Laura Valero

Biomimetic polypyrrole based all three-in-one triple layer sensing actuators exchanging cations

Simultaneous actuation and sensing properties of a triple layer actuator interchanging cations are presented for the first time. Thick polypyrrole (pPy)/dodecylbenzenesulfonate (DBS) films (36 μm) were electrogenerated on stainless steel electrodes. Sensing characteristics of pPy-DBS/tape/pPy-DBS triple layer artificial muscle were studied as a function of electrolyte concentration, temperature and driving current using lithium perchlorate (LiClO4) aqueous solution as electrolyte. The chronopotentiometric responses were studied by applying consecutive square waves of currents to produce angular movements of ±45° by the free end of the triple layer. The evolution of the muscle potential (anode film versus cathode film) during current flow is a function of the studied chemical and physical variables. The electrical energy consumed to describe a constant angle is a linear function of the working temperature or of the driving electrical current, and a double logarithmic function of the electrolyte concentration. Those are the sensing functions. The cation exchanging bending triple layer actuator senses the working conditions. Similar sensing functions were described in the literature for devices interchanging anions. Irrespective of the reaction mechanism, a single electrochemo–mechanical device comprised of two reactive polymer electrodes (oxidation film and reduction film) works simultaneously as both sensor and actuator (self-sensing actuators). These are the general sensing properties of dense and biomimetic reactive gels of conducting polymers. Thus, any reactive device based on the same type of materials and reactions (batteries, smart windows, actuators, electron–ion transducers) is expected to sense surrounding conditions, as biological organs do.

Structural Electrochemistry from Freestanding Polypyrrole Films: Full Hydrogen Inhibition from Aqueous Solutions

Free-standing polypyrrole fi lms, being the metal‐polymer contact located several millimeters outside the electrolyte, give stationary closed coulovoltammetric (charge/potential) loop responses to consecutive potential sweeps from ‐2.50 V to 0.65 V in aqueous solutions. The continuous and closed charge evolution corroborates the presence of reversible fi lm reactions (electroactivity), together high electronic and ionic conductivities in the full potential range. The closed charge loop demonstrates that the irreversible hydrogen evolution is fully inhibited from aqueous solutions of different salts up to ‐2.5 V vs Ag/AgCl. The morphology of the closed charge loops shows abrupt slope changes corresponding to the four basic components of the structural electrochemistry for a 3D electroactive gel: reduction-shrinking, reduction-compaction, oxidation-relaxation, and oxidation-swelling. Freestanding fi lms of conducting polymers behave as 3D gel electrodes (reactors) at the chain level, where reversible electrochemical reactions drive structural conformational and macroscopic (volume variation) changes. Very slow hydrogen evolution is revealed by coulovoltammetric responses at more cathodic potentials than ‐1.1 V from strong acid solutions, or in neutral salts self-supported blend fi lms of polypyrrole with large organic acids. Conducting polymers overcome graphite, mercury, lead, diamond, or carbon electrodes as hydrogen inhibitors, and can compete with them for some electro-analytical and electrochemical applications in aqueous solutions.

Exchanged cations and water during reactions in polypyrrole macroions from artificial muscles.

The movement of the bilayer (polypyrrole-dodecylbenzenesulfonate/tape) during artificial muscle bending under flow of current square waves was studied in aqueous solutions of chloride salts. During current flow, polypyrrole redox reactions result in variations in the volumes of the films and macroscopic bending: swelling by reduction with expulsion of cations and shrinking by oxidation with the insertion of cations. The described angles follow a linear function, different in each of the studied salts, of the consumed charge: they are faradaic polymeric muscles. The linearity indicates that cations are the only exchanged ions in the studied potential range. By flow of the same specific charge in every electrolyte, different angles were described by the muscle. The charge and the angle allow the number and volume of both the exchanged cations and the water molecules (related to a reference) between the film to be determined, in addition to the electrolyte per unit of charge during the driving reaction. The attained apparent solvation numbers for the exchanged cations were: 0.8, 0.7, 0.6, 0.5, 0.5, 0.4, 0.25, and 0.0 for Na(+), Mg(2+), La(3+), Li(+), Ca(2+), K(+), Rb(+), and Cs(+), respectively.

One Actuator and Several Sensors in One Device with only Two Connecting Wires: Mimicking Muscle/Brain Feedback

Artificial muscles based on conducting polymers, fullerene derivatives, carbon nanotubes, graphenes or other carbon derivative molecular structures are electrochemomechanical actuators. Electrochemical reactions drive most of the volume variation and the concomitant actuation. So under flow of a constant current, any working or surrounding variable influencing the reaction rate will be sensed by the muscle potential, or by the consumed energy, evolution during actuation. Experimental results and full theoretical description will be presented. The muscle potential is a well defined function of: driving current, volume variation (external pressure or hanged masses), temperature and electrolyte concentration. While working artificial muscles detect any change of whatever of those variables by changing either its potential or its consumed energy evolution. Experimental changes fit those predicted by the theoretical description. Only two connecting wires contain, simultaneously, actuating (current) and sensing (potential) signals. Those constitute new feedback intelligent and biomimetic devices opening new technological borders and mimicking natural muscles/brain communication.

Polymeric artificial muscles are linear faradaic motors

Engineers, physicists and robot designers use to consider polymeric bilayer actuators (or artificial muscles) as low reliable devices for soft tools or soft robotic developments. Here we present the mechanical (movement rate and position) characterization of a polypyrrole/tape bilayer bending actuator. The polypyrrole film was synthesized in presence of dodecyl-benzene-sulphonate (DBS-) and ClO4- anions: it exchanges cations during subsequent oxidation/reduction reactions. The angular rate of the movement results a linear function of the applied current and the described angle is a linear function of the consumed charge. The correlation coefficients overcame 0.99: electro-chemo-mechanical polymeric motors are full reliable for technological applications. The electrochemical model explaining the relationships between charge, film volume variation, mechanical work, force and displacement, strain and stress is also presented.