Electroactive polymers
Encyclopedia
Electroactive Polymers, or EAPs, are polymer
s that exhibit a change in size or shape when stimulated by an electric field
. The most common applications of this type of material are in actuator
s and sensor
s. A typical characteristic property of an EAP is that they will undergo a large amount of deformation while sustaining large force
s. The majority of historic actuators are made of ceramic
piezoelectric materials. While these materials are able to withstand large forces, they commonly will only deform a fraction of a percent. In the late 1990s, it has been demonstrated that some EAPs can exhibit up to a 380% strain, which is much more than any ceramic actuator. Another one of the most common applications for EAPs is in the field of robotics
in the development of artificial muscles. Due to this being one of the most common and attractive applications, EAPs are often referred to as artificial muscles. A cartoon drawing of a gripping device using EAPs as "fingers" to grip a ball is depicted in Figure 1.
. The rubber band was fixed at one end and was attached to a mass
at the other. It was then charged and discharged to study the change in length with electrical current. M.P. Sacerdote followed up on Roentgen’s experiment by formulating a theory on strain response to an applied electric field
in 1899. It wasn’t until the year 1925 that the first piezoelectric polymer was discovered (Electret
). Electret was formed by combining carnauba wax
, rosin
and beeswax
, and then cooling the solution while it is subject to an applied DC
electrical bias. The mixture would then solidify into a polymeric material that exhibited a piezoelectric effect.
Polymers that respond to environmental conditions other than an applied electrical current have also been a large part of this area of study. In 1949, Katchalsky et al. demonstrated that when collagen
filaments are dipped in acid
or alkali
solutions they would respond with a change in volume
. The collagen filaments were found to expand in an acidic solution and contract in an alkali
solution. Although other stimuli (such as pH) have been investigated, due to its ease and practicality most research has been devoted to developing polymers that respond to electrical stimuli in order to mimic biological systems.
It wasn’t until the late 1960s when the next major breakthrough in EAPs was observed. In 1969, Kawai was able to demonstrate that polyvinylidene fluoride
(PVDF) exhibits a large piezoelectric effect. This sparked research interest in developing other polymers systems that would show a similar effect. In 1977, the first electrically conducting polymers were discovered by Hideki Shirakawa
et al. Shirakawa along with Alan MacDiarmid
and Alan Heeger demonstrated that polyacetylene
was electrically conductive, and that by doping it with iodine
vapor, they could enhance its conductivity by 8 orders of magnitude. Thus the conductance was close to that of a metal. By the late 1980s a number of other polymers had been shown to exhibit a piezoelectric effect or were demonstrated to be conductive.
In the early 1990s, ionic polymer-metal composite
s were developed and shown to exhibit electroactive properties far superior to previous EAPs. The major advantage of IPMCs was that they were able to show activation (deformation) at voltage
s as low as 1 or 2 volt
s. This is orders of magnitude less than any previous EAP. Not only was the activation energy for these materials much lower, but they could also undergo much larger deformations. IMPCs were shown to exhibit anywhere up to 380% strain, orders of magnitude larger than previously developed EAPs. In 1999, one of the pioneers of the field of robotics
and artificial muscles, Yoseph Bar-Cohen
, proposed the Armwrestling Match of EAP Robotic Arm Against Human
Challenge. This was a challenge in which research groups around the world competed to design a robotic arm consisting of EAP muscles that could defeat a human in an arm wrestling match. The first challenge was held at the Electroactive Polymer Actuators and Devices Conference in 2005. Another major milestone of the field is that the first commercially developed device including EAPs as an artificial muscle was produced in 2002 by Eamex in Japan
. This device was a fish that is able to swim on its own, moving its tail using an EAP muscle
. But the progress in practical development is not satisfactory, probably due to misleading in fundamental research.
are a group of crystalline polar polymers that are also ferroelectric, meaning that they maintain a permanent electric polarization that can be reversed, or switched, in an external electric field
. Ferroelectric polymers, such as polyvinylidene fluoride
(PVDF), are used in acoustic transducers and electromechanical actuators because of their inherent piezoelectric response, and as heat sensors because of their inherent pyroelectric response.
ic groups linked to each other by a flexible spacer. The mesogens within a backbone form the mesophase structure causing the polymer itself to adopt a conformation compatible with the structure of the mesophase. The direct coupling of the liquid crystalline order with the polymer conformation has given main-chain liquid crystalline elastomers a large amount of interest. The synthesis of highly oriented elastomers leads to have a large strain thermal actuation along the polymer chain direction with temperature variation resulting in unique mechanical properties and potential applications as mechanical actuators.
