A linear electrochemical actuator is described where at least one electrically conductive (between 400 and 1000 S/cm), doped polyaniline solid fiber or a yarn produced from such fibers is disposed in an electrolyte inside of a electrically conductive polyaniline hollow fiber, thereby allowing 2-electrode operation without a metal backing. This is an example of the electrochemical devices of the present invention having a solid-in-hollow polymer fiber configuration. In a propylene carbonate electrolyte, the electrochemical and actuation behavior of the fibers was found to be influenced by the solubility and size of the polymer dopants. That is, solubility of the dopant in the electrolyte resulted in high electroactivity and strain in the fibers. Actuation of fibers was also affected by electrolyte anions, small anions resulting in anion-exchange dominated actuation, while large anions resulted in cation-exchange dominated actuation. Isotonic strains of 0.9% and isometric stresses of 0.9 MPa were realized for the solid-in-hollow polyaniline fiber actuators.