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Large Strain Electroactive Polymers for Use in Artificial Muscle Applications MEMS 1082 Department of Mechanical Engineering and Materials Science – Electromechanical Sensors and Actuators Fall 2018 Swanson School of Engineering, University of Pittsburgh Large Strain Electroactive Polymers for use in Artificial Muscle Applications Corey May, Wesley Keck, Ryan Black 12/9/2018 Abstract Electroactive polymers (EAPs) are synthetic materials that undergo strain when a voltage or charge is applied, therefore causing deformation. Because of their potential for large strains, EAPs are perfect candidates for applications in both sensors and actuators. Electroactive polymers can undergo deformation when electrical currents are applied because the semi-crystalline structure is polarized causing the positive and negative poles to orient within the material. The deformation and strain within the EAP materials can be determined using the piezoelectric effect and electrostrictive effect equations. The Gibb’s Free Energy equations can be used to determine several transduction effects with EAP materials. The research into EAPs became popular in the early 1990’s, but their use to date has been limited due to power inefficiencies, lack of force production, inability for precision control, and quick degradation of the materials with use. Advances to improve these issues in the last decade have made great strides toward one day being able to use these in many applications. Electroactive polymers have the potential to repair degraded muscles, work as pumps, and give robotics more lifelike movement with better dexterity. MEMS 1082 Department of Mechanical Engineering and Materials Science – Electromechanical Sensors and Actuators Fall 2018 Swanson School of Engineering, University of Pittsburgh Introduction: Humans have always been fascinated with making art inspired by the biological form. One key biological aspect to replicate is the intricacy of human movement. Movement based around the design of pumps, motors, hydraulics, pneumatics, and electrics fall short with micro-movement because of the need for bulky gears, a need for a compressor, and metal casings. None of these power generation methods could capture micro-scale movement seen in the human form effectively. Any device needing intricate movement could benefit from a replacement actuation method. Electroactive Polymers may be a solution. These are polymers that respond to electric fields by contracting or expanding according to an induced polarization. These polymers have extremely high positioning accuracy and self-sensing capability. They require a large actuation voltage, but have the ability to generate large forces and strains. Because of their pliable, light, moldable shape they could be used to create unique intricate strain configurations, like to repair muscle, actuate a robot, or to micro-adjust sensors onboard a drone. One student from Virginia Tech used EAPs to drive an arm-wrestling robot. Currently, these sensors are still in a design stage and are not widely available commercially. While great research has been made in the last 20 years, EAPs are still fragile and prone to fatigue. Definitions Electroactive polymers (EAPs) are synthetic materials that undergo strain when a voltage or charge is applied, therefore causing deformation. Because of their potential for large strains, EAPs are perfect candidates for applications in both sensors and actuators. Sensors are defined as components that have the ability to detect changes (e.g. temperature change, strain, force, pressure, etc.) and communicate that information electronically with a computer, typically through the use of a data acquisition system (DAQ). Conversely, actuators are defined as components which cause mechanical changes, such as an actuator which opens and closes a valve or causes the movement of a diaphragm. Actuators require an electrical signal to operate. History In a world constantly advancing and innovating, electroactive polymers have made massive strides over the years. Today, EAPs are being explored as possible substitutes for muscles tissue, as they are able to expand and contract with varying applied voltages. The study of Electroactive polymers (EAP) gained traction in the early 1990s but the origins of experiments in EAPs date back to 1880 by a scientist named Roentgen. In his experiment, Roentgen applied a voltage to the ends of strips of 16 by 100cm natural MEMS 1082 Department of Mechanical Engineering and Materials Science – Electromechanical Sensors and Actuators Fall 2018 Swanson School of Engineering, University of Pittsburgh rubber and placed weights on the other end to see how the weights would move [1]. As the popularity of these materials started to pick up in the early 1990s, they were not able to produce much strain, they were energy inefficient and the materials broke down rapidly with use. By March of 1999 the first EAP conference was held to help expand the cooperation between organizations working with the materials and to showcase the materials capabilities. At this conference Yoseph Bar-Cohen posed a challenge to the engineering and scientific community to produce an electroactive powered arm that could beat a person at arm wrestling. In 2005, the first arm wrestling competition was held. Three teams faced off with a local high school girl. This challenge proved to be too ambitious for the time when the student easily beat two of the teams and the third broke. The idea has since become a long-term goal for the community. It is believed that once this goal is reached, the technology is at a point it can simulate human functions in robotics. Since no teams were able to provide a design up to the challenge in 1999, a device was created to test how well designs performed against each other. Figure 1: Device used to measure the output force from EAP activated arm wrestling designs. Figure reproduced from World Wide Electroactive Polymer website [2] The first commercial use of EAPs came in late 2002 when a Japanese company by the name of Eamex created robotic fish. The fish are powered by electro-magnetic induction coils in the upper and lower portion of the fish tank [3] General background Types of materials EAPs are generally broken down into two sub categories, electrically stimulated polymers known as dielectric polymers, and ionically stimulated polymers. MEMS 1082 Department of Mechanical Engineering and Materials Science – Electromechanical Sensors and Actuators Fall 2018 Swanson School of Engineering, University of Pittsburgh Within the dielectric polymers there are subclasses including ferro electric polymers, electrostrictive polymers, and liquid crystal polymers. Ferroelectric polymers are materials that, when exposed to a significant electric field, can spontaneously repolarize in the direction of the electric field [4]. Ionic polymer-metal composites (IPMCs) and stimuli responsive gels are two types of ionically stimulated polymers. IPMCs are polymers such as nafion and flemion which are plated in noble metals like gold or platinum. When a voltage is applied the cations and water molecules migrate to one side of the material causing deformation. Stimuli responsive gels Comparing the two types, dielectric polymers can hold their shape when a DC voltage is applied. Ionically activated materials show better strain abilities and require much less power, but they must stay wet in order to work. How EAPs Function: Electroactive polymers contain arranged poles within their structural chains. Natural semicrystaline materials generally have randomlly arranged polar orientation. Through a variety of methods, material scientists have developed methods to arrange the poles in specific orientations. When exposed to an electric field the poles of EAPs rearrange causing a strain in the material. By producing polymer chains in different configurations, and getting better polar organization, newly developed materials are displaying more strain with less applied electric fields. The method to arrange the poles in a way that cause strain varies depending on the class and type of material, but it usually involves applying a large amount of power into the material at a specific stage of production. Polyvinylideneflouride (PVDF) is a common and promising electrically stimulated EAP. It’s a semicrytaline poled ferroelectric polymer. To create the polarized structure in PVDT the materials are placed in a strong electric field. Often, this is done while the material is still in a liquid state, then it is cooled to a solid state before being removed from the electric field. Another method is to use coronal discharges into the material from a needle electrode [4]. MEMS 1082 Department of Mechanical Engineering and Materials Science – Electromechanical Sensors and Actuators Fall 2018 Swanson School of Engineering, University of Pittsburgh Figure 2: Representation of the polymer structure of PVDF when an electric field is applied. Figure reproduced from Dielectric Elastomers as Electromechanical Transducers [5] Theory: Solid State Materials Professor Robert Newnham produced work in solid state field. When a solid-state material responds to electric field it is said to electro-constrict. Any E field will induce polarization in a solid-state material. For instance, plastic is not a piezoelectric but applying a DC voltage we will induce polarization and turn plastic into a piezoelectric. Vibration response during electrostriction is around 1/10 nm, but is dependent on thickness and field. Charges and stresses are generated in specific directions, producing longitudinal and
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