The activation of ionic polymers, on the other hand, requires only 1-2 volts. They however need to maintain wetness, though some polymers have been developed as self contained encapsulated activators which allows their use in dry environments. Ionic polymers also have a low electromechanical coupling. They are however ideal for the bio-mimetic devices.
and damping characteristics are obtained (assuming the polymer is a damped harmonic oscillator). Elastic materials take the mechanical energy of the stress and convert it into potential energy which can later be recovered. An ideal spring will use all the potential energy to regain its original shape (no dampening), while a liquid will use all the potential energy to flow, never returning to its original position or shape (high dampening). A viscoeleastic polymer will exhibit a combination of both types of behavior.
can be measured from the change in amplitude and resolved into dielectric storage and loss components. The electric displacement field can also be measured by following the current. Once the field is removed, the dipoles will relax back into a random orientation.
(MEMS) to produce smart actuators. As the most prospective practical research direction, EAPs have been utilized in artificial muscles. Their ability to emulate the operation of biological muscles with high fracture toughness
, large actuation strain and inherent vibration damping draw the attention of scientists in this field.
In recent years, “electro active polymers for refreshable Braille
displays” has emerged to aid the visually impaired in fast reading and computer assisted communication. This concept is based on using an EAP actuator configured in an array form. Rows of electrode
s on one side of an EAP film and columns on the other activate individual elements in the array. Each element is mounted with a Braille dot and is lowered by applying a voltage across the thickness of the selected element, causing local thickness reduction. Under computer control, dots would be activated to create tactile patterns of highs and lows representing the information to be read.
Small pumps can also be achieved by applying EAP materials. These pumps could be used for drug delivery
, microfluidic devices, active flow control, and a multitude of consumer applications. The most likely configuration for a pump based on actuators would be a dual diaphragm device. The advantages that an ionomeric pump could offer would be low voltage (battery
) operation, extremely low noise signature, high system efficiency, and highly accurate control of flow rate.
Another technology that can benefit from the unique properties of EAP actuators is optical membranes. Due to their low modulus, the mechanical impedance of the actuators, they are well-matched to common optical membrane
materials. Also, a single EAP actuator is capable of generating displacements that range from micrometers to centimeters. For this reason, these materials can be used for static shape correction and jitter suppression. These actuators could also be used to correct for optical aberrations due to atmospheric interference.
Since these materials exhibit excellent electroactive character, EAP materials show potential in biomimetic-robot research, stress sensors and acoustics
field, which will make EAPs become a more attractive study topic in the near future. They have been used for various actuators such as face muscles and arm muscles in humanoid robots.
surface. This will prevent the evaporation
of water contained in the EAP, and also reduce the potential loss of the positive counter ions when the EAP is operating submerged in an aqueous environment. Improved surface conductivity should be explored using methods to produce a defect-free conductive surface. This could possibly be done using metal vapor deposition or other doping methods. It may also be possible to utilize conductive polymers to form a thick conductive layer. Heat resistant EAP would be desirable to allow operation at higher voltages without damaging the internal structure of the EAP due to the generation of heat in the EAP composite. Development of EAPs in different configurations (e.g., fibers and fiber bundles), would also be beneficial, in order to increase the range of possible modes of motion.
