|So what else can be made to move by sheer chemical force. In what must be part 10 in our Making-It-Move sequel - Ma, Guo, Anderson and Langer are moving around a piece of water-responsive polymer film (DOI).|
A polymer film was created by Pyrrole electropolymerisation in a pentaerythritol ethoxylate / boron trifluoride etherate / isopropyl alcohol system. In this way polypyrrole (PPy) gets embedded in a polyol / borate interpenetrating network according to the idealised equation 2ROH + 2BF3 -> RO-BF2--OR + BF4- + H+. In this system both phases act as each other's counterion.
The polyol/borate phase is good at reversibly absorbing water. When a flat film is left on a moist surface the bottom part, absorbing water, swells more than the top part making the film buckle and move away from the surface. Due to mechanical instability the buckled film will eventually fall over and in a next phase the the other side of the film faces the wet surface and the process repeats itself. Hence, locomotion!.
The numbers: a cycle takes 5 minutes and can repeat itself hundreds of times. The force exerted is 80 times higher than mammalian skeletal muscle. The film can lift a 9.5 g object by 2 mm in 3 seconds. Optimal film thickness ranges between 15 and 40 micron.
In another exploit the film was slammed against a piezoelectric polyvinylidene difluoride film and again exposed to the wet surface. The repeated bending and stretching made the film generate a cool 5 nano Watt of power. This does not sound like much but the researchers are confident their invention will one day power a euphemistically called ultralow-power device. movie!