Toribio F. Otero

Soft and Wet Conducting Polymers for Artificial Muscles

Humans try to use their technology to reproduce devicesthat have been optimized by nature over thousands ofyears during the evolution of living creatures. Among thesesystems, muscles are elegant devices developed by evolu-tion to transform chemical energy to mechanical energyand heat at constant temperature, far away from the limita-tions imposed by Carnot’s theorem on our thermal motors.Proteins (polymers), small organic molecules, and inorgan-ic and organic ions in aqueous media constitute the mixedmaterials chosen by nature from which to construct mus-cles. The increase in Ca

Bilayer dimensions and movement in artificial muscles

Abstract A bilayer structure (polypyrrole/flexible and inactive polymer) was constructed. Reverse conformational changes promoted along polypyrrole chains during electrochemically induced oxidation-reduction processes were transformed to macroscopic angular movement of the bilayer. The movement of devices having different areas or different thicknesses of polypyrrole were quantified at different current densities. At all different device dimensions the movement rate was a linear function of the applied current per milligram of polypyrrole. The consumed electrical charge per milligram of polypyrrole to deflect through a constant angle was independent of the applied current density. At the same time the consumed electrical energy per milligram of polypyrrole increases linearly as a function of the applied current.Thus the movement rates, consumed charges and energies are not controlled by dimensions but by the masses of electroactive material.

A new model for electrochemical oxidation of polypyrrole under conformational relaxation control

Abstract The electrochemical behaviour of conducting polymers was modelled using a conformational relaxation treatment, defined as a rearrangement of the chain conformations along the polarization time. After relaxation other electrochemical processes are controlled and completed under diffusion control. The relaxation process is quantified by a relaxation time, which depends on the anodic overpotential (oxidation level) and the cathodic overpotential (closure and compaction of the structure related to the neutral state). Under these conditions and assuming nucleation-like kinetics for the advance of the oxidation front, anodic chronoamperograms for polypyrrole films in 0.1 M LiClO 4 + propylene carbonate solutions obtained by applying potential steps from different positive potentials to several negative values were simulated and fitted to experimental results.

Artificial muscles from bilayer structures

Abstract A different electrochemical behaviour of polypyrrole in different solvents is observed. Differences are attributed to the presence of interactions between solvent and polymer. These interactions change when the polymer is oxidized or reduced. The high polarity of water promotes an intense interaction with polarons and bipolarons. During oxidation counterions and water molecules penetrate in the polymer promoting an important swelling and molecular strain. Counterions and water molecules are expelled from the polymer during reduction promoting its contraction. The effect is lighter when different solvents are used. A bilayer structure polypyrrole-flexible and inactive polymer was constructed to transform molecular strain in macroscopic movements. An artificial muscle (electrochemomechanical device) was constructed. Electrical currents trigger oxidation or reduction (chemical) processes developing a mechanical movement. The movement can be stopped or inverted by stopping or inverting the current flow.

Electrochemical generation of polythiophene films on platinum electrodes

Abstract The formation and growth of polythiophene films on platinum electrodes has been undertaken using cyclic voltammetry, a square-wave potential and polarization at constant potential. A nucleation process promotes the formation of a polymer layer. The growth of the polymer layer initially causes an increase in the total conductivity of the polymer in spite of the continuous decrease in ionic conductivity necessary to retain film electroneutrality. At high polymer thickness, the lower ionic conductivity and the chemical activity loss of the film cause a drop in conductivity.

Electrochemical control of the morphology, adherence, appearance and growth of polypyrrole films

Abstract The influence of the electrochemical method employed to electrogenerate polymer films on morphology, appearance and adherence to a platinum electrode was studied. The use of a potential step, or cyclic voltammetry and square waves of potential at different frequencies, respectively, allows one to obtain a dendritic structure, with low adheence and presence of supporting electrolyte between the polymer and the metal, or flat, shiny, homogeneous and extremely adherent polymer films. Variations in height or width of the potential steps, and limits of the potential sweeps and sweep rate allowed the physical properties of the layer to be controlled progressively.

ARTIFICIAL MUSCLES BASED ON CONDUCTING POLYMERS

Abstract A general description of artificial muscles based on the reversible electrochemical oxidation/reduction reactions in conducting polymers and the concomitant reversible change of volume is presented. The construction of bilayers and multilayers of conducting polymer/flexible adherent polymer is presented. The reproducibility of the macroscopic movement was very high using bilayers showing deviations

A sensing muscle

The variation of the consumed electric energy by an electrochemo-mechanical triple layer actuator (polypyrrole film/non-conducting and adhesive film/polypyrrole film) can be used as sensor of the physical or chemical variables controlling the electrochemical reaction. Amongst others, the electrolyte concentration and the temperature of cell can be sensed. A physical variable that not participates in the electrochemical reaction, as the trailed weight by the triple layer during a bent can also be determined. The electric energy consumed during a constant movement of the triple layer changes linearly as a function of these variables. So, this artificial muscle works as an actuator and sensor simultaneously. This macroscopic muscle with sensing and actuating capabilities only require two wires connecting equipment and the device.

Fabrication of conductive electrospun silk fibroin scaffolds by coating with polypyrrole for biomedical applications

Scaffolds constituted by micro and nanofibers of silk fibroin were obtained by electrospinning. Fibers of fibroin meshes were coated with polypyrrole (pPy) by chemical polymerization; chemical linkages between polymers were observed by SEM and IR spectroscopy. Mechanical resistance of the meshes was improved by polypyrrole coating. Furthermore, coated meshes present a high electroactivity allowing anion storage and delivery during oxidation/reduction reactions in aqueous solutions. Uncoated and pPy coated materials support the adherence and proliferation of adult human mesenchymal stem cells (ahMSCs) or human fibroblasts (hFb). The bioactivity of fibroin mesh overcomes that of the polypyrrole coated meshes.

Biomimetic Conducting Polymers: Synthesis, Materials, Properties, Functions, and Devices

Electro-generations of conducting polymers are fast processes through a complex mechanism of parallel reactions giving mixed materials. The films are three-dimensional electrochemical reactors: reactive macromolecules, ions, and solvent. The film composition and its related properties reviewed here change by oxidation/reduction along several orders of magnitude under control. One reaction shifts different properties (multifunctionality). In one device several reaction driven tools, actuators and sensor, work simultaneously (full integration). The activation energy, the coefficient, and the reaction order shift with the initial state of packing conformations. Trapping charges and conformational energetic states constitute new multivalent chemo-conformational memories, close to brain memory.