Electro active polymers (EAP) are actuation materials that are used to drive mechanisms and are fastly replacing conventional methods. Several investigations are in its way to utilize the excellent properties of the polymer. These materials are now applied in various fields including robotics, medicine, defense etc:-and are effective alternatives for conventional sensors and actuators such as motors, gears, piezoelectric crystals, bearings, screws etc: - These are unique materials capable of soft actuation under low applied voltages. They have been called by some researchers ‘artificial muscles’ due to their large strain characteristics and electro-mechanical-chemical muscle- like behavior. They
have been shown to be quite capable of low temperature actuation as well as being quite durable when compared to other actuators in their class. This leads to the belief that there is great potential for use in space applications. EAP’s can change all the paradigm of design and they show great potential as soft robotic actuators, artificial muscles and dynamic sensors in macro-to-micro size range.
2. MICROSTRUCTURE AND COMPOSITION
The bending actuator is composed mainly of perflourinated ion exchange membrane metallic composite backbone called ionic polymer metallic composite or IPMC (0.18µm). IPMC have commercial name Nafion®. The ionomer background or matrix is coated on both sides with metallic electrodes made of noble metals such as Pt, Au or Pt/Au (5-10µm). It is then neutralized with a certain amount of counter-ions such as monovalent cations of alkali metals such as Li+, Na+, K+ and Rb+. A finishing layer of gold is provided to increase surface conductivity. It is then fully solvated. The most common solvent used is water but we can also use organic solvents like Ethylene glycol or Glycerol. An IPMC has to be kept moist continuously for long working (4 months) and it is done by providing a polysilicon coating.
The preparation of ionic polymer metallic composites (IPMC’s) requires involved laboratory work. The current state of the art of IPMC manufacturing technique incorporates two distinct preparation processes:
1. Initial compositing process
2. Surface electroding process
Rectangular silicone wells with size 1cm by 3cm by 2-5mm is used as containers for
IPMC fabrication. It is deposited onto glass microscopic slides. We use a multiple material freeform fabrication system with Cartesian gantry positioning system and multiple changeable deposition tools. A low vapour pressure solvent (N-N- dimethyl formamide,DMF) is added to the ionomer dispersion to help control cracking on drying.
3.1. Initial Compositing process
It is to metallize the inner surface of polymer by a chemical reduction process. Ionic polymer is soaked in salt solution of a complex salt Pt (NH3)4HCl to allow platinum containing cations to diffuse through via the ion exchange process. A proper reducing agent such as LiBH4 or NaBH4 is introduced to metallize the polymer by a chemical reduction means. The metallic platinum particles are not homogeneously formed across the membrane but concentrate predominantly near the interface boundaries. It has been experimentally observed that the platinum particulate layer is buried microns deep (typically 1–20 µm) within the IPMC surface and is highly dispersed.
The primary reaction for platinum composites is:
LiBH4 + 4[Pt (NH3)4]2+ + 8OH- ==> 4Pt + 16NH3 + LiBO2 + 6H2O
3.2. Surface Electroding process
In the subsequent surface electroding process, multiple reducing agents are introduced (under optimized concentrations) to carry out the reducing reaction similar to previous equation in addition to the initial platinum layer formed by the initial compositing process. The roughened surface disappears. Platinum will deposit predominately on top of initial Pt layer. Other metals which are also successfully used include palladium, silver, gold, carbon, graphite etc: - After the upper electrode material is deposited and allowed to air dry, the glass slide is placed in an oven and annealed at 7000C for 45minutes. The silicone well is filled with deionized water for 30 minutes to saturate the IPMC. The device is lifted from the well with tweezers and tested. To maintain the actuation capability of IPMC, the material needs to be kept moist continuously providing the media that is necessary for ion mobility that is responsible for actuation. Without coating material, the IPMC can work in air for less than 5 minutes. So a low modulus poly silicon coating is applied to the surface to trap the solvent inside IPMC.