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
s that exhibit a change in size or shape when stimulated by an electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
. The most common applications of this type of material are in actuator
Actuator
An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...
s and sensor
Sensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s. A typical characteristic property of an EAP is that they will undergo a large amount of deformation while sustaining large force
Force
In physics, a force is any influence that causes an object to undergo a change in speed, a change in direction, or a change in shape. In other words, a force is that which can cause an object with mass to change its velocity , i.e., to accelerate, or which can cause a flexible object to deform...
s. The majority of historic actuators are made of ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
piezoelectric materials. While these materials are able to withstand large forces, they commonly will only deform a fraction of a percent. In the late 1990s, it has been demonstrated that some EAPs can exhibit up to a 380% strain, which is much more than any ceramic actuator. Another one of the most common applications for EAPs is in the field of robotics
Robotics
Robotics is the branch of technology that deals with the design, construction, operation, structural disposition, manufacture and application of robots...
in the development of artificial muscles. Due to this being one of the most common and attractive applications, EAPs are often referred to as artificial muscles. A cartoon drawing of a gripping device using EAPs as "fingers" to grip a ball is depicted in Figure 1.
History of EAPs
The field of EAPs emerged back in 1880, when Wilhelm Roentgen designed an experiment in which he tested the effect of an electrical current on the mechanical properties of a rubber bandRubber band
A rubber band is a short length of rubber and latex formed in the shape of a loop and is commonly used to hold multiple objects together...
. The rubber band was fixed at one end and was attached to a mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
at the other. It was then charged and discharged to study the change in length with electrical current. M.P. Sacerdote followed up on Roentgen’s experiment by formulating a theory on strain response to an applied electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
in 1899. It wasn’t until the year 1925 that the first piezoelectric polymer was discovered (Electret
Electret
Electret is a dielectric material that has a quasi-permanent electric charge or dipole polarisation. An electret generates internal and external electric fields, and is the electrostatic equivalent of a permanent magnet. Oliver Heaviside coined this term in 1885...
). Electret was formed by combining carnauba wax
Carnauba wax
Carnauba , also called Brazil wax and palm wax, is a wax of the leaves of the palm Copernicia prunifera, a plant native to and grown only in the northeastern Brazilian states of Piauí, Ceará, and Rio Grande do Norte. It is known as "queen of waxes" and usually comes in the form of hard yellow-brown...
, rosin
Rosin
.Rosin, also called colophony or Greek pitch , is a solid form of resin obtained from pines and some other plants, mostly conifers, produced by heating fresh liquid resin to vaporize the volatile liquid terpene components. It is semi-transparent and varies in color from yellow to black...
and beeswax
Beeswax
Beeswax is a natural wax produced in the bee hive of honey bees of the genus Apis. It is mainly esters of fatty acids and various long chain alcohols...
, and then cooling the solution while it is subject to an applied DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
electrical bias. The mixture would then solidify into a polymeric material that exhibited a piezoelectric effect.
Polymers that respond to environmental conditions other than an applied electrical current have also been a large part of this area of study. In 1949, Katchalsky et al. demonstrated that when collagen
Collagen
Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...
filaments are dipped in acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
or alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
solutions they would respond with a change in volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
. The collagen filaments were found to expand in an acidic solution and contract in an alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
solution. Although other stimuli (such as pH) have been investigated, due to its ease and practicality most research has been devoted to developing polymers that respond to electrical stimuli in order to mimic biological systems.
It wasn’t until the late 1960s when the next major breakthrough in EAPs was observed. In 1969, Kawai was able to demonstrate that polyvinylidene fluoride
Polyvinylidene fluoride
Polyvinylidene fluoride, or PVDF is a highly non-reactive and pure thermoplastic fluoropolymer.PVDF is a specialty plastic material in the fluoropolymer family; it is used generally in applications requiring the highest purity, strength, and resistance to solvents, acids, bases and heat and low...
