MUSCLING INTO MOTION

In the movies, a person with bionic muscles is portrayed as someone with strength and speeds far superior to human capabilities. Bionic muscles, however, aren't necessarily science fiction: Recent developments in electroactive polymers (EAPs) that can stretch, contract, or bend based on the electrical stimulus may one day make such bionics possible.¹

Visco-elastic EAP materials can potentially provide actuation with a life-like response and more flexible configurations, as well as vibration and shock dampening. Some biological functions that might be adapted include landing softly like cats or traversing distances by hopping like a grasshopper. EAP-based actuators could eliminate the need for gears, bearings, and other components that complicate the construction of robots, which would reduce cost, weight, and premature failures. As this technology evolves, EAPs will very likely be part of other novel biologically inspired mechanisms in commercial products and medical devices.

Active polymers initially received relatively little attention due to their limited actuation capability. In the last 10 years, however, more effective EAPs have emerged. Current EAPs can be divided into two major groups: ionic materials, which are actuated by ionic mobility, and electronic materials, which are actuated by electric fields. Ionic EAPs have the advantage of requiring only 1 to 3 V and milliampere currents for operation, but they are slower and face electrochemical and mechanical challenges. In contrast, electronic EAPs are more robust, but they require several milliampere currents and voltages of 100 to 150 V/µm, which means thousands of volts because the films are at least 10 to 20 µm thick. Researchers are gaining ground in reducing the needed voltage, but there is still progress to be made. The electronic EAPs induce larger forces than the ionic ones (except for the carbon nanotubes), up to 300 MPa. Most of the ionic types are made as bending actuators; whereas the electronic ones are stretched upon activation.

The ionic EAP gripper grabs objects using 2 to 3 V to open and close the fingers. (JPL)

Researchers have demonstrated a number of unique mechanisms and devices using EAPs, including a catheter steering element used to guide a catheter tip into an artery, a robotic arm, a gripper (see figure), and a device designed to wipe dust off a window of a small robot rover. When stimulated by a potential difference of less than 3 V, the ion-exchange membrane metallic composite (IPMC) wiper blade bends in a complete loop.

Conducting polymers, including those used for electroactive polymer actuators, are remarkably versatile. Designers could make entire devices, including sensors, actuators, light-emitting diodes (LEDs), batteries, electronic components, and structural elements using just one class of materials. Combining the photonic, transducing, sensing, and other characteristics of polymers with actuation capabilities offers enormous potential.

Unfortunately, the force actuation and mechanical energy density of EAPs are relatively low, limiting current applications. To foster global activity in this area, I have challenged the worldwide community of EAP experts to develop an EAP-actuated robotic arm to win an arm-wrestling match against a human opponent. Progress toward this goal will lead to significant benefits in a number of areas but particularly for medicine, including effective prosthetics. Through EAP technology, a person confined to a wheelchair today may someday be able to jog to the grocery store. oe

References

1. Y. Bar-Cohen (Ed.), Electroactive Polymer (EAP) Actuators as Artificial Muscles—Reality, Potential and Challenges, ISBN 0-8194-4054-X, SPIE Press, Vol. PM98, (March 2001).