NPG Asia Materials featured highlight | doi:

Biomimetic materials: Moveable muscles

Imagine being able to roll out a sheet of moveable machines as easily as one prints a newspaper.  Using carbon nanotube technology, Kinji Asaka and colleagues¹ have developed a thin polymer sheet that can be controllably bent back and forth using a low voltage signal.  Flexible, strong, and operable in air, these new polymer actuators show promise as low-cost manufacturing of various biomimetic materials, including artificial muscles.

Previous attempts at creating polymer actuators have relied upon oxidation/reduction chemistry and a liquid electrolyte to convert an electrical signal into mechanical motion.  In a different approach, Asaka’s team altered the actuator’s internal volume to make it bend back and forth.

The researchers mixed millimeter-long single-walled carbon nanotubes with an ionic liquid known as EMITFSI (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) to produce a gel that dried into highly conductive, free-standing sheets.  The completely ionized solvent associated strongly with the nanotubes, forming a mechanically strong material that readily transmits electrical signals.

The actuators were made by cutting two 15 × 1 mm-wide strips from the nanotube sheets to use as electrodes.  Next, the researchers sandwiched a soft polymer electrolyte film—containing EMITFSI and an inert fluoro-polymer—between the two electrodes.  Applying a voltage across the electrodes caused the ionic liquid to move within the actuator, thereby bending the material

“We assume that the large cations and small anions of the ionic liquid migrate across the electrolyte layer to the opposite electrode,” says Asaka.  “This leads to volume expansion or contraction of the electrode, resulting in bending motion.”

Using standard AAA batteries, the actuator was bent back and forth in air by changing the voltage polarity (Fig. 1).  The movement was varied from frequencies of 1 to 100 Hz without any deterioration of the actuator structure or its range of motion.

“The ion size determines the bending motion of the actuator,” says Asaka. “EMITFSI gave the best results in terms of actuation speed. Also since the precursors of our actuator are gelatinous mixtures, one may take advantage of printing techniques for the fabrication of actuator films, and eventually extend to, for example, a roll-to-roll manufacturing process.”

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.

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