Bipolar electrochemical tweezers using pristine carbon fibers with intrinsically asymmetric features

Structures that can be stimulated to change shape may be utilized for a variety of applications, but they frequently need to be processed and modified. We propose here a simple, straightforward strategy of actuation based on bipolar electrochemistry driving asymmetric reactions at the surface grooves of pristine carbon fibers. In the first set of proof-of-principle experiments, a free-standing carbon fiber is polarized in a closed bipolar cell to trigger asymmetric benzoquinone/hydroquinone redox reactions in the two distinct compartments. Beyond a particular threshold potential, ion transfer occurs, and the part of the fiber involved in the anodic reaction exhibits reversible directional motion. Elemental surface characterization of the polarized carbon fiber indicates that the deflection is due to the intercalation/deintercalation of ions accompanying the oxidation/reduction of the fiber. The simultaneous local surface ionic adsorption/desorption is responsible for the fiber deflection. The length of the fiber part exposed to the electrochemical reduction reaction in the opposite compartment of the closed bipolar cell, as well as the groove orientation, determines the motion's intensity and direction, respectively. Effective bending is achieved by optimization of fiber alignment and stimuli parameters. Actuation of two parallel fibers, oriented in opposite directions, leads to microtweezer-type behavior. We anticipate that these results will enrich the tool case for research in the field of soft robotics and micromechanics.