Architecture and Building Technology Department

A first time report on the reactive sensing capabilities of polyaniline (PANI) is presented. Microfibrous silkfibroin mats were coated with polyaniline (PANI) through in situ chemical polymerization. The mat givesclosed coulovoltammetric responses in acidic aqueous solution, indicating that only reversible PANI oxi-dation/reduction reactions occur. Inside the reversible range the chronopotentiometric responses changewith (sense) the reaction variables: electrolyte concentration, pH, temperature and driving current. Thepotential of the materials, or the consumed electrical energy, for a constant reaction time follow the linearor semilogarithmic relationships with each of the experimental variables predicted by the electrochem-ical kinetics. The reversible charge from the closed coulovoltammetric loop also senses the chemical orthermal energetic conditions of the reaction acting on the conformational movements getting deeperoxidation states for rising energetic working conditions.

The reactivity of conducting polymers can be exploited to generate new sensors of their working conditions.
Here, we verified that the reactive (redox) nature of a chemically generated polypyrrole (PPy) on a
polyurethane (PU) microfibrous mat senses the surrounding (physical and chemical) conditions. The (PU/
PPy) microfibrous mat was obtained by in situ chemical polymerization of pyrrole from aqueous medium
using tetra fluoromethane sulfonate as dopant and a polyurethane microfibrous mat, produced by electrospinning
of a PU solution, as template. SEM images showed the homogenous growth of nanostructured
PPy on each fiber of the PU microfibrous mat. The cyclic voltammogram (CV) revealed an oxidation peak
at 0.22 V and a reduction peak at 0.08 V vs Ag/AgCl, indicating that the electrochemical activity of the
material is imparted by PPy component. Sensing abilities with regard to the surrounding conditions were
monitored through chronopotentiometric (CP) responses between two defined oxidation states. The consumed
electrical energy during the electrochemical reactions was a linear function of the applied currents
or of the working temperature (the reaction material is a current sensor and a temperature sensor) and a
semi-logarithmic dependence of the electrolyte concentration (concentration sensor). These sensing abilities
are explained on the basis of recently developed theories on the reactivity of conducting polymers.

Conducting polymers are soft, wet and reactive gels capable of mimicking biological functions. They are the
electrochemomechanical actuators having the ability to sense the surrounding variables simultaneously. The
sensing and actuating signals are sent/received back through the same two connecting wires in these materials. The
sensing ability is a general property of all conducting polymers arises from the unique electrochemical reaction
taking place in them. This sensing ability is verified for two different conducting polymers here – for an
electrochemically generated polypyrrole triple layer bending actuator exchanging cations and for a chemically
generated polytoluidine linear actuator exchanging anions. The configuration of the polypyrrole actuator device
corresponds to polypyrrole-dodecyl benzene sulfonate (pPy-DBS) film/tape/ pPy-DBS film in which the film on
one side of the triple layer is acted as anode and the film on the other side acted as cathode simultaneously, and the
films interchanged their role when move in the opposite direction. The polytoluidine linear actuator was
fabricated using a hydrgel microfiber through in situ chemical polymerization. The sensing characteristics of these
two actuators were studied as a function of their working conditions: applied current, electrolyte concentration and
temperature in aqueous electrolytes. The chronopotentiometric responses were studied by applying square electrical
currents for a specified time. For the pPy actuator it was set to produce angular movement of ± 45º by the free end
of the actuator, consuming constant charges of 60 mC. In both the actuators the evolution of the muscle potential
along the electrical current cycle was found to be a function of chemical and physical variables acting on the
polymer reaction rates: electrolyte concentration, temperature or driving electrical current. The muscle potential
evolved decreases with increasing electrolyte concentrations, increasing temperatures or decreasing driving
electrical currents. The electrical energy consumed during reaction was a linear function of the working
temperature or of the driving electrical current and a double logarithmic function of the electrolyte concentration.
Thus, the conducting polymer based actuators exchanging cations or anions during electrical current flow is a
sensor of the working physical and chemical conditions which is a general property of all conducting polymers.

The electrochemical activity of self-supported thick (70 µm) polypyrrole films electrogenerated in a mixed electrolyte of dodecylbenzenesulfonic acid and LiClO 4 was studied in aqueous solutions of LiClO 4 . This material inhibits hydrogen evolution up to cathodic potentials of −3.0 V versus Ag/AgCl. During oxidation or reduction reactions under galvanostatic conditions, the electrical energy consumed for a constant charge is a function of either the working temperature or the flowing current. As a consequence any electrochemical device, moreover artificial muscles, constructed with reactive conducting polymers is expected to work simultaneously as a sensor of the working temperature.

Films of conducting polymers can be oxidized and reduced in a reversible way. Any intermediate oxidation state determines an electrochemical equilibrium. Chemical or physical variables acting on the film may modify the equilibrium potential, so that the film acts as a sensor of the variable. The working potential of polypyrrole/DBSA (Dodecylbenzenesulfonic acid) films, oxidized or reduced under constant currents, changes as a function of the working conditions: electrolyte concentration, temperature or mechanical stress. During oxidation, the reactive material is a sensor of the ambient, the consumed electrical energy being the sensing magnitude. Devices based on any of the electrochemical properties of conducting polymers must act simultaneously as sensors of the working conditions. Artificial muscles, as electrochemical actuators constituted by reactive materials, respond to the ambient conditions during actuation. In this way, they can be used as actuators, sensing the surrounding conditions during actuation. Actuating and sensing signals are simultaneously included by the same two connecting wires.

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.

Devices formed by two triple layer artificial muscles of polypyrrole, working face to face in aqueous solution, producing longitudinal and valve movements are presented. Linear movements describing up to a 60% the length of the device were attained. Both, rate and sense of the movement are under control of the driving current. The position of the device is under control of the consumed charge. Any attained position is maintained after switching off the device. The device works simultaneously as an actuator and as a sensor of: the electrolyte concentration, the current flowing by the device, or the trailed weight.

The chemical synthesis of an EDOT derivative endowed with an electron acceptor anthraquinone moiety (AQ-EDOT) is described. The electrochemical polymerization of the monomer has been studied by cyclic voltammetry, chronoamperometry and chronopotentiometry. The monomer oxidation-polymerization takes places on platinum at potentials more positive than 1.3 V vs. Ag/AgCl. The polymer film presents a stable redox process with E 0 = 0.22 V, that can be assigned to the characteristic exchange process of the parent unsubstituted PEDOT polymer. An unstable redox process at E 0 = −1.00 V, present decreasing charges on the consecutive cycles despite that the lost reduction charge is recovered by two irreversible oxidation processes taking place at high anodic potentials 0.00 and 0.16 V. A structural charge trapping effects occurring by reduction at −1.1 V and re-oxidation at 0.16 V of the anthraquinone moiety is suggested. The stable redox process is not affected by cycling allowing the obtention of the oxidation empirical kinetics, kinetic coefficients and reaction orders. Different initial states attained by reduction at different cathodic potentials for a constant time were explored for the kinetic study.