IT IS difficult to imagine a world without electric motors. They are everywhere: spinning discs inside DVD players, powering washing machines and fans, keeping trains and escalators moving, starting car engines, waving windscreen wipers and making phones vibrate when new messages arrive. If you want to transform electrical energy into physical motion, you use a motor. But that could be about to change as a lighter, more flexible and less power-hungry alternative starts to muscle in, quite literally, on the motor's patch.

Researchers have been experimenting with artificial muscles—electroactive polymers that can expand or contract when a voltage is applied—for decades. But until recently researchers assumed that they would mainly be used, as their name suggests, to move the limbs of robots. It turns out, however, that with a few design twists they can be applied to many other tasks. Artificial muscles can provide force-feedback wobbles and clicks in mobile devices, for example, or move components inside autofocus camera lenses. With the right configuration of muscles it is even possible to produce rotary motion. And, just like motors, artificial muscles can also be used in reverse as generators, transforming movement into electricity.

There are two main types of electroactive polymer (EAP), which are usually referred to as ionic and dielectric (see diagram). An ionic EAP consists of a sponge-like polymer, soaked in a liquid electrolyte containing free-floating positive ions, and sandwiched between two electrodes. When a voltage is applied, the positively charged ions move towards the negative electrode, carrying electrolytic fluid with them. The sponge swells up on one side and shrinks on the other, which causes it to move. This kind of EAP can bend a lot, producing a large movement when a voltage is applied. A single muscle of this kind can be used to power a windscreen wiper, for example, by making it bend one way and then the other. Two or more such muscles can be configured as a gripper to pick up objects. But the electrolyte can evaporate, causing the muscle to dry out when exposed to the air. And ionic EAPs revert to their original shape as soon as the applied voltage is removed. Dielectric EAPs, by contrast, are easier to work with and have much more pulling and pushing power. They consist of a flexible polymer sandwiched between two electrodes. When a voltage is applied, positive charge builds up on one electrode and negative charge on the other, creating an attractive force that squeezes the polymer. As the polymer contracts in one direction, it expands in another, and these movements can do useful work. Unlike a rotary motor, which requires a crank arrangement to produce linear motion, an artificial muscle can do it directly, for example in a smartphone's autofocus mechanism, which requires tiny, precise linear movements. Moreover, a stack of several dielectric EAPs can generate substantial forces using little power. They are also light, which is one reason why America's space agency, NASA, is so interested in them, says Yoseph Bar-Cohen, a physicist and pioneer in the field at NASA's Jet Propulsion Laboratory in Pasadena, California. Brain and brawn Using artificial muscles in place of motors could reduce the size and weight of space probes and robots. “The next-generation space telescope will have to be significantly bigger,” says Dr Bar-Cohen. But doubling the diameter of the telescope's mirror, without changing its design, would increase its weight fourfold. So one idea being explored is to make inflatable space telescopes that use artificial muscles to unfurl the mirror and then, once it is fully extended, to adjust its optics, much like the muscles in an eye, he says.

Just how strong are these muscles? In 2005 Dr Bar-Cohen decided to find out by pitting a human against a trio of robotic arms, based on EAPs, in an arm-wrestling contest. In theory it should be possible to make EAPs as strong as human muscle, he says. (By weight, EAPs are 40 times stronger than human muscle, according to researchers at the Auckland Bioengineering Institute.) Even so, the person representing humanity in the contest—a 17-year-old high-school student from San Diego called Panna Felsen—took a matter of seconds to crush each of her three robotic opponents. To Dr Bar-Cohen this humiliating defeat is a clear sign that it is not how big your muscles are that counts, it is how you use them.

And it is precisely by using dielectric EAPs in a clever way that, earlier this year, a group of researchers in New Zealand managed to get muscles to do something that no living creature has managed to pull off: turn a wheel. With the possible exception of the propeller-like flagella of bacteria, there are no examples of organisms that use muscles to create continuous rotary motion. But a team led by Iain Anderson, the head of the Auckland Bioengineering Institute's Biomimetics Lab, has built an entire menagerie of muscle-powered motors. Although there are differences between them, the basic principle for each is the same. “The muscles manipulate the shaft of the motor like a finger and thumb would hold a pencil,” says Dr Anderson.

