If a film of volume-changing material (e.g. the conjugated polymer) is attached to a film that does not change volume (such as a metal electrode or a film of an inert polymer), then this bilayer will bend. In the schematic below, the polymer (we use polypyrrole, PPy) is shown as green and the second layer (we uses gold, which then also serves as the working electrode) is shown as yellow. The volume of the polymer is changed by an applied electrochemical potential.

Here are some PPy/Au bilayers microfabricated on a silicon substrate (using surface micromachining and the differential adhesion method (link to patent), shown looking straight down from overhead.   I made these devices while I was at Linköping University in Sweden, working with Prof. Olle Inganäs.   The bilayers are 1 mm long and 30 µm wide, and they are attached on the bottom edge.   They are shown going from straight to curled up.   The cartoons on the left show side views of what is happening in the photographs.   The entire wafer is immersed in an aqueous electrolyte (0.1 M NaDBS) and serves as the working electrode; the counter and reference electrodes are not shown.

For further information see:

E. Smela, Olle Inganäs, and Ingemar Lundström, "Controlled folding of micrometer-size structures," Science, 268 (23 June), 1735-1738, 1995.

Photos in the figure are from E. Smela, Olle Inganäs, and Ingemar Lundström, “Controlled folding of micrometer-size structures," Science, 268 (23 June), 1735-1738, 1995.

To see a video of these bilayers in action, double-click on the image on the left

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For a Flash animation of a plate rotating (kindly made by Peter Warkentin), click here (this will take you off of my site but open in a separate page).

To see a video of such plates, microfabricated on Si, moving, double-click the image on the left, or click the play button.   In this video, the plates are 150 µm on each side, and the hinges are 30 x 30 µm. Hinges of that size were also able to rotate plates that were 1 mm on a side – these bilayers are strong (for measurements on these actuators, see JMEMS 1999.)

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By using multiple hinges and plates, more complex structures can be created. This is a schematic of a self-opening box. (For MEMS people, follow this link for the processing sequence.)

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For a video showing this self-folding box opening and closing in real time, double-click the image below.

These 2 videos show boxes folding around grains of sand.

This video shows a whole array of self-folding boxes.

Edwin Jager, who was co-supervised by me and Olle Inganäs at Linköping University, used this approach to realize a micro-robot arm. It was able to pick up and reposition a bead entirely under electrical control.

To our knowledge, this is still the only microfabricated robot arm ever made. You can see a video here.

The example on the left was made by surface micromachining, and the rigid plates were polymeric. Silicon plates defines by bulk micromachining can also be made (follow link for fabrication). After etching, this plate was attached to the wafer only by the PPy/Au bilayer. In the figure the plate was rotated out of the plane of the wafer by ~120 degrees.

Figure is from "Electrochemically Driven Polypyrrole Bilayers for Moving and Positioning Bulk Micromachined Silicon Plates," E. Smela, M. Kallenbach, and J. Holdenried, J. Microelectromechanical Systems, 8(4), 373-383, 1999.

The sequence above shows a second plate that has been held stationary at various angles by potentials at which the polymer is intermediate between the fully oxidized and reduced states.

Figure is from "Electrochemically Driven Polypyrrole Bilayers for Moving and Positioning Bulk Micromachined Silicon Plates," E. Smela, M. Kallenbach, and J. Holdenried, J. Microelectromechanical Systems, 8(4), 373-383, 1999.

This video shows this actuator holding a couple of fixed positions.

These actuators are robust: they operate even when they come into contact with macro-scale objects from the outside world.

For a video of earlier-generation bulk Si actuators, click below.

It is interesting to put an active device on a moving silicon plate; an example is presented here of a moving electrochromic device.

Figure is from "A Microfabricated Movable
Electrochromic 'Pixel' Based on Polypyrrole,"
E. Smela, Advanced Materials, 11(16) 1343-1345, 1999.

Polypyrrole changes color as well as volume when the oxidation level changes. The outer electrodes (black in the figure above) change the potential applied to the two PPy bilayer actuators (top to bottom), and a second electrode (yellow in the figure above) changes the potential of the PPy film on the plate (side to side, center row).

Figure is from "A Microfabricated Movable Electrochromic 'Pixel' Based on Polypyrrole," E. Smela, Advanced Materials, 11(16) 1343-1345, 1999.

For a video showing this device in action, click below.

For further information, see:

E. Smela, "A Microfabricated Movable Electrochromic
'Pixel' Based on Polypyrrole," Advanced Materials,
11(16) 1343-1345, 1999.

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For a video of the “flower”,
double-click below.

For a video of a hand moving a hair
(and then dropping it), double-click below.