Yahya A. Ismail

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.

Hydrogel-Assisted Polyaniline Microfiber as Controllable Electrochemical Actuatable Supercapacitor

Flexible, controllable, and stable electrochemical supercapacitors serving as actuators at low operating voltage combining the advantages of the high power of the dielectric capacitors and the high specific energy of rechargeable batteries are important in artificial muscle technology, hybrid electric vehicles, and in short-term power sources for mobile electronic devices [Baughman, Science, 300, 268 (2003); Winter and Brodd, Chem. Rev. (Washington, D.C.), 104, 4245 (2004); Ebron, et al., Science, 311, 1580 (2006)]. High capacitance, a surprising 99% inner charge contribution and actuation in the hydrogel-assisted actuatable electrochemical supercapacitor (HAES) microfiber fabricated through wet spinning of a chitosan solution, followed by the in situ chemical polymerization of aniline was made possible through the perfect utilization of the large surface area provided by the nanostructured polyaniline grown inside as well as on the surface of the fiber. The HAES electrodes with an actuation strain of 0.33% showed 703 F/g specific capacitance in 1 M methane sulfonic acid, and more than 3000 cycles durability. The change in impedance as well as capacitance was achieved by the controlled strain as a function of applied stress, which can establish a direct relationship between the actuation strain and specific capacitance of electrochemical supercapacitors.

The fabrication of polyaniline/single-walled carbon nanotube fibers containing a highly-oriented filler

Highly uniform composite nanofibers composed of well-oriented single-walled carbon nanotubes (SWCNTs) wrapped in a conducting polymer have been fabricated using electrospinning. Water-soluble polyaniline (WS-PANI) was used as a conducting material to improve the processability during electrospinning. The WS-PANI formed a homogeneous dispersion with the SWCNTs and poly(vinyl alcohol), and good compatibility of the WS-PANI with the SWCNTs was demonstrated by data showing interactions between two components and the wrapping of the SWCNTs by the WS-PANI. Through transmission electron microscopy, atomic force microscopy, and polarized Raman spectroscopy, we confirmed that the WS-PANI plays an important role as a conducting polymer matrix to achieve aligned SWCNTs in composite nanofibers and to form uniform nanofibers.

Synthesis, Electrical, Electronic and Charge Transport Properties of Poly(aniline‐co‐p‐toluidine)

Copolymers of aniline with p‐toluidine were synthesized for different molar ratios of the respective monomers in acid medium. The electrical conductivity, charge transport and spectral characteristics upon incorporation of p‐toluidine units into the polyaniline backbone were investigated. The electrical conductivity of the copolymers showed frequency dependence which became more prominent with an increase in the number of p‐toluidine units in the polyaniline backbone. A direct relationship between the frequency dependence and electron localization was observed in the copolymers. Electronic spectra showed blue shifts in the π→π*and benzenoid→quinoid transitions revealing a decrease in the extent of conjugation in the copolymers. The protonated forms of the copolymers were soluble in DMSO giving polaron band around 400 nm. The decrease in electrical conductivity was attributed to the greater electron localizations as revealed from the broader ESR signals. Temperature dependence of electrical conductivity showed that charge transport was mainly through variable range hopping though a mixed conduction behavior was observed at higher temperature range.

Biomimetic Sensing – Actuators Based on Conducting Polymers

Thirty years after the discovery of conducting polymers (CP) and their electrochemical behavior (Chiang et al. 1977; Shirakawa et al. 1977), CP have been proven to be useful for applications in a variety of areas such as batteries and supercapacitors, photovoltaics, memory devices, light emitting diodes, artificial muscles (actuators), biomedical devices, biosensors, corrosion protection etc., thanks to their unique physical and electrochemical properties. Their physical characteristics are similar to those of commodity polymers; but unlike commodity polymers, CP can be subjected to oxidation and reduction just as in the case of metals and inorganic semiconductors. These reactions yield a complex material combination: polymer-ions-solvent. In the field of actuators, conductive polymers have been used with great success. During its oxidation/reduction the volume of the CP film changes. The volume variation can be used to perform a linear movement or an angular movement. The first CP actuators date back to 1992 (Baughman & Shacklette 1991; Otero et al. 1992a; Pei & Inganas 1992b). Since then, much progress has been achieved in materials generation and design and characterization of devices. Different configurations have been designed and studied, and different models were proposed to understand the properties and to improve the performance of those devices. In addition, due to the electrochemical nature of the actuation, any variable acting on the reaction will be detected and quantified by the actuator response; it means that working under constant current, the potential evolution will detect the change. This is a unique device: artificial muscles based on conducting polymers are also sensors, while working, of the environmental variables (Otero 2009). They are sensors of temperature, electrolyte concentration and the current flowing through them. Since they are capable of detecting an obstacle along its route and moving it to performing a work, they possess tactile properties (Otero & Cortes 2003). These unique features, coupled with the easy construction of actuators in many different sizes, from macro to microscopic devices, constitute a technological challenge. Some multinational companies, like Bayer, became involved in this new technology after acquisition of startup companies. The European Union is supporting the European Scientific

Synthesis, Electrical Conductivity, Spectral and Thermal Stability Studies on Poly(aniline-co-o-nitroaniline)

o-Nitroaniline units were incorporated in the polyaniline backbone through copolymerization with aniline. The copolymers were synthesized for 1:3 and 1:1 molar ratios of aniline and o-nitroaniline in acidic medium using potassium persulphate as oxidant and their properties were compared with that of polyaniline. The polymers showed less electrical conductivity than polyaniline. Unlike polyaniline, the presence of nitro group caused higher frequency dependence of electrical conductivity. Electronic spectra showed a blue shift in both the band of the copolymers due to the decrease in the extent of conjugation leading to lower conductivity, which could also be explained in terms of a decrease of delocalization of electron as evinced from electron para magnetic resonance (EPR) data. Thermo gravimetric analysis (TGA) revealed that copolymer derived from 1:1 molar ratio showed comparable thermal stability with polyaniline and the one derived from 1:3 molar ratios is thermally less stable than polyaniline. Activation energies for thermal degradation were estimated using Broido equation. The temperature dependence of electrical conductivity suggested charge transport is mainly through variable range hopping.

Polypyrrol/chitosan hydrogel hybrid microfiber as sensing artificial muscle

An electrochemical actuator demands that it should act as a sensor of the working conditions for its efficient application in devices. Actuation and sensing characteristics of a biopolymer/conducting polymer hybrid microfiber artificial muscle fabricated through wet spinning of a chitosan solution followed by in situ chemical polymerization with pyrrol employing bis(triflouro methane sulfonyl) imide as dopant and ferric chloride as a catalyst is presented. The polypyrrol/chitosan hybrid microfiber was investigated by FTIR, scanning electron microscopy (SEM), electrical conductivity measurement, cyclic voltammetric and chronopotentiometric methods. The electrochemical measurements related to the sensing abilities were performed as a function of applied current, concentration and temperature keeping two of the variables constant at a given time using NaCl as electrolyte. Cyclic voltammograms confirmed that the electro activity is imparted by polypyrrol (pPy). The fiber showed an electrical conductivity of 3.21x10-1 Scm-1and an average linear electrochemical actuation strain of 0.54%. The chronopotentiometric responses during the oxidation/reduction processes of the microfiber for the different anodic/cathodic currents and the linear fit observed for the consumed electrical energy during the reaction for various applied currents suggested that it can act as a sensor of applied current. The chronopotentiometric responses and the linear fit of consumed electrical energy at different temperatures suggested that the actuator can act as a temperature sensor. Similarly a semi logarithmic dependence of the consumed electrical energy with concentration of the electrolyte during reaction is suggestive of its applicability as a concentration sensor. The demand that an electrochemical actuator to be a sensor of the working conditions, for its efficient application in devices is thus verified in this material.

Conducting polymers are simultaneous sensing actuators

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 ± 45° 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.

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.