The Fascination of Technology Thread-like Motor Powers Soft Robots

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Ecole Polytechnique Fédérale de Lausanne (EPFL)

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In our “Fascination with Technology” section, we showcase impressive research and development projects for design engineers every week. Today: how a flexible motor, just a few millimeters thick, generates enough power to drive soft robots.

Engineers at the Soft Transducers Laboratory at EPFL developed the world’s first fiber-based pump as early as 2023. Its flexible tube can generate fluid pressure and flow on its own. It can be sewn into smart textiles to circulate fluids for thermoregulation and haptic feedback, or to power wearable assistive devices.
Now, with FiberMotor, the team has further developed its technology for use in textiles. It is a flexible motor that is only a few millimeters thick. The component, consisting of two nested fibers wrapped with electrodes, generates enough power to drive soft robots.

Our FiberMotor is the first fiber-format sliding motor. It generates enough force to drive devices for soft robotics applications, and is quiet, flexible, and bidirectional.

Herbert Shea, Head of the Soft Transducers Lab

The fiber-shaped motor converts electrical energy directly into motion. FiberMotor has a diameter of 1 to 3 mm (0.04 to 0.12 inches) and consists of two concentric hollow fibers. Each fiber is wrapped in tightly insulated copper wire electrodes that are only slightly thicker than a human hair. When a voltage is applied to these electrodes, electrostatic forces repeatedly align the fibers with one another. This causes the inner fiber to slide inside the outer one—similar to a telescope. “Our FiberMotor is the first sliding motor in fiber format,” says Herbert Shea, director of the Soft Transducers Lab. “It generates enough force to drive devices for soft robotics applications, and it is quiet, flexible, and bidirectional.” The work was published in *Advanced Materials*.

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Smooth, Natural Movements

Many everyday movements, such as lifting, pushing, and pulling, involve linear rather than rotational motion. Replicating this type of motion in wearable robotics has long been a challenge: motors with sufficient power often require rigid gears, transmission systems, and other bulky components.Although so-called artificial muscles have been developed, their range of motion is limited by how much their materials can stretch, contract, or bend. In contrast, the FiberMotor’s range of motion is limited only by the length of the fibers themselves.Since the device contains neither gears nor transmission systems, it can also be driven from the outside. This means that a counteracting force can move it safely. As a result, the motor adapts more naturally to human movements than conventional motors. For example, if the wearer moves unexpectedly or against the direction of the FiberMotor’s drive, the fibers simply slide past one another instead of jamming.

Soft Exosuits to Support Mobility

In laboratory demonstrations, individual FiberMotors were able to support stationary loads of about 75 grams (2.6 ounces). Four motors bundled together generated enough force to lift a 46-gram (1.6-ounce) chocolate bar and bend a cable-actuated robot finger.The team also integrated the FiberMotor into a prototype garment shaped to fit a knee. Since the motors are thin and lightweight, the researchers envision distributing them across a smart textile rather than concentrating the force in a single rigid motor unit.As a next step, the researchers aim to improve the FiberMotor’s performance and durability. To do this, they will use even thinner electrodes, consisting primarily of insulating materials, and optimize the materials used. In the long term, the combination of power and flexibility is expected to enable wearable assistive devices, including soft exosuits to support mobility, wearable haptic systems for virtual reality, and lightweight prosthetics. Lead author Sylvain Schaller has already founded the startup Elecsyor to commercialize the technology.Another research goal is to integrate feedback data on the wearer’s position and movement. This would allow the motor to quickly adapt its function to the wearer’s intentions.

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