4. MECHANISM FOR ELECTRICAL ACTUATION
The electrical-chemical-mechanical response of the IPMCs depends on the neutralizing cation, the nature and the degree of solvent saturation, the electrode morphology, and the chemical structure of the ionomer backbone.When a strip of solvated Nafion based IPMC sample is subjected to an electric potential of several volts (1-3 V) across its faces, it bends towards the anode. The speed and magnitude of this actuation towards the anode depends on the type of solvent. The actuation towards the anode is relatively slow with ethylene glycol comparing to water, and it is comparatively much slower with glycerol than with water or ethylene glycol as solvents. For Nafion-based IPMCs with alkali metals as counter ions, the actuation towards the anode is followed by a slow back relaxation towards the cathode .The back relaxation speed also depends on the type of solvent. The duration of the back relaxation phase can vary, from less than about 60 seconds (e.g., with most alkali metals and with water), to about 300 seconds (e.g., in K+-form with ethylene glycol), and to about 2000 seconds (e.g., in Na+-form with glycerol).
The sample eventually reaches an equilibrium state (while the electric potential is still on), which is generally different from its initial equilibrium position
When an external voltage is applied on an IPMC film, it causes bending towards the anode. The IPMC strip bends due to these ion migration induced hydraulic actuation and redistribution. Nafion IPMC has the ability to absorb considerable amount of water, which increases the cations mobility and conductivity. The cations will get hydrated while the anions sulfonate (SO3-) group remains fixed to the polymer matrix. When a voltage (1-3V) is applied the hydrated cations will move towards the cathode side. The swelling or expansion at the cathode side results due to the increase in volume at the cathode side of IPMC, as a result of the transfer of hydrated cations.This swelling is followed by a slow back relaxation towards cathode. This is because that the weak bonds associated with the hydrated cations break after prolonged exposure to the applied electric field causing the inherent ‘relaxation.’ This will cause the re-orientation of the cations in the boundary layer. Finally the EAP will come to an equilibrium position.
5. FACTORS AFFECTING ACTUATION
When a voltage is applied the IPMC’s behavior is clearly a function of several parameters such as:
5.1. Counter ion species
For neutralizing counter ions we have used Li+, Na+, K+ and Rb+. The properties of the bare ionomer as well as that of IPMC change with the cation type for the same membrane. It has been shown that using Li+ as cationic base we can get greater displacement and force density per volt.
The speed and magnitude of the actuation towards the anode depends on the type of solvent. The actuation towards the anode is relatively slow with ethylene glycol comparing to water, and it is comparatively much slower with glycerol than with water or ethylene glycol as solvents.
Its frequency dependence shows that as frequency increases the beam displacement decreases. However, it must be realized that, at low frequencies (0.1–1 Hz), the effective elastic modulus of the IPMC cantilever strip under an imposed voltage is also rather small. On the other hand, at high frequencies (5–20 Hz) such moduli are larger and displacements are smaller. This is due to the fact that at low frequencies water and hydrated ions have time to gush out of the surface electrodes while at high frequencies they are rather contained inside the polymer.
The deflection increases as voltage is increased and reaches saturation as the voltage rises. Under an AC voltage, the film undergoes swinging movement and the displacement level depends not only on the voltage magnitude but also on the frequency. Generally, activation at lower frequencies (down to 0.1 or 0.01 Hz) induces higher displacement and it reaches saturation as the voltage increases.
Recent tests of the performance of the ionomers at low temperatures showed that while the response decreases with temperature, a sizeable displacement was still observed at the temperature of -140oC. This displacement decrease can be compensated by increasing the voltage and it is interesting to point out that, at low temperatures, the response reaches saturation at much higher voltage levels. The deflection level and the consumed power for the ionomer films was tested at both room temperature and -100oC .Tests of the current as a function of voltage response indicate that the resistance rises with the decrease in temperature, which is in contrast to the behavior of metals and conductive materials.
