature DEA fabrication process, including silicone membrane casting on high quality PET foil coated with a sacrificial layer, followed by the release of the membrane in a water bath and its prestretching, as well as electrode application. For small-size devices (electrodes typically smaller than 10mm), it is important to be able to precisely pattern the electrodes on the dielectric membrane. Two methods will be discussed: 1) Gold ion implantation for high-conductivity compliant electrodes that can be patterned down to 100 um with a shadow mask, or down to 1 um with a lift-off process, and 2) Stamped conductive rubber, for electrodes that can be very rapidly and precisely applied on an elastomeric membrane. Finally different applications using this fabrication technology will be presented, such as a rotary motor, a rolling robot, an energy-harvesting device, and tuneable lenses

Towards fast, reliable, and manufacturable DEAs: miniaturized motor and Rupert the rolling robot

Dielectric elastomer transducers (DETs) are known for their large strains, low mass and high compliance, making them very attractive for a broad range of applications, from soft robotics to tuneable optics, or energy harvesting. However, 15 years after the first major paper in the field, commercial applications of the technology are still scarce, owing to high driving voltages, short lifetimes, slow response speed, viscoelastic drift, and no optimal solution for the compliant electrodes. At the EPFL's Microsystems for Space Technologies laboratory, we have been working on the miniaturization and manufacturability of DETs for the past 10 years. In the frame of this talk, we present our fabrication processes for high quality thin-_lm silicone membranes, and for patterning compliant electrodes on the sub mm-scale. We use either implantation of gold nano-clusters through a mask, or pad-printing of conductive rubber to precisely shape the electrodes on the dielectric membrane. Our electrodes are compliant, time stable and present strong adhesion to the membrane. The combination of low mechanical- loss elastomers with robust and precisely-defined electrodes allows for the fabrication of very fast actuators that exhibit a long lifetime. We present different applications of our DET fabrication process, such as a soft tuneable lens with a settling time smaller than 175 microseconds, a motor spinning at 1500 rpm, and a self-commutating rolling robot

Fabrication process of silicone-based dielectric elastomer actuators

This contribution demonstrates the fabrication process of dielectric elastomer transducers (DETs). DETs are stretchable capacitors consisting of an elastomeric dielectric membrane sandwiched between two compliant electrodes. The large actuation strains of these transducers when used as actuators (over 300% area strain) and their soft and compliant nature has been exploited for a wide range of applications, including electrically tuneable optics, haptic feedback devices, wave-energy harvesting, deformable cell-culture devices, compliant grippers, and propulsion of a bio-inspired fish-like airship. In most cases, DETs are made with a commercial proprietary acrylic elastomer and with hand-applied electrodes of carbon powder or carbon grease. This combination leads to non-reproducible and slow actuators exhibiting viscoelastic creep and a short lifetime. We present here a complete process flow for the reproducible fabrication of DETs based on thin elastomeric silicone films, including casting of thin silicone membranes, membrane release and prestretching, patterning of robust compliant electrodes, assembly and testing. The membranes are cast on flexible polyethylene terephthalate (PET) substrates coated with a water-soluble sacrificial layer for ease of release. The electrodes consist of carbon black particles dispersed into a silicone matrix and patterned using a stamping technique, which leads to precisely-defined compliant electrodes that present a high adhesion to the dielectric membrane on which they are applied

Pad printing 1-10 mm thick elastomer membranes for DEAs

We present a technique for stamping patterned silicone elastomer membranes with thicknesses ranging from 1 to 10 um. Silicone elastomers are becoming the material of choice for dielectric elastomer transducers. The variety of readily available materials, their versatility in terms of film thicknesses and their excellent mechanical properties have made them a very appealing alternative to the widely used acrylic elastomer VHB from 3M. Silicone films are typically blade casted or spin coated, two complementary techniques allowing for large-area (> 10 cm x 10 cm) and ultra-thin (< 1 µm) membranes respectively. By comparison, membranes up to 5 cm x 5 cm in area and with thicknesses ranging from 1 to 10 µm can be fabricated with a stamping technique. Unlike blade casting and spin coating this technique can be used to directly pattern (in-plane) the membrane to any desired shape, thus providing great design flexibility. We demonstrated in prior work that stretchable electrodes can also be patterned by stamping. Combined with the ability to pattern silicone membranes, it enables the stamping of functional structures such as dielectric elastomer actuators (DEAs) with high level of integration (vertical integration). In this contribution we detail our fabrication process and highlight the important parameters. As a proof of concept we characterized a stamped DEA, as well as a stamped vertical electrical connection for layers interconnection

Mm-size bistable zipping dielectric elastomer actuators for integrated microfluidics

We report on a new structure of Dielectric Elastomer Actuators (DEAs) called zipping DEAs, which have a set of unique characteristics that are a good match for the requirements of electrically-powered integrated microfluidic pumping and/or valving units as well as Braille displays. The zipping DEAs operate by pulling electrostatically an elastomer membrane in contact with the rigid sidewalls of a sloped chamber. In this work, we report on fully functional mm-size zipping DEAs that demonstrate a complete sealing of the chamber sidewalls and a tunable bistable behavior, and compare the measurements with an analytical model. Compared to our first generation of devices, we are able vary the sidewall angle and benefit therefore from more flexibility to study the requirements to make fully functional actuators. In particular, we show that with Nusil CF19 as membrane material (1.2 MPa Young’s modulus), it is possible to zip completely 2.3 mm diameter chambers with 15° and 21° sidewalls angle equibiaxially prestretched to λ0=1.12 and 15° chambers with λ0=1.27