(PVDF) exhibits a large piezoelectric effect. This sparked research interest in developing other polymers systems that would show a similar effect. In 1977, the first electrically conducting polymers were discovered by Hideki Shirakawa
Hideki Shirakawa
Hideki Shirakawa is a Japanese chemist and winner of the 2000 Nobel Prize in Chemistry for his discovery of conductive polymers together with physics professor Alan J. Heeger and chemistry professor Alan G...
et al. Shirakawa along with Alan MacDiarmid
Alan MacDiarmid
Alan Graham MacDiarmid ONZ was a chemist, and one of three recipients of the Nobel Prize for Chemistry in 2000.-Early life:He was born in Masterton, New Zealand as one of five children - three brothers and two sisters...
and Alan Heeger demonstrated that polyacetylene
Polyacetylene
Polyacetylene is an organic polymer with the repeat unit n. The high electrical conductivity discovered for these polymers beginning in the 1960's accelerated interest in the use of organic compounds in microelectronics...
was electrically conductive, and that by doping it with iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
vapor, they could enhance its conductivity by 8 orders of magnitude. Thus the conductance was close to that of a metal. By the late 1980s a number of other polymers had been shown to exhibit a piezoelectric effect or were demonstrated to be conductive.
In the early 1990s, ionic polymer-metal composite
Ionic polymer-metal composite
Ionic polymer–metal composite are synthetic composite materials that display artificial muscle behaviour under an applied voltage. IPMCs are composed of an ionic polymer like Nafion or Flemion whose planar surfaces are coated with conductors such as platinum or gold. Under an applied voltage ,...
s were developed and shown to exhibit electroactive properties far superior to previous EAPs. The major advantage of IPMCs was that they were able to show activation (deformation) at voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
s as low as 1 or 2 volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s. This is orders of magnitude less than any previous EAP. Not only was the activation energy for these materials much lower, but they could also undergo much larger deformations. IMPCs were shown to exhibit anywhere up to 380% strain, orders of magnitude larger than previously developed EAPs. In 1999, one of the pioneers of the field of robotics
Robotics
Robotics is the branch of technology that deals with the design, construction, operation, structural disposition, manufacture and application of robots...
and artificial muscles, Yoseph Bar-Cohen
Yoseph Bar-Cohen
Yoseph Bar-Cohen is a physicist at the Jet Propulsion Laboratory who specializes in electroactive materials and ultrasonic nondestructive evaluation , and is responsible for the Nondestructive Evaluation and Advance Actuators lab at JPL. Dr...
, proposed the Armwrestling Match of EAP Robotic Arm Against Human
Armwrestling Match of EAP Robotic Arm Against Human
The Armwrestling Match of EAP Robotic Arm Against Human is a challenge posed by Yoseph Bar-Cohen of the JPL in 1999. The initial challenge is to create a simple human-like robotic arm which, using electroactive polymers as artificial muscles, can beat a human opponent in an arm wrestling match...
Challenge. This was a challenge in which research groups around the world competed to design a robotic arm consisting of EAP muscles that could defeat a human in an arm wrestling match. The first challenge was held at the Electroactive Polymer Actuators and Devices Conference in 2005. Another major milestone of the field is that the first commercially developed device including EAPs as an artificial muscle was produced in 2002 by Eamex in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
. This device was a fish that is able to swim on its own, moving its tail using an EAP muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
. But the progress in practical development is not satisfactory, probably due to misleading in fundamental research.
Types of Electroactive Polymers
EAP can have several configurations, but are generally divided in two principal classes: Dielectric and Ionic.Dielectric EAPs
- Dielectric EAPsDielectric elastomersDielectric elastomers are smart material systems which produce large strains and belong to the group of electroactive polymers . Based on their simple working principle dielectric elastomer actuators transform electric energy directly into mechanical work. DE are lightweight, have a high elastic...
, are materials in which actuation is caused by electrostatic forces between two electrodes which squeeze the polymer. Dielectric elastomers are capable of very high strains and are fundamentally a capacitor that changes its capacitance when a voltage is applied by allowing the polymer to compress in thickness and expand in area due to the electric field. This type of EAP typically requires a large actuation voltage to produce high electric fields (hundreds to thousands of voltVoltThe volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s), but very low electrical powerPower (physics)In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
consumption. Dielectric EAPs require no power to keep the actuator at a given position. Examples are electrostrictive polymers and dielectric elastomersDielectric elastomersDielectric elastomers are smart material systems which produce large strains and belong to the group of electroactive polymers . Based on their simple working principle dielectric elastomer actuators transform electric energy directly into mechanical work. DE are lightweight, have a high elastic...
.