5.6 Platinum penetration and dispersion
Deeper the platinum penetration lower is the surface resistance and greater is the force density. The incipient particles coagulate during the chemical reduction process .One can then realize that there is a significant potential for controlling this process in terms of platinum particle penetration, size and distribution. This could be achieved by introducing effective dispersing agents (additives) during the chemical reduction process. One can anticipate that the additives should enhance dispersal of platinum particles within the ionic polymer and thus reduce coagulation. As a result, a better platinum particle penetration in the polymer with a smaller average particle size and more uniform distribution could be obtained. This uniform distribution makes it more difficult for water to pass through. Thus; the water leakage out of the surface electrode could be significantly reduced.
6. THERMODYNAMIC EFFICIENCY OF IPMC
The bending force of the IPMC is generated by the effective redistribution of hydrated ions and water. The IPMC strip bends due to these ions migration-induced hydraulic actuation and redistribution. The total bending force, Ft, can be approximated as:
where f is the force density per unit length. Assuming a uniformly distributed load over the length of the IPMC, the mechanical power produced by the IPMC can be estimated from:
Pout = Ft v
Notation v is the average tip velocity of the IPMC in action. Finally, the thermodynamic efficiency, Eff,em, can be obtained as:
where Pin is the electrical power input to the IPMC.
7. APPLICATIONS OF EAP
For many years, electroactive polymers (EAP) received relatively little attention due to the small number of available materials and their limited actuation capability. The recent emergence of EAP materials with large displacement response changed the paradigm of these materials and their potential capability. Their main attractive characteristic is their operational similarity to biological muscles, particularly their resilience and ability to induce large actuation strains. Unique robotic components and miniature devices are being explored, where EAPs serve as actuators to enable new capabilities. Now EAP’s are revolutionizing the industrial and bio-medical field.
7.1 Industrial Applications
7.1.1 EAP actuating a dust wiper in NASA mission
Using a bending EAP material, a dust-wiper has been produced for the NASA/NASDA MUSES-CN mission. This dust-wiper is being developed for the infrared camera window of the mission's Nanorover. The MUSES-CN mission is a joint effort between NASA and the Japanese Space Agency, to explore the surface of a small near-Earth asteroid. The team is testing the use of highly effective ion-exchange membrane metallic composites (IPMC) made of perfluorocarboxylate-gold composite with two types of cations, tetra-n-butylammonium and lithium. Under a potential difference of less than 3-V, these IPMC materials are capable of bending beyond a complete circle. The use of highly effective IPMC materials, mechanical modeling, unique components and a protective coating increases the probability of success for the EAP actuated dust-wiper.
7.1.2. Miniature Robotic arm
Another application of EAP actuators is the development of a miniature robotic arm with closed-loop control. A longitudinal EAP is used to lift and drop the arm, whereas a 4-finger gripper is used to grab rocks and other objects. To date, multi-finger grippers that consist of two, four, and eight fingers have been produced. This gripper was driven by a 5 V square wave signal at a frequency of 0.1 Hz and could lift a mass of 10.3 gm. The demonstration of this gripper capability to lift a rock paved the way for a future potential application of the gripper to planetary sample collection tasks.
7.1.3. Robotic swimming fish
A robotic swimming structure can be made by cutting and packaging strips of IPMC’s to the desired size and shape and consequently placing an alternating low voltage (a few volts-peaks per strip) across the muscle assembly. In this manner, robotic swimming fishes and submarine structures containing a sealed signal and power generating module (preferably in the head assembly) can be made to swim at various depths by varying the buoyancy of the structure by conventional means. Remote commands via radio signals can then be sent to modulate propulsion speed and buoyancy. By varying the frequency of the applied voltage, the speed of muscle-bending oscillation of the membranes, and therefore propulsion of the swimming structure can be modulated. A maximum speed of approximately 2 m/min was achieved under an applied voltage of 2 V. This remotely controllable stealthy, noiseless, biomimetic swimming robotic fish made with IPMC’s can be used for several naval applications.
7.1.4 Linear Actuators
Linear actuators can be made of IPMC’s to produce a variety of robotic manipulators including platform type or parallel platform type. Also, multiple degrees of freedom of motion can be obtained by controlling each IPMC with a robotic controller.
