Auckland Bioengineering Institute


Artificial muscle technology

Ben O'Brien introduces artificial muscle technology and explains why the Biomimetics Group is interested in this area of research.

The Biomimetics Group at the Auckland Bioengineering Institute draws inspiration from nature to develop new technology for use in the healthcare and prosthetic industries. A key focus of the group's research is dielectric elastomer artificial muscles. This article answers the common question: 'why investigate artificial muscles'?

Mimicking muscle contraction with dielectric elastomers

Dielectric elastomers consist of a compliant electrical insulator sandwiched between two electrically conducting electrodes. The insulator is typically made of silicone rubber and the electrodes of carbon grease. When a high voltage is applied across the electrodes, charge accumulates in each electrode leaving positive charges in one and negative charges in the other. These positive and negative charges attract, squashing the insulating membrane, and, because the membrane is incompressible, they cause it to expand in area. (see Figure 1)

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Figure 1 Dielectric elastomer showing expansion in area when a high voltage is applied

Linear actuators

A linear actuator is a device that converts energy into a straight line motion. This is compared with “rotary actuators”, such as a car engine, that convert energy into rotational motion (the piston in a car engine would be a linear actuator). Most actuators excel in one area, but are weak in others. For example, the car engine is good at producing high power, but is fuel-inefficient, heavy and noisey.

Dielectric elastomers are called artificial muscles because they have good all-round characteristics not demonstrated by other linear actuators. They are fast, strong, can achieve large extensions, quiet, and very light. In fact, by weight, they are 40 times more powerful than human muscle.

Historically if a designer wanted something to move in a straight line, she would need to use a rotary actuator and convert the rotational motion into a straight line motion. This conversion process requires extra components in the form of gears, or cranks, or cams; adds mass, volume, cost and complexity to the actuator; introduces more places for wear or failure; and reduces efficiency.

By using an artificial muscle (linear actuator) this conversion process can be eliminated, improving the efficiency, weight, volume, complexity, price, reliability, and robustness of the system. However, with artificial muscles it can get even better than that.

Real-world applications

Due to the viscoelastic nature of artificial muscles they can act as suspension systems, removing the need for external suspension. As they are soft they can be used around humans without fear of injury. Their miniturisability makes them ideal for small, portable equipment like cameras.

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<p> <strong>Figure 2</strong> The ctenophore propeles itself through the water by the action of millions of cilia on its surface [1]</p>

Our group has conducted research into making the muscles smart and self-sensing. This means that the muscle can sense its own extension, eliminating the need for external sensors and further reducing the cost and complexity of the systems. Just like human muscles, our artificial muscles can feel where they are with their eyes closed.

Artificial muscles can also be run in reverse to generate power. Imagine being able to walk up a hill with a leg-assisting device, helping you overcome your disability, or carry a heavy load. On the way down the hill the device could run in reverse to recharge its batteries, just like an electric car.

The applications of artificial muscles are many and varied including use in prosthetics and strength-assist devices. This author's research focuses on the development of large arrays of micro-sized artificial muscles all working together to achieve larger goals. Example outcomes of this research might include: carrying objects like a crowd surfer; moving fluid like cilia; propelling robots through water like ctenophore; or carrying a robot like a centipede.

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<p> <strong>Figure 3</strong> Prototype cilia array: Inspired by the ctenophore. These artificial muscle cilia wave and could one day propel underwater micro-robots.</p>

Current research

The Biomimetics Group is currently focusing on the fine control of artificial muscles to generate power. Group members are also investigating human machine interfacing where nerve signals are interpreted from a human and used to drive artificial muscle prosthetic devices.

A few artificial muscle products are already on the market, over the next 10 years you can expect to see a lot more.

Further reading

For more information on this topic please see Todd Gisby's technical overview, 'What are artificial muscles and how do they work?'.

References

  1. Ctenophore photo courtesy of Dr Iain Anderson, i.anderson@auckland.ac.nz