Ferroelectric Polymers
Ferroelectric PolymersFerroelectric polymers
Ferroelectric Polymersare a group of crystalline polar polymers that are also ferroelectric, meaning that they maintain a permanent electric polarization that can be reversed, or switched, in an external electric field....
are a group of crystalline polar polymers that are also ferroelectric, meaning that they maintain a permanent electric polarization that can be reversed, or switched, in an external electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
. Ferroelectric polymers, such as polyvinylidene fluoride
Polyvinylidene fluoride
Polyvinylidene fluoride, or PVDF is a highly non-reactive and pure thermoplastic fluoropolymer.PVDF is a specialty plastic material in the fluoropolymer family; it is used generally in applications requiring the highest purity, strength, and resistance to solvents, acids, bases and heat and low...
(PVDF), are used in acoustic transducers and electromechanical actuators because of their inherent piezoelectric response, and as heat sensors because of their inherent pyroelectric response.
Electrostrictive Graft Polymers
Electrostricive graft polymers consist of flexible backbone chains with branching side chains. The side chains on neighboring backbone polymers cross link and form crystal units. The backbone and side chain crystal units can then form polarized monomers, which contain atoms with partial charges and generate dipole moments, shown in Figure 2. When an electrical field is applied, a force is applied to each partial charge and causes rotation of the whole polymer unit. This rotation causes electrostrictive strain and deformation of the polymer.Liquid Crystalline Polymers
Main-chain liquid crystalline polymers have mesogenMesogen
Mesogen is the fundamental unit of a liquid crystal that induces structural order in the crystals.Typically, a liquid-crystalline molecule consists of a rigid moiety and one or more flexible parts. The rigid part aligns molecules in one direction, whereas the flexible parts induce fluidity in the...
ic groups linked to each other by a flexible spacer. The mesogens within a backbone form the mesophase structure causing the polymer itself to adopt a conformation compatible with the structure of the mesophase. The direct coupling of the liquid crystalline order with the polymer conformation has given main-chain liquid crystalline elastomers a large amount of interest. The synthesis of highly oriented elastomers leads to have a large strain thermal actuation along the polymer chain direction with temperature variation resulting in unique mechanical properties and potential applications as mechanical actuators.
Ionic EAPs
- IonIonAn ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ic EAPs, in which actuation is caused by the displacement of ions inside the polymer. Only a few volts are needed for actuation, but the ionic flow implies a higher electrical power needed for actuation, and energy is needed to keep the actuator at a given position. Examples of ionic EAPS are conductive polymerConductive polymerConductive polymers or, more precisely, intrinsically conducting polymers are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymers is their processability, mainly by dispersion. Conductive...
s, ionic polymer-metal compositeIonic polymer-metal compositeIonic polymer–metal composite are synthetic composite materials that display artificial muscle behaviour under an applied voltage. IPMCs are composed of an ionic polymer like Nafion or Flemion whose planar surfaces are coated with conductors such as platinum or gold. Under an applied voltage ,...
s (IPMCs), and responsive gels. Yet another example is a Bucky gel actuator, which is a polymer-supported layer of polyelectrolytePolyelectrolytePolyelectrolytes are polymers whose repeating units bear an electrolyte group. These groups will dissociate in aqueous solutions , making the polymers charged. Polyelectrolyte properties are thus similar to both electrolytes and polymers , and are sometimes called polysalts. Like salts, their...
material consisting of an ionic liquidIonic liquidAn ionic liquid is a salt in the liquid state. In some contexts, the term has been restricted to salts whose melting point is below some arbitrary temperature, such as . While ordinary liquids such as water and gasoline are predominantly made of electrically neutral molecules, ILs are largely made...
sandwiched between two electrodeElectrodeAn electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
layers consisting of a gelGelA gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state...
of ionic liquid containing single-wall carbon nanotubeCarbon nanotubeCarbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...
s. As the name implies, these gel actuators can also be made with bucky balls instead of nanotubes.
