Electroactive Polymer


Electroactive Polymers as Artificial Muscles - A Primer
Friday, 04 February 2011 15:56
PDF Print E-mail

1. Overview

Electroactive polymers (EAPs) are touted as the basis for future artificial muscles. EAPs can be deformed repetitively by applying external voltage across the EAP, and they can quickly recover their original configuration upon reversing the polarity of the applied voltage. To explore the potential use of EAPs as artificial muscles, a brief evaluation is presented of an ionic-based EAP composite as a candidate artificial muscle material. The electromechanical properties of the EAP under dry and moist conditions are presented along with the EAPs performance under load conditions. AS shown through a series of simple tests, the EAP has a high load bearing capacity to mass ratio, short response time, and nearly linear deformation response with respect to applied voltage.

In the present demonstration, the electrical and physical properties of a 0.02 cm thick electroactive polymer (EAPs) strips (5 cm x 0.6 cm) were recorded under dry and moist conditions. Different controlled DC voltages were applied across the EAP composite and the current (mA) and voltage (V) were recorded. In addition, the electrical resistance of the EAP samples was recorded along both axis of the EAP. The extent of deformation of the EAP and deformation response time were measured, for a given specimen length and width, from a vertical line in degrees. These measurements for both positive and negative polarity of the applied DC voltage in order to determine if there are any physical memory effects. In order to assess the strength the EAP, the lift capability to EAP mass ratio, were evaluated at different electrical field strengths. Resistance measurements were also conducted to evaluate the rate of conductance loss upon the evaporation of water from the EAP. Finally, muscle-like behavior is illustrated through a number of simple demonstrative experiments.


2. EAP Performance Testing

2.1 Test System

2.2 Setup for Simple Deflection Tests

The EPA strip (4"x0.6") is held between two small flat copper electrodes)

2.3 EAP Strength: Simple Lift

A small weight is placed on the EPA (at the trailing end)

2.4 Forceps Simulation

A forceps can be constructed by fixing a rigid plastic strip of equal length to that of the EAP to one side of the dual electrode clamp. The EAP is then placed between the electrodes. Upon the application of DC voltage across the EAP strip, it will bend and allow the capture of objects using this EAP forceps.

Illustration of an EAP-powered forceps. (a) forceps open; (b) forceps closes upon polarity reversal; (c) and (d) lift action.

3. Discussion

3.1 EAP Deflection

The EAP can be easily deformed upon the application of low voltage (approximately 1-3.5 V). Deflection varies linearly at low applied voltages (<1 V) with nonlinear behavior observed at higher voltages. At the linear range the EAP deforms at a rate of about 20-35 degrees/volt. The magnitude of deflection of the EAP strip (measured in degrees of deflection) is similar in both directions (upon reversing the polarity of the electrical field). This suggests that the EAP surfaces have similar conductivity and that the EAP composition is reasonably uniform. However, the EAP strip can at times deflect significantly more in one direction and the change in deflection variation with voltage is non-linear. In the above cases resistance measurements can be used to verify if the less conductive side of the EAP is contact with the negative electrode which would result in the observed reduction in bending. Such a behavior is believed to be due to either loss of positive counter ions in the matrix (due to repeated soaking of the EAP in water) or imperfections in the EAP conductive surface. For the specific EAP tested in this illustration the change in deflection with applied voltage was greater above about 2.5 V. In other words, at higher voltages the EAP the applied voltage causes a greater deflection per volt than at low voltages.

The EAP performs well when immersed in water. The deflection is somewhat less than in air given the additional work that the EAP strip has to perform in order to displace water as it deforms. The deflection of the EAP in water is more consistent and the electromechanical response does not change significantly over 20-30 minutes.  In contrast, the performance of the EAP in air declines over time, requiring re-wetting of the EAP after a period of about 3-5 minutes.

3.2 EAP Response Time

EAP response time can be measured to as the time it takes an EAP strip to deform to its final equilibrium position under different applied voltages. For example, the response time (for a 5"x0.6" EAP strip) was determine to increase with increasing voltage at a rate of about 5.2 seconds/volt. However, the rate at which the response time increases decreases as the voltage increases. The speed (or rate) of deformation can also be evaluated in terms of degrees of deflection/second. FOr the above EPA strip the deformation speed increased with the applied voltage up to about 7 degrees of deflection/sec; however, the increase in deflection speed was progressively less as the applied voltage increased above about 1.5 V. The EAP deformation is governed by attraction of the positive counter ions to the negative electrode (Cathode). This attractive force increases with increasing applied voltage. As a result, the the EAP strip bends at a faster rate as the applied voltage is increased.

3.3 EAP Electrical Resistance

The decrease in electromechanical response of the EAP, when operating in air, is attributed to evaporation of water from the EAP strip. Therefore, it is expected that the resistance of the electrical resistance of the EAP would increase with time (after re-wetting and then exposure to air). THis can be confirmed by measuring the electrical resistance of the EAP strip in the middle and near the edges of the EPA strip. The resistance of the EAP will increases with time as the water in the strip is squeezed away from the the region held by the electrodes. Subsequent water evaporation from the electrode area, already depleted of water, eventually results in a jump in the resistance likely due to loss of mobile water. As a result the mobility of counter ions inside the strip ,near the electrodes, is virtually eliminated. As time progresses, water which was squeezed away from the compressed electrode region diffuses back to that region thereby allowing for some restoration of counter ion mobility as suggested by the slight decline of the resistance after the jump.

