Artificial muscle for soft robotics

Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (HSEAS) have developed a dielectric elastomer with a broad range of motion that requires relatively low voltage and no rigid components. The team says this type of actuator could be used in everything from wearable devices to soft grippers, laparoscopic surgical tools, entirely soft robots or artificial muscles in more complex robotics.

The actuators that currently move soft robots tend to rely on hydraulics or pneumatics, which are slow to respond and difficult to store.

Dielectric elastomers, soft materials that have good insulating properties, could offer an alternative to pneumatic actuators but they currently require complex and inefficient circuitry to deliver high voltage as well as rigid components to maintain their form— both of which defeat the purpose of a soft robot. The HSEAS researchers’ dielectric elastomer is claimed to overcome these obstacles.

“We think this has the potential to be the holy grail of soft robotics,” said Mishu Duduta, a graduate student at HSEAS.“Electricity is easy to store and deliver but until now, the electric fields required to power actuators in soft robots has been too high.This research solves a lot of the challenges in soft actuation by reducing actuation voltage and increasing energy density, while eliminating rigid components.”

In building its dielectric elastomer, the team combined two known materials that worked well individually — an elastomer based on one developed at UCLA that eliminated the need for rigid components and an electrode of carbon nanotubes. The complementary properties of these two materials enabled the new device to outperform standard dielectric elastomer actuators.

Most dielectric elastomers have limited range of motion and need to be pre-stretched and attached to a rigid frame. Starting with an elastomer that doesn’t need to be pre-stretched, the modified materials begin as liquids and can be cured rapidly under UV light to produce paper-thin sheets. These sheets are sticky so they can adhere well to each other, and to the electrodes.

For the electrodes, the team replaced carbon grease, which is typically used as an electrode in dielectric elastomers, with a mat of thin carbon nanotubes. The nanotubes neither increase the stiffness of the elastomer nor decrease the energy density, meaning the elastomer can still stretch and provide significant force.The team fabricated the elastomers and electrodes one on top of the other, creating a multilayer sandwich.In this way, each electrode gets double usage, powering the elastomer above and below.

“The voltage required to actuate dielectric elastomers is directly related to the thickness of the material, so you have to make your dielectric elastomer as thin as possible,” explained Duduta. “But really thin elastomers are flimsy and can’t produce force. A multilayer elastomer is much more robust and can actually provide significant force.”

“Actuation is one of the most difficult challenges in robotics,” said Robert Wood, Professor of engineering and applied sciences at HSEAS. “The vast majority of existing robots rely on conventional electromagnetic rotary motors. In cases where we cannot use such motors, for example in soft robots, there are few alternatives for high performance actuation. This breakthrough in electrically-controlled soft actuators brings us much closer to muscle-like performance in an engineered system and opens the door for countless applications in soft robotics.”