Electrorheological Fluid
Electrorheological fluids change the viscosity of a solution with the application of an electric field. The fluid is a suspension of polymers in a low dielectric-constant liquid. With the application of a large electric field the viscosity of the suspension increases. Potential applications of these fluids include shock absorbers, engine mounts and acoustic dampers.Ionic polymer-metal composite
Ionic polymer-metal composites consist of a thin ionomeric membrane with noble metal electrodes plated on its surface. It also has cations to balance the charge of the anions fixed to the polymer backbone. They are very active actuators that show very high deformation at low applied voltage and show low impedance. Ionic polymer-metal composites work through electrostatic attraction between the cationic counter ions and the cathode of the applied electric field, a schematic representation is shown in Figure 3. These types of polymers show the greatest promise for bio-mimetic uses as collagen fibers are essentially composed of natural charged ionic polymers. Nafion and Flemion are commonly used ionic polymer metal composites.Comparison of Electronic and Ionic EAPs
Dielectronic polymers are able to hold their induced displacement while activated under a DC voltage. This allows dielectronic polymers to be considered for robotic applications. These types of materials also have high mechanical energy density and can be operated in air without a major decrease in performance. However, dielectronic polymers require very high activation fields (>10 v/um) that are close to the breakdown level.The activation of ionic polymers, on the other hand, requires only 1-2 volts. They however need to maintain wetness, though some polymers have been developed as self contained encapsulated activators which allows their use in dry environments. Ionic polymers also have a low electromechanical coupling. They are however ideal for the bio-mimetic devices.
Characterization
While there are many different ways electroactive polymers can be characterized, only three will be addressed here: stress-strain curve, dynamic mechanical thermal analysis, and dielectric thermal analysis.Stress-Strain Curve
Stress strain curves provide information about the polymer’s mechanical properties such as the brittleness, elasticity and yield strength of the polymer. This is done by providing a force to the polymer at a uniform rate and measuring the deformation that results. An example of this deformation is shown in Figure 4. This technique is useful for determining the type of material (brittle, tough, etc.), but it is a destructive technique as the stress is increased until the polymer fractures.Dynamic mechanical thermal analysis (DMTA)
Both dynamic mechanical analysis is a non destructive technique that is useful in understanding the mechanism of deformation at a molecular level. In DMTA a sinusoidal stress is applied to the polymer, and based on the polymer’s deformation the elastic modulusElastic modulus
An elastic modulus, or modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically when a force is applied to it...
and damping characteristics are obtained (assuming the polymer is a damped harmonic oscillator). Elastic materials take the mechanical energy of the stress and convert it into potential energy which can later be recovered. An ideal spring will use all the potential energy to regain its original shape (no dampening), while a liquid will use all the potential energy to flow, never returning to its original position or shape (high dampening). A viscoeleastic polymer will exhibit a combination of both types of behavior.
Dielectric thermal analysis (DETA)
DETA is similar to DMTA, but instead of an alternating mechanical force an alternating electric field is applied. The applied field can lead to polarization of the sample, and if the polymer contains groups that have permanent dipoles (as in Figure 2), they will align with the electrical field. The permittivityPermittivity
In electromagnetism, absolute permittivity is the measure of the resistance that is encountered when forming an electric field in a medium. In other words, permittivity is a measure of how an electric field affects, and is affected by, a dielectric medium. The permittivity of a medium describes how...
can be measured from the change in amplitude and resolved into dielectric storage and loss components. The electric displacement field can also be measured by following the current. Once the field is removed, the dipoles will relax back into a random orientation.
Applications of EAP
EAP materials can be easily manufactured into various shapes due to the ease in processing many polymeric materials, making them very versatile materials. One potential application for EAPs is that they can potentially be integrated into microelectromechanical systemsMicroelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
(MEMS) to produce smart actuators. As the most prospective practical research direction, EAPs have been utilized in artificial muscles. Their ability to emulate the operation of biological muscles with high fracture toughness
Fracture toughness
In materials science, fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for virtually all design applications. The fracture toughness of a material is determined from the...
, large actuation strain and inherent vibration damping draw the attention of scientists in this field.