7.1.5 Slithering device
Snake-like locomotion can be accomplished by arranging appropriate segments of the IPMC in series and controlling each segment’s bending by applying sequential input power to each segment in a cascade mode
7.1.6 Metering valves
Metering valves can be can be manufactured from IPMC. By applying a calibrated amount of direct voltage/current to the IPMC metering valve attached to any tubes and, consequently, varying the degree of bending displacement of the IPMC, the control of aqueous fluid flow can be attained.
7.1.7 Diaphragm pumps
Bellows pumps can be made by attaching two planar sections of slightly different sizes of IPMC sections and properly placing electrodes on the resulting cavity. This permits modulation of the volume trapped between the IPMC’s. The applied voltage amplitude and frequency can be adjusted to control the flow and volume of fluid being pumped. Single or multiple IPMC’s can function as the diaphragms that creates positive volume displacement. Such a pump produces no noise and has a controllable flow rate in the range of a few micro liters per minute.
7.1.8 Electromechanical relay switches
Non-magnetic, self-contained, electromechanical relay switches can be made from IPMC’s by utilizing their good conductivity and bending characteristics in small applied voltages to close a circuit. In this manner, several of these IPMC actuators can be arranged to make a multipole-multithrow relay switch.
7.1.7 Resonant flying machines
The IPMC can be packed for application as a resonant flying machine. In this configuration, the treated IPMC’s (‘muscles’) can flap like a pair of wings and create a flying machine. ‘Resonant’ means excitation at the resonant frequency of the membrane, which causes the most violent vibration of the membrane. Each body of mass has a resonant frequency at which it will attain its maximum displacement when shaken by some input force or power. If one applies the input voltage signal at or near the resonant frequency of the wing structure, large deformations can be obtained which will vibrate the wing structure in a resonant mode ,similar to a humming bird’s or insect’s wing-flap motion.
7.1 Bio-medical Applications
The softness and flexibility of IPMC’s are definite advantages that can be used in biomedical applications. In this section, we present a number of potential biomedical applications that have been or are currently being developed.
7.2.1 Artificial EAP heart
In the natural heart, the outer container and energy conversion element is, on a macroscopic scale, a single tissue - myocardium - with embedded blood supply and conduction system. The natural heart gets its energy from the working fluid. While the function of the natural heart can be modulated by neurohormonal signals, it is capable of operating and responding on its own. The traditionally designed electromechanical heart passes energy from a storage element (usually a battery) to an electronic controller, which in turn activates an electric motor, solenoid, or similar electrical-to-mechanical element. The motor applies forces to the blood pump through a linkage such as a cam, screw or hydraulic pump. The blood pump itself is passive.
One of our goals in the Electro active Polymers program is to develop pumps in which, as in the natural heart, the container that holds the blood is contractile, thus integrating energy conversion, actuation and structure into one part of the system.
7.2.2 Heart compression device
These devices are made for heart patients with heart abnormalities associated with cardiac muscle functions. In particular ionic polymeric metal composite (IPMC) biomimetic sensors, actuators and artificial muscles integrated as a heart compression device which can be implanted external to the patient’s heart, and partly sutured to the heart without contacting or interfering with the internal blood circulation. Thus, the potential IPMC device thereby can avoid thrombosis and similar complications, which are common to current artificial heart, or heart-assist devices, which may arise when the blood flow makes repeated contacts with non-biological or non-self surfaces. The device is implanted essentially in the ribcage of the patient but is supported on a slender flexible stem that extends to the abdomen.
7.2.3 Surgical tool
The IPMC actuator can be adopted for use as a guide wire or a micro-catheter in biomedical applications for intra-cavity endoscopic surgery and diagnostics. Small internal cavities in the body can be navigated by using small strip or fiber like IPMC actuators.
7.2.4 Peristaltic pumps
Peristaltic pumps can be made from tubular sections of the membrane of IPMC and placement of the electrodes in appropriate locations. Modulating the volume trapped in the tube is possible by applying appropriate input voltage at the proper frequency.
