The unique capabilities of soft robots are to bend, deform, stretch, twist, or squeeze in all the ways that conventional rigid robots cannot. Today, it is easy to envision a world in which humans and robots collaborate — in close proximity — in many realms. Emerging soft robots may help to ensure that this can be done safely, and in a way that syncs to human environments, or even interfaces with humans themselves. Soft robotic systems can easily adapt to unstructured environments, or to irregular or soft surfaces such as the human body.

But despite their promise, to date, most soft robots move slowly and clumsily when compared with many conventional robots. The gap is narrowing thanks to new developments in the fundamental unit of robotic motion: the actuator. Responsible for the mechanical movement of a mechanism or a machine, actuators do their work in various ways, relying on electromagnetic, piezoelectric, pneumatic, or other forces.

Researchers have married the electromagnetic drives used in most conventional robotic systems with soft materials in order to achieve both speed and softness. The main challenge was to build an actuator that could achieve speeds greater than what have typically been possible with soft robotic actuators, many of which depend on slow processes such as airflow or thermal effects.

A small, soft actuator made of liquid metals and flexible polymers is the soft analog of an electromagnetic motor. (UCSB)

The work was based on the electromagnetic motor, a common type of fast and low-voltage actuator that is used in everything from electric cars to appliances, but has seen little effective application in soft robotic systems. The result is a type of actuator that is fast, low-voltage and soft, and also remarkably small — just a few millimeters in size. Using unique, liquid-metal alloy conductors encased in hollow polymer fibers and magnetized polymer composites, the researchers created patterned, three-dimensional components that form the basis of soft analogs of standard electrical motors. The fibers themselves are polymer composites that were engineered to have high thermal conductivity, greatly improving their performance.

The components are each soft and stretchable, and were combined to create the motor-like structures that can move things. To demonstrate, researchers created a tiny, millimeters-wide gripper that can close in just milliseconds, and a soft tactile stimulator that can operate at frequencies of hundreds of cycles per second.

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