In recent years, “electro active polymers for refreshable Braille
Braille
The Braille system is a method that is widely used by blind people to read and write, and was the first digital form of writing.Braille was devised in 1825 by Louis Braille, a blind Frenchman. Each Braille character, or cell, is made up of six dot positions, arranged in a rectangle containing two...
displays” has emerged to aid the visually impaired in fast reading and computer assisted communication. This concept is based on using an EAP actuator configured in an array form. Rows of electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s on one side of an EAP film and columns on the other activate individual elements in the array. Each element is mounted with a Braille dot and is lowered by applying a voltage across the thickness of the selected element, causing local thickness reduction. Under computer control, dots would be activated to create tactile patterns of highs and lows representing the information to be read.
Small pumps can also be achieved by applying EAP materials. These pumps could be used for drug delivery
Drug delivery
Drug delivery is the method or process of administering a pharmaceutical compound to achieve a therapeutic effect in humans or animals. Drug delivery technologies modify drug release profile, absorption, distribution and elimination for the benefit of improving product efficacy and safety, as well...
, microfluidic devices, active flow control, and a multitude of consumer applications. The most likely configuration for a pump based on actuators would be a dual diaphragm device. The advantages that an ionomeric pump could offer would be low voltage (battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
) operation, extremely low noise signature, high system efficiency, and highly accurate control of flow rate.
Another technology that can benefit from the unique properties of EAP actuators is optical membranes. Due to their low modulus, the mechanical impedance of the actuators, they are well-matched to common optical membrane
Membrane (selective barrier)
A membrane is a layer of material which serves as a selective barrier between two phases and remains impermeable to specific particles, molecules, or substances when exposed to the action of a driving force...
materials. Also, a single EAP actuator is capable of generating displacements that range from micrometers to centimeters. For this reason, these materials can be used for static shape correction and jitter suppression. These actuators could also be used to correct for optical aberrations due to atmospheric interference.
Since these materials exhibit excellent electroactive character, EAP materials show potential in biomimetic-robot research, stress sensors and acoustics
Acoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...
field, which will make EAPs become a more attractive study topic in the near future. They have been used for various actuators such as face muscles and arm muscles in humanoid robots.
Future Directions
The field of EAPs is far from mature, which leaves several issues that still need to be worked on. The performance and long-term stability of the EAP should be improved by designing a water impermeableSemipermeable membrane
A semipermeable membrane, also termed a selectively permeable membrane, a partially permeable membrane or a differentially permeable membrane, is a membrane that will allow certain molecules or ions to pass through it by diffusion and occasionally specialized "facilitated diffusion".The rate of...
surface. This will prevent the evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
of water contained in the EAP, and also reduce the potential loss of the positive counter ions when the EAP is operating submerged in an aqueous environment. Improved surface conductivity should be explored using methods to produce a defect-free conductive surface. This could possibly be done using metal vapor deposition or other doping methods. It may also be possible to utilize conductive polymers to form a thick conductive layer. Heat resistant EAP would be desirable to allow operation at higher voltages without damaging the internal structure of the EAP due to the generation of heat in the EAP composite. Development of EAPs in different configurations (e.g., fibers and fiber bundles), would also be beneficial, in order to increase the range of possible modes of motion.
See also
- Armwrestling Match of EAP Robotic Arm Against HumanArmwrestling Match of EAP Robotic Arm Against HumanThe Armwrestling Match of EAP Robotic Arm Against Human is a challenge posed by Yoseph Bar-Cohen of the JPL in 1999. The initial challenge is to create a simple human-like robotic arm which, using electroactive polymers as artificial muscles, can beat a human opponent in an arm wrestling match...
- Pneumatic artificial muscle
External links
- EAP Webhub
- Artificial Muscle, inc. - High Volume Manufacturer - Dielectric Elastomer
- Strategic Polymer Sciences, Inc. - Electroactive Polymer (EAP) - Films - High Energy Density Capacitors - High Strain Actuators
- Article from YCES electrochemistry encyclopedia
- Optotune LLC
- Electroactive Polymers as Artificial Muscles
- Creepy Concept Videos of EAP´s performing
- Danfoss PolyPower
- An SPIE article on EAP science and technology
- Polymers & Liquid Crystals
- Piezotech Piezoelectric & electroactive polymers