The steadily growing field of soft robotics introduces the need to design entirely new components to replace those used in traditional, rigid robotic systems. One of the most immediate requirements is for innovative actuators that produce motion that is difficult or impossible to produce using standard components like motors, owing to the conceivably infinite number of degrees of freedom possible in soft systems.

The Whitesides Research Group at Harvard addressed the need for new actuation methods by developing PneuNets Bending Actuators [1], which are driven by pressurized fluid and are capable of widely varying types of motion depending on geometry and material. Actuators like these have properties that are heavily advantageous in robotics because of their extraordinary level of compliance, allowing interaction with objects of unknown geometry as well as safe interaction with humans; for example, one of the many demonstrations of this actuator involved grasping and lifting an egg without breaking it (see reference [1]).

However, pneumatic actuation introduces a multitude of constraints for application of the PneuNets technology. The Zhao Research Group, in working with small-scale robotics, cannot feasibly include the components required for PneuNets actuators (such as an air compressor and its power source) in the projects we work on; however, we saw the clear benefits of using actuators with similar characteristics to PneuNets in our systems.

We turned to another emerging technology, introduced in 2014 by Ray Baughman's group at the University of Texas at Dallas, [2] which is an artificial muscle that can be fabricated by coiling commercially available, inexpensive threads. Thermal expansion of the thread in its coiled state results in linear extension or contraction, depending on chirality, and can produce a force per weight superior to that of human muscle. Choice of an electrically conductive thread permits actuation by applying a voltage across the muscle, where Joule heating in the thread provides the driving force for thermal expansion.

Our idea was to combine the two technologies--PneuNets and the coiled-thread artificial muscle--to produce a soft actuator free from dependence on pneumatics. This allows the actuator to be used in systems in which weight reduction is critical (especially small robotic systems), where pneumatic components are not feasible or desired. Embedding the coiled muscles in a soft elastomer allows actuation similar to that of PneuNets simply by applying a voltage, with simpler and less expensive fabrication than what PneuNets require.


[1] F. Ilievski, A. D. Mazzeo, R. F. Shepherd, X. Chen, and G. M. Whitesides, “Soft Robotics for Chemists,” Angew. Chem. Int. Ed., vol. 50, no. 8, pp. 1890–1895, 2011.

[2] R. H. Baughman et al. "Artificial Muscles from Fishing Line and Sewing Thread." Science 343.6173 (2014): 868-72. Web.