Artificial Muscles [Preview]

Novel motion-producing devices--actuators, motors, generators--based on polymers that change shape when stimulated electrically are reaching commercialization.
Building Products

HAVING DEVELOPED a basic mechanism, the SRI team soon began work on a flood of application concepts:

Linear actuators. To make what they call spring rolls, the engineers wrap several layers of prestrained laminated dielectric elastomer sheet around a helical spring. The tension spring supports the circumferential prestrain, whereas the lengthwise prestrain of the film holds the spring compressed [see box on page 68]. Voltage makes the film squeeze in thickness and relax lengthwise so that the device extends. The spring rolls can therefore generate high force and stroke in a compact package. Kornbluh reports that automakers are interested in these mechanisms as replacements for the many small electric motors found in cars, such as in motorized seat-position controls and in the valve controls of high-efficiency camless engines.

Bending rolls. Taking the same basic spring roll, engineers can connect electrodes to create two or more distinct, individually addressed sections around the circumference. Electrically activating that section makes its side of the roll extend, so the entire roll bends away from that side [see box on page 68]. Mechanisms based on this design could engage in complicated motions that would be difficult to accomplish using conventional motors, gears and linkages. Possible uses would be in steerable medical catheters and in so-called snake robots.

Push-pull actuators. Pairs of dielectric elastomer films or of spring rolls can be arranged in a “push-pull” configuration so that they work against each other and thus respond in a more linear (“one input yields one output”) fashion. Shuttling voltage from one device to the other can shift the position of the whole assembly back and forth; activating both devices makes the assembly rigid at a neutral point. In this way, the actuators act like the opposing bicep and tricep muscles that control movements of the human arm.

Loudspeakers. Stretch a dielectric elastomer film over a frame that has an aperture in it. Expanding and contracting rapidly according to the applied voltage signal, the diaphragm will then emit sound. This configuration can yield a lightweight, inexpensive flat-panel speaker whose vibrating medium is both the driver and sound-generating panel. Current designs offer good performance in the mid- and high-frequency ranges. The speaker configuration has not yet been optimized as a woofer, although no obstacle prevents it from operating well at low frequencies [see box on preceding page].

Pumps. The design of a dielectric elastomer diaphragm pump is analogous to that of a low-frequency loudspeaker to which engineers have added a fluid chamber and two one-way check valves to control the flow of liquid. Artificial muscles are well suited to powering microfluidic pumps, for example, on the lab-on-a-chip devices prized by medicine and industry.

Sensors. Because of their nature, all SRI's dielectric elastomer devices exhibit a change in capacitance when they are bent or stretched. Thus, it is possible to make a sensor that is compliant and operates at low voltage. According to Kornbluh, the team came close to getting an automaker to adopt the technology as a sensor for measuring the tension of a seat belt. Such sensors could similarly be incorporated in fabrics and other materials as fibers, strips or coatings, he says.

Surface texturing and smart surfaces. If the polymers are imprinted with patterns of electrodes, arrays of dots or shapes can be raised on a surface on demand. This technology might find use as an active camouflage fabric that can change its reflectance as desired or as a mechanism for making “riblets” that improve the aerodynamic drag characteristics of airplane wings [see box on opposite page].

Power generators. Again, because these materials act as soft capacitors, variable-capacitance power generators and energy harvesters can be built from them. DARPA and the U.S. Army funded development of a heel-strike generator, a portable energy source that soldiers and others in the field could use to power electronic devices in place of batteries. An average-size person taking a step each second can produce about a watt of power using a device now under development [see box above]. Von Guggenberg says this concept has caught the interest of footwear companies. The devices could similarly be attached to backpack straps or car-suspension components. In principle, this approach could also be applied to wave generators or wind-power devices.

SRI researchers have tested a more radical concept—“polymer engines.” Propane fuel was burned inside a chamber, and the pressure from the resulting combustion products distorted a dielectric elastomer diaphragm, generating electricity. Such designs might eventually lead to efficient, extremely small generators in the centimeter-or-less size range.

But truly marketable products are still to come. “At this point we’re building turnkey devices that we can place in the hands of engineers so they can play with them and get comfortable with the technology,” von Guggenberg notes. “We hope it's just a matter of time before every engineer will consider this technology as they design new products.”

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