Additive manufacturing When the muscle comes from the printer

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Empa researchers have developed a method to additively manufacture the soft, elastic but also powerful structures of artificial muscles. One day, they could be used in medicine or robotics - and anywhere else where things need to move at the touch of a button.

Developing artificial muscles that are in no way inferior to the real thing has been a major technical challenge to date. In order to keep up with their biological counterparts, artificial muscles must not only be strong, but also elastic and soft. Basically, artificial muscles are so-called actuators: Components that convert electrical impulses into movement. Actuators are used wherever something moves at the push of a button, whether at home, in a car engine or in highly developed industrial plants. However, these hard mechanical components do not yet have much in common with muscles.

A team of researchers from the Empa Laboratory for Functional Polymers is therefore working on actuators made of soft materials. Now, for the first time, they have developed a method for producing such complex components using a 3D printer. The so-called dielectric elastic actuators (DEA) consist of two different silicone-based materials: a conductive electrode material and a non-conductive dielectric. These materials interlock in layers. "It's a bit like interlacing your fingers," explains Empa researcher Patrick Danner. If an electrical voltage is applied to the electrodes, the actuator contracts like a muscle. When the voltage is switched off again, it relaxes to its original position.

Additively manufacturing such a structure is not trivial, Danner knows. Despite their very different electrical properties, the two soft materials should behave very similarly during the printing process. They must not mix, but still have to hold together well in the finished actuator. The printed "muscles" must be as soft as possible so that an electrical stimulus can lead to the required deformation. Added to this are the requirements that all 3D printable materials have to fulfill: They must liquefy under pressure so that they can be pressed out of the printer nozzle. Immediately afterwards, however, they must be viscous enough to retain the printed shape. "These properties are often in direct contradiction to each other," says Danner. "If you optimize one of them, three others change, usually to the detriment of the other."

Reconciling contradictory characteristics

In collaboration with researchers from ETH Zurich, Danner and Dorina Opris, head of the "Functional Polymeric Materials" research group, have succeeded in reconciling many of these contradictory properties. Two special inks, developed at Empa, are printed into functioning soft actuators using a nozzle developed by ETH researchers Tazio Pleij and Jan Vermant. The collaboration is part of the large-scale project "Manufhaptics", which is part of the ETH Domain's strategic focus area "Advanced Manufacturing". The aim of the project is to develop a glove that makes virtual worlds tangible. The artificial muscles are designed to simulate the gripping of objects through resistance.

If we make them a little thinner, we come pretty close to the way real muscle fibers work.

Dorina Opris

However, the soft actuators have far more potential applications. They are lightweight, noiseless and, thanks to the new 3D printing process, can be shaped as required. They could replace conventional actuators in cars, machines and robotics. If they are developed even further, they could also be used for medical applications. Dorina Opris and Patrick Danner are already working on this: their new process can be used to print not only complex shapes, but also long elastic fibers. "If we make them a little thinner, we can get pretty close to how real muscle fibers work," says Opris. The researcher believes that in the future it may be possible to print an entire heart from such fibers. However, there is still a lot to do before such a dream becomes a reality.