Fiber Actuators/Artificial Muscles

Also known as muscle-like actuators are devices/materials that can expand/shrink in length, bend, or rotate in response to an external stimulus such as heat, electric field, magnetic field, light, fluid pressure, redox reactions, etc. I have worked on the following types of artificial muscles at a system level and also device level.

Thermal actuators: As the name suggests, thermal actuators actuate upon heating. The heat can be generated via different mechanisms such as passing a current through a conductive coating (i.e., Joule heating), light/matter interaction (i.e., high power laser), convective flow, etc. For the first time, I introduced niobium nanowire yarns as a promising alternative to carbon nanotube yarns. My colleagues and I showed that by infiltrating twisted niobium nanowire yarns with paraffin wax, we could make a torsional/linear actuator. Upon Joule heating, the paraffin wax melts and expands in volume by almost 30% which generates the driving force for the actuation (Figure A, B). Later, I showed the torsional actuation is not unique to paraffin-infiltrated nanowire yarns and we can generate torsional actuation from a shape memory alloy fine fiber twisted-yarn as well (Figure C). In a different project, my colleagues and I showed that by twist-coiling nylon 6,6 filaments and fibers we could make a very low-cost linear actuator that beats human’s muscles in terms of strain, output power, and energy density (figure D). Aside from linear and torsional actuators, I made a bending actuator from nylon 6,6 which can be controlled to bend in different directions (Figure E-G).

Figure – (A) Twisted niobium nanowire yarn actuator. (B) Niobium twist spun with a twist angle of 13°. (C) Diagram illustrating the working mechanism of an SMA torsional actuator. The bottom yellow part of the yarn represents the gold-coated section of the yarn. (D) SEM image of a twisted silver-plated nylon linear artificial muscle. (E) Schematic of the nylon multi-directional actuator. By heating one side of the actuator, the amorphous chains (red lines) shrink in length and the crystalline regions (blue lines) expand in volume. The result is the surface contraction of the beam at its heated surface which creates the bending motion. (F-G) The coordinate of the tip of the actuator by applying the periodic inputs.

Pneumatic actuators: These actuators work based on fluid pressure inside a compliant/expandable bladder. I have demonstrated torsional pneumatic actuators which can reversibly rotate by three full rotations and generate peak torsional speed of 1,800 RPM with a specific torque of 3.6 N·m/kg (similar to that of ungeared electric motors).

Electrochemical actuators: As side projects, I have made bending actuators with PEDOT and torsional actuators with niobium nanowire yarn. For PEDOT actuators, the electrochemical redox reaction is responsible for the actuation while for the niobium nanowire yarns, charge injection in the double layer.

More on the actuation mechanisms, applications, and limitations of the mentioned actuators can be found in our recent invited review paper.

Publications on this topic:

Some of our artificial muscle works were on news outlets including:

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