These rotary motors (see picture) resemble bicycle wheels that have had their spokes replaced with thin, black slivers of EAP. With at least six per motor, working as opposing pairs, these muscles are positioned between the outer rim and the central driveshaft. To make the shaft turn, the muscles work in concert, rhythmically contracting, one pair after another. As they do so, each pair applies pressure to a soft ring around the driveshaft. The pulsating muscles collectively and continuously pinch the shaft and apply a rotational force, causing it to turn.

Strictly speaking this is not the first time artificial muscles have been used to turn a wheel, says Dr Anderson. But it was previously done using a ratcheting mechanism which required heavy solid parts such as a clutch. His team's new design eliminates the need for anything rigid, such as bearings or gears. The breakthrough idea was to grip the shaft from both sides, which made bearings redundant, he says.

Dr Anderson concedes that these muscle motors are unlikely to start powering cars or trains any time soon. They cannot come close to electric motors in revolutions per minute, for a start. “But it has opened up a new design space,” he says. For example, by using multiple motors on the same shaft it is possible to get more than one degree of freedom. “Not only can they turn the shaft but they can lift it as well,” says Dr Anderson. This is hard to achieve with rigid bearings and could lead to strange new modes of locomotion, allowing robots to walk or roll, depending upon which is more appropriate.

“Using artificial muscles instead of motors could cut the size and weight of space probes and robots.”

As well as working on rotary motion, Dr Anderson's group is also investigating the use of EAPs as electrical generators. The pioneering work in this field was done by researchers at SRI International, formerly known as the Stanford Research Institute, a non-profit research outfit spun out of Stanford University that is based in Menlo Park, California, and holds many of the patents on EAPs.

Although SRI researchers have shown that it is possible to use artificial muscles to convert kinetic energy into electricity, getting the process started has always been a bit of a problem, because dielectric EAPs need a small initial charge in each cycle. SRI solved this problem using an external priming circuit, but Dr Anderson's team has found a way to integrate the primer into the EAP itself, making the device both soft and self-contained. These could be used to produce energy on a large scale, in wave-power generators, or on a small scale, by integrating small, lightweight devices into the heels of shoes. Previous research by SRI estimated that this arrangement could generate around one watt of power from normal walking.

Pressing forward

Using your shoes to recharge your phone as you walk has obvious appeal, though quite how the two would be connected is unclear. But the first product intended to put artificial muscles into consumers' hands will be aimed at video gamers. A company called Artificial Muscle based in Sunnyvale, California, which was spun out of SRI International, is about to launch a product called the Mophie Pulse, a “haptic” case for the iPod touch. It uses a technology called ViviTouch which is designed to provide gamers with realistic tactile feedback, letting them feel every tremor and explosion in a game or conveying more subtle sensations, like a heartbeat, says Marcus Rosenthal of Artificial Muscle.

Existing haptic devices use rotary motors with off-centre weights to generate such tremors. But this approach only allows vibrations within a narrow band of frequencies to be generated, says Mr Rosenthal. By using a pair of artificial muscles to move a weight and generate tremors, his firm's technology provides far more subtle control. “It's the difference between a doorbell and a speaker,” he says. And it uses one-third of the energy of a conventional haptic device. Beyond gaming, the technology can also be used to provide better sensory feedback when using touch screens. Existing devices can emulate a clicking sensation when the user touches an on-screen button, for example, but Artificial Muscle's approach can create many different types of click, depending on the kind of control being pressed.

Artificial muscles seem unlikely ever to be able to compete with hydraulic actuators for strength, or combustion engines for speed and torque. But in some applications, it seems, they could give the venerable electric motor a run for its money. And although they may yet find uses in Mars rovers and space telescopes, in the short term they would appear to have more potential inside mobile phones, cameras, game controllers and other consumer-electronics products. Having originated in the field of robotics, artificial muscles' real strength, it seems, may lie elsewhere.