It’s a creature made for the tropics: it creeps and loves humid air. Researchers in China have now created a new polymer-based device that moves along like a caterpillar when exposed to moisture in the air.
The device was developed by Junqi Sun and colleagues at Jilin University in Chanchun. As they described in their research paper, they took a hydrogel polymer film that absorbs water and stuck it to a flexible polymer support that didn’t respond to water. When the hydrogel layer absorbed water, it expanded as the humidity increased. The underlying polymer support stayed the same.
The idea of the polymeric caterpillar is similar to the bimetallic strip we learned about in high school physics to sense temperature. When the hydrogel expanded in the polymeric caterpillar, the device bent into an upside down “V.” When the humidity dropped, the hydrogel contracted and caused the two layers to relax into a straight line.
Sun and his colleagues controlled the humidity by blowing rounds of dry and moist nitrogen over the polymeric device. The relative humidity went as high as 40% and dropped as low as 11%. By attaching “claws” to each end of the two polymers, and putting the device on a rough source, the bilayer could inch along a straight line, bearing a load up to 120 times its weight.
As science writer James Mitchell Crow explains in his Chemistry World article:
A particular set of properties are needed for the hydrogel to form a powerful actuator, says Sun. It must retain its stiffness when swollen with water, have a high rate of moisture expansion, and rapidly absorb and desorb water. To achieve this, Sun used a polyelectrolyte multilayer layer (PEM) film, which is made from alternating layers of oppositely charged polymers. ‘We believe that many PEM films can be used to fabricate powerful actuators because the layer-by-layer assembly enables precise control of compositions, structures and properties of PEM films,’ Sun says.
Because relative humidity is hard to control outside laboratory conditions, Mitchell reports that Sun and colleagues are exploring other types of polymers that respond to different conditions, such as light or temperature.
Source: “Polymer caterpillar crawls in humid weather,” Chemistry World, 05/26/11
Rajendrani "Raj" Mukhopadhyay is a science writer and editor who contributes news stories and feature articles on scientific advances to a variety of magazines. Raj holds Ph.D. in biophysics from Johns Hopkins University.