A synthetic muscle has been developed that uses inflatable balloons to produce a tensile force from a positive pressurization. Mechanical supply and ducting is used to provide the required pressurization. However, the mechanical supply and ducting that is currently used is limited in several respects. To optimize rate of actuation, it is desirable to minimize the cell volume and to maximize the area through which fluid is forced into the cell. Ideally, a fully closed cell that is inflated through some mass transfer process that takes place through the cell boundary would lead to a maximum cell response rate.

With such a system, the transfer rate would scale with the cell surface area, i.e. square of diameter, and the fill-volume of the cell would scale with the cube of the diameter. The time required to fill the cell is the volume divided by the rate, which scales with the diameter of the cell. It follows that as the dimensions of the individual cells are reduced, the time required to fill the cell is proportionately reduced, i.e. very small cells fill very rapidly.

An ionic-selective membrane pump was developed to inflate balloons attached end-to-end in a chain. This actuation system offers advantages over the mechanical and chemical reaction actuation methods because this actuation system has a very simple design, no moving parts, and can be readily miniaturized for optimal system performance.

The synthetic muscle has an outer layer forming a closed shape with an interior. The outer layer is a non-permeable encapsulation material. An electrolyte — for example, a proton-containing electrolyte — fills the interior. A first electrode passes through the outer layer and extends into the interior. In addition, a second electrode passes through the outer layer and extends through the interior. The second electrode is fixed to the outer layer at two points. The second electrode does not move with respect to the outer layer at these two points. The first electrode may be an anode, and the second electrode comprises a cathode. In another embodiment, the first and second membranes are electrochemically reversible materials to facilitate reversal of charged particle flow through an ion exchange membrane.

The synthetic muscle also includes an ion exchange membrane disposed within the interior between the first and second electrodes. The ion exchange membrane is attached to the second electrode at multiple locations along a length of the second electrode between the two points of attachment of the second electrode to the outer layer. The ion exchange membrane may be a cationic selective membrane or a microporous membrane containing a polymer having charged pendant groups to provide a wall charge within each microchannel. The charged pendant groups include pendent sulfonic acid groups. The polymer is a sulfonated tetrafluorethylene copolymer.

The ion exchange membrane is a tubular sleeve, and the second electrode extends through the tubular sleeve. The number of locations at which the ion exchange membrane is attached to the second electrode defines a number of distinct pockets of the ion exchange membrane. The ion exchange membrane extends through the interior of the outer layer and is attached to the outer layer at the points of attachment of the second electrode.

The synthetic muscle also includes a power source in communication with both the first and second electrodes to provide the necessary power to the electrodes.

This work was done by Thomas J. Gieseke of the Naval Undersea Warfare Center. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp  under the Mechanics/Machinery category. NUWC-0010

This Brief includes a Technical Support Package (TSP).
Membrane Pump for Synthetic Muscle Actuation

(reference NUWC-0010) is currently available for download from the TSP library.

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