Wiggly gel makes a muscle.

Japanese scientists have made a polymer gel that moves in a lifelike way. Yoshihito Osada and his fellow chemists at Ibaraki University first made a gel of negatively charged polymers. They cut out a strip about the size of a matchstick, attached small hooks to it and suspended the gel strip in water. Because of its charged insides, the polymer swelled. The water also contained a surfactant, whose molecules had a positively charged end.

The researchers set up an electric field across the solution, changing its direction every 2 seconds. The polyment bent and straingthened with each change, wiggling forward at 10 inches per minute, they report in the Jan. 16 NATURE.

The polymer wiggles because alternate sides of the strip lose, and then regain, the ability to absorb water. The electric field causes surfactant molecules to migrate toward one end of the container. As these molecules encounter the polymer, they coat the near side of the strip. The positive charge of the surfactant cancels some of the gel's negative charge, causing that side of the polymer to lose osmotic pressure, purge water and shrink inward. When the scientists reverse the electric field, the surfactant molecules head in the opposite direction; thuthus, those attached to the polymer leave, while others link up to the other side of the gel, causing that side to shorten. As a result, the polymer straightens.