7.2.5 Artificial smooth muscle actuator
Artificial smooth muscle actuators similar to biological smooth muscles can be made by attaching several segments of tubular sections of IPMC and employing a simple control scheme to sequentially activate each segment to produce a traveling wave of volume change in the combined tube sections. This motion can be used to transport material or liquid contained in the tube volume. Artificial veins, arteries, intestines made with the IPMC can be fabricated and packaged in variety of sizes depending on the application. Figure 25 shows an artificial smooth muscle actuator that mimics a human hand. It is made with IPMC.
Another method of using IPMC actuators is to package them as human skeletal joint mobility and power augmentation systems in the form of wearable, electrically self-powered, exoskeletal prostheses, orthoses and integrated muscle fabric system components such as jackets, trousers, gloves and boots. These features are intended to improve the quality of a human system and can be extended to power augmentation of attire for advanced soldier and astronaut systems, and prosthetic devices which would empower paraplegics, quadriplegics and disabled and elderly people, as well as a variety of other robotic and medical applications. The essence of the operation of such prostheses, orthoses and wearable attire (smart muscle fabric) is that, for example, a skeletal joint such as the elbow will be equipped with a flexible strip-like bending muscle made from a family of IPMC’s.
7.2.6 Correction of refractive errors of the human eyes and bionic eyes and vision
Various configurations of IPMC may be used in medical applications involving dynamic or static surgical corrections of the refractive errors of the mammalian eyes. Described here is an apparatus and method to create either an automatic or on-demand correction of refractive errors in the eye by the use of an active and smart (computer-controllable) scleral band equipped with composite IPMC artificial muscles. The scleral band is an encircling band around the middle of the eye’s globe to provide relief of intraretinal tractional forces, in the case of retinal detachment or buckle surgery, by indentation of the sclera as well as repositioning of the retina and choroids. It can also induce myopia, depending on how much tension is placed on the buckle, by increasing the length of the eye globe in the direction of the optical axis and changing the corneal curvature. By using the same kind of encircling scleral band, even in the absence of retinal detachment, one can actively change the axial length of the scleral globe and the corneal curvature in order to induce refractive error correction. Figure 37 depicts the proposed surgical correction of refractive errors by active scleral bands to create bionic eyes. The band has a built-in coil to be energized remotely by magnetic induction and thus provide power for the activation of IPMC muscles. The active composite artificial muscle will deactivate on command, returning the axial length to its original position and vision back to normal.
• Soft and flexible, hence find wide application in bio-medical field
• EAP’s can be mass produced. Hence it results in low cost.
• EAP’s can be easily fabricated in various shapes.
• Inherent vibration damping.
• Lighter compared to other actuators and sensors.
• Response speed is significantly higher.
• Superior fatigue characteristics
• Large actuation strains
• Can withstand extreme conditions esp. up to -1400C.This suits EAP in planetary applications.
• No effective and robust EAP material is currently commercially available.
• Selection of suitable and satisfying materials poses a problem as new and new materials emerge.
• A compromise between stress and strain needed
Smart materials such as EAP’s are the foundation of current state-of-the-art devices to convert energy from chemical or electrical into mechanical energy to perform useful work. In the field of sensing, these devices can provide an efficient way of converting mechanical energy into electrical or chemical forms. This seminar had summarized efforts on a number of potential applications of ionic polymer– metal composite that have proven to be a viable alternative to conventional means.
Electroactive polymers have emerged with great potential enabling the development of unique biomimetic devices. As artificial muscles, EAP actuators are offering capabilities that are currently considered science fiction. Developing such actuators is requiring development on all fronts of the field infrastructure. Enhancement of the performance of EAP will require advancement in related computational chemistry models, comprehensive material science, electro-mechanics analytical tools, and improved material processing techniques.
Making robots that are actuated by EAP, as artificial muscles that are controlled by artificial intelligence would create a new science and technology realities. While such capabilities are expected to significantly change future robots, significant research and development effort is needed to develop robust and effective EAP-based actuators.