3.4 EAP Lift & Strength

The EAP strength is an important parameter robotics, artificial muscles and actuator applications. The maximum lifting ability of the EAP strip typically shows a linear increase of the maximum weight lifted with the applied voltage, at a rate of about 1.2 g/V for the strip tested in the present study. The force output of the EAP, defined here as the ratio of the maximum weight lifted by the EAP relative to its own weight, also increases linearly with the applied voltage. The rate at which the force output increases is nearly constant at 20 (g lifted/g EAP)/V. When the EAP strip length is halved (wing configuration), the maximum weight lifted, at a given voltage, should be similar to that obtained by the longer strip; however, the force output to voltage ratio would be doubled. The above behavior indicates that the flexural strength of a shorter strip is greater, although the extent of deflection is smaller.

3.5 EAP Deformation Mechanism

A definitive theory to explain the mechanism of EAP deformation is yet to emerge. However, based on the composition of the EAP, its performance when subjected to an electrical field, and the requirement for the presence of positive counter ions and water for its operation suggest a possible operational mechanism. Upon the application of an electrical field across a moist EAP, which is held between metal electrodes attached across a partial section of an EAP strip, bending of the EAP is induced. Positive counter ions move towards the negative electrode (cathode), while negative ions that are fixed (or immobile) to the polymer (e.g. SO3) experience an attractive force from the positive electrode (anode). At the same time, water molecules in the EAP matrix diffuse towards the region of high positive ion concentration (near the negative electrode) to equalize the charge distribution. As a result, the region near the anode swells and the region near the cathode de-swells, leading to stresses which cause the EAP strip to bend towards the positive anode.

4. Conclusions

 

  • Perfluorsulfonate polymers can be used to form electroactive polymer composites that deform in response to an applied electrical potential across an EAP composite. These polymers can deform both in an air and while immersed in water.
  • The EAP must remain moist in order to preserve its electromechanical properties.
  • The EAP is capable of lifting weight many times its own weight (up to a ratio of 70 gm lifted/g EAP) for the EAP studied in the current project.
  • The increase in the EAP response time was higher as the applied voltage increased reaching about 7 deflection degrees/sect at an applied voltage of 3.5 V.
  • A mechanism for the deformation of the EAP is postulated and is the basis for a number of suggestions for optimizing future EAPs.
  • It is anticipated that over the next decade EAP artificial muscles will have a wide range of engineering, biomedical and industrial uses.

 

5. Future Directions

 

  • The performance and long-term stability of the EAP should be improved by designing a water impermeable surface. This will prevent the evaporation of water contained in the EAP and also reduce the potential loss of the positive counter ion when the EAP is operating while 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 think conductive layer.
  • Heat resistant EAP would be desirable to allow operation at higher voltages without damaging the internal structure of the EAP due to thee generation of heat in the EAP composite.
  • Develop EAP in different configurations (e.g., fibers and fiber bundles) in order to increase the range of possible modes of motion.

 

6. References

 

  1. Bar-Cohen, S. Leary, A. Yavrouian, K. Oguro, S. T. Tadokoro, J. Harrison, J. Smith and J. Su, "Challenges to the Transition of IPMC Artificial Muscle Actuators to Practical Applications" Material Research Society (MRS) Symposium: FF: Electroactive Polymers, Nov. 29- December 1, 1999, MS Document ID: 31295, MRS (1999).
  2. Bar-Cohen, Y., T. Xue, M. Shahinpoor, J. O. Simpson and J. Smith, "Low-Mass Muscle Actuators using Electroactive Polymers (EAP)", Proceedings of SPIE's 5th Annual International Symposium on Smart Structures and Materials, 1-5 March, 1998, San Diego, CA, Paper No. 3324-32.
  3. Escoubes, M. and M. Pineri, in "Perfluorinated Ionomer Membranes," A. Eisenberg and H. L. Yeager (EDs.), American Chemical Society, Washington (1982)
  4. Jager, E. W. H., O. Ingan's, and I. Lundstrom, "Microrobots for Micrometer-Size Objects in Aqueous Media: Potential Tools for Single Cell Manipulation,"Science, 30, Vol. 288, 2335-2338 (2000).
  5. "NASA Builds Muscles," Time Magazine, March 22, 1999, page 88.
  6. Otero, T.F., J.M.Sansitena, "Soft and wet conducting polymers for artificial muscle", J. of Advanced Materials, 10, 491 - 494 (1998).
  7. Shahinpoor, M. and K. J. Kim, "The Effect of Surface-Electrode Resistance on the Performance of Ionic Polymer-Metal Composite (IPMC) Artificial Muscles", Smart Materials and Structures Journal, Vol. 9, 543-551 (2000).
Last Updated ( Wednesday, 16 February